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Shrubs
Kinds of Shrubs Terms modified by Shrubs Selected AbstractsPARENTAL EFFECTS AND GENDER SPECIALIZATION IN A TROPICAL HETEROSTYLOUS SHRUBEVOLUTION, Issue 3 2000Germá;n Avila-Sakar Abstract., Male sterility in hermaphroditic species may represent the first step in the evolution toward dioecy. However, gender specialization will not proceed unless the male-sterile individuals compensate for fitness lost through the male function with an increase in fitness through the female function. In the distylous shrub Erythroxylum havanense, thrum plants are partially male-sterile. Using data collected throughout eight years, we investigated whether thrum individuals have an increased performance as female parents, thereby compensating for their loss of male fitness. We found that thrum plants outperformed pins in the probabilities of seed maturation and germination and long-term growth of the seedlings. In turn, pollen from pin plants achieved greater pollen tube growth rates. Our results suggest that the superior performance of the progeny of thrum maternal plants is a consequence of better seed provisioning via effects of the maternal environment, cytotype or nuclear genes. Overall, our results suggest that E. havanense is evolving toward a dioecious state through a gynodioecious intermediate stage. This evolutionary pathway is characterized by an unusual pattern of gender dimorphism with thrums becoming females and pins becoming males. We propose that this pattern may be better explained by the interaction between male-sterility cytoplasmic genes and the heterostyly supergene. [source] Threatened Peripheral Populations in Context: Geographical Variation in Population Frequency and Size and Sexual Reproduction in a Clonal Woody ShrubCONSERVATION BIOLOGY, Issue 3 2007SARAH B. YAKIMOWSKI especies en riesgo; límites de distribución; poblaciones periféricas; reproducción sexual; Vaccinium stamineum Abstract:,Geographically peripheral populations of widespread species are often the focus of conservation because they are locally rare within political jurisdictions. Yet the ecology and genetics of these populations are rarely evaluated in a broader geographic context. Most expectations concerning the ecology and evolution of peripheral populations derive from the abundant-center model, which predicts that peripheral populations should be less frequent, smaller, less dense, and have a lower reproductive rate than central populations. We tested these predictions and in doing so evaluated the conservation value of peripheral populations for the clonal shrub Vaccinium stamineum L. (Ericaceae, deerberry), which is listed as threatened in Canada. Based on 51 populations sampled from the center to the northern range limits over 2 years, population frequency and size declined toward the range limit, but ramet density increased. Sexual reproductive output varied widely among populations and between years, with many populations producing very few seeds, but did not decline toward range margins. In fact seed mass increased steadily toward range limit, and this was associated with faster germination and seedling growth, which may be adaptive in seasonal northern environments. Our results did not support the prediction that clonal reproduction is more prevalent in peripheral populations or that it contributed antagonistically to the wide variation in seed production. Peripheral populations of V. stamineum are as productive as central populations and may be locally adapted to northern environments. This emphasizes the importance of a broad geographical perspective for evaluating the ecology, evolution, and conservation of peripheral populations. Resumen:,Las poblaciones geográficamente periféricas de una especie de amplia distribución a menudo son el foco de conservación porque son raras localmente dentro de jurisdicciones políticas. Sin embargo, la ecología y genética de estas poblaciones son evaluadas poco frecuentemente en un contexto geográfico más amplio. La mayoría de las expectaciones relacionadas con la ecología y evolución de las poblaciones periféricas se derivan del modelo centro-abundante, que predice que las poblaciones periféricas son menos frecuentes, más pequeñas, menos densas y menor tasa reproductiva que poblaciones centrales. Probamos estas predicciones y al hacerlo evaluamos el valor de conservación de poblaciones periféricas de una especie de arbusto clonal (Vaccinium stamineum L., Ericaceae), que está enlistada como amenazada en Canadá. Con base en 51 poblaciones muestreadas del centro hacia los límites norteños de su distribución durante 2 años, la frecuencia y tamaño poblacional declinó hacia los límites de su distribución, pero la densidad de rametos aumentó. La reproducción sexual varió ampliamente entre las poblaciones y entre años, con muchas poblaciones produciendo muy pocas semillas, pero no declinó hacia los límites de su distribución. De hecho, la masa de semillas incrementó sostenidamente hacia los límites, y esto se asoció a una acelerada germinación y crecimiento de plántulas, lo cual puede ser adaptativo en ambientes norteños estacionales. Nuestros resultados no sustentaron la predicción de que la reproducción clonal es más prevaleciente en poblaciones periféricas o que contribuye antagónicamente a la amplia variación en la producción de semillas. Las poblaciones periféricas de V. stamineum son tan productivas como las poblaciones centrales y pueden estar adaptadas localmente a ambientes norteños. Esto enfatiza la importancia de una perspectiva geográfica amplia cuando se evalúa la ecología, evolución y conservación de poblaciones periféricas. [source] Restoration of a Forest Understory After the Removal of an Invasive Shrub, Amur Honeysuckle (Lonicera maackii)RESTORATION ECOLOGY, Issue 2 2004Kurt M. Hartman Abstract The recruitment of native seedlings is often reduced in areas where the invasive Amur honeysuckle (Lonicera maackii) is abundant. To address this recruitment problem, we evaluated the effectiveness of L. maackii eradication methods and restoration efforts using seedlings of six native tree species planted within eradication and unmanipulated (control) plots. Two eradication methods using glyphosate herbicide were evaluated: cut and paint and stem injection with an EZ-Ject lance. Lonicera maackii density and biomass as well as microenvironmental characteristics were measured to study their effects on seedling growth and survivorship. Mean biomass of Amur honeysuckle was 361 ± 69 kg/ha, and density was 21,380 ± 3,171 plants/ha. Both eradication treatments were effective in killing L. maackii (, 94%). The injection treatment was most effective on large L. maackii individuals (>1.5 cm diameter), was 43% faster to apply than cutting and painting and less fatiguing for the operator, decreased operator exposure to herbicide, and minimized impact to nontarget vegetation. Deer browse tree protectors were used on half of the seedlings, but did not affect survivorship or growth. After 3 years, survival of native seedlings was significantly less where L. maackii was left intact (32 ± 3%) compared with the eradication plots (p < 0.002). Seedling survival was significantly different between cut (51 ± 3%) and injected (45 ± 3%) plots. Species had different final percent survival and rates of mortality. Species survival differed greatly by species (in descending order): Fraxinus pennsylvanica > Quercus muehlenbergii , Prunus serotina, Juglans nigra > Cercis canadensis > Cornus florida. Survivorship and growth of native seedlings was affected by a severe first-year drought and by site location. One site exhibited greater spring soil moisture, pH, percent open canopy, and had greater survivorship relative to the other site (55 ± 2 vs. 30 ± 2%). Overall, both L. maackii eradication methods were successful, but restorationists should be aware of the potential for differential survivorship of native seedlings depending on species identity and microenvironmental conditions. [source] Changes in Sexual Expression as Result of Defoliation and Environment in a Monoecious Shrub in Mexico: Implications for PollinationBIOTROPICA, Issue 4 2009Gerardo Arceo-Gómez ABSTRACT Folivory may indirectly impact plant reproduction through changes in sexual expression (i.e., number or proportion of male and female flowers produced), which influence plant,pollinator interactions via changes in pollinator preference or efficiency. This study is an experimental evaluation of the effect that defoliation has on sex expression in the monoecious shrub Cnidoscolus aconitifolius, how such effect varies across sites, as well as how such changes indirectly affect pollinator visitation rates. The present study used three populations of C. aconitifolius, each one located in a different site in Yucatán (México): pasture, deciduous forest and subdeciduous medium height forest and three levels of defoliation: 50 percent, 100 percent, and a control (no damage). Results showed that defoliation reduced significantly the total number of male flowers produced in two of the sites. Defoliation did not impact female flower production or the proportion of female flowers produced. Finally, floral visit rates were not affected by defoliation via changes in sexual expression and neither by site or by the interaction site × defoliation. Findings showed that defoliation had an effect on sex expression in C. aconitifolius, although apparently this change did not affect the plant,pollinator interactions. [source] Implications of Climatic Warming for Conservation of Native Trees and Shrubs in FloridaCONSERVATION BIOLOGY, Issue 4 2001David W. Crumpacker Climatic-envelope models are useful for simultaneous investigation of many plant species whose range-limiting mechanisms are poorly known. They are most effectively applied in regions with strong temperature and moisture gradients and low relief. Their required databases are often relatively easy to obtain. We provide an example involving the effect of six annual warming scenarios, ranging from +1° C to +2° C and from +10% to ,20% annual precipitation (some have greater warming in winter than in summer), on 117 native woody species in Florida (U.S.A.). Tree species at their southern range boundaries in several parts of Florida are likely to be negatively affected by as little as 1° C warming if it is greater in winter than in summer or is accompanied by a 20% decrease in annual precipitation. Potential species responses to an identical type of 1° C warming may be different for some conservation areas in the same region of Florida. Potentially extensive disruption of some major woody ecosystems is predicted under certain types of 1° C annual warming and under all types of 2° C annual warming that were investigated. Additional consideration of nonclimatic factors suggests that many potential effects on species and ecosystems are not underestimates of actual effects over a 100-year period of warming. We recommend monitoring for decreased fertility and viability of ecologically important, temperate woody species near their southern range limits in Florida. Early detection of such changes in fitness might then provide time for mitigations designed to alleviate more serious subsequent effects on biodiversity. Control of invasive, non-native plant species and prevention of their additional introduction, human-assisted translocation of native subtropical plant species into previously temperate parts of Florida, and restoration of more natural hydrological regimes are examples of potentially useful mitigations if climatic warming continues. Resumen: Los modelos de procesos ecológicos y los modelos empíricos han sido usados para relacionar predicciones de cambio climático con los efectos en especies de plantas y vegetación. Los modelos climáticos son útiles para la investigación simultánea de muchas especies de plantas cuyos mecanismos limitantes de rango son poco conocidos. Estos modelos son más eficientemente aplicados en regiones con gradientes de temperatura y humedad fuertes y con relieve bajo. Las bases de datos requeridas son a menudo relativamente fáciles de adquirir. Proveemos un ejemplo que involucra el efecto de seis escenarios anuales de calentamiento con un rango de +1° C a +2° C y de +10% a ,20% de precipitación anual (algunos con rangos de calentamiento mayores en el invierno que en el verano), en 117 especies leñosas nativas de Florida ( E.U.A.). Las especies de árboles en sus límites de rango al sur en diversas partes de Florida son más factibles de ser negativamente afectadas por tan poco como 1° C de calentamiento, si este es mayor en el invierno que en el verano o si es acompañado por una disminución de un 20% de precipitación anual. Las respuestas potenciales de las especies a un tipo idéntico de calentamiento de 1° C puede ser diferente para algunas áreas de conservación en la misma región de Florida. Se predicen perturbaciones potencialmente extensivas en algunos ecosistemas leñosos principales investigados bajo ciertos tipos de calentamiento anual de 1° C y bajo todos los tipos de calentamiento anual de 2° C. Las consideraciones adicionales de factores no climáticos sugieren que muchos efectos potenciales sobre las especies y ecosistemas no son subestimaciones de los efectos actuales sobre un período de calentamiento de 100 años. Se recomienda el monitoreo de la disminución de la fertilidad y viabilidad de especies leñosas templadas ecológicamente importantes cerca de los límites sureños de sus rangos en la Florida. La detección temprana de estos cambios en adaptabilidad pueden proveer tiempo para mitigaciones diseñadas para aliviar efectos posteriores más serios en la biodiversidad. Algunos ejemplos de mitigaciones potencialmente útiles en caso de que el calentamiento global continúe incluyen el control de especies de plantas invasoras no nativas y la prevención de su introducción adicional, la translocación asistida por humanos de plantas nativas subtropicales en partes previamente templadas de Florida y la restauración de regimenes hidrológicos más naturales. [source] Xylem root and shoot hydraulics is linked to life history type in chaparral seedlingsFUNCTIONAL ECOLOGY, Issue 1 2010Robert B. Pratt Summary 1.,Shrubs in fire prone chaparral communities have evolved different life history types in response to fire. A key to understanding the evolution of life history type differences is to understand how physiological traits are linked to differences in life history type. Vascular adaptations are important for delivering an efficient and stable water supply to evergreen chaparral shrub leaves. This study tested for a link between vascular physiology and life history type in chaparral shrubs. 2.,Chaparral shrub species along the south-western coast of North America survive wildfire by three different life histories. Non-sprouters are killed by fire and re-establish exclusively through germination of fire-stimulated seeds, facultative sprouters re-establish by a combination of vegetative sprouting and fire-stimulated seeds, and obligate sprouters re-establish exclusively by vegetative sprouting because their seeds do not survive fire. Non-sprouters and facultative sprouters establish seedlings in the open canopy post fire environment, whereas obligate sprouters establish seedlings in the shady understory of the mature chaparral canopy. 3.,Seedlings of nine species (Rhamnaceae) representing three each of the different life history types were grown in deep containers in a common garden under treatments of sun and shade. Hydraulic conductance was measured using a high-pressure flow meter for all organs, and a vacuum technique was used to measure conductance of fine and woody roots. We predicted that non-sprouters would exhibit greater hydraulic efficiency than the sprouting species, and that facultative sprouters would be more efficient than the shade tolerant obligate sprouters. 4.,Non-sprouters had the greatest hydraulic conductance per unit leaf and sapwood area at the whole seedling level, whereas facultative and obligate sprouters were not different. Comparing hydraulic conductance across major organs (from fine roots to leaves) showed that the hydraulic system was well coordinated. At the whole seedling level, the root system was more of a bottleneck than the shoot system. This pattern was consistent with high resistance extraxylary pathways in roots and differences in root architecture. 5.,The greater hydraulic efficiency of the non-sprouter life history type is attributed to its post-fire pioneering habit and may partially explain the relatively high speciation in the non-sprouters. Lower hydraulic efficiency is associated with a sprouting life history and greater shade tolerance. The seedling root systems represent a hydraulic bottleneck that may place roots under especially intense selection. [source] Climate change in the Arctic: using plant functional types in a meta-analysis of field experimentsFUNCTIONAL ECOLOGY, Issue 1 2002C. F. Dormann Summary 1,The effects of global climate change are predicted to be strongest in the Arctic. This, as well as the suitability of tundra as a simple model ecosystem, has led to many field experiments investigating consequences of simulated environmental change. 2,On the basis of 36 experiments reviewed here, minor light attenuation by clouds, small changes in precipitation, and increases in UV-B radiation and atmospheric CO2 concentrations will not affect arctic plants in the short term. However, temperature elevation, increases in nutrient availability and major decreases in light availability will cause an immediate plant-growth response and alter nutrient cycling, possibly creating positive feedbacks on plant biomass. The driver of future change in arctic vegetation is likely to be increased nutrient availability, arising for example from temperature-induced increases in mineralization. 3,Arctic plant species differ widely in their response to environmental manipulations. Classification into plant functional types proved largely unsatisfactory for generalization of responses and predictions of effects. 4,Nevertheless, a few generalizations and consistent differences between PFTs were detected. Responses to fertilization were the strongest, particularly in grasses. Shrubs and grasses were most responsive to elevated temperature. 5,Future studies should focus on interactive effects of environmental factors, investigate long-term responses to manipulations, and incorporate interactions with other trophic levels. With respect to plant functional types, a new approach is advocated, which groups species according to their responses to environmental manipulations. [source] Shrub effects on herbs and grasses in semi-natural grasslands: positive, negative or neutral relationships?GRASS & FORAGE SCIENCE, Issue 1 2008A. Pihlgren Abstract The present study investigated how the abundance and sexual reproduction of herbs and grasses relates to the presence of shrubs of Rosa dumalis in three semi-natural pastures in Sweden. Shrubs may affect grassland plants negatively by competition, positively by serving as grazing refuge, or neutrally. At different distances from shrubs of R. dumalis, data were collected on plant abundance, frequency of reproductive shoots, vegetation height and litter depth. In one grassland, data were collected on seedling density and frequency of reproductive shoots in the presence and absence of grazing. The shrubs functioned as grazing refuges with taller vegetation, deeper litter and higher probability of reproduction by plants. The overall number of plant species remained the same at all distances from shrubs. Most species showed a neutral relationship with shrubs. Proportionately, 0·08,0·26 of the species showed a negative pattern to shrubs and 0·14,0·30 a positive pattern. Seedling density was negatively correlated with litter depth and peaked at 60,90 cm from shrubs. Establishment of seedlings of small-seeded species was negatively related to shrubs probably because of thicker litter layer close to shrubs. The observed patterns were compared with different functional traits, such as Ellenberg values, plant height, growth form and Raunkiaer life form. Plant height from data in the literature was the trait that best explained the relationship of plant species to shrubs because tall species were more common in proximity to shrubs. It was concluded that shrubs increase the heterogeneity in grasslands and that intensive shrub-clearing may negatively affect biodiversity. [source] Relay Cropping of Sorghum and Legume Shrubs for Crop Yield Improvement and Striga Control in the Subsistence Agriculture Region of Tigray (Northern Ethiopia)JOURNAL OF AGRONOMY AND CROP SCIENCE, Issue 1 2005F. Reda Abstract Striga hermonthica is a major constraint in the subsistence agriculture regions of northern Ethiopia. Low soil fertility and overall environmental degradation has contributed to the build up of the parasitic weed infestation. Improved cropping systems have to be introduced to address the interrelated problems of Striga and soil fertility decline. Thus, relay cropping of sorghum with legume shrubs was investigated at two locations representing different environments. Results showed that the output of the improved cropping system was dependent on ecological endowments. Relay cropping led to significant improvement in yield at Sheraro, at the site with relatively better weather and soil conditions. The legume shrubs resulted in significantly lower sorghum yield in a dryland location (Adibakel). Overall Striga infestation declined over the 3-year period; however, treatment differences were not apparent. Among the two legume shrubs, Sesbania sesban was better adapted to the dryland areas. Relay cropping could provide a viable option for farmers in both types of environments that are characterized by accelerated decline in natural resource base. However, it could mean compromising the yield of non-fertilized sorghum in the interest of long-term benefits of low incidence of Striga and more rewarding crop enterprise in dry areas. [source] Pollen Morphology of Tundra Shrubs and Submarginal Plants from Barrow, AlaskaJOURNAL OF INTEGRATIVE PLANT BIOLOGY, Issue 7 2006Ling-Yu Tang Abstract Investigation of plant morphological features, pollen, and habitat have been made for two shrub species from Barrow, Alaska, namely Dryas integrifolia M. Vahl and Salix rotundifolia Trautv., both of which are endemic to the Arctic floristic area. The former species has small lanceolate or plate leaves, whereas the latter has rounded leaves with distinct veins, rich in vitamin C. Both have dwarf and sprawling habits. Pollen studies showed that the pollen grains of the two species are spheroidal to sub-spheroidal or prolate. The type of aperture was tricolporate; pollen size 26.3,31.3 ,m; ornamentation finely reticulate under a light microscope (LM) and striate-reticulate under a scanning electron microscope (SEM) for D. integrafolia and finely reticulate under the LM and SEM for S. rotundifolia. Comparisons were made between the pollen from the same species from Arctic collections with those from China and Japan. Investigation of pollen morphology of tundra plants can provide significant data for comparative studies of fossil pollen and for the reconstruction of paleovegetation and paleoclimate in the Barrow area. (Managing editor: Ya-Qin Han) [source] Shrubs as ecosystem engineers in a coastal dune: influences on plant populations, communities and ecosystemsJOURNAL OF VEGETATION SCIENCE, Issue 5 2010J. Hall Cushman Abstract Question: How do two shrubs with contrasting life-history characteristics influence abundance of dominant plant taxa, species richness and aboveground biomass of grasses and forbs, litter accumulation, nitrogen pools and mineralization rates? How are these shrubs , and thus their effects on populations, communities and ecosystems , distributed spatially across the landscape? Location: Coastal hind-dune system, Bodega Head, northern California. Methods: In each of 4 years, we compared vegetation, leaf litter and soil nitrogen under canopies of two native shrubs ,Ericameria ericoides and the nitrogen-fixing Lupinus chamissonis, with those in adjacent open dunes. Results: At the population level, density and cover of the native forb Claytonia perfoliata and the exotic grass Bromus diandrus were higher under shrubs than in shrub-free areas, whereas they were lower under shrubs for the exotic grass Vulpia bromoides. In contrast, cover of three native moss species was highest under Ericameria and equally low under Lupinus and shrub-free areas. At community level, species richness and aboveground biomass of herbaceous dicots was lower beneath shrubs, whereas no pattern emerged for grasses. At ecosystem level, areas beneath shrubs accumulated more leaf litter and had larger pools of soil ammonium and nitrate. Rates of nitrate mineralization were higher under Lupinus, followed by Ericameria and then open dune. At landscape level, the two shrubs , and their distinctive vegetation and soils , frequently had uniform spatial distributions, and the distance separating neighbouring shrubs increased as their combined sizes increased. Conclusions: Collectively, these data suggest that both shrubs serve as ecosystem engineers in this coastal dune, having influences at multiple levels of biological organization. Our data also suggest that intraspecific competition influenced the spatial distributions of these shrubs and thus altered the distribution of their effects throughout the landscape. [source] Facilitation of tree saplings by nurse plants: Microhabitat amelioration or protection against herbivores?JOURNAL OF VEGETATION SCIENCE, Issue 2 2008Lorena Gómez-Aparicio Abstract Question: Positive interactions are predicted to be common in communities developing under high physical stress or high herbivory pressure due to neighbour amelioration of limiting physical and consumer stresses, respectively. However, when both stress sources meet in the same community, the relative importance of the two facilitation mechanisms is poorly understood. We ask: What is the relative importance of abiotic vs. biotic mechanisms of facilitation of tree saplings by shrubs in Mediterranean mountain forests? Location: Sierra Nevada, SE Spain (1800,1850 m a.s.l.) Methods: Saplings of four tree taxa (Acer opalus ssp. grana-tense, Quercus ilex, Pinus nigra ssp. salzmanii and P. sylvestris var. nevadensis) were planted following a 2 × 2 factorial design: two levels of herbivory (control and ungulate exclusion) and two microhabitats (under shrubs and in open areas). Sapling survival and growth were monitored for five years. Results: Shrubs had positive effects on sapling survival both in control and ungulate excluded plots. This effect was species-specific, with shrubs increasing the survival of Acer opalus and Quercus ilex three and twofold, respectively, but having a minor effect on the Pinus species. Herbivory damage was also species-specific, being much higher for Acer opalus than for any other species. Shrubs did not protect saplings of any species against ungulates. Thus, all Acer saplings (the most damaged species) suffered herbivory outside the exclosures, which largely reduced sapling height. Conclusions: Protection from abiotic stress (summer drought and winter frost) was much more relevant than protection from biotic stress (herbivory). However, we propose that the final balance between the two mechanisms can be expected to vary strongly between sites, depending on the relative magnitude of the different sources of stress and the intrinsic traits (e.g. palatability) of the species interacting. [source] Forest Restoration in Urbanizing Landscapes: Interactions Between Land Uses and Exotic ShrubsRESTORATION ECOLOGY, Issue 2 2005Kathi L. Borgmann Abstract Preventing and controlling exotic plants remains a key challenge in any ecological restoration, and most efforts are currently aimed at local scales. We combined local- and landscape-scale approaches to identify factors that were most closely associated with invasion of riparian forests by exotic shrubs (Amur honeysuckle [Lonicera maackii] and Tatarian honeysuckle [L. tatarica]) in Ohio, U.S.A. Twenty sites were selected in mature riparian forests along a rural,urban gradient (<1,47% urban land cover). Within each site, we measured percent cover of Lonicera spp. and native trees and shrubs, percent canopy cover, and facing edge aspect. We then developed 10 a priori models based on local- and landscape-level variables that we hypothesized would influence percent cover of Lonicera spp. within 25 m of the forest edge. To determine which of these models best fit the data, we used an information-theoretic approach and Akaike's information criterion. Percent cover of Lonicera was best explained by the proportion of urban land cover within 1 km of riparian forests. In particular, percent cover of Lonicera was greater in forests within more urban landscapes than in forests within rural landscapes. Results suggest that surrounding land uses influence invasion by exotic shrubs, and explicit consideration of land uses may improve our ability to predict or limit invasion. Moreover, identifying land uses that increase the risk of invasion may inform restoration efforts. [source] Use of Shrubs as Nurse Plants: A New Technique for Reforestation in Mediterranean MountainsRESTORATION ECOLOGY, Issue 2 2002Jorge Castro Abstract Common techniques currently used for afforestation in the Mediterranean basin consider the pre-existing vegetation (mainly shrubs) as a source of competition for trees, and consequently it is generally eliminated before planting. Nevertheless, it has been demonstrated that woody plants can facilitate the establishment of understory seedlings in environments that, like the Mediterranean area, are characterized by a pronounced dry season. In this study, we experimentally analyze the usefulness of shrubs as nurse plants for afforestation of two native conifers, Pinus sylvestris L. (Scots pine) and Pinus nigra Arnold (black pine). Two-year-old seedlings were planted in four microhabitats: (1) open interspaces without vegetation (which is the usual method used in afforestation programs), (2) under individuals of Salvia lavandulifolia, (3) under the north side of spiny shrubs, and (4) under the south side of spiny shrubs. Pine survival was remarkably higher when planted under individuals of the shrub S. lavandulifolia (54.8% for Scots pine, 81.9% for black pine) compared with open areas (21.5% for Scots pine, 56.8% for black pine; chi square, p < 0.05). The survival of both pines was also higher when planted on the north side of spiny shrubs, although the survival on the south side was similar to that found in open areas. In addition, pine growth was not inhibited when planted in association with shrubs. This pattern appears to result from the combination of abiotic conditions imposed by the presence of a nurse shrub, which leads to improvement in seedling water status and therefore reduced summer mortality by drought. The results show that the use of shrubs as nurse plants is a technique that offers both economic and ecological advantages, in terms of savings in labor and plant material and reduced and even negligible impact on the pre-existing vegetation. [source] The effect of single vegetation elements on wind speed and sediment transport in the Sahelian zone of Burkina FasoEARTH SURFACE PROCESSES AND LANDFORMS, Issue 10 2007J. K. Leenders Abstract Soil loss caused by wind erosion is a widespread phenomenon in the Sahelian zone of West Africa. According to Sahelian farmers, scattered vegetation standing in amongst the crop has the potential for a wind erosion control strategy. This study was conducted to study the effect of single vegetation elements on the pattern of average wind speed and sediment transport. This was done by two experiments that were carried out during the rainy seasons of 2002 and 2003 in north Burkina Faso, West Africa. Wind speeds were measured using three sonic anemometers, at a sampling frequency of 16 Hz. Sediment transport was determined by calculating the mass fluxes from 17 MWAC catchers. In this study, a shrub was defined as a vegetation element with branches until ground and a tree as a vegetation element with a distinctive trunk below a canopy. Behind shrubs wind speed near the soil surface was reduced up to approximately seven times the height of the shrub. The observed reduction in wind speed in the area where wind speed was reduced was 15 per cent on average. At the sides of the shrub, wind speed was increased, by on average 6 per cent. As the area of increase in wind speed is one-third of the area of decrease in wind speed, the net effect of a shrub is a reduction in wind speed. A similar pattern was visible for the pattern of sediment transport around a shrub. Downwind of a shrub, sediment transport was diminished up to seven times the height of the shrub. Probably most of this material was trapped by the shrub. Trees showed a local increase of wind around the trunk, which is expected to relate to an increase in sediment transport around the trunk. Mass flux measurements of sediment transport were not made, but visual observations in the field substantiate this. Behind the canopy of a tree, a tree acts similarly to a shrub regarding its effects on average wind speed, but as a tree is generally a larger obstacle than a shrub the extent of this effect is larger than for shrubs. Thus, whereas shrubs are more effective than trees regarding their direct effect on soil loss by trapping sand particles near the soil surface, trees are more effective in affecting soil loss indirectly by reducing the wind speed downwind more effectively than shrubs. Therefore, to reduce soil loss in an area, the presence of both trees and shrubs is crucial. Copyright © 2007 John Wiley & Sons, Ltd. [source] Long-distance dispersal of seeds in the fire-tolerant shrub Banksia attenuataECOGRAPHY, Issue 4 2009Tianhua He Long-distance dispersal (LDD) of seeds enables alleles, individuals and species of plants to (re)colonize suitable but remote habitats. Banksia attenuata is a long-lived resprouting shrub restricted to dune crests in fire-prone sclerophyll shrublands of the Eneabba sandplain, southwestern Australia. Highly polymorphic microsatellite DNA genetic markers and population assignment tests were employed to identify LDD immigrants among 788 individuals from 27 stands of B. attenuata comprising a metapopulation. Of the 487 (61.8% of the total) individuals unambiguously assigned to a unique source population, 27 (5.5%) were identified as immigrants by assignment to a known population other than that from which they were sampled, while the remaining 460 were assigned to the population from which they were sampled. The distance between source and sink populations for these immigrants ranged from 0.2 to 2.6,km, averaging 1.4,km, and broadly trending in the direction of seasonal winds. These results suggest that B. attenuata has similar long-distance seed dispersal properties as its co-occurring shorter-lived and fire-sensitive congener, B. hookeriana, despite fewer, larger and less mobile seeds. The frequency and distance of LDD for seeds observed in both species (5.5,6.8%) helps explain the persistence of populations on these geographically isolated dunes, where they are subject to local extinction from recurrent fire and severe summer drought, and will remain important under predicted climate change conditions. Analysis also revealed that species richness of the functional group to which B. attenuata belongs was positively correlated with the number of immigrants identified per dune, and such correlation was likely driven by environmental properties of the dunes, particularly water availability. [source] Age-related change in canopy traits shifts conspecific facilitation to interference in a semi-arid shrublandECOGRAPHY, Issue 4 2007Orna Reisman-Berman Shifts between facilitation and interference and their importance in shaping plant population and community dynamics have received wide recognition. Nevertheless, the causes and spatio-temporal scales of these shifts are poorly understood, yet strongly debated. This study tested the hypothesis that age-related changes in canopy structure shift the effect of a nurse shrub on their protégé from facilitation to interference, using as a model system the interaction between the dwarf shrub Sarcopoterium spinosum and conspecific new recruits, in the shrubland of the transition area between the Mediterranean and the semi-arid climatic zones of Israel. Foliation level (i.e. the percentage of canopy surface area covered with leaves), a measure of shrub canopy structure, increased with age. Shading level was significantly and positively related to foliation level. Densities of new recruits in the shrubland showed a unimodal response to canopy structure and cover: the highest densities were associated with canopies presenting low and medium foliation levels (providing 71 and 82% shade, respectively), while high foliation levels (93% shade) and open spaces among canopies were characterized by very low densities. A related field experiment using shading nets revealed that seedling survival rates followed a similar unimodal pattern, with the highest survival (ca 60%) detected in moderate shade (70%), twice as much as in full sun, and the lowest survival (ca 10%) observed in extreme shade (90%). These results support the study hypothesis on age-dependent interactions. Thus, in a semi-arid shrubland ecosystem, the transition of the "nurse shrub" from "young" to "old" stage can shift facilitation to interference. Hence, the age structure of established shrub populations determines a) the availability of suitable sites for seedling recruitment and b) the balance between facilitation versus interference effects on seedling establishment. [source] The relative importance of landscape and community features in the invasion of an exotic shrub in a fragmented landscapeECOGRAPHY, Issue 2 2006Anne M. Bartuszevige Although invasive plants are recognized as a major ecological problem, little is known of the relative importance of plant community characteristics versus landscape context in determining invasibility of communities. We determined the relative importance of community and landscape features of 30 woodlots in influencing the invasion of Lonicera maackii. We sampled woodlots using the point-quarter method and calculated canopy openness and basal areas and densities of shrub, sapling and tree species, as well as woody species richness. We used aerial photos and ArcView GIS to calculate landscape parameters from the same woodlots using a buffer distance of 1500,m. We used logistic and linear regression analyses to determine the community and landscape factors that best explain L. maackii presence and density. We also tested whether woodlot invasion by L. maackii begins at woodlot edges. Presence of L. maackii was significantly explained only by distance from the nearest town (logistic regression, p=0.017); woodlots nearer town were more likely to be invaded. Among invaded woodlots, density of L. maackii was positively related to the amount of edge in the landscape (partial R2=0.592) and negatively related to total tree basal area (partial R2=0.134), number of native woody species (partial R2=0.054), and sapling shade tolerance index (partial R2=0.054). Lonicera maackii in woodlot interiors were not younger than those on the perimeters, leading us to reject the edge-first colonization model of invasion. Our findings reveal that landscape structure is of primary importance and community features of secondary importance in the invasion of L. maackii. This shrub is invading from multiple foci (towns) rather than an advancing front. Connectivity in the landscape (i.e. the number of corridors) did not promote invasion. However, edge habitat was important for invasion, probably due to increased propagule pressure. The community features associated with L. maackii invasion may be indicators of past disturbance. [source] Vegetation impacts on near bank flowECOHYDROLOGY, Issue 4 2009Leslie Hopkinson Abstract Vegetation is an important component of stream restoration designs used to control streambank retreat, but vegetation effects on near bank flows need to be quantified. The goal of this research was to evaluate how three-dimensional velocity structure and turbulence characteristics vary with three vegetation treatments: tree, shrub and grass. A second order prototype stream (Tom's Creek in Blacksburg, Virginia, USA) with individual reaches dominated by each vegetation treatment was modelled in a research flume using a fixed-bed Froude-scale modelling technique. One model streambank of the prototype stream was constructed for each vegetation type and compared to a bare control (only grain roughness). Velocity profiles perpendicular to the flume model boundary were measured using a three-dimensional acoustic Doppler velocimeter. Three-dimensional velocity records, turbulent kinetic energy characteristics, and Reynolds stresses were analysed. The addition of vegetation on a sloping streambank increased the free stream streamwise velocity as compared to a bare streambank. Velocity in the downstream direction decreased in the area close to the streambank boundary for all vegetation treatments. Tree turbulence intensity and Reynolds stress distributions were similar to the bare condition due to the sparse tree placement characteristic of mature forests. The turbulence caused by the upright shrub treatment increased turbulent kinetic energy and Reynolds stresses near the streambank, particularly at the toe. The flexible grass vegetation folded and protected the streambank, reducing shear stress near the boundary. Copyright © 2009 John Wiley & Sons, Ltd. [source] Converting visual census data into absolute abundance estimates: a method for calibrating timed counts of a sedentary insect populationECOLOGICAL ENTOMOLOGY, Issue 4 2003Ho Jung S. Yoo Abstract., 1.,Visual surveys for small organisms on complex substrates often yield serious underestimates of true counts. When both visual counts (relative estimates of abundance) and absolute counts can be obtained from the same sample, however, the visual counts can be calibrated such that absolute estimates can be obtained in the future from visual surveys alone. 2.,A method is presented for converting quick, timed, visual counts of a sedentary insect on a shrub into absolute estimates of abundance. 3.,Analogies were drawn from simple, well-known predation theories to develop a two-parameter non-linear model. Parameter estimates were obtained by both inverse prediction and direct estimation methods; the latter were found to yield more accurate predictions of absolute abundance. 4.,The calibration model is mechanistic in its approach, and thus has potential for application in other systems in which all individuals are visible, but able to be missed during timed counts. [source] Comparative seed ecology of the endangered shrub, Pimelea spicata and a threatening weed, Bridal Creeper: Smoke, heat and other fire-related germination cuesECOLOGICAL MANAGEMENT & RESTORATION, Issue 1 2003Anthony J. Willis SummaryPimelea spicata R. Br. is a nationally listed endangered Australian shrub threatened with extinction by habitat fragmentation and environmental weed invasion. Bridal Creeper (Asparagus asparagoides L. W. Wight) is the primary weed threat to the largest remaining populations of P. spicata in the Cumberland Plain. Fire, as part of an integrated pest management program, offers the potential to stimulate P. spicata populations while controlling Bridal Creeper. It is important, therefore, to understand how the components of fire affect the germination and growth of both species. Using laboratory experiments we investigated the effects of smoke, heat, ash and/or light on the germination of P. spicata and Bridal Creeper. We found a significant promotive effect of smoke and indication of an inhibitory heat shock (90°C for 10 min) effect on the germination of P. spicata seeds. The response of Bridal Creeper seeds to the same factors was complex; while the results of one experiment suggested an inhibitory effect of smoke and a promotive effect of heat, subsequent trials were contradictory, implying that Bridal Creeper, like many weeds, is able to germinate under a wide range of environmental conditions. Other experiments investigated the optimal germination temperature and innate dormancy of P. spicata in the absence of fire-related germination cues. Of the incubation temperatures investigated, the optimal diurnally fluctuating regime for P. spicata germinations was 10°C and 20°C in the night and day, respectively. The innate dormancy of freshly produced seeds disappeared after 3 months. In contrast to Bridal Creeper, we found a persistent germinable seed bank of about 97 P. spicata seeds/m2 located in the top 5 cm of the soil profile. While fire alone is unlikely to kill Bridal Creeper plants, fire may help to manage local infestations of the weed by limiting germination and providing opportunity for herbicide treatment of regrowth. [source] Absorption of copper(II) by creosote bush (Larrea tridentata): Use of atomic and x-ray absorption spectroscopyENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 11 2001Jorge Luis Gardea-Torresdey Abstract Larrea tridentata (creosote bush), a common North American native desert shrub, exhibits the ability to take up copper(II) ions rapidly from solution. Following hydroponic studies, U.S. Environmental Protection Agency method 200.3 was used to digest the plant samples, and flame atomic absorption spectroscopy (FAAS) was used to determine the amount of copper taken up in different parts of the plant. The amount of copper(II) found within the roots, stems, and leaves was 13.8, 1.1, and 0.6 mg/g, respectively, after the creosote bush was exposed to a 63.5-ppm copper(II) solution for 48 h. When the plant was exposed to a 635-ppm copper(II) solution, the roots, stems, and leaves contained 35.0, 10.5, and 3.8 mg/g, respectively. In addition to FAAS analysis, x-ray microfluorescence (XRMF) analysis of the plant samples provided further confirmation of copper absorption by the various plant parts. X-ray absorption spectroscopy (XAS) elucidated the oxidation state of the copper absorbed by the plants. The copper(II) absorbed from solution remained as copper(II) bound to oxygen-containing ligands within the plant samples. The results of this study indicate that creosote bush may provide a useful and novel method of removing copper(II) from contaminated soils in an environmentally friendly manner. [source] Spatial patterns and environmental factors affecting the presence of Melampsorella caryophyllacearum infections in an Abies alba forest in NE SpainFOREST PATHOLOGY, Issue 3 2006A. Solla Summary The presence of trunk swellings caused by the rust fungus Melampsorella caryophyllacearum was systematically surveyed in an Abies alba forest (Irati, NE Spain), using 1237 circular plots (diameter = 18 m). The relationship between fungal presence and several abiotic (aspect, elevation, distance to the nearest river and slope) and biotic factors (basal area of A. alba and/or Fagus sylvatica, shrub, fern and herb cover) was assessed through correlation and ordination analyses. Additionally, the spatial pattern of the presence of diseased trees was described using Ripley's K function. Southern-aspect plots had a significantly lower presence of diseased trees than plots-oriented north, east and west. Plots with diseased trees were located at a significantly lower elevation, and at a shorter distance to the river than plots without infections. Plots with diseased trees had almost twice the average A. alba basal area, and less average F. sylvatica basal area than plots without diseased trees. However, similar mean values of slope and shrub, fern and herb cover were found in both types of plots. The disease showed spatial aggregation in patches with a mean radius of ca. 900 m. The implications of the results for disease management are discussed. Résumé La présence de renflements sur les troncs causés par l'agent de la rouille, Melampsorella caryophyllacearum, a étéétudiée de façon systématique dans une forêt d'Abies alba (Irati, NE Espagne), en utilizant 1237 placettes circulaires (diamètre de 18 m). Les relations entre la présence du champignon et divers facteurs abiotiques (orientation, altitude, distance à la rivière la plus proche, pente) et biotiques ( surface terrière de A. alba et/ou Fagus sylvatica, abondance de la couverture herbacée et abondance d'arbustes et fougères) ont étéétudiées par analyses de corrélation et d'ordination. D'autre part, la structure spatiale de la présence d'arbres infectés a été décrite en utilizant la fonction K de Ripley. Les placettes exposées au sud présentent moins fréquemment des arbres malades que celles exposées au nord, à l'est ou à l'ouest. Les placettes avec des arbres malades sont situées à une altitude significativement plus faible et à une distance plus faible d'une rivière que les placettes sans infections, et elles présentent une surface terrière 2 fois plus forte en moyenne pour A. alba, et plus faible pour F. sylvatica, que les placettes non-infectées. Toutefois, des valeurs moyennes équivalentes pour la pente, la couverture herbacée et l'abondance d'arbustes et fougères, sont observées pour les deux types de placettes. La maladie montre une agrégation spatiale en foyers d'un rayon moyen de 900 m. Les résultats sont discutés dans une perspective de gestion de la maladie. Zusammenfassung Das Vorkommen von durch den Rostpilz Melampsorella caryophyllacearum verursachten Stammdeformationen wurde in einem Abies alba - Wald (Irati, NO-Spanien) auf 1237 kreisförmigen Probeflächen (Durchmesser 18 m) systematisch erfasst. Die Beziehung zwischen dem Pilzvorkommen und mehreren abiotischen (Exposition, Meereshöhe, Distanz zum nächsten Fluss, Hangneigung) und biotischen Faktoren (Deckungsgrad von A. alba und/oder Fagus sylvatica, Strauch-, Farn- und Krautschicht) wurden durch Korrelations- und Ordinations-Analysen überprüft. Zudem wurden räumliche Muster der befallenen Bäume mit Hilfe von Ripley's K-Funktion beschrieben. In südexponierten Probeflächen kamen signifikant weniger erkrankte Bäume vor als in nach Norden, Osten und Westen orientierten Standorten. Flächen mit Befall lagen in signifikant geringerer Meereshöhe und kürzerer Distanz zum nächsten Fluss als solche ohne Befall. Zudem hatten sie beinahe die doppelte Basalfläche mit A. alba und eine durchschnittliche geringere Basalfläche mit F. sylvatica. Die durchschnittlichen Werte für die Strauch-, Farn- und Krautschicht sowie die Hangneigung unterschieden sich jedoch nicht in den Flächen mit und ohne Befall. Erkrankte Bäume waren räumlich aggregiert mit einem mittleren Radius von ca. 900 m. Die Bedeutung dieser Befunde für das Krankheitsmanagement wird diskutiert. [source] Local and ecoregion effects on fish assemblage structure in tributaries of the Rio Paraíba do Sul, BrazilFRESHWATER BIOLOGY, Issue 12 2009BENJAMIN CARVALHO TEIXEIRA PINTO Summary 1.,We examined the effects of physical and chemical habitat variables and ecoregions on species occurrence and fish assemblage structure in streams of the Paraíba do Sul basin, in southeast Brazil. 2.,Fish and environmental data were collected from 42 sites on 26 first to fourth order streams (1 : 50 000 map scale) in three ecoregions. The sites occurred in one valley and two plateau ecoregions at altitudes of 40,1080 m and distances of 0.1,188 km from the main channel of the Rio Paraíba do Sul. Physical habitat (substratum, riparian cover, habitat types) and water quality (dissolved oxygen, pH, temperature and conductivity) variables were measured at each sampling site. 3.,A total of 2684 individuals in 16 families and 59 species were recorded. 4.,Ecoregion was a better predictor of the fish assemblage than the other environmental variables, according to the differences between the mean within-class and mean between-class similarities in assemblage data. 5.,Differing landscape characteristics were associated with differing local variables and thereby with differing fish assemblage structures. Riffles, shrub, grass, dissolved oxygen, conductivity and temperature were closely related to fish assemblage structure. 6.,Fish assemblages in sites far from the main river and at higher altitudes also differed from those near the Paraíba do Sul main channel, presumably as a result of differences in connectivity, covarying environmental factors and anthropogenic influence. 7.,These results reinforce the importance of understanding how stream communities are influenced by processes and patterns operating at local and regional scales, which will aid water resource managers to target those factors in their management and rehabilitation efforts. [source] Allometry, growth and population regulation of the desert shrub Larrea tridentataFUNCTIONAL ECOLOGY, Issue 2 2008A. P. Allen Summary 1Quantifying the effects of individual- and population-level processes on plant-community structure is of fundamental importance for understanding how biota contribute to the flux, storage and turnover of matter and energy in ecosystems. 2Here we synthesize plant-allometry theory with empirical data to evaluate the roles of individual metabolism and competition in structuring populations of the creosote Larrea tridentata, a dominant shrub in deserts of southwestern North America. 3At the individual level, creosote data support theoretical predictions with regard to the size dependence of total leaf mass, short-term growth rates of leaves and long-term growth rates of entire plants. Data also support the prediction that root,shoot biomass allocation is independent of plant size. 4At the population level, size,abundance relationships within creosote stands deviate strongly from patterns observed for steady-state closed-canopy forests due to episodic recruitment events. This finding highlights that carbon storage and turnover in water-limited ecosystems can be inherently less predictable than in mesic environments due to pronounced environmental forcing on demographic variables. 5Nevertheless, broad-scale comparative analyses across ecosystems indicate that the relationship of total abundance to average size for creosote populations adhere to the thinning rule observed and predicted by allometry theory. This finding indicates that primary production in water-limited ecosystems can be independent of standing biomass due to competition among plants for resources. 6Our synthesis of theory with empirical data quantifies the primary roles of individual-level metabolism and competition in controlling the dynamics of matter and energy in water-limited ecosystems. [source] Disentangling effects of an experimentally imposed extreme temperature event and naturally associated desiccation on Arctic tundraFUNCTIONAL ECOLOGY, Issue 6 2006F. L. MARCHAND Summary 1Climate projections suggest that extreme events will increase in frequency during this century. As tundra is recognized to be among the most vulnerable biomes, we exposed patches of arctic tundra vegetation to an experimental heatwave (by infrared irradiation), followed by a recovery period. The heating increased the surface temperature with an average of 7·6 °C during 13 days, which slightly exceeded the longest climatic episode with such a temperature deviation since 1961. 2The heatwave decreased stomatal conductance (gs) and PSII maximum efficiency (Fv/Fm), although there were differences in response among the four target species. Salix arctica Pall. (shrub) was affected during the heatwave and could not recover. In Carex bigelowii Tor. ex Schwein (sedge) and Pyrola grandiflora Radius (forb), on the other hand, the effects on gs and Fv/Fm became clear, particularly in the aftermath of the heatwave, whereas Polygonum viviparum L. (forb) was never stressed. 3Effects of the heat on gs were mainly indirect, through increased desiccation, whereas effects on Fv/Fm were more related to leaf temperature (although not in all species). The observed changes can therefore probably be ascribed to a combination of heat and drought causing dysfunctions that ultimately led to senescence. 4Two conclusions of this study, species-specific responses and increased leaf mortality, indicate that more frequent extreme temperature events accompanied by desiccation might alter/endanger tundra communities in a future climate. Predictions of global change effects on arctic ecosystems should therefore take into account the impact of extremes. [source] Physiological responses of two contrasting desert plant species to precipitation variability are differentially regulated by soil moisture and nitrogen dynamicsGLOBAL CHANGE BIOLOGY, Issue 5 2009LISA D. PATRICK Abstract Alterations in global and regional precipitation patterns are expected to affect plant and ecosystem productivity, especially in water-limited ecosystems. This study examined the effects of natural and supplemental (25% increase) seasonal precipitation on a sotol grassland ecosystem in Big Bend National Park in the Chihuahuan Desert. Physiological responses , leaf photosynthesis at saturating light (Asat), stomatal conductance (gs), and leaf nitrogen [N] , of two species differing in their life form and physiological strategies (Dasylirion leiophyllum, a C3 shrub; Bouteloua curtipendula, a C4 grass) were measured over 3 years (2004,2006) that differed greatly in their annual and seasonal precipitation patterns (2004: wet, 2005: average, 2006: dry). Precipitation inputs are likely to affect leaf-level physiology through the direct effects of altered soil water and soil nitrogen. Thus, the effects of precipitation, watering treatment, soil moisture, and nitrogen were quantified via multivariate hierarchical Bayesian models that explicitly linked the leaf and soil responses. The two species differed in their physiological responses to precipitation and were differentially controlled by soil water vs. soil nitrogen. In the relatively deeply rooted C3 shrub, D. leiophyllum, Asat was highest in moist periods and was primarily regulated by deep (16,30 cm) soil water. In the shallow-rooted C4 grass, B. curtipendula, Asat was only coupled to leaf [N], both of which increased in dry periods when soil [N] was highest. Supplemental watering during the wet year generally decreased Asat and leaf [N] in D. leiophyllum, perhaps due to nutrient limitation, and physiological responses in this species were influenced by the cumulative effects of 5 years of supplemental watering. Both species are common in this ecosystem and responded strongly, yet differently, to soil moisture and nitrogen, suggesting that changes in the timing and magnitude of precipitation may have consequences for plant carbon gain, with the potential to alter community composition. [source] Canopy recovery after drought dieback in holm-oak Mediterranean forests of Catalonia (NE Spain)GLOBAL CHANGE BIOLOGY, Issue 12 2004Francisco Lloret Abstract Climate change is likely to produce more frequent and longer droughts in the Mediterranean region, like that of 1994, which produced important changes in the Quercus ilex forests, with up to 76% of the trees showing complete canopy dieback. At the landscape level, a mosaic of responses to the drought was observed, linked to the distribution of lithological substrates. Damage to the dominant tree species (Q. ilex) and the most common understorey shrub (Erica arborea) was more noticeable on the compact substrates (breccia) than on the fissured ones (schist). This result was consistent with observations documenting deeper root penetration in schist than in breccia materials, allowing the plants growing on fissured substrates to use water from deeper soil levels. Smaller plants were more vulnerable to drought than larger plants in the trees, but not in the shrubs. Overall, Q. ilex was more affected than E. arborea. The resilience of the system was evaluated from the canopy recovery 1 year after the episode. Stump and crown resprouting was fairly extensive, but the damage pattern in relation to substrate, plant size, and species remained similar. The effect of recurrent drought episodes was studied on vegetation patches of Q. ilex located on mountain slopes and surrounded by bare rock. We observed that plants that resprouted weakly after a previous drought in 1985 were more likely to die or to produce poor regeneration in 1995 than plants that had resprouted vigorously. Vegetation patches located on the lower part of the slope were also less damaged than patches situated uphill. The study provides evidence of relevant changes in forest canopy as a consequence of extreme climate events. The distribution of this effect across the landscape is mediated by lithological substrate, causing patchy patterns. The results also support the hypothesis that recurrent droughts can produce a progressive loss of resilience, by depleting the ability of surviving plants to regenerate. [source] Historical shrub,grass transitions in the northern Chihuahuan Desert: modeling the effects of shifting rainfall seasonality and event size over a landscape gradientGLOBAL CHANGE BIOLOGY, Issue 10 2003Qiong Gao Abstract We use a spatially explicit landscape model to investigate the potential role of rainfall on shrub,grass transitions in the Jornada Basin of southern New Mexico during the past century. In long-term simulations (1915,1998) along a 2700 m transect running from a dry lake bed to the foothills of a small mountain, we test two hypotheses: (i) that wetter winters and drier summers may have facilitated shrub encroachment in grasslands, and (ii) that increases in large precipitation events may have increased soil water recharge at deeper layers, thus favoring shrub establishment and growth. Our model simulations generally support the hypothesis that wetter winters and drier summers may have played a key role, but we are unable to reproduce the major shifts from grass- to shrub-domination that occurred in this landscape during the early part of the 1900s; furthermore, the positive shrub response to wetter winters and drier summers was only realized subsequent to the drought of 1951,1956, which was a relatively short ,window of opportunity' for increased shrub establishment and growth. Our simulations also generally support the hypothesis that an increase in the number of large precipitation events may also have favored shrub establishment and growth, although these results are equivocal, depending upon what constitutes a ,large' event and the timing of such events. We found complex interactions among (i) the amount/seasonality of rainfall, (ii) its redistribution in the landscape via run-on and runoff, (iii) the depth of the soil water recharge, and (iv) subsequent water availability for the growth and reproduction of shrubs vs. herbaceous plants at various landscape positions. Our results suggest that only a mechanistic understanding of these interactions, plus the role of domestic cattle grazing, will enable us to elucidate fully the relative importance of biotic vs. abiotic factors in vegetation dynamics in this semiarid landscape. [source] An empirical model of carbon fluxes in Russian tundraGLOBAL CHANGE BIOLOGY, Issue 2 2001Dmitri G. Zamolodchikov Summary This study presents an empirical model based on a GIS approach, which was constructed to estimate the large-scale carbon fluxes over the entire Russian tundra zone. The model has four main blocks: (i) the computer map of tundra landscapes; (ii) data base of long-term weather records; (iii) the submodel of phytomass seasonal dynamics; and (iv) the submodel of carbon fluxes. The model uses exclusively original in situ diurnal CO2 flux chamber measurements (423 sample plots) conducted during six field seasons (1993,98). The research sites represent the main tundra biome landscapes (arctic, typical, south shrub and mountain tundras) in the latitudinal diapason of 65,74°N and longitudinal profile of 63°E,172°W. The greatest possible diversity of major ecosystem types within the different landscapes was investigated. The majority of the phytomass data used was obtained from the same sample plots. The submodel of carbon fluxes has two dependent [GPP, Gross Respiration (GR)] and several input variables (air temperature, PAR, aboveground phytomass components). The model demonstrates a good correspondence with other independent regional and biome estimates and carbon flux seasonal patterns. The annual GPP of Russian tundra zone for the area of 235 × 106 ha was estimated as ,485.8 ± 34.6 × 106 tC, GR as +474.2 ± 35.0 × 106 tC, and NF as ,11.6 ± 40.8 × 106 tC, which possibly corresponds to an equilibrium state of carbon balance during the climatic period studied (the first half of the 20th century). The results advocate that simple regression-based models are useful for extrapolating carbon fluxes from small to large spatial scales. [source] | |||||||||||||||||||||||||||||||||||||||||||