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Stem Density (stem + density)

Distribution by Scientific Domains

Life Sciences	72%
Earth and Environmental Science	19%
2 Other Domains	9%

Selected Abstracts

Intraspecific diversity and dominant genotypes resist plant invasions

ECOLOGY LETTERS, Issue 1 2008
Gregory M. Crutsinger
Abstract Numerous studies have asked whether communities with many species deter invasions more so than do species-poor communities or whether dominant species deter invasion by colonizing species. However, little is known about whether high intraspecific diversity can deter biological invasions or whether particular genotypes might deter invasions. In this study, we present experimental evidence that intraspecific diversity and particular genotypes of tall goldenrod, Solidago altissima, can act as a barrier to colonization by new species. We found that biomass of colonizing species was negatively correlated with genotypic diversity, and particular genotypes affected the richness, cover, and biomass of colonizing species. Stem density of S. altissima increased with genotypic diversity and varied among genotypes, suggesting that stem density is a key mechanism in limiting colonization dynamics in this system. Our results indicate that the loss of intraspecific diversity within a dominant plant species can increase susceptibility to plant invasions. [source]

Effects of natural disturbance and selective logging on Nothofagus forests in south-central Chile

JOURNAL OF BIOGEOGRAPHY, Issue 7 2002
William Pollmann
Aim Changes in stand structure, floristic composition and tree population dynamics during the last four centuries were described in southern temperate rain forests. The impacts of natural and anthropogenic disturbance since the late 1560s were examined for old-growth and logged forests. Location The study was conducted in montane Nothofagus alpina forests in the Andean Range of south-central Chile. Study sites were located at a range of altitudes between 1000 and 1250 m a.s.l. Methods Temporal variation in species recruitment and annual dendroecological data were used to determine the historical development and disturbance history of three old-growth forests, and three stands after selective logging in the late 1880s to early 1900s. Considering the spatial structure of evergreen vs. deciduous elements, the forests are a mixture of deciduous and evergreen tree species. Results Stem density, maximum stem diameter and basal area differed between the old-growth and logged stands, but species composition did not. At lower altitudes, N. alpina was the dominant canopy species in both old-growth and logged stands, but regeneration of N. alpina was significantly different in these stands. At higher altitudes, N. alpina and N. dombeyi were the dominant canopy species in both old-growth and logged stands, and here regeneration patterns of these Nothofagus species were similar. After selective logging, in mixed forests of shade-intolerant Nothofagus and more shade-tolerant trees (such as Laurelia philippiana) there has been a shift in regeneration from more shade-tolerant resprouting species towards Nothofagus. Major and moderate releases in radial growth, indicative of disturbance, occurred in most of the older trees during the last four centuries, and especially during the last 100 years. Growth rates of N. alpina are higher than those of associated shade-tolerant trees, and apparently increase after disturbances. Main conclusions Results suggest that under disturbance regimes dominated by treefall gaps, and additionally canopy openings by selective logging, maintenance of Nothofagus species appears to be associated with complementary differences in growth rate, sprouting capacity, canopy residence time, and longevity. Such small-scale canopy openings may help explain the relative abundance of N. alpina in montane mixed rain forests in the Andean Range, where the maintenance of Nothofagus species in many stands has been attributed to a high frequency of coarse-scale disturbance. My results serve to emphasize that understanding the species coexistence and forest dynamics in Nothofagus forests may require attention to interspecific differences in life-history characteristics. [source]

Does designation of protected areas ensure conservation of tree diversity in the Sudanian dry forest of Burkina Faso?

AFRICAN JOURNAL OF ECOLOGY, Issue 2 2010
Souleymane Paré
Abstract The importance of conservation status of the forest (protected versus unprotected) at two sites with differing human population density (high versus low) on the tree diversity of a Sudanian dry forest in Burkina Faso was studied. All woody species were recorded in 127 circular plots (area = 456.16 m2), and density, dominance, frequency, importance value indices and a variety of diversity measures were calculated to assess the species composition, structure and heterogeneity. A total of 69 species, representing 26 families and 52 genera, were found. Combretaceae, Leguminosae subfamily Caesalpinioideae and Rubiaceae were the dominant families. Neither human pressure nor forest conservation status significantly influenced the tree species richness. Stem density and basal area were significantly higher at the site with high population density than otherwise. Fisher's diversity index revealed the unprotected forest at the site with low population density as the most diverse. We identified species with high conservation importance that should be enriched to maintain a viable population size. In conclusion, the current designation of protected areas seems inefficient at ensuring the conservation of tree diversity in the forest reserve. Thus, participatory conservation programme should be initiated. Résumé À deux endroits où la densité de population humaine est différente (forte versus faible), nous avons étudié l'importance du statut de conservation de la forêt (protégée ou non) pour la diversité d'une forêt soudanienne sèche du Burkina Faso. On a enregistré toutes les espèces ligneuses sur 127 parcelles circulaires (superficie = 456,16 m²), et on a calculé la densité, la dominance, les indices de valeurs d'importance value et une variété de mesures de diversité pour évaluer la composition, la structure et l'hétérogénéité des espèces. On a découvert un total de 69 espèces, représentant 26 familles et 52 genres. Les familles dominantes étaient Combretaceae, Leguminosae de la sous-famille Caesalpinioideae, et Rubiaceae. Ni la pression humaine, ni le statut de conservation des forêts n'influençaient significativement la richesse en espèces d'arbres. La densité des troncs et la surface terrière à la base étaient significativement plus élevées sur le site à forte densité de population. L'indice de diversité de Fisher a permis de révéler que la forêt non protégée du site à faible densité de population était la plus diversifiée. Nous avons identifié des espèces de haute valeur de conservation qui devraient être enrichies pour préserver une taille de population viable. En conclusion, la désignation actuelle d'aires protégées semble inefficace pour assurer la conservation de la diversité des arbres dans la réserve forestière. Donc, il faudrait initier un programme de conservation participative. [source]

A long-term record of Nothofagus dominance in the southern Andes, Chile

AUSTRAL ECOLOGY, Issue 1 2005
William Pollmann
Abstract The general model of regeneration dynamics in Nothofagus forests of southern South America could have value in community ecology if predictive relationships between disturbance history, functional traits and site attributes could be identified. Examined here is the proposal that on favourable sites shade-intolerant Nothofagus are likely not to survive in competition with shade-tolerant, broad-leaved evergreen taxa of temperate rain forests, and persistence, thus, is dependent on periodic coarse-scale disturbance. Comparison of stand dynamics of three old-growth Nothofagus forests at different elevations in the southern Andes, Chile where deciduous Nothofagus alpina dominates the upper canopy, and examination of the life history trade-offs of this variation were made. Stem density of all stems ,5.0 cm d.b.h. was 233,303 stems per hectare, and basal area was 123.9,171.0 m2ha,1. Maximum lifespan of N. alpina was found to be greater than ca 640 years, exceeding all previously reported ages for this species in the region. Forests had a stable canopy composition for this long-term, but some appeared to lack effective regeneration of N. alpina in recent years. Regeneration of N. alpina was generally greater in disturbed stands and higher elevation than in undisturbed stands and at lower elevation. Recruitment emerged to be strongly affected by competitive over- and understorey associates. There was a gradient of increasing dependence of N. alpina on disturbance towards the more productive end of the environment gradients, and hence less dependence of N. alpina on disturbance for its regeneration towards higher elevation. The study confirms that changes in forest composition may be explained by processes occurring in accordance with the predictions of the existing model of Nothofagus regeneration dynamics, providing stronger evidence specifically directed at mid-tolerant N. alpina, and by factoring out regeneration dynamics on favourable sites. Thus, for N. alpina, trait differences probably contribute to the competitive advantage over its associates in productive habitats, and may be linked to small-to-intermediate-sized disturbances which inevitably occur as older trees die, enabling N. alpina to persist in forests and therefore maintain species coexistence for the long-term. [source]

Diversity and composition of trees and shrubs in Kasagala forest: a semiarid savannah woodland in central Uganda

AFRICAN JOURNAL OF ECOLOGY, Issue 1 2010
Samson Gwali
Abstract The diversity and composition of trees and shrubs of ,5 cm diameter at breast height (DBH) were investigated in Kasagala woodland in central Uganda using 1 ha permanent sample plots. A total of 2745 trees and shrubs with a mean stem density of 686 ha,1 were recorded. These included 69 tree species belonging to 28 families and 47 genera. There was a larger number of small stems compared with that of larger stems. There was significant variation in stem size class distribution between the plots (F = 3.14, P = 0.027). The variation in stem densities (counts) across different size classes was significant (F = 8.31, P < 0.001). Species diversity was higher in the low lands compared with that in the elevated sites in the woodland. The species encountered were unevenly distributed across the plots. Species abundance was not significantly different across the sample plots (F = 2.63, P = 0.053). We suggest that the structure of the forest is typical of any regenerating forest, but other human influences may have played a part in the dominance of size classes <10 cm DBH. The causes of the present status and composition of the woodland require further investigation. Résumé La diversité et la composition des arbres et arbustes de plus de 5 cm dbh ont étéétudiées dans la forêt de Kasagala, au centre de l'Ouganda, en utilisant des parcelles échantillons permanentes d'un hectare. On a relevé la présence de 2745 arbres et arbustes, avec une densité moyenne de 686 troncs ha,1. Ceux-ci comprenaient 69 espèces d'arbres appartenant à 28 familles et à 47 genres. Il y avait un plus grand nombre de petits troncs que de gros. Il y avait une variation significative de la distribution des classes de taille entre les parcelles (F = 3.14, P = 0.027). La variation de la densité des troncs (comptages) entre les différentes classes de taille était significative (F = 8.31, P < 0.001). La diversité des espèces était plus grande dans les terres basses que dans les sites plus élevés dans la forêt. Les espèces rencontrées étaient distribuées de façon inégale entre les parcelles. L'abondance des espèces n'était pas significativement différente selon les parcelles échantillons (F = 2.63, P = 0.053). Nous suggérons que la structure de la forêt est typique de toute forêt en voie de régénération, mais que d'autres influences humaines peuvent avoir joué un rôle dans la dominance des classes de taille <10 cm dbh. Les raisons du statut et de la composition actuels de la forêt requièrent de nouvelles investigations. [source]

Estimating the potential role of commercial over-harvesting in resource viability: a case study of five useful tree species in South Africa

LAND DEGRADATION AND DEVELOPMENT, Issue 3 2005
C. M. Shackleton
Abstract There is a growing commercialization of non-timber forest products (NTFPs) as a means of livelihood by rural communities throughout the developing world. This often occurs in the absence of any clear understanding of or guidelines regarding sustainable yields and ecological impacts, which may undermine the success of NTFP enterprises, especially from arid regions. This paper reports on the use of size class profiles and three quantitative indices to examine population profiles of five potentially useful tree species used as NTFPs in the semiarid lowveld of South Africa. We also contrast the population densities of the five tree species in 2003 with data from 1992. Low stem densities and population profiles indicated that three of the five species would preclude the establishment of NTFP enterprises based on their products. The other two species seem to have sufficient densities for some harvesting to take place, within an adaptive management framework. However, the longitudinal data indicated that the density of both these species had significantly declined over an eleven-year period, highlighting the need for appropriate management institutions. Additionally, the proportion of mature stems cut, and the degree of cutting per stem, had increased for all five species over the eleven years. The three quantitative indices of population stability were not correlated with one another, and hence provided a useful suite of measures sensitive to different aspects of size class profiles and their interpretation. Copyright © 2005 John Wiley & Sons, Ltd. [source]

Sowing density and harvest time affect fibre content in hemp (Cannabis sativa) through their effects on stem weight

ANNALS OF APPLIED BIOLOGY, Issue 2 2009
W. Westerhuis
Abstract Sowing density and harvest time are considered important crop management factors influencing fibre quantity and quality in hemp (Cannabis sativa). We investigated whether the effects of these factors are essentially different or that both factors affect stem weight and thereby total and long-fibre content. The effects of all combinations of three sowing densities and three harvest times were studied for six different stem parts. Almost 500 samples consisting of stem parts from 50 plants and with a length of 50 cm were tested. Fibres were extracted by a controlled warm-water retting procedure, followed by breaking and scutching. The initial sample weight was fractionated into retting losses, wood, tow and long fibre. In both Italy and the Netherlands, crops were successfully established with different stem densities (99,283 m,2), plant heights (146,211 cm) and stem diameters (4.5,8.4 mm) at harvest. Stem dry matter yields (6.8,11.7 Mg ha,1) increased with a delay in harvest time but were not affected by sowing density. Retting loss percentages were lower in lower stem parts and decreased with later harvest because maturation was associated with increasing amounts of fibre and wood. Within a certain stem part, however, the absolute retting losses were constant with harvest time. Multiple linear regression analyses showed that the amount of fibre in a hemp stem is almost completely determined by the weight and the position of that stem part. When the plant grows, the increase in dry matter is split up into fibres and wood in a fixed way. This total fibre/wood ratio was highest in the middle part of the stem and lower towards both bottom and top. Sowing density and harvest time effects were indirect through stem weight. The long-fibre weight per stem increased with the total fibre weight and hence with stem weight. Stem weight increased with harvest time; as harvest time did not affect plant density, the highest long-fibre yields were obtained at the last harvest time. The long fibre/total fibre ratio was lowest in the bottom 5 cm of the stems but similar for all other parts. Sowing density and harvest time effects again were indirect. Fibre percentages in retted hemp decreased with increasing stem weights towards a level that is presumably a variety characteristic. The dry matter increase between harvests, however, is much more important with respect to total and long-fibre yield. [source]

Impact of landscape spatial pattern on liana communities in tropical rainforests at Los Tuxtlas, Mexico

APPLIED VEGETATION SCIENCE, Issue 3 2009
Víctor Arroyo-Rodríguez
Abstract Questions: What are the species composition and species and stem densities of liana communities in tropical landscapes of different deforestation levels? Which spatial attributes (forest cover, patch area, shape and isolation) have the strongest influence on liana communities in these landscapes? Location: Forty-five rainforest patches in Los Tuxtlas Biosphere Reserve, Mexico. Methods: In three landscapes with different deforestation levels (HDL=4%; IDL=11%; and LDL=24% of remaining forest cover) liana communities (DBH ,2.5 cm) were characterized in 15 randomly selected patches per landscape (10 50 m × 2 m transects per patch=0.1 ha), and evaluated the effects of patch area, shape and isolation on liana species and stem density (number of species and stems per 0.1 ha). Results: A total of 64 taxa and 24 families were sampled. Species composition differed highly among landscapes, with HDL being the most dissimilar landscape. The response of lianas to landscape spatial pattern differed significantly among landscapes. Proximity to villages had a strong positive effect on species and stem densities in LDL and IDL. There was a sharp decrease in liana stem density in HDL, with four patches (27%) found to be unoccupied by lianas. Conclusions: Fragmentation may have a positive effect on lianas, partly because of edge effects. This positive effect seems to be limited by the proportion of remaining forest cover in the landscape, as the liana communities had collapsed in the most deforested landscape. [source]

Changes in Structure, Composition, and Nutrients During 15 Yr of Hurricane-Induced Succession in a Subtropical Wet Forest in Puerto Rico

BIOTROPICA, Issue 4 2010
Tamara Heartsill Scalley
ABSTRACT The trajectory of hurricane-induced succession was evaluated in a network of forest plots measured immediately before and 3 mo, 5, 10, and 15 yr after the direct impact of a Category 4 hurricane. Comparisons of forest structure, composition, and aboveground nutrients pools were made through time, and between species, life-history groups and geomorphic settings. The hurricane reduced aboveground biomass by 50 percent, causing an immediate decrease in stem density and diversity indices among all geomorphic settings. After 15 yr, basal area and aboveground biomass returned to pre-hurricane levels, while species richness, diversity indices, and stem densities exceeded pre-hurricane levels. Differences in species composition among geomorphic settings had not returned after 15 yr but differences in stem densities and structure were beginning to emerge. Significant differences were observed in the nutrient concentration of the three species that comprised the most aboveground biomass, and between species categorized as secondary high-light species and primary, low-light species. Species whose abundance was negatively correlated with the mature forest dominant also had distinct nutrient concentrations. When total aboveground nutrient pools were compared over time, differences in leaf nutrients among species were hidden by similarities in wood nutrient concentrations and the biomass dominance of a few species. The observed successional trajectory indicates that changes in species composition contributed to fast recovery of aboveground biomass and nutrient pools, while the influence of geomorphic setting on species composition occurs at time scales >15 yr of succession. Abstract in Spanish is available at http://www.blackwell-synergy.com/loi/btp [source]

Resource regulation by a twig-girdling beetle has implications for desertification

ECOLOGICAL ENTOMOLOGY, Issue 2 2008
B. D. DUVAL
Abstract 1.,Resource regulation by insects is the phenomenon by which herbivory enhances resources for the progeny of the herbivore. This report provides an example of resource regulation with implications for desertification in the Chihuahuan Desert of North America. 2.,Female Oncideres rhodosticta beetles chew girdles around mesquite (Prosopis glandulosa) stems before ovipositing in those stems. The mesquite plants respond by producing compensatory stems below the girdle. Mesquite volume was significantly correlated with the total number of beetle girdles across a suite of low shrub density grassland and high shrub density dune sites, and plants in dune sites had more old and new girdles than mesquite in grasslands. 3.,Smaller, younger shrubs in grassland responded more vigorously to girdling than did larger, older shrubs in dune landscapes. Stems on shrubs within grassland produced significantly more and longer compensatory stems per girdle than stems on shrubs in dunes. Soil capture by individual plants positively correlated with stem density, and stem density is increasing in the younger plants as a response to beetle damage. 4.,This study suggests that the interaction between O. rhodosticta and mesquite is an example of resource regulation that increases the stem density and soil capture ability of mesquite. Because the conversion of productive grasslands to mesquite dune landscapes is one of the most important drivers of desertification in the Chihuahuan Desert, feedbacks between organisms that promote an increase in the size and soil capture ability of mesquite may exacerbate desertification. [source]

Intraspecific diversity and dominant genotypes resist plant invasions

Measurements of transpiration in four tropical rainforest types of north Queensland, Australia

HYDROLOGICAL PROCESSES, Issue 26 2007
David McJannet
Abstract Transpiration of four different rainforest types in north Queensland, Australia, was determined using the heat pulse technique for periods ranging between 391 and 657 days. Despite the complexity of the natural rainforest systems being studied, the relationship between sample tree size and daily water use was found to be strong, thus providing a robust means by which to scale transpiration from individual trees to the entire forest stand. Transpiration was shown to be dependent on solar radiation and atmospheric demand for moisture with little evidence of limitation by soil moisture supply. Total stand transpiration was controlled by forest characteristics such as stem density, size distribution and sapwood area. Annual transpiration for each of the four sites ranged between 353 mm for cloud forest and 591 mm for montane rainforest. In comparison with the international literature, transpiration from Australian rainforests is low; the reasons for this could be related to a combination of differences in forest structure, climatic conditions, canopy wetness duration and tree physiology. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Influence of Emergent and Submerged Macrophytes on the Structure of Planktonic Ciliate Communities in Shallow Freshwater Lakes (Eastern Poland)

INTERNATIONAL REVIEW OF HYDROBIOLOGY, Issue 3 2008
Tomasz Mieczan
Abstract Data from two shallow macrophyte-dominated lakes (Eastern Poland) sampled with standardized methods, were evaluated in order to examine the effects of various stands of macrophytes in predicting protozooplankton community structure. Differences in macrophyte structure led to two distinct groups of habitats having different patterns of ciliate distribution. The first group consists of two vegetated habitats of sparse stem density and of the open water zone, and the second of submerged macrophyte species, which were more dense and complex. The number of significant correlations was different in the studied habitats. In central zones of macrophyte habitats the number of ciliates had the strongest correlation with concentrations of total organic carbon and Ptot. On the other side in the border zone a significant correlation between the number of ciliates and the chlorophyll a concentration was found. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Effect of selective logging on forest structure and nutrient cycling in a seasonally dry Brazilian Atlantic forest

JOURNAL OF BIOGEOGRAPHY, Issue 3 2006
Dora Maria Villela
Abstract Aim, The Brazilian Atlantic forest covers c. 10% of its original extent, and some areas are still being logged. Although several ecological studies in Atlantic forest have been published over the past two to three decades, there has been little research on forest dynamics and there is a particular lack of information on the effects of disturbance. The aim of the present study was to assess the impact of selective logging on forest structure, floristic composition soil nutrients, litterfall and litter layer in a seasonally dry Atlantic forest. Location, The Mata do Carvão is located in the Guaxindiba Ecological Reserve in São Francisco do ltabapoana district (21°24, S, 41°04, W), Rio de Janeiro, Brazil. Methods, Four plots (50 × 50 m) were set up in 1995 in each of two stands: unlogged and logged. In each plot, all trees , 10 cm d.b.h. were enumerated, identified and measured. Vouchers were lodged at UENF Herbarium. Five surface soil samples were collected in each plot in the dry season (in October 1995). Litterfall was collected in eight traps (0.50 m2) in each plot over a year from 14 November 1995 to 11 November 1996. The litter layer was sampled in eight quadrats (0.25 m2) in each plot in the dry and wet seasons. Soils were air-dried, sieved, and chemically analysed. The litter was dried (80 °C), sorted into six fractions, weighed and bulked samples analysed for nutrients. Results, Forest stands did not differ in stem density and total basal area, with a total of 1137 individuals sampled in 1996 (564 unlogged and 573 logged), and a total basal area of 15 m2 (unlogged) and 13.0 m2 (logged). However, unlogged stands had more large trees (, 30 cm in d.b.h.) and greater mean canopy height. Among the families, Rutaceae and Leguminosae were the most abundant families in both sites, although the Rutaceae had a higher density in unlogged and Leguminosae in the logged stand. The species diversity index was similar between stands. Late-successional species, such as Metrodorea nigra var. brevifolia and Paratecoma peroba, were less abundant in the logged stand. Selective logging did not affect nutrient concentrations in the soil or in the litter. However, quantities of the nutrients in the total litterfall and in the leaf litterfall and litter layer were higher in unlogged than in logged stands, mainly as a result of fallen M. nigra leaves. Metrodorea nigra was considered a key species in the nutrients dynamics in Carvão forest. Main conclusions, Despite the fact that effects on tree diversity and soil nutrients were not clear, selective logging in this Atlantic forest altered canopy structure, increased the relative abundance of some early-secondary species and decreased the litter input and stock of nutrients. Detailed information on the influence of logging on the distribution and structure of plant populations and in nutrient processes is fundamental for a sustainable logging system to be developed. [source]

Impacts of selective logging on population structure and dynamics of a canopy tree (Olea capensis) in Kakamega forest

AFRICAN JOURNAL OF ECOLOGY, Issue 3 2010
Mugatsia Tsingalia
Abstract This study assessed the impacts of logging on the population structure and dynamics of Elgon teak (Olea capensis), a canopy dominant in Kakamega forest. The objective was to ascertain and determine the regeneration status of this tree species in view of its commercial value. A comparison was made between logged and unlogged sections at three sites in the forest. Pairwise comparisons of the differences in the population structure among three sites and the differences in the mean tree size revealed significant differences in all three study sites. Results also revealed a variation in stem density that was dependent on the block of forest under study. Results of the spatial contagion revealed that O. capensis had a varied clumped distribution in all the study sites. Clumping was highest in the Kisere forest. These results point to logging as the major factor that explains the differences in population structure and stem density in different parts of Kakamega forest. And while intermittent recruitment may explain clumped distribution, logging appears to explain the variation in the degree of clumping. Résumé Cette étude a évalué les impacts des coupes de bois sur la structure et la dynamique de la population d'un olivier sauvage Olea capensis, arbre dominant de la canopée dans la forêt de Kakamega. L'objectif était de vérifier et de déterminer le statut de régénération de cette espèce dans l'optique de sa valeur commerciale. Nous avons fait une comparaison entre des sections exploitées ou non de trois sites de la forêt. Des comparaisons par paires des différences de structure de la population entre les sites et les différences de la taille moyenne des arbres des trois sites étudiés ont révélé des différences significatives entre les trois sites. Des résultats ont aussi révélé une variation de la densité des troncs qui dépendait du bloc forestier étudié. Des résultats de la contagion spatiale ont révélé qu'O. capensis avait une distribution en bouquets différente dans tous les sites. Le regroupement était le plus élevé dans la forêt de Kisere. Ces résultats indiquent que l'exploitation forestière serait le facteur le plus important expliquant les différences de structure de population et de densité des troncs observées dans différentes parties de la forêt de Kakamega. Et si un recrutement intermittent peut expliquer la distribution en bouquets, les coupes de bois semblent expliquer la variation du taux de rassemblement en bouquets. [source]

Diversity and composition of trees and shrubs in Kasagala forest: a semiarid savannah woodland in central Uganda

Patterns of elephant impact on woody plants in the Hluhluwe-Imfolozi park, Kwazulu-Natal, South Africa

AFRICAN JOURNAL OF ECOLOGY, Issue 1 2010
Roger Patrick Boundja
Abstract This study identifies patterns of elephant Loxodonta africana africana impacts upon tree species and woody plant communities in Hluhluwe-Imfolozi Park, a South African savannahs/woodlands area. Elephants were reintroduced there from 1981, following more than 80 years of absence. Data were collected in 2003 on elephant impact on woodland in the Park. Different vegetation types were susceptible to different types and levels of damage by elephants, suggesting that elephants will not homogenize the vegetation. Elephants targeted larger stems for all types of damage, with a strong preference for some of the less abundant species such as Albizia versicolor (breaking and toppling) and Cordia caffra and Schotia brachypetala (debarking). Elephant impacts tended to be distributed evenly across the park landscape, irrespective of stem density or proximity to permanent water. Overall, elephants have little impact on slowing or reversing the spread of undesirable woody species, but are having a marked impact on certain less common tree species and larger tree size-classes in the Hluhluwe-Imfolozi Park. Résumé Cette étude identifie le schéma des impacts des éléphants Loxodonta africana africana sur des espèces d'arbres et sur des communautés végétales du Parc de Hluhluwe-Imfolozi, une zone de forêts et de savanes d'Afrique du Sud. Des éléphants y furent réintroduits à partir de 1981, après une absence de plus de 80 ans. En 2003, on a récolté des données sur l'impact des éléphants sur les forêts du parc. ON a vu que les différents types de végétation étaient sensibles à des types et à des niveaux différents de dommages causés par les éléphants, ce qui laisse entendre que les éléphants ne vont pas uniformiser la végétation. Les éléphants visaient les plus gros troncs pour tous les types de dommages, avec une préférence marquée pour certaines des espèces les moins abondantes, comme Albizia versicolor (cassés et renversés) et Cordia caffra and Schotia brachypetala (écorcés). Les impacts des éléphants avaient tendance àêtre répartis également dans tout le paysage du parc, quelles que soient la densité des troncs ou la proximité de points d'eau permanents. En général, les éléphants avaient peu d'impact sur le ralentissement ou l'inversion de la dispersion des espèces ligneuses indésirables, mais ils ont un impact réel sur certaines espèces d'arbres moins fréquentes et sur les arbres de classes de taille plus grandes dans le Parc de Hluhluwe-Imfolozi. [source]

Sacoglottis gabonensis, a keystone fruit for forest elephants in the Réserve de Faune du Petit Loango, Gabon

AFRICAN JOURNAL OF ECOLOGY, Issue 2 2009
Bethan J. Morgan
Abstract This study suggests that the fruits of Sacoglottis gabonensis (Baill.) Urb. (Humiriaceae) are a keystone resource for forest elephants (Loxodonta cyclotis Matschie) in a coastal rain forest, the Réserve de Faune du Petit Loango, Gabon (now part of Loango National Park). Faecal counts demonstrated that forest elephants used Sacoglottis -dominated forest more when Sacoglottis was abundant and electivity indices suggest that Sacoglottis is a preferred food. The flora of Petit Loango is characterized by the absence of herbaceous vegetation such as Marantaceae and Zingiberaceae, and during the prolonged dry season few fleshy fruits are present other than Sacoglottis fruits, which are produced in a glut during this time. While inter-annual fruiting reliability remains to be confirmed, fruit production in 1998 and high stem density relative to other study sites provide indirect evidence that Sacoglottis fruits are a reliable inter-annual resource at Petit Loango. It is thus proposed that Sacoglottis gabonensis fruits fulfil an important role as a keystone ,fallback' resource for forest elephants during the dry season at Petit Loango. Résumé Cet article suggère que les fruits de Sacoglottis gabonensis (Baill.) Urb. (Humiriaceae) sont une ressource primordiale pour l'éléphant de forêt (Loxodonta cyclotis Matschie) dans une forêt pluviale côtière, la Réserve de Faune du Petit Loango, au Gabon, qui fait partie du Parc National de Loango. Le comptage des excréments a montré que les éléphants de forêt fréquentent les forêts dominées par Sacoglottis davantage lorsque les Sacoglottis sont abondants, et les indices d'électivité suggèrent que c'est leur nourriture préférée. La flore de Petit Loango se caractérise par l'absence de végétation herbacée comme les Marantaceae et les Zingiberaceae, et pendant la longue saison sèche, il y a peu d'autres fruits pulpeux que les Sacoglottis qui sont produits en abondance à cette saison. Même s'il faut encore confirmer que les fruits sont produits en continu d'une année à l'autre, la production des fruits en 1998 et la densité de hautes pousses, comparées aux autres sites étudiés, montrent indirectement que les fruits de Sacoglottis sont une ressource fiable tout au long de l'année à Petit Loango. On pose donc l'hypothèse que les fruits de Sacoglottis gabonensis jouent un rôle important comme aliment ressource pour les éléphants de forêt de Petit Loango pendant la saison sèche. [source]

Ecological recovery of an afromontane forest in south- western Uganda

AFRICAN JOURNAL OF ECOLOGY, Issue 2004
J. B. Lejju
Abstract A study of the regeneration of an afromontane forest was carried out in Mgahinga Gorilla National Park (MGNP), south-western Uganda. The area landscape has been subjected to agricultural encroachment for the last 50 years. The landscape was changed by terracing and removing the indigenous vegetation and replacing it with exotic tree species. Stratified random sampling was employed in sampling the vegetation. There was a significant difference in species richness and density in the three habitat types. The natural forest supported the highest stem density (75%) and the lowest stem density (4%) was recorded under exotic woodlots. Seedlings (<2 cm, diameter at breast height) accounted for the majority of juveniles in the three habitats. The natural forest had the highest density (24,625 seedlings ha,1) and exotic woodlots supported the lowest stem density (1350 seedlings ha,1). The level of regeneration in the encroachment area is influenced by the intensity of cultivation and soil nutrients. The advanced growth beneath the exotic woodlots, especially black wattle (Acacia mearnsii) and Eucalyptus sp. stands is relatively impoverished. This condition beneath the exotic species suggests that a low diverse community of native species is able to exploit this environment. [source]

Forest succession in Kibale National Park, Uganda: implications for forest restoration and management

AFRICAN JOURNAL OF ECOLOGY, Issue 1 2003
Jeremiah S. Lwanga
Abstract Forest succession was studied in four plots in former grasslands at the Ngogo study area in Kibale National Park, Uganda. The plots were located in areas that had been protected from fire for 0.58, 25, 9 and ,30 years for plots 1, 2, 3 and 4, respectively. Species richness reflected the length of time that the plot had been protected from fire; it was highest in plot 4 and lowest in plot 1. Species density, stem density and basal area were all highest in plot 4 and lowest in plot 1. The species densities of plots 2 and 3 were not different. Similarly, plots 2 and 4 did not differ with regard to stem density or basal area. Animal seed dispersers played a vital role in the colonization of grasslands by forest tree species. Résumé On a étudié la succession forestière dans quatre plots d'anciennes prairies dans la zone de recherche de Ngogo, dans le Parc National de Kibale, en Ouganda. Ces plots étaient situés dans des endroits qui avaient été protégés du feu pendant 0, 58, 25, 9 et ± 30 ans pour les plots 1, 2, 3 et 4 respectivement. La richesse en espèces reflétait la durée pendant laquelle le plot avait été protégé du feu ; elle était maximale dans le plot 4 et minimale dans le plot 1. La densité des espèces n'était pas différente dans les plots 2 et 3. De même, les plots 2 et 4 ne différaient pas en ce qui concernait la densité des pousses ou la surface basale. Les animaux qui dispersent les semences ont joué un rôle vital dans la colonisation des prairies par les espèces d'arbres de forêts. [source]

The regulation of ecosystem functions by ecotypic variation in the dominant plant: a Spartina alterniflora salt-marsh case study

JOURNAL OF ECOLOGY, Issue 1 2002
Denise M. Seliskar
Summary 1Genetic differences among populations of a keystone species may affect ecosystem functional properties. We tested this by planting Spartina alterniflora from different geographical regions in a newly created salt marsh in Delaware, USA. 2Spartina alterniflora plants from morphologically distinct short-form (back marsh) populations were originally collected from Massachusetts (41°34, N), Delaware (38°47, N), and Georgia (31°25, N) in the USA and vegetatively propagated for 6 years in a salt water-irrigated common garden in Delaware before transfer to a newly created salt marsh. 3The magnitude of the expression of marsh functions in the created marsh, measured over 5 years, remained distinct in patches of each ecotype. End of season aerial biomass, below-ground biomass, root and rhizome distribution, canopy height, stem density, and carbohydrate reserves were closer to values reported for the plants' native sites than to those typical of Delaware. Thus, many of the plant features characteristic of particular latitudes appear to be under genetic control. Such ecotypic differentiation influences ecosystem function through keystone resource and keystone modifier activities. 4Respiration of the microbial community associated with either dead shoots or the soil varied with plant ecotype in the created wetland and the patterns reflected those reported for their native sites. High edaphic respiration under the Massachusetts ecotype was correlated with the high percentage of sugar in the rhizomes. Edaphic chlorophyll was greater under the canopies of the Massachusetts and Delaware ecotypes than under the Georgia canopy and exhibited a relationship similar to that of algal production rates reported for the native sites. Larval fish were most abundant in pit traps in the Massachusetts ecotype. [source]

Effects of the past and the present on species distribution: land-use history and demography of wintergreen

JOURNAL OF ECOLOGY, Issue 2 2000
Kathleen Donohue
Summary 1,Past land use can have long-term effects on plant species' distributional patterns if alterations in resources and environmental conditions have persistent effects on population demography (environmental change) and/or if plants are intrinsically limited in their colonization ability (historical factors). 2,We evaluated the role of environmental alteration vs. historical factors in controlling distributional patterns of Gaultheria procumbens, a woody, clonal understorey species with a pronounced restriction to areas that have never been ploughed, and near absence from adjoining areas that were ploughed in the 19th century. The demographic study was conducted in scrub oak and hardwood plant communities on an extensive sand plain, where it was possible to control for the effect of variation in environment prior to land use. 3,The observed demographic effects were contrary to the hypothesis that persistent environmental alteration depressed demographic performance and limited the distribution of G. procumbens. We observed no overall effect of land-use history on stem density, stem recruitment or flower production. In fact, some aspects of performance were enhanced in previously ploughed areas. Populations in previously ploughed areas exhibited less stem mortality in scrub oak transitions, an increase in germination, seedling longevity and proportion of potentially reproductive stems in both plant communities, a trend for slower observed rates of population decline in both plant communities, and a higher projected rate of population growth in the scrub oak transitions. Thus, particularly in scrub oak communities, the lower abundance of G. procumbens in formerly ploughed than in unploughed areas contrasted with its performance. 4,The limited occurrence of G. procumbens in formerly farmed areas was explained instead by its slow intrinsic growth rate, coupled with limited seedling establishment. Lateral population extension occurred exclusively through vegetative growth, allowing a maximum expansion of 43 cm year,1. 5,We conclude that inherent limitations in the colonizing ability of some plant species may present a major obstacle in the restoration or recovery of plant communities on intensively disturbed sites, even in the absence of persistent environmental effects that depress population growth. [source]

Density and habitat associations of Henslow's Sparrows wintering in saline soil barrens in southern Arkansas

JOURNAL OF FIELD ORNITHOLOGY, Issue 4 2008
William C. Holimon
ABSTRACT Although the habitat requirements of breeding populations of Henslow's Sparrow (Ammodramus henslowii) have been examined, less is known about their habitat requirements and ecology during the nonbreeding season. We estimated population densities and quantified habitat associations of Henslow's Sparrows wintering in saline soil barrens in southern Arkansas. Densities of Henslow's Sparrows in the saline soil barrens were similar to those in the Longleaf Pine (Pinus palustris) Ecosystem of the southeastern United States, considered by many to be their primary wintering habitat. Henslow's Sparrows were closely associated with open areas with greater cover of Aristida spp. and globe beaksedge (Rhynchospora globularis), greater stem density at 11,20 cm above ground, more lichens, more herbaceous cover, more bare ground, greater occurrence of little bluestem (Schizacyrium scoparium) as the tallest vegetation, less moss, and less shrub cover than randomly selected sites. In contrast to the results of studies conducted in the Longleaf Pine Ecosystem, the presence of Henslow's Sparrows in our study was not correlated with the height of the tallest vegetation. Our results indicate that saline soil barrens of southern Arkansas support a high density of wintering Henslow's Sparrows and do so for longer postdisturbance periods than longleaf pine savanna. We also found that stem density near the ground was similar to that reported from longleaf pine savanna, but only about half that observed on their breeding grounds. Areas used by Henslow's Sparrows had more lichen and less moss cover, suggesting that those areas were drier than random sites within the barrens. Further research is needed to determine if large populations of Henslow's Sparrows winter in other saline soil barrens and if fire influences habitat associations and densities in the barrens. SINOPSIS Aunque los requisitos de hábitat para las poblaciones reproductivas de Ammodramus henslowii han sido determinadas, se conoce muy poco sobre su ecología y requerimientos de hábitat durante la temporada no-reproductiva. Estimamos la densidad poblacional y cuantificamos el hábitat asociado a Gorrión de Henslow que pasan el invierno en un salitral con suelo empobrecido en el sur de Arkansas. La densidad de las aves en el salitral resultó similar a lo encontrado en Ecosistemas de Pinos (Pinus palustris) en el sureste de los EUA, considerado por muchos como el principal hábitat invernal para la especie. Los gorriones estuvieron altamente asociados a áreas abiertas con covertura de Aristida spp. y Rhynchospora globularis, con mayor densidad de tallos, altura entre 11,20 cm sobre el suelo, mayor cantidad de líquenes, mayor cubierta herbácea, más suelo desnudo, mayor presencia de Schzacyrium scoparium (como la vegetación de mayor tamaño), menos musgos, y menos arbustos que localidades seleccionadas al azar. En contraste a los resultados de estudios conducidos en Ecosistemas de Pinos, la presencia del gorrión en nuestra área de estudio no estuvo correlacionada con la altura de la vegetación de mayor tamaño. Nuestros resultados indican que las salinas en Arkansas sostienen una alta densidad de aves invernales, y lo hacen por periodos más largos, después de disturbios, que en las savanas de pinos. También encontramos que la densidad de tallos, cerca del suelo, era similar a la informada en savanas de pinos, pero tan solo la mitad de lo indicado para lugares en donde las aves se reproducen. Las áreas utilizadas tienen más líquenes, pero menos musgos, lo que sugiere que dichas áreas son más secas que lugares con suelo empobrecido muestreados al azar. Se necesitan más trabajos para determinar si otras grandes poblaciones del gorrión de Henslow pasan el invierno en otras salinas con suelos empobrecidos y si eventos como fuegos incluyen en la asociación del hábitat y densidades en los lugares con suelo empobrecido. [source]

Landscape context and microenvironment influences on liana communities within treefall gaps

JOURNAL OF VEGETATION SCIENCE, Issue 5 2008
Agustina Malizia
Abstract Questions: How do gap aspect, openness, age and gap density in the surroundings affect diversity and composition of lianas within treefall gaps? Are the variation patterns in liana communities within treefall gaps associated with their climbing mechanisms? Location: Subtropical montane forests in northwestern Argentina. Methods: We used ordination analyses (NMDS) and multiple regressions to describe liana communities (species and climbing groups, stems , 1 cm) in 35 gaps located in a 6-ha plot and to assess relationships with aspect, canopy and subcanopy openness, age and density of recent (three to eight years old) and old (> 8 years old) gaps. Results: Treefall gaps segregated in the ordination diagrams based on their liana species and climbing mechanisms composition: gaps surrounded by a higher density of recent gaps showed higher densities of lianas species using tendrils, spines/hooks and voluble stems. Liana density and richness (independent of stem density) were also positively associated with the density of recent gaps in the surroundings, and negatively associated to gap age. The number of liana species corrected per area was negatively associated to gap aspect and subcanopy openness, and absolute number of liana species was positively associated to gap canopy openness. Conclusions: This study integrates the analysis of different factors influencing liana communities within treefall gaps, and shows that spatial context of gap density (a variable largely neglected in previous studies) is particularly important on lianas composition and diversity, probably by increasing propagule input and the availability of small trellises for support. [source]

Relationships between soil hydrology and forest structure and composition in the southern Brazilian Amazon

JOURNAL OF VEGETATION SCIENCE, Issue 2 2007
Stefan Jirka
Abstract Question: Is soil hydrology an important niche-based driver of biodiversity in tropical forests? More specifically, we asked whether seasonal dynamics in soil water regime contributed to vegetation partitioning into distinct forest types. Location: Tropical rain forest in northwestern Mato Grosso, Brazil. Methods: We investigated the distribution of trees and lianas , 1 cm DBH in ten transects that crossed distinct hydrological transitions. Soil water content and depth to water table were measured regularly over a 13-month period. Results: A detrended correspondence analysis (DCA) of 20 dominant species and structural attributes in 10 × 10 m subplots segregated three major forest types: (1) high-statured upland forest with intermediate stem density, (2) medium-statured forest dominated by palms, and (3) low-statured campinarana forest with high stem density. During the rainy season and transition into the dry season, distinct characteristics of the soil water regime (i.e. hydro-indicators) were closely associated with each vegetation community. Stand structural attributes and hydro-indicators were statistically different among forest types. Conclusions: Some upland species appeared intolerant of anaerobic conditions as they were not present in palm and campinarana sites, which experienced prolonged periods of saturation at the soil surface. A shallow impermeable layer restricted rooting depth in the campinarana community, which could heighten drought stress during the dry season. The only vegetation able to persist in campinarana sites were short-statured trees that appear to be well-adapted to the dual extremes of inundation and drought. [source]

Effects of repeated burning on species richness in a Florida pine savanna: A test of the intermediate disturbance hypothesis

JOURNAL OF VEGETATION SCIENCE, Issue 1 2000
Brian Beckage
Wunderlin (1982) except for Aristida beyrichiana; which follows Peet (1993). Vouchers for each species were collected and deposited at the University of Central Florida herbarium Abstract. We studied the effect of burning frequency on the density and species richness of understory flowering stems in a Florida sandhill. Flowering stems were censused weekly for 54 weeks in six sites that had been burned one to six times in the previous 16 years. We concurrently measured overstory characteristics such as species composition, density and basal area. We used maximum likelihood and Akaike's Information Criterion to compare linear, quadratic, saturating, and null models of community response to repeating burning. We did not find a relationship between species richness, diversity or flowering stem density and fire frequency. Tree density was related to fire frequency and may represent an indirect pathway for fire effects on understory characteristics. While we found no support for the Intermediate Disturbance Hypothesis, an analysis of our experimental design indicated that we had low statistical power. We develop the hypothesis that a saturating model of response to fire best describes understory species richness in our system. We test this hypothesis using the most extensive published fire data set we are aware of and find support for a saturating model. [source]

Heritability of morphophysiological traits and inbreeding effects in grazing-type lucerne

PLANT BREEDING, Issue 2 2005
L. Pecetti
Abstract To estimate heritability and inbreeding in grazing-type lucerne, 14 parent genotypes and their half-sib and selfed progeny were grown under spaced-plant conditions. Dry-matter weight (four cuts), plant height, basal plant diameter, stem density, and late-autumn vegetation were recorded on a plant basis. Genetic coefficients of variation for the three germplasm groups, and broad-sense heritability on a plot basis for parents were computed from variance component estimates; narrow-sense heritability was estimated from progeny-parent regression. Genetic variation was generally greater among selfed than half-sib progeny. Broad-sense heritability was high, while narrow-sense heritability was much lower for all traits except plant diameter. Inbreeding effects, assessed with comparison between parents and selfed progeny, were notable for plant weight and late-autumn vegetation only. The high correlation coefficients computed between parents and either progeny suggested the equivalent value of self- and polycross-progeny tests for selecting superior parents. [source]

Impact of landscape spatial pattern on liana communities in tropical rainforests at Los Tuxtlas, Mexico

Impacts of cyclone Larry on the vegetation structure of timber plantations, restoration plantings and rainforest on the Atherton Tableland, Australia

AUSTRAL ECOLOGY, Issue 4 2008
JOHN KANOWSKI
Abstract We examined the impact of severe cyclone ,Larry' on the vegetation structure of monoculture and mixed species timber plantations, restoration plantings and reference sites in upland rainforests on the Atherton Tableland, north Queensland, Australia. Sites were initially assessed in 2000 and resurveyed in 2006, 6,8 months after the cyclone traversed the region. In both surveys, timber plantations had a relatively open canopy, grassy understorey and few shrubs or small-sized trees; whereas restoration plantings had a relatively closed canopy, an understorey of bare ground, leaf litter and rainforest seedlings, a high density of small-diameter trees and a moderate representation of special life forms characteristic of rainforest. Cyclone damage varied with tree size, site type, proximity to the cyclone and stem density. First, the proportion of trees that were severely damaged by the cyclone (major branches broken, stem snapped or pushed over) increased with the diameter of trees across all site types. Second, damage to larger-sized trees (>10 cm d.b.h., >20 cm d.b.h.) was proportionally highest in monoculture plantations, intermediate in mixed species plantations and rainforest, and lowest in restoration plantings. Third, within site types, damage levels decreased with distance from the cyclone track and with stem density. There was no evidence that topographical position influenced damage levels, at least for timber plantations. We tentatively attribute the high levels of damage experienced by timber plantations to their relatively open structure and the large size of stems in plantations. Restoration plantings generally escaped severe damage by the cyclone, but their continued development towards rainforest conditions may require a coordinated monitoring and maintenance programme to address the potential threat of weed invasion. [source]

Flowering phenology of myrtaceous trees and their relation to climatic, environmental and disturbance variables in northern New South Wales

AUSTRAL ECOLOGY, Issue 2 2000
B. Law
Abstract Patterns of flowering phenology, which represent a partial description of food availability for nectarivores, are described for 20 species of myrtaceous trees on the mid-north coast of New South Wales (NSW), Australia. Data were recorded monthly between 1982 and 1992 across 23 sites that comprise a variety of local environmental conditions and disturbance histories. Flowering periodicity and intensity were highly variable between species and sites, ranging from annual flowering to no flowering over the 10-year period. Cool temperatures prior to floral budding was a strong predictor of flowering for nine species. Extraordinary climatic events also influenced flowering. The period of greatest flowering for all species combined occurred 9 months after the highest monthly rainfall recorded in the survey (March 1985). An 18-month extreme drought led to poor flowering in Corymbia variegata, Eucalyptus acmenoides, Eucalyptus grandis and Eucalyptus resinifera, but recovery after the drought broke was rapid. In contrast to climate, few site-based environmental variables explained the intersite variation in flowering performance of the tree species. Site disturbance from logging at two sites during the survey did not influence flowering in the remaining canopy over following years. No species showed a negative correlation with a history of recent logging, and direct comparisons, between large- and medium-sized trees, of the percentage of foliage in flower showed no differences for any species. At the scale of a timber production forest, the negligible effect of tree size (if >10 cm diameter at breast height over bark), and high stem density resulting from selective logging, leave about half of the net harvestable area producing flowers at a similar density to unlogged forest. However, larger trees flowered more frequently than medium-sized trees in C. variegata (medium: every 5.9 years; large: every 2.3 years) and there was a trend in this direction for 13 of 17 species. Low-intensity burns and wildfires caused differing amounts of crown scorch, sometimes resulting in bud loss, but most species flowered at prefire levels 1,3 years after the disturbance. Eucalyptus microcorys and Angophora costata flowered poorly at sites that experienced frequent low-intensity burns. Regionally, blossom shortages occur through a combination of spatial and temporal patchiness in flowering and the clearing of those species that occurred on soils preferred for agriculture. On the north coast of NSW, these shortages commonly occur from late winter to spring. Species that flower reliably in this period include Eucalyptus robusta, Eucalyptus tereticornis and Eucalyptus siderophloia in late winter and E. siderophloia and E. acmenoides in spring. [source]