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Arid Ecosystems (arid + ecosystem)

Distribution by Scientific Domains

Life Sciences	66%
Earth and Environmental Science	25%

Selected Abstracts

Pulse dynamics and microbial processes in aridland ecosystems

JOURNAL OF ECOLOGY, Issue 3 2008
Scott L. Collins
Summary 1Aridland ecosystems cover about one-third of terrestrial environments globally, yet the extent to which models of carbon (C) and nitrogen (N) cycling, developed largely from studies of mesic ecosystems, apply to aridland systems remains unclear. 2Within aridland ecosystems, C and N dynamics are often described by a pulse-reserve model in which episodic precipitation events stimulate biological activity that generate reserves of biomass, propagules and organic matter that prime the ecosystem to respond rapidly to subsequent precipitation events. 3,The role of microbial C and N processing within the pulse-reserve paradigm has not received much study. We present evidence suggesting that fungi play a critical and underappreciated role in aridland soils, including efficient decomposition of recalcitrant C compounds, N-transformations such as nitrification, and nutrient storage and translocation of C and N between plants and biotic soil crusts. While fungi may perform some of these functions in other ecosystems, this ,fungal loop' assumes particular importance in the N cycle in aridlands because water availability imposes even greater restrictions on bacterial activity and physicochemical processes limit accumulation of soil organic matter (SOM). 4We incorporate these findings into a Threshold-Delay Nutrient Dynamics (TDND) model for aridland ecosystems in which plant responses to pulsed precipitation events are mediated by a fungal loop that links C and N cycling, net primary production (NPP) and decomposition in aridland soils. 5Synthesis. Arid ecosystems are highly sensitive to global environmental change including N deposition and altered precipitation patterns; yet, models from mesic ecosystems do not adequately apply to aridland environments. Our ,fungal loop' N cycle model integrates spatial structure with pulse dynamics and extends the pulse-reserve paradigm to include the key role of microbial processes in aridland ecosystem dynamics. [source]

Patterns of ant species richness along elevational gradients in an arid ecosystem

GLOBAL ECOLOGY, Issue 2 2003
Nathan J. Sanders
ABSTRACT Aim In this study, we examine patterns of local and regional ant species richness along three elevational gradients in an arid ecosystem. In addition, we test the hypothesis that changes in ant species richness with elevation are related to elevation-dependent changes in climate and available area. Location Spring Mountains, Nevada, U.S.A. Methods We used pitfall traps placed at each 100-m elevational band in three canyons in the Spring Mountains. We compiled climate data from 68 nearby weather stations. We used multiple regression analysis to examine the effects of annual precipitation, average July precipitation, and maximum and minimum July temperature on ant species richness at each elevational band. Results We found that patterns of local ant species richness differed among the three gradients we sampled. Ant species richness increased linearly with elevation along two transects and peaked at mid-elevation along a third transect. This suggests that patterns of species richness based on data from single transects may not generalize to larger spatial scales. Cluster analysis of community similarity revealed a high-elevation species assemblage largely distinct from that of lower elevations. Major changes in the identity of ant species present along elevational gradients tended to coincide with changes in the dominant vegetation. Regional species richness, defined here as the total number of unique species within an elevational band in all three gradients combined, tended to increase with increasing elevation. Available area decreased with increasing elevation. Area was therefore correlated negatively with ant species richness and did not explain elevational patterns of ant species richness in the Spring Mountains. Mean July maximum and minimum temperature, July precipitation and annual precipitation combined to explain 80% of the variation in ant species richness. Main conclusions Our results suggest that in arid ecosystems, species richness for some taxa may be highest at high elevations, where lower temperatures and higher precipitation may support higher levels of primary production and cause lower levels of physiological stress. [source]

Vegetation Change and Soil Nutrient Distribution along an Oasis-Desert Transitional Zone in Northwestern China

JOURNAL OF INTEGRATIVE PLANT BIOLOGY, Issue 11 2007
Bao-Ming Chen
Abstract Many studies have focused on soil nutrient heterogeneity and islands of fertility in arid ecosystems. However, few have been conducted on an oasis-desert transitional zone where there is a vegetation pattern changing from shrubs to annual herbs. The goal of the present study was to understand vegetation and soil nutrient heterogenity along an oasis-desert transitional zone in northwestern China. Three replicated sampling belts were selected at 200 m intervals along the transitional zone. Twenty-one quadrats (10 × 10 m) at 50 m intervals were located along each sampling belt. The vegetation cover was estimated through the quadrats, where both the soil under the canopy and the open soil were sampled simultaneously. The dominated shrub was Haloxylon ammodendron in the areas close to the oasis and Nitraria tangutorum dominated the areas close to the desert. In general, along the transitional zone the vegetation cover decreased within 660 m, increased above 660 m and decreased again above 1 020 m (close to the desert). The soil nutrients (organic matter, total N, NO3, and NH4+) showed significant differences along the zone. The soil nutrients except the soil NH4+ under the canopy were higher than those in open soil, confirming "islands of fertility" or nutrient enrichment. Only a slight downward trend of the level of "islands of fertility" for soil organic matter appeared in the area within 900 m. Soil organic matter both under canopy and in interspace showed a positive correlation with the total vegetation cover, however, there was no significant correlation between the other soil nutrients and the total vegetation cover. We also analyzed the relationship between the shrubs and annuals and the soil nutrients along the zone. Similarly, there was no significant correlation between them, except soil organic matter with the annuals. The results implied that annual plants played an important role in soil nutrient enrichment in arid ecosystem. [source]

Grazing without grasses: Effects of introduced livestock on plant community composition in an arid environment in northern Patagonia

APPLIED VEGETATION SCIENCE, Issue 1 2006
Mariana Tadey
Abstract Question: How does grazing intensity affect plant density, cover and species richness in an Patagonian arid ecosystem? Location: Monte steppe ecoregion, SW Argentina. Methods: I analysed the effect of grazing on plant density, cover and species richness using a stocking rate gradient within the same habitat. Six paddocks were used with stocking rates ranging between 0.002 , 0.038 livestock/ha. Plant density, species richness, plant cover and percentage of grazed branches were determined by sampling plots within each paddock. The percentage of grazed branches was used as an independent measurement of grazing intensity. Results: Higher stocking rates were related to lower plant density, species richness and plant cover. The paddock with the lowest grazing intensity had 86% more plants per unit area, 63% more plant cover and 48% higher species richness. The percentage of grazed branches and the quantity of dung increased with stocking rate. Conclusions: Introduced livestock seriously affect native vegetation in the Patagonian Monte. The damage observed in this xerophytic plant community suggests that plant adaptations to aridity do not provide an advantage to tolerate or avoid grazing by vertebrate herbivores in this region. Plant degradation in this arid environment is comparable to the degradation found in more humid ecosystems. [source]

Patterns of ant species richness along elevational gradients in an arid ecosystem

Community organization and species richness of ants (Hymenoptera/Formicidae) in Mongolia along an ecological gradient from steppe to Gobi desert

JOURNAL OF BIOGEOGRAPHY, Issue 12 2003
Martin Pfeiffer
Abstract Aim, Ants (Hymenoptera/Formicidae) have strong influences on ecosystems especially in arid regions. However, little is known about ants of the vast steppe and desert regions of Central Asia. Here we provide the first comprehensive study of ant communities in Mongolia, conducted along a north-to-south gradient in climate. We examined ants' distribution patterns, assessed the impact of climatic parameters on community structure and species diversity and investigated the influence of the corresponding communities of plants. Location, Mongolia (Central Asia). Methods, We observed 31,956 ants at seed baits at 11 study sites along a transect from steppe to Gobi desert for which we attained meteorological data (mean yearly precipitation: 197 to 84 mm). Extra sampling was conducted at sugar and protein baits and by the inspection of different microhabitats. Vegetation patterns of each plot were recorded. Statistical evaluation comprised ordination and correlation. Results, We observed 15 species of ants at seed baits. Three faunal complexes of ants could be distinguished by detrended correspondence analysis (DCA): (1) in steppe baits were dominated by Formica - and Myrmica -species, (2) in semi desert we found mostly species of Tetramorium, Myrmica, Proformica, Plagiolepis, and Leptothorax, and (3) in desert Cataglyphis aenescens and Messor aciculatus dominated, and Lasius was exclusively found there. Another 11 rare ant species were sampled by hand and at sugar baits. Altogether five ant species were new to the Mongolian fauna: Cardiocondyla koshewnikovi, Myrmica koreana, Myrmica pisarskii, Polyergus nigerrimus, and Proformica kaszabi. Assignment of taxa to functional groups showed that in steppe cold climate specialists dominated, in semi desert we found mainly opportunists, and in desert hot climate specialists. Several functional groups know from arid zones in other parts of the world were missing. In desert certain species were highly dominant. First DCA scores of ant- and plant-communities were highly correlated with each other and with climatic parameters. While plant species diversity was positively correlated with increasing northern latitude, ant diversity and ant species richness were not correlated with latitude and responded neither to precipitation, nor to any other climatic parameter. Semi desert was a transition zone between steppe and desert, with high species richness. Ant genus composition of the ecotone overlapped with both other regions. However, beta diversity between pairs of plots within this zone was low, indicating a small-scale mosaic pattern. Main conclusions, The ant communities in the Mongolian steppe and desert zones were strongly influenced by low temperatures and differed in many aspects from the ant fauna in other arid ecosystems, especially in terms of species richness, diversity of feeding guilds, and richness of functional groups. [source]

Extending point pattern analysis for objects of finite size and irregular shape

JOURNAL OF ECOLOGY, Issue 4 2006
THORSTEN WIEGAND
Summary 1We use a grid- and simulation-based approach to extend point pattern analysis to deal with plants of finite size and irregular shape, and compare the results of our approach with that of the conventional point approximation. The plants are approximated by using an underlying grid and may occupy several adjacent grid cells depending on their size and shape. Null models correspond to that of point pattern analysis but need to be modified to account for the finite size and irregular shape of plants. 2We use a mapped area of a grass-shrub steppe in semi-arid Patagonia, Argentina, to show that the shrub community is essentially randomly structured, but that shrubs facilitate grasses in their immediate neighbourhood. 3The occurrence of this random spatial structure provides important new information on the biology of shrub populations. In general, previous data from semi-arid and arid ecosystems have shown that adult shrubs tend to show over-dispersed patterns, whereas juveniles are clumped. 4We find that the point approximation may produce misleading results (i) if plant size varies greatly, (ii) if the scale of interest is of the same order of magnitude as the size of the plants, and (iii) if the plants of a given pattern are constrained through competition for space by the presence of other plants. The point approximation worked well in all other cases, but usually depicted weaker significant effects than when the size and shape of plants were taken into account. 5Our approach to quantifying small-scale spatial patterns in plant communities has broad applications, including the study of facilitation and competition. Ecologists will be able to use the software available to take advantage of these methods. [source]

Vegetation Change and Soil Nutrient Distribution along an Oasis-Desert Transitional Zone in Northwestern China

Spatial patterns of desert annuals in relation to shrub effects on soil moisture

JOURNAL OF VEGETATION SCIENCE, Issue 2 2010
J. Li
Abstract Questions: What are the effects of a shrub (Haloxylon ammodendron) on spatial patterns of soil moisture in different seasons? How does productivity of understorey annuals respond to these effects? Are such effects always positive for annuals under shrubs? Location: South Gurbantunggut Desert, northwest China. Methods: Using geostatistics, we explored seasonal patterns of topsoil moisture in a 12 × 9-m plot over the growing season. To determine spatial patterns of understorey annuals in response to H. ammodendron presence, biomass of annuals was recorded in four 0.2 × 5.0-m transects from the centre of a shrub to the space between shrubs (interspace). We also investigated vertical distribution of root biomass for annuals and soil moisture dynamics across soil profiles in shrub-canopied areas and interspaces. Results: Topsoil moisture changed from autocorrelation in the wet spring to random structure in the dry season, while soil moisture below 20 cm was higher in shrub-canopied areas. Across all microhabitats, soil moisture in upper soil layers was higher than in deeper soil layers during the spring wet season, but lower during summer drought. Topsoil was close to air-dry during the dry season and developed a ,dry sand layer' that reduced evaporative loss of soil water from deeper layers recharged by snowmelt in spring. Aboveground biomass of understorey annuals was lowest adjacent to shrub stems and peaked at the shrub margin, forming a ,ring' of high herbaceous productivity surrounding individual shrubs. To acclimate to drier conditions, annuals in interspaces invested more root biomass in deeper soil with a root/shoot ratio (R/S) twice that in canopied areas. Conclusions: Positive and negative effects of shrubs on understorey plants in arid ecosystems are commonly related to nature of the environmental stress and tested species. Our results suggest there is also microhabitat-dependence in the Gurbantunggut Desert. Soil water under H. ammodendron is seasonally enriched in topsoil and deeper layers. Understorey annuals respond to the effect of shrubs on soil water availability with lower R/S and less root biomass in deeper soil layers and develop a ,ring' of high productivity at the shrub patch margin where positive and negative effects of shrubs are balanced. [source]

Role of host identity in effects of phytogenic mounds on plant assemblages and species richness on coastal arid dunes

JOURNAL OF VEGETATION SCIENCE, Issue 5 2007
Magdy I. El-Bana
Abstract Question: Phytogenic mounds (nebkhas) - the natural accumulation of wind-blown sediments within or around the canopies of plants - have been proposed as important structures for locally maintaining high species richness in coastal and arid ecosystems. Nebkhas are assumed to increase habitat heterogeneity, but what is the importance of the nebkha host species relative to other nebkha characteristics in determining the associated plant assemblages? Are some host species more effective in creating diversity hotspots, or does a single species-area relationship apply to all nebkhas, regardless of host species? Can the influence of the host be ascribed to its indirect effects on abiotic attributes of the nebkha complex? Methods and location: We investigated plant species richness and composition on nebkhas around six psammophytic species on Mediterranean coastal dunes of the Sinai Peninsula. Results: Plant species richness was significantly related to nebkha size by the single power function according to the general prediction of island biogeography theory, but this relationship was modified - though to a limited degree - by nebkha host species identity. Canonical Correspondence Analysis revealed that nebkha host species identity and nebkha environmental and non-environmental factors significantly explained species composition on the nebkhas, but host species identity did so to a greater extent. The latter might reflect differences in seed trapping ability or free space for colonization between host species. Conclusion: Differences in community composition and richness among nebkhas formed by different host species represent a key factor in the maintenance of plant diversity on arid coastal dunes. [source]

The ecological value of Eryngium horridum in maintaining biodiversity in subtropical grasslands

AUSTRAL ECOLOGY, Issue 5 2009
ALESSANDRA FIDELIS
Abstract The role of facilitation in the structuring of plant communities has been often demonstrated in environments under high abiotic stress, especially in semi-arid and arid ecosystems and high elevations. Few studies, however, analysed facilitation in systems that are highly productive and rich in species, which are thought to be theoretically unlikely to demonstrate strong effects of facilitation. Here, we investigate the importance of Eryngium horridum, a rosette species, on the maintenance of plant diversity in subtropical grasslands in southern Brazil. We evaluated facilitation in areas under two different types of management: abandonment and grazing. Plots were established in areas with and without individuals of E. horridum and all species were identified and had their cover estimated. The Relative Neighbour Effect index was calculated in order to verify the presence of competition or facilitation. Our results indicated facilitation in both abandoned and in grazed grasslands, but apparently through different mechanisms. In the first case, the plant's architecture opens the canopy and allows more light to reach small forbs in the grass matrix. In the second case, E. horridum appears to protect more palatable species from herbivores. Otherwise considered an obnoxious species, E. horridum plays an important ecological role in subtropical grasslands in southern Brazil by facilitating other species and consequently, increasing local richness. Areas with this rosette species are important sources of diaspores, which are able to colonize new open sites and thus, maintain biodiversity. [source]