The central role of Clark's nutcracker in the dispersal and establishment of whitebark pine

Summary Plant and soil water relationships in a typical nebraska Sandhills prairie were examined to 1) explain the observed distribution patterns of several dominant grasses along a topographic gradient, and 2) show how spatial and temporal variations in soil moisture are critical to community organization on a sandy substrate. An experimental transect encompassing the major community and soil types along a steep, west-facing vegetated dune was established. Maximum available water was shown to be significantly higher in the fine textured surface soils of the lowland sites than the coarse textured sands of the dune sites. Seasonal (1979) patterns of available soil moisture of the sampling sites on the transect showed that in the upper elevation dune sands, moisture was available in the entire profile with surface depletions not occurring until mid to late summer. In contrast, moisture in the surface 60–80 cm in the fine textured lowland soils was exhausted by early to mid-summer with the entire profile nearly dry by late summer. Deep-rooted, C₄ species, Andropogon hallii and Calamovilfa longifolia which are common on upper, coarser sandy soils showed significantly greater water stress on fine textured soils than on dune sands. C₃, shallowrooted species, Agropyron smithii, Stipa comata, and Koeleria cristata always experienced lower mid-day and predawn leaf water potentials than the C₄ species. The C₃ species, with the exception of Koeleria are most abundant on finer textured soils that provide substantial moisture during their peak activity in the spring. It appears that the C₄ species show more conservative water use patterns than the C₃ species as significantly lower leaf conductances in the C₄'s were measured when soil water was abundant. The C₃ species appear to be opportunistic with available water and rapidly deplete surface soil moisture as a result of high transpiration rates. These data suggest that the temporal and spatial distribution of available water along this gradient controls species distribution according to rooting morphology, photosynthetic physiology, and water deficits, incurred by transpirational losses. Competitive interactions between species that utilize soil moisture differently may be an important factor in community organization.     

荒漠草原-灌丛镶嵌体的植被稳态转变特征

以柠条锦鸡儿（Caragana korshinskii）灌丛引入形成荒漠草原-灌丛地镶嵌体为研究对象,选择其内部荒漠草地（DG）、草地边缘（GE）、灌丛边缘（SE）、灌丛地（SL）为试验样地,开展荒漠草原向灌丛地人为稳态转变过程土壤水分与植被变化特征研究。结果如下：各样地0-200 cm土层水分含量随着转变过程呈显著降低趋势（P<0.05）,其中荒漠草地、草地边缘和灌丛边缘样地的土壤水分均在秋季雨水补充期增加,灌丛地由于深层土壤水分过度消耗而愈加降低,且灌丛边缘和灌丛地深层水分显著低于荒漠草地、草地边缘（P<0.05）,垂直动态不显著;地上植被随样地转变除优势度指数表现为灌丛地多样性、均匀度、丰富度指数均显著低于各样地（P<0.05）,多年生优势草本蒙古冰草（Agropyron mongolicum）、短花针茅（Stipa breviflora）逐渐被一年生草本刺藜（Dysphania aristata）、猪毛蒿（Artemisia scoparia）所代替;土壤种子库萌发种类随样地转变逐渐减少,种子库密度则显著升高（P<0.05）,灌丛地以一年生草本植物为主（占总密度的97.51%）,荒漠草地则以多年生草本萌发为主,且各样地土壤萌发种子库及多年生草本多集中于0-5 cm土层（P<0.05）;转变过程荒漠草原和灌丛地植被地上地下相似度分别为0.14和0.19,均显著低于两边缘样地0.35,较两边缘样地群落更为稳定,呈现草原灌丛化过程中草地-灌丛共存的植被双稳态特征。     

Response of photosynthesis of different plant functional types to environmental changes along Northeast China Transect

Net photosynthesis (P_n), transpiration (E), stomatal conductance (g_s), internal CO₂ concentration (C_i), and water use efficiency (WUE) were examined on 215 species from eight plant functional types (PFTs) along a precipitation gradient in northeast China (the Northeast China Transect, or NECT). Among the eight PFTs, meadow steppe grasses had the highest rates of net photosynthesis and forest grasses the lowest and the following order of P_n was noted: meadow steppe grasses >typical steppe grasses >steppe shrubs >desert grasses >forest trees >forest shrubs >desert shrubs >forest grasses (P<0.05). Transpiration tended to be the highest in the steppe grasses and lowest in forest shrubs. Transpiration also decreased rapidly with the appearance of C₃ desert species at the desert end. The forest tree PFT had lower P_n, E, g_s than the steppe PFTs, whereas WUE values were somewhat greater in the forest tree PFT than the desert shrubs and grasses. Low C_i values along the steppe section (from 400 to 1100 km, east to west) indicated the presence of C₄ species. Of all the PFTs, only shrubs and herbs were noted at all points along the transect. No clear relationship between P_n, E, g_s, WUE of herb and shrub PFTs and annual precipitation was noted – low values were found at both the high and low precipitation ends of the transect. Highest values were noted when precipitation was intermediate. Received: 28 October 1998 / Accepted: 10 May 1999     

Resource Islands Predict the Distribution of Heterotrophic Bacteria in Chihuahuan Desert Soils

The resource island hypothesis predicts that soil resources such as nitrogen, phosphorus, and water will be distributed evenly in grasslands but have a patchy distribution focused around plants in shrublands. This hypothesis predicts that microorganism numbers will follow resources and be (i) evenly distributed in grasslands, (ii) concentrated around individual plants in shrublands, and (iii) higher where resources are higher when comparing the same vegetation type. This study enumerated total heterotrophic bacteria and a subset of these, the nitrogen-efficient guild (NEG), in three shrublands (playa fringe mesquite, tar bush, and creosote) and two grasslands (playa and bajada). Both heterotrophs and NEG members followed the distribution pattern predicted by the resource island hypothesis. There were no significant differences in heterotroph or NEG numbers comparing at-plant and interplant samples for both the playa and bajada grasslands. Furthermore, populations were generally higher in nutrient-rich playa grasslands than nutrient-poor bajada grasslands. In contrast, both heterotroph and NEG numbers were higher at shrubs than between shrubs in all three shrub sites. These results suggest that resource abundance in resource islands predicts the distribution of heterotrophic bacterial numbers in desert soils.     

Distributions of C4 plants along environmental and compositional gradients in southeastern Arizona

Distributional patterns of C₄ plants were investigated in 4 study areas located in se Arizona: granite slopes in the Mule Mountains, limestone slopes in the Mule Mountains, calcareous bajada (alluvial plain) below the Mule Mountains, and limestone slopes in the Huachuca Mountains. Cover data for all vascular species were obtained from 238 0.1 ha (20×50 m) sample quadrats located over ranges of elevation and topographic position within the study areas. Overall, 69 C₄ species representing 6 angiosperm families were encountered. C₄ species accounted for 13.5% to 22.3% of vascular species within the study areas. C₄ species frequency in quadrats (on the basis of all species or of grasses only) increased from mesic to xeric community types in all study areas except the calcareous bajada. Similar, but less consistent, trends were evident in the relative cover contributed by C₄ species. In two of the study areas (granite slopes in the Mule Mountains, limestone slopes in the Huachuca Mountains) regression analyses revealed statistically significant trends of C₄ species frequency and relative cover along environmental (elevation/solar-irradiation scalar) and compositional (reciprocal averaging ordination) gradients. A lack of consistent trends on limestone slopes in the Mule Mountains may be the result of grazing and/or recent invasion of low-elevation limestone areas by a Chihuahuan Desert flora dominated by C₃ dicot shrubs. The calcareous bajada below the Mule Mountains was studied less intensively, but its flora was found to contain the highest frequency of C₄ species of the 4 study areas. In contrast, C₄ cover on the bajada was low, presumably as a consequence of heavy grazing pressure on the grasses. The results of the present investigation support the prediction that C₄ species should be proportionally more successful in habitats characterized by high temperatures, high irradiance and low moisture.     

Effects of an African grass invasion on Hawaiian shrubland nitrogen biogeochemistry

African perennial C₄ grasses are highly successful invaders in Hawaiian ecosystems. We examined the effects of African molasses grass (Melinis minutiflora Beauv.) on Hawaiian shrubland nitrogen (N) dynamics without the influence of fire disturbance. Vegetation tissue carbon and nitrogen chemistry, soil inorganic N pools, net N mineralization rates, and total soil N were studied in three adjacent areas: a monospecificMelinis grassland, a mixed grass/shrubland mosaic, and an un-invaded shrubland.Melinis plots within the mosaic area exhibited the largest inorganic N pools and fastest net N mineralization rates, but were temporally variable with grass phenology. Un-invaded shrubland plots contained the smallest inorganic N pools and lowest net N mineralization rates. Grass foliar C:N and litter C:N were lower than those of common shrubland species, providing one possible link between species and ecosystem N dynamics at this site. The combined effects of N cycle modification, successful light competition, and fire-cycle enhancement make the invasion ofMelinis a significant perturbation to Hawaiian shrubland ecosystem function and successional dynamics. ei]Section editor H Lambers     

Aboveground net primary production dynamics in a northern Chihuahuan Desert ecosystem

Aboveground net primary production (ANPP) dynamics are a key element in the understanding of ecosystem processes. For semiarid environments, the pulse-reserve framework links ANPP to variable and unpredictable precipitation events contingent on surficial hydrology, soil moisture dynamics, biodiversity structure, trophic dynamics, and landscape context. Consequently, ANPP may be decoupled periodically from processes such as decomposition and may be subjected to complex feedbacks and thresholds at broader scales. As currently formulated, the pulse-reserve framework may not encompass the breadth of ANPP response to seasonal patterns of precipitation and heat inputs. Accordingly, we examined a 6-year (1999–2004), seasonal record of ANPP with respect to precipitation, soil moisture dynamics, and functional groups in a black grama (Bouteloua eriopoda) grassland and a creosotebush (Larrea tridentata) shrubland in the northern Chihuahuan Desert. Annual ANPP was similar in the grassland (51.1 g/m²) and shrubland (59.2 g/m²) and positively correlated with annual precipitation. ANPP differed among communities with respect to life forms and functional groups and responses to abiotic drivers. In keeping with the pulse-reserve model, ANPP in black grama grassland was dominated by warm-season C₄ grasses and subshrubs that responded to large, transient summer storms and associated soil moisture in the upper 30 cm. In contrast, ANPP in creosotebush shrubland occasionally responded to summer moisture, but the predominant pattern was slower, non-pulsed growth of cool-season C₃ shrubs during spring, in response to winter soil moisture accumulation and the breaking of cold dormancy. Overall, production in this Chihuahuan Desert ecosystem reflected a mix of warm-temperate arid land pulse dynamics during the summer monsoon and non-pulsed dynamics in spring driven by winter soil moisture accumulation similar to that of cool-temperate regions. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Species turnover of monkey beetles (Scarabaeidae: Hopliini) along environmental and disturbance gradients in the Namaqualand region of the succulent Karoo, South Africa

Species turnover of monkey beetle (Scarabaeidae: Hopliini) assemblages along disturbance and environmental gradients was examined at three sites within the arid, winter rainfall Namaqualand region of the succulent Karoo, South Africa. At each site two study plots with comparable vegetation and soils but contrasting management (grazing) histories were chosen, the disturbed sites having fewer perennial shrubs and generally more annuals and bare ground. Beetles collected using coloured pan-traps showed a consistently higher abundance in disturbed sites. Lepithrix, Denticnema and Heterochelus had higher numbers in disturbed plots, while Peritrichia numbers were lower in disturbed areas. Measures of species richness and diversity were consistently higher in the undisturbed sites. Distinctive assemblages of monkey beetles and plants occurred at each site. A high compositional turnover ( diversity) was recorded for both monkey beetles and plants along a rainfall gradient; between-site diversity values ranged from 0.7 to 0.8 (out of a maximum of 1.0). Species turnover of beetles was higher between the disturbed sites along the environmental gradient than the corresponding undisturbed sites. The high monkey beetle species turnover is probably linked to the high plant species turnover, a distinctive feature of succulent Karoo landscapes. Monkey beetles are useful indicators of overgrazing disturbance in Namaqualand, as their pollinator guilds are apparently disrupted by overgrazing. A shift away from perennial and bulb pollinator guilds towards those favouring weedy annuals was observed in disturbed areas. The consequences to ecosystem processes due to the effects of disturbance on monkey beetle communities and the role of monkey beetles as indicators of disturbance is discussed, as well as the implications of disturbance on monkey beetle pollination guilds.     

Vegetation patterns,microtopography, and soils on a Chihuahuan desert playa

Abstract. Spatial patterns of vegetation on an ephemeral lake bed (playa) in the northern Chihuahuan desert were closely correlated with elevation. Distributions of species along a 265 m transect from the center to the edge of the playa showed abrupt boundaries at 110 m and 220 m. These boundaries seem related to the maximum elevation covered with standing water following complete flooding, and to an edaphic boundary between heavy clay soils at the center of the playa and adjacent soils. The complete turnover in species composition across this transect, with an elevation change of only 85 cm, indicates that environmental gradients resulting from flooding and soil development are steep. Vegetative patterns within the lowest part of the playa were patchy, with patch diameters ranging from 2 to 5 m. These patches appear to be produced by differences in the duration and frequency of flooding between small knolls and depressions resulting from gilgai microtopography.     

Altitudinal change in soil and foliar nutrient concentrations and in microclimate across the tree line on the subtropical island mountain Mt. Teide (Canary Islands)

Mt. Teide (Tenerife, Spain) is a high volcanic island mountain with an unusually low tree line elevation (2000–2100 m). While searching for the causes of this tree line depression, we analysed the concentrations of total N, available P, and salt-exchangeable Ca, K and Mg in three soil horizons, and the foliar N, P, Ca, K and Mg concentrations in six abundant plant species (trees, shrubs, forbs) along a transect from 1400 (1600) to 3100 m a.s.l. The objective of the study was to detect altitudinal trends in soil and plant nutrient status below and above the tree line. For characterising elevational changes in microclimate and hydrology, we also conducted measurements of air and soil temperatures (T_a and T_s), atmospheric water vapour saturation deficit (D), potential evaporation (E) and soil moisture (θ) on 3–4 occasions along the transect.T_a and T_s did not linearly decrease with elevation but were highest at or immediately below the tree line and were relatively low in the closed Pinus canariensis forest at 1600 m. T_s reached maxima at about 70 °C near the tree line. The absence of a linear temperature decrease with elevation was caused by a stable temperature inversion at about 2000 m and by canopy shading in the forest canopy below the tree line. In June, the topsoil (10 cm) dried out almost completely between 1800 and 3100 m, but remained moist in the pine forest at 1600 m. This gradient reflects the transition from the montane cloud belt to the dry alpine belt higher upslope. The subsoil (30 cm) contained >30 vol% of soil water at all elevations even in June except for the uppermost site (3100 m). Potential evaporation increased with elevation despite a decrease in D. We assume that this is mainly due to the air pressure-dependent increase with elevation in the diffusion coefficient for water vapour in air.The concentrations of N and ‘available’ P (after Olsen) in the mineral topsoil were by far smaller than in alpine soils of other humid mountains which is thought to be a consequence of a very dry and biologically inactive topsoil in the semi-arid alpine belt of Mt. Teide. In contrast to many other mountains, foliar N, P and cation concentrations in the plants did not increase with elevation but either remained unchanged, or decreased as in the tree line species P. canariensis. Nevertheless, P. canariensis probably is not limited by nutrient deficiency at the tree line despite rather low N and P needle concentrations. Rather, drought and heat stress effects on seedling establishment are thought to be the causes of the tree line depression.     

Vegetation variation within and among palustrine wetlands along an altitudinal gradient in KwaZulu-Natal,South Africa

The variation in graminoid species composition and diversity and the distribution of photosynthetic pathways among 66 wetlands in KwaZulu-Natal, South Africa, and within six of these wetlands was described and related to measured physical parameters, using multivariate and univariate techniques. Altitude, which ranged from 550 m to 2120 m, accounted for most variation among wetlands, with an almost complete turnover of species along this gradient. Landform setting was less important in explaining overall species composition, but relationships of individual species were revealed (e.g. Eleocharis dregeana showed an affinity for depressions). Within a wetland there was an almost complete turnover of species along a gradient of wetness, as described using soil morphological criteria. Most species were consistently associated with the same wetness zones across different wetland sites, e.g., Phragmites australis with the wettest zone, Pycreus macranthus with the intermediate zone, and Eragrostis plana with the least wet zone. The occurrence and abundance of different photosynthetic pathway types depended on altitude and degree of wetness. At high altitudes, C₃ sedges, notably Carex acutiformis, dominated the wettest zone and C₃ and C₄ grasses and sedges dominated the intermediate and least wet zones. At mid altitudes, C₃ and C₄ sedges and C₃ grasses dominated the wettest zone, C₃ and C₄ grasses and sedges dominated the intermediate zone and C₄ grasses dominated the least wet zone. Low altitude sites showed a similar distribution of photosynthetic pathways as mid-altitude sites, but C₃ species were less abundant. Species richness was positively associated with the log of wetland size and, at the level of an individual wetland, species richness and evenness were found to be consistently greater in the intermediate and least wet zone compared with the wettest zone. The management implications of the results are discussed in the light of continuing anthropogenic loss of wetlands in the study area and global climate change.     

The Influence of Spatial Patterns of Soil Moisture on the Grass and Shrub Responses to a Summer Rainstorm in a Chihuahuan Desert Ecotone

The cycling of surface water, energy, nutrients, and carbon is different between semiarid grassland and shrubland ecosystems. Although differences are evident when grasslands are compared to shrublands, the processes that contribute to this transition are more challenging to document. We evaluate how surface redistribution of precipitation and plant responses to the resulting infiltration patterns could contribute to the changes that occur during the transition from grassland to shrubland. We measured soil water potential under grasses (Bouteloua eriopoda), shrubs (Larrea tridentata) and bare soil and changes in plant water relations and gas exchange following a 15 mm summer storm in the grassland–shrubland ecotone at the Sevilleta National Wildlife Refuge in central New Mexico USA. Following the storm, soil water potential (Ψ_s) increased to 30 cm depth beneath both grass and shrub canopies, with the greatest change observed in the top 15 cm of the soil. The increase in Ψ_s was greater beneath grass canopies than beneath shrub canopies. Ψ_s under bare soil increased only to 5 cm depth. The substantial redistribution of rainfall and different rooting depths of the vegetation resulted in high Ψ_s throughout most of the rooting volume of the grasses whereas soil moisture was unchanged throughout a large portion of the shrub rooting volume. Consistent with this pattern, predawn water potential (Ψ_PD) of grasses increased more than 5 MPa to greater than −1 MPa whereas Ψ_PD of shrubs increased to −2.5 MPa, a change of less than 2 MPa. Transpiration increased roughly linearly with Ψ_PD in both grasses and shrubs. In grasses, assimilation was strongly correlated with Ψ_PD whereas there was no relationship in shrubs where assimilation showed no significant response to the pulse of soil moisture following the storm. These data show that preferential redistribution of water to grass canopies enhances transpiration and assimilation by grasses following large summer storms. This process may inhibit shrubland expansion at the ecotone during periods without extreme drought.     

Local population differentiation in <Emphasis Type="Italic">Bromus tectorum</Emphasis> L. in relation to habitat-specific selection regimes

A central question of invasion biology is how an exotic species invades new habitats following its initial establishment. Three hypotheses to explain this expansion are: (1) the existence of ‘general purpose’ genotypes, (2) the in situ evolution of novel genotypes, and (3) the dispersal of existing specialized genotypes into habitats for which they are pre-adapted. Bromus tectorum is a selfing exotic winter annual grass that has achieved widespread dominance in semiarid western North America and that is actively invading salt desert habitats. We examined mechanisms driving this invasion in three complementary studies. In reciprocal seeding experiments with ten populations from saline playa, salt desert shrubland, and upland sagebrush communities along a salinity gradient in western Utah, we found that seeds from the playa population were able to establish better than those of most other populations across all habitats, including two highly saline sites. Seeds of one of the two seed sources from upland big sagebrush communities established best on the site of origin, while the seeds of the other source established well across a range of non-saline sites. In a greenhouse experiment with playa and upland seed sources, we found evidence of adaptation to high salinity in the playa source. Playa plants were larger than upland plants in control treatments and invested less in root. They maintained vigor and seed production at high salinity better than upland plants. Molecular genetic analysis showed that the playa population was dominated by a single SSR genotype absent from upland big sagebrush populations, which were comprised of SSR genotypes generally rare or absent in populations from salt desert shrubland sites. Our results demonstrate local population differentiation in B. tectorum, resulting at least in part from differential selection on pre-adapted genotypes with characteristic marker fingerprints. We found little evidence for selection favoring novel genotypes.     

Shrub facilitation drives tree establishment in a semiarid fog‐dependent ecosystem

Questions

The exceptional occurrence of tall rain forest patches on foggy coastal mountaintops, surrounded by extensive xerophytic shrublands, suggests an important role of plant–plant interactions in the origin and persistence of these patches in semi‐arid Chile. We asked whether facilitation by shrubs can explain the growth and survival of rain forest tree species, and whether shrub effects depend on the identity of the shrub species itself, the drought tolerance of the tree species and the position of shrubs in regard to wind direction.

Location

Open area–shrubland–forest matrix, Fray Jorge Forest National Park, Chile.

Methods

We recorded survival after 12 years of a ~3600 tree saplings plantation (originally ~30‐cm tall individuals) of Aextoxicon punctatum, Myrceugenia correifolia and Drimys winteri placed outside forests, beneath the shrub Baccharis vernalis, and in open (shrub‐free) areas. We assessed the effects of neighbouring shrubs and soil humidity on survival and growth along a gradient related to the direction of fog movement.

Results

B. vernalis had a clear facilitative effect on tree establishment and survival since, after ~12 years, saplings only survived beneath the shrub canopy. Long‐term survival strongly depended on tree species identity, drought tolerance and position along the soil moisture gradient, with higher survival of A. punctatum (>35%) and M. correifolia (>14%) at sites on wind‐ and fog‐exposed shrubland areas. Sites occupied by the shrub Aristeguietia salvia were unsuitable for trees, presumably due to drier conditions than under B. vernalis.

Conclusions

Interactions between shrubs and fog‐dependent tree species in dry areas revealed a strong, long‐lasting facilitation effect on planted tree's survival and growth. Shrubs acted as benefactors, providing sites suitable for tree growth. Sapling mortality in the shrubland interior was caused by lower soil moisture, the consequence of lower fog loads in the air and thus insufficient facilitation. While B. vernalis was a key ecosystem engineer (nurse) and intercepted fog water that dripped to trees planted underneath, drier sites with A. salvia were unsuitable for trees. Consequently, nurse effects related to water input are strongly site and species specific, with facilitation by shrubs providing a plausible explanation for the initiation of forest patches in this semi‐arid landscape.     

Plant species richness and diversity along an altitudinal gradient in the Sierra Nevada, Mexico

This article presents an analysis of plant species richness and diversity and its association with climatic and soil variables along a 1300‐m elevation gradient on the Cerro Tláloc Mountain in the northern Sierra Nevada in Mexico. Two 1000‐m² tree sampling plots were created at each of 21 selected sampling sites, as well as two 250‐m² plots for shrubs and six 9‐m² plots for herbaceous plants. Species richness and diversity were estimated for each plant life form, and beta diversity between sites was estimated along the gradient. The relationship between species richness and diversity and environmental variables was modelled using simple linear correlation and regression trees. Species richness and diversity showed a unimodal pattern with a bias towards high values in the lower half of the elevation gradient under study. This response was consistent for all three life forms. Beta diversity increased steadily along the elevation gradient, being lower between contiguous sites at intermediate elevations and high – the species replacement rate was nearly 100%– between sites at the extremes of the gradient. Few species were adapted to the full spectrum of environmental variation along the elevation gradient studied. The regression tree suggests that differences in species richness are mainly influenced by elevation (temperature and humidity) and soil variables, namely A₂ permanent wilting point, organic matter and horizon field capacity and A₁ horizon Mg²⁺.     

Plant nutrient changes in a semi-arid Mediterranean shrubland after fire

Abstract. Alkaline xeric soils from SE Spain weathered from kakiritized dolomitic rocks and subjected to recurrent fires, show phosphorus immobilization, low nutrient availability and high cation content, which might affect plant nutrition. Currently, Ulex shrubland, with a variety of dolomitophilous endemics, is colonizing eroded areas and replacing the original shrubland communities. Variation in nutrient content during post-fire regeneration was studied in six species which differ widely in their successional status. All species showed a high consumption of nutrients immediately after the fire. A pattern of decline in internal nutrient levels, especially P, with time since fire was detected in mid-to-late successional shrubs: Juniperus oxycedrus, Genista spartioides, and to a lesser extent in Rosmarinus officinalis, a shrub species of intermediate successional status. This pattern of decline may represent a progressive change towards a metabolism with a low turnover along the age gradient. Early successional shrub species behaved in a different way. Cistus clusii showed a nutrient-cumulative strategy, especially for P, across the fire-age gradient. Ulex rivasgodayanus maximized internal N concentrations due to its symbiotic activity with N₂-fixing bacteria. Cistus and Rosmarinus held high cation levels in their tissues. The nutritional characteristics of these shrub species are seen as advantageous adaptations to the specific soil properties and disturbance regime of the area.     

The desert vegetation of El Pinacate,Sonora, Mexico

The Pinacate region is part of the Gran Desierto, one of the driest deserts in North America. The presence-absence of perennial plant species, together with soil and landform characteristics, were registered in 110 sampling sites within this region. A classification and ordination of plant communities showed soil and landform units to be good predictors of plant variation. Plant distribution and species richness were also strongly related to altitude and rockiness. A Generalised Linear Model was used to fit the response curves of individual species.The environmental factors related with plant distribution are indicators of the water regime within a given site. Plant communities repeat along topographic gradients the large-scale biogeographic variation of the Sonoran Desert. Microphyllous shrubs colonise the drier bajada slopes, while more diverse communities, dominated by cacti and drought-deciduous trees, grow on the wetter pediments and in pockets within rocky soils.     

Variation in the species composition and mean body size of an avian foliage-gleaning guild along an elevational gradient: correlation with arthropod body size

The composition of an avian foliage-gleaning guild was analyzed with respect to body size at nine sites along an elevational gradient in the Oregon Cascades. Mean body size decreased from 20.5 g near the lower forest boundary where it meets the grassland at about 775 m to 9.3 g near timberline at about 1720 m. Both the loss of larger species and the gain of smaller species contributed to the change. Mean volume of the foliage-dwelling arthropods also decreased with increasing elevation by two orders of magnitude along the same gradient. A significant decrease in body size occurred in three arthropod groups, larval Lepidoptera, Homoptera, and spiders, and of these, larval Lepidoptera dominated the overall size trend among arthropods. Both developmental differences (higher elevation sites are delayed seasonally on the same calendar date) and taxonomic differences contributed to the change in mean arthropod size. Mean bird size was positively correlated (r=0.93) with the body size of foliage-dwelling arthropods. A similar pattern was suggested for other avian guilds dependent directly or indirectly upon foliage-dwelling arthropods, but not for guilds independent of foliage-dwelling arthropods.     

Positive effects of shrubs on plant species diversity do not change along a gradient in grazing pressure in an arid shrubland

Facilitative or positive interactions among species are driven mainly by the environmental amelioration or protection from grazing provided by nurse plants. Some studies have suggested that protection from grazing is inconsequential in water-limited environments because of low herbivore densities and their grazing effects. Others, however, argue that herbivores have a major effect on semi-arid plant communities, and that protection from grazing is a significant factor driving positive plant–plant interactions in such environments. We identified a gradient in grazing pressure in a semi-arid shrubland in south-eastern Australia along which we compared soil condition, incident radiation and plant composition beneath two nurse shrub species with open (shrub-free) interspaces. Our aim was to assess the degree of microclimatic amelioration provided by both shrubs, and changes in the interactions (intensity, importance and frequency) between both nurse shrubs and their understorey species, and their effects on species richness at the community level. Both the relative interaction intensity (RII) and interaction importance (I_imp) indices of plant–plant interactions were generally positive and independent of grazing pressure. Soil beneath both nurse plants had significantly greater indices of nutrient cycling and infiltration, and contained more C and N than soil in the open. Almost twice as many species occurred under the canopies of both shrubs (44 species) than in the open (23 species), and the composition of species differed significantly among microsites. Fifty-four percent of all perennial plant species occurred exclusively under shrubs. Our results suggest that environmental amelioration is a stronger driver of the facilitatory effect of shrubs on their understorey species than protection from grazing. Our conclusions are based on the fact that the substantial effect of plant–plant interactions on plant species richness was largely independent of grazing pressure. Irrespective of the underlying mechanism for this effect, our study illustrates the ecological role of shrubs as refugia for understorey plants in semi-arid environments and cautions against management practices aimed at reducing shrub populations.     

Multiscale soil and vegetation patchiness along a gradient of herbivore impact in a semi-arid grazing system in West Africa

We studied the degree and scale of patchiness of vegetation and selected soil variables along a gradient of herbivore impact. The gradient consisted of a radial pattern of `high', `intermediate' and `low' herbivore impact around a watering point in a semi-arid environment in Burkina Faso (West Africa). We hypothesised that, at a certain range of herbivore impact, vegetated patches alternating with patches of bare soil would occur as a consequence of plant-soil feedbacks and run-off-run-on patterns. Indeed, our transect data collected along the gradient showed that vegetated patches with a scale of about 5–10 m, alternating with bare soil, occurred at intermediate herbivore impact. When analysing the data from the experimental sites along the gradient, however, we also found a high degree of patchiness of vegetation and soil variables in case of low and high herbivore impact. For low herbivore impact, most variation was spatially explained, up to 100% for vegetation biomass and soil temperature, with a patch scale of about 0.50 m. This was due to the presence of perennial grass tufts of Cymbopogon schoenanthus. Patterns of soil organic matter and NH₄-N were highly correlated with these patterns of biomass and soil temperature, up to r=0.7 (P<0.05) for the in situ correlation between biomass and NH₄-N. For high herbivore impact, we also found that most variation was spatially explained, up to 100% for biomass and soil temperature, and 84% for soil moisture, with three distinct scales of patchiness (about 0.50 m, 1.80 m and 2.80 m). Here, microrelief had a corresponding patchy structure. For intermediate herbivore impact, again most variation was spatially explained, up to 100% for biomass and soil temperature, and 84% for soil moisture, with a patch scale of about 0.95 m. Here, we found evidence that vegetated patches positively affected soil moisture through less run-off and higher infiltration of rainwater that could not infiltrate into the bare soil elsewhere, which was not due to microrelief. Thus, we conclude that our findings are in line with our initial hypothesis that, at intermediate herbivore impact, vegetated patches alternating with patches of bare soil persist in time due to positive plant-soil feedbacks.