Shrub Microsites Research Articles

Spatial distribution and physiology of biological soil crusts from semi-arid central Spain are related to soil chemistry and shrub cover

Despite the critical role of biological soil crusts (BSCs) in arid and semi-arid ecosystem function, few studies are found concerning the most important environmental variables affecting their distribution and physiology. This study seeks to determine soil and microenvironmental factors affecting the spatial distribution and pigment production of BSC-forming lichens and mosses in open patches of a semi-arid Mediterranean kermes oak thicket. We measured late-successional BSC cover, shrub cover, distance to nearest kermes oak (to test for effects of kermes oak thicket microenvironment on BSC), and pigment concentration of one lichen ( Cladonia foliacea) and one moss ( Pleurochaete squarrosa) species in the Nature Reserve El Regajal-Mar de Ontígola (Central Spain). At the macroscale (>0.5 m), results showed that BSC distribution and pigments were tightly coupled to a suite of soil properties, in particular soil pH, Fe, and Ca. Specifically, soil pH had a positive relationship with the cover of five individual BSC-forming lichen species and was negatively related to pigment production in C. foliacea. When pH was excluded from the analysis, Ca appeared as the main soil variable and was correlated with total BSC cover and total lichen cover. The micronutrient Fe had a significant positive relationship with the concentration of eight pigments in P. squarrosa and was also coupled with the cover of two BSC-forming lichens. Manganese, previously proposed as a key limiting micronutrient for BSCs, affected lichen diversity in a negative way. At the microscale (∼0.5 m), kermes oak microenvironment, shrub cover, and moss cover were determinants of BSC distribution, and total lichen and total BSC cover were overrepresented on N and E-facing shrub microsites. Our findings suggest that soil chemical variability and microsite diversity created by neighbouring vegetation affect BSC distribution in complex and essential ways and that studies aiming to explore BSC-environment relationships should be conducted at various spatial scales. Studies based on species- or group-specific responses are, thus, inadequate to unveil the main factors determining the distribution of the diverse organisms that constitute BSCs and/or to propose potential tools aiming to restore BSC in arid and semiarid ecosystems.

Soil water availability and microsite mediate fungal and bacterial phospholipid fatty acid biomarker abundances in Mojave Desert soils exposed to elevated atmospheric CO2

[1] Changes in the rates of nitrogen (N) cycling, microbial carbon (C) substrate use, and extracellular enzyme activities in a Mojave Desert ecosystem exposed to elevated atmospheric CO2 suggest shifts in the size and/or functional characteristics of microbial assemblages in two dominant soil microsites: plant interspaces and under the dominant shrub Larrea tridentata. We used ester-linked phospholipid fatty acid (PLFA) biomarkers as a proxy for microbial biomass to quantify spatial and temporal differences in soil microbial communities from February 2003 to May 2005. Further, we used the 13C signature of the fossil CO2 source for elevated CO2 plots to trace recent plant C inputs into soil organic matter (SOM) and broad microbial groups using δ13C (‰). Differences between individual δ13CPLFA and δ13CSOM for fungal biomarkers indicated active metabolism of newer C in elevated CO2 soils. Total PLFA-C was greater in shrub microsites compared to plant interspaces, and CO2 treatment differences within microsites increased under higher soil water availability. Total, fungal, and bacterial PLFA-C increased with decreasing soil volumetric water content (VWC) in both microsites, suggesting general adaptations to xeric desert conditions. Increases in fungal-to-bacterial PLFA-C ratio with decreasing VWC reflected functional group-specific responses to changing soil water availability. While temporal and spatial extremes in resource availability in desert ecosystems contribute to the difficulty in identifying common trends or mechanisms driving microbial responses in less extreme environments, we found that soil water availability and soil microsite interacted with elevated CO2 to shift fungal and bacterial biomarker abundances in Mojave Desert soils.

Facilitation vs. apparent competition: insect herbivory alters tree seedling recruitment under nurse shrubs in a steppe–woodland ecotone

Summary 1. Facilitation of recruitment by ‘nurse’ plants can play a major role in harsh environments. Yet the extent to which consumer‐mediated apparent competition from habitat‐forming plants may counteract facilitative interactions remains largely unexplored. 2. We examined whether seedling predation by tenebrionid beetles seeking refuge under nurse shrubs may prevent tree recruitment facilitation in a Patagonian steppe–woodland ecotone. Newly emerged seedlings of Austrocedrus chilensis were planted in shrub canopy, off‐shrub shelter and bare soil microsites, and monitored for causes of early mortality and for overall survival under ambient and elevated beetle densities. 3. Most seedlings in open microsites died from abiotic stress, whereas shrub cover and artificial shelters decreased desiccation mortality. Herbivory was the main cause of mortality in shrub microsites. Beetle addition increased predation beneath shrubs and in off‐shrub shelters, indicating that apparent competition ‘spilled over’ from shrubs with high insect densities. 4. Litter removal from shrubs prevented seedling predation suggesting that nurse plants altered recruitment by providing food as well as shelter to insects. Herbivory rates did not depend on seedling patch density but decreased with seedling age, with 1‐week old plants being most vulnerable to beetle predation. 5. Synthesis. Apparent competition from nurse plants can strongly reduce recruitment facilitation in stressful environments, although weak herbivore density dependence and seedling growth into ontogenetic refuges may limit the strength of apparent competition. An explicit consideration of negative, as well as positive, herbivore‐mediated indirect effects from habitat‐forming plants would broaden the understanding of community dynamics along stress gradients.

Factors affecting exotic annual plant cover and richness along roadsides in the eastern Mojave Desert, USA

Roads are often considered a critical point of entry for invasive exotic plant species. We evaluated the cover and richness of exotic plants relative to road type (gravel or paved) and distance from roads in the eastern Mojave Desert where exotic annual species are increasing the flammability of the desert. Exotic and native annual plant cover and richness were sampled from five to 45m from roads in three prevalent microsite types for this system: Larrea tridentata microsites, Ambrosia dumosa microsites, and interspaces between shrubs. Six exotic and 58 native annual species were detected during sampling. There was no distinction between the behavior of exotics and natives relative to road type or distance from the roads. A. dumosa and L. tridentata microsites had more exotic cover than interspaces. Likewise, exotic richness was significantly higher in the shrub microsites. A taxonomically controlled analysis involving three plant families revealed no significant differences in cover of native or exotic species relative to their distance from the road. Our results suggest that exotic plant species in the Mojave Desert are not necessarily more prevalent near roadsides than in adjacent undisturbed desert. Therefore, roadside surveys alone may not be adequate to detect exotic species presence.

Sheep Grazing Decreases Organic Carbon and Nitrogen Pools in the Patagonian Steppe: Combination of Direct and Indirect Effects

We explored the net effects of grazing on soil C and N pools in a Patagonian shrub–grass steppe (temperate South America). Net effects result from the combination of direct impacts of grazing on biogeochemical characteristics of microsites with indirect effects on relative cover of vegetated and unvegetated microsites. Within five independent areas, we sampled surface soils in sites subjected to three grazing intensities: (1) ungrazed sites inside grazing exclosures, (2) moderately grazed sites adjacent to them, and (3) intensely grazed sites within the same paddock. Grazing significantly reduced soil C and N pools, although this pattern was clearest in intensely grazed sites. This net effect was due to the combination of a direct reduction of soil N content in bare soil patches, and indirect effects mediated by the increase of the cover of bare soil microsites, with lower C and N content than either grass or shrub microsites. This increase in bare soil cover was accompanied by a reduction in cover of preferred grass species and standing dead material. Finally, stable isotope signatures varied significantly among grazed and ungrazed sites, with δ15N and δ13C significantly depleted in intensely grazed sites, suggesting reduced mineralization with increased grazing intensity. In the Patagonian steppe, grazing appears to exert a negative effect on soil C and N cycles; sound management practices must incorporate the importance of species shifts within life form, and the critical role of standing dead material in maintaining soil C and N stocks and biogeochemical processes.

Small-scale habitat use and assemblage structure of ground-dwelling beetles in a Patagonian shrub steppe

Studies in arid systems have often focussed on the origin and maintenance of vegetation patchiness, but little is yet known about how microhabitat structure created by woody plants affects soil macroarthropods. We studied patterns of small-scale habitat use by darkling and ground beetles (Coleoptera: Tenebrionidae and Carabidae) in a two-phase shrub steppe mosaic, in NW Patagonia, Argentina. Beetles were sampled using pitfall traps during the early (December–January) and late (February–March) activity season (3675 trap-days). Traps were placed in three microsites: shrub centre, shrub periphery and off-shrub bare-soil areas. We also experimentally tested effects of aerial cover and litter accumulation, two key features of shrub patches, on beetle activity and assemblage structure. We captured 1415 individuals, the majority being omnivorous tenebrionids (67%) and predaceous carabids (28%). All recorded species (five tenebrionids plus two carabids) occurred in all three microsites and in both trapping seasons. Overall beetle activity, biomass and diversity were consistently greater in shrub microsites than in bare-soil areas. Mean diurnal soil temperatures were higher in bare soil (37.6 °C) than beneath shrubs (23.6 °C). Tenebrionid activity decreased markedly in the late season, coinciding with the increased frequency of daily maximum soil temperatures over 30 °C (shrub) and 50 °C (bare soil). Carabid activity did not vary with season, reflecting a shift in species dominance not found among tenebrionids. Addition of artificial shelters to bare-soil patches revealed differing perceptions of microhabitat cover by beetle groups, carabids being less selective than tenebrionids. In contrast, litter manipulation did not significantly alter microhabitat use by beetles. We conclude that shrubs act as ‘keystone structures’ providing sheltered habitat to ground-dwelling macroarthropods. Thus, disturbances and management actions altering shrub cover may have unwanted, community-wide impacts on biodiversity and ecosystem processes.

Effects of elevated CO2 on fine root dynamics in a Mojave Desert community: a FACE study

AbstractFine roots (≤1 mm diameter) are critical in plant water and nutrient absorption, and it is important to understand how rising atmospheric CO2 will affect them as part of terrestrial ecosystem responses to global change. This study's objective was to determine the effects of elevated CO2 on production, mortality, and standing crops of fine root length over 2 years in a free‐air CO2 enrichment (FACE) facility in the Mojave Desert of southern Nevada, USA. Three replicate 25 m diameter FACE rings were maintained at ambient (∼370 μmol mol−1) and elevated CO2 (∼550 μmol mol−1) atmospheric concentrations. Twenty‐eight minirhizotron tubes were placed in each ring to sample three microsite locations: evergreen Larrea shrubs, drought‐deciduous Ambrosia shrubs, and along systematic community transects (primarily in shrub interspaces which account for ∼85% of the area). Seasonal dynamics were similar for ambient and elevated CO2: fine root production peaked in April–June, with peak standing crop occurring about 1 month later, and peak mortality occurring during the hot summer months, with higher values for all three measures in a wet year compared with a dry year. Fine root standing crop, production, and mortality were not significantly different between treatments except standing crop along community transects, where fine root length was significantly lower in elevated CO2. Fine root turnover (annual cumulative mortality/mean standing crop) ranged from 2.33 to 3.17 year−1, and was not significantly different among CO2 treatments, except for community transect tubes where it was significantly lower for elevated CO2. There were no differences in fine root responses to CO2 between evergreen (Larrea) and drought‐deciduous (Ambrosia) shrubs. Combined with observations of increased leaf‐level water‐use efficiency and lack of soil moisture differences, these results suggest that under elevated CO2 conditions, reduced root systems (compared with ambient CO2) appear sufficient to provide resources for modest aboveground production increases across the community, but in more fertile shrub microsites, fine root systems of comparable size with those in ambient CO2 were required to support the greater aboveground production increases. For community transects, development of the difference in fine root standing crops occurred primarily through lower stimulation of fine root production in the elevated CO2 treatment during periods of high water availability.

Technical Note: A Technique for Conducting Small-Plot Burn Treatments

An experimental design required burn treatments for 10-m2 circular plots. We constructed a fire enclosure for the plots using sheetmetal, electrical conduit, and other commonly available materials. We field tested the enclosure in sagebrush-grass ecosystems in central Nevada and central Utah, and evaluated peak fire temperatures using small metal tags striped with temperature sensitive paint. We obtained average peak surface temperatures of 310, 307, and 381 C in bare ground, under grass, and under shrub microsites, respectively, for the Nevada sites and 253, 299, and 337 C for the same microsites, respectively, in Utah. Subsurface (2-cm depth) temperatures rarely exceeded 79 C, the lowest temperature detectable by our method. The enclosure contained the fire and did not permit escape of any embers or firebrands. The fire enclosure, burn technique and temperature monitoring method used are inexpensive, easily deployed, and desirable for experiments where larger-scale burns are impractical. DOI:10.2458/azu_jrm_v56i3_korfmacher

Technical note: a technique for conducting small-plot burn treatments

An experimental design required burn treatments for 10-m2 circular plots. We constructed a fire enclosure for the plots using sheetmetal, electrical conduit, and other commonly available materials. We field tested the enclosure in sagebrush-grass ecosystems in central Nevada and central Utah, and evaluated peak fire temperatures using small metal tags striped with temperature sensitive paint. We obtained average peak surface temperatures of 310, 307, and 381 degrees C in bare ground, under grass, and under shrub microsites, respectively, for the Nevada sites and 253, 299, and 337 degrees C for the same microsites, respectively, in Utah. Subsurface (2-cm depth) temperatures rarely exceeded 79 degrees C, the lowest temperature detectable by our method. The enclosure contained the fire and did not permit escape of any embers or firebrands. The fire enclosure, burn technique and temperature monitoring method used are inexpensive, easily deployed, and desirable for experiments where larger-scale burns are impractical.

Soil resource heterogeneity in the Mojave Desert

Heterogeneity of soil resources was investigated in the Mojave Desert to better understand spatial variability of soil attributes in a Larrea tridentata–Ambrosia dumosa community. Six soil samples were collected at each of eight microsites, with this sampling scheme replicated for three sites that were 0·7 km apart on a 10-km long bajada. The eight microsites were: five different shrub types (individual shrubs of L. tridentata, individual shrubs of Lycium pallidum , individual shrubs of A. dumosa, mixtures of shrub species with small mammal burrows, and mixtures of shrubs without small mammal burrows), a grass microsite (around individual plants of the tussock grass Pleuraphis rigida), and two microsites without perennial vegetation (one within small washes and the other within the interspace between perennial plants). Fifteen soil parameters were assessed, and all were found to differ among microsites. Nutrients were assessed as the pools extractable by vegetation. Mineral nutrients except for Ca, organic matter, and litter were higher in soils collected from shrub microsites compared to the others. Shrub microsites with small mammal burrows had significantly more N and P than those without burrows. Non-vegetated microsites in washes and plant interspaces contained very low N and P levels. Mg, Na, pH and cation exchange capacity were highest in soils under Lycium shrubs, while Ca levels were highest in wash soils. Arbuscular mycorrhizal spore densities were generally low (0–0·2 spores g −1 soil) except for Lycium soils, which contained 1·8 spores g −1 soil. Wash and interspace soils lacked spores but had mycorrhizal inoculum potential values similar to the other microsites, indicating that mycorrhizal propagules were present. Thus, the presence of perennial shrubs strongly influenced microsite soil characteristics and resulted in higher nutrient levels. Small mammal burrows further enhanced the mineral nutrient content of soils. In addition, all soil characteristics except for spore density differed among the three sites across the bajada, and interactions between microsite and site were common. Clearly, soil resources for plants are spatially heterogeneous in this desert ecosystem and differ greatly between adjacent micro- and macrosites in this L. tridentata–A. dumosa community.

The impact of elephant and goat grazing on the endemic flora of South African succulent thicket

Succulent thicket is an important centre of endemism for succulents and geophytes. Both inside and outside reserves, this component is threatened by megahervibore and goat grazing, respectively. Goat grazing has greatly reduced the cover and diversity of endemic geophytes and succulent shrubs in succulent thicket has in the Addo Elephant National Park. Elephant grazing has had a more moderate effect on these plants and has not reduced the rich Crassulaceae flora in shrub microsites. All endemics noted in the study were present in ungrazed sites but fewer were recorded in the elephant- (63%) and goat- (53%) grazed treatments. There are several solutions to this problem: more botanical (ungrazed) reserves should be established; reserves should be enlarged to reduce the stocking density of elephants; and elephant and other game farming should be encouraged as an alternative to goat-farming outside reserves.