Impacts of altered precipitation regimes on soil communities and biogeochemistry in arid and semi‐arid ecosystems

Altered precipitation patterns resulting from climate change will have particularly significant consequences in water‐limited ecosystems, such as arid to semi‐arid ecosystems, where discontinuous inputs of water control biological processes. Given that these ecosystems cover more than a third of Earth's terrestrial surface, it is important to understand how they respond to such alterations. Altered water availability may impact both aboveground and belowground communities and the interactions between these, with potential impacts on ecosystem functioning; however, most studies to date have focused exclusively on vegetation responses to altered precipitation regimes. To synthesize our understanding of potential climate change impacts on dryland ecosystems, we present here a review of current literature that reports the effects of precipitation events and altered precipitation regimes on belowground biota and biogeochemical cycling. Increased precipitation generally increases microbial biomass and fungal:bacterial ratio. Few studies report responses to reduced precipitation but the effects likely counter those of increased precipitation. Altered precipitation regimes have also been found to alter microbial community composition but broader generalizations are difficult to make. Changes in event size and frequency influences invertebrate activity and density with cascading impacts on the soil food web, which will likely impact carbon and nutrient pools. The long‐term implications for biogeochemical cycling are inconclusive but several studies suggest that increased aridity may cause decoupling of carbon and nutrient cycling. We propose a new conceptual framework that incorporates hierarchical biotic responses to individual precipitation events more explicitly, including moderation of microbial activity and biomass by invertebrate grazing, and use this framework to make some predictions on impacts of altered precipitation regimes in terms of event size and frequency as well as mean annual precipitation. While our understanding of dryland ecosystems is improving, there is still a great need for longer term in situ manipulations of precipitation regime to test our model.     

Regional productivity mediates the effects of grazing disturbance on plant cover and patch‐size distribution in arid and semi‐arid communities

Patch‐size distribution and plant cover are strongly associated to arid ecosystem functioning and may be a warning signal for the onset of desertification under changes in disturbance regimes. However, the interaction between regional productivity level and human‐induced disturbance regime as drivers for vegetation structure and dynamics remain poorly studied. We studied grazing disturbance effects on plant cover and patchiness in three plant communities located along a regional productivity gradient in Patagonia (Argentina): a semi‐desert (low‐productivity community), a shrub‐grass steppe (intermediate‐productivity community) and a grass steppe (high‐productivity community). We sampled paddocks with different sheep grazing pressure (continuous disturbance gradients) in all three communities. In each paddock, the presence or absence of perennial vegetation was recorded every 10 cm along a 50 m transect. Grazing effects on vegetation structure depended on the community and its association to the regional productivity. Grazing decreased total plant cover while increasing both the frequency of small patches and the inter‐patch distance in all communities. However, the size of these effects was the greatest in the high‐productivity community. Dominant species responses to grazing explained vegetation patch‐ and inter‐patch‐size distribution patterns. As productivity decreases, dominant species showed a higher degree of grazing resistance, probably because traits of species adapted to high aridity allow them to resist herbivore disturbance. In conclusion, our findings suggest that regional productivity mediates grazing disturbance impacts on vegetation mosaic. The changes within the same range of grazing pressure have higher effects on communities found in environments with higher productivity, markedly promoting their desertification. Understanding the complex interactions between environmental aridity and human‐induced disturbances is a key aspect for maintaining patchiness structure and dynamics, which has important implications for drylands management.     

Impacts of tourism on threatened plant taxa and communities in Australia

Summary Many Australian plant species and communities appear to be threatened by tourism. A review of management plans, recovery plans and a survey of experts found that tourism was considered to be a direct or indirect threatening process for 72 plant taxa. This is one fifth of threatened species for which threats have been identified. In addition, many more species are listed as threatened by weeds, trampling, pathogens, clearing and collecting. These are often indirect impacts of tourism, particularly in conservation reserves where tourism is the only commercial activity permitted. Tourism was also considered to be a threatening process for several plant communities. A lack of recognition of the importance of direct and indirect impacts of tourism may potentially hinder the conservation of plant species and communities both in Australia and overseas. It may also limit the effectiveness of sustainable tourism policies, particularly in conservation reserves.     

Impacts of inundation and drought on eukaryote biodiversity in semi‐arid floodplain soils

Floodplain ecosystems are characterized by alternating wet and dry phases and periodic inundation defines their ecological character. Climate change, river regulation and the construction of levees have substantially altered natural flooding and drying regimes worldwide with uncertain effects on key biotic groups. In southern Australia, we hypothesized that soil eukaryotic communities in climate change affected areas of a semi‐arid floodplain would transition towards comprising mainly dry‐soil specialist species with increasing drought severity. Here, we used 18S rRNA amplicon pyrosequencing to measure the eukaryote community composition in soils that had been depleted of water to varying degrees to confirm that reproducible transitional changes occur in eukaryotic biodiversity on this floodplain. Interflood community structures (3 years post‐flood) were dominated by persistent rather than either aquatic or dry‐specialist organisms. Only 2% of taxa were unique to dry locations by 8 years post‐flood, and 10% were restricted to wet locations (inundated a year to 2 weeks post‐flood). Almost half (48%) of the total soil biota were detected in both these environments. The discovery of a large suite of organisms able to survive nearly a decade of drought, and up to a year submerged supports the concept of inherent resilience of Australian semi‐arid floodplain soil communities under increasing pressure from climatic induced changes in water availability.     

Large‐scale environmental flow results in mixed outcomes with short‐term benefits for a semi‐arid floodplain plant community

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Predicting bait uptake by feral cats,Felis catus,in semi‐arid environments

Feral cat control using aerial broadcasting of toxic baits continues to be used in the rangelands of Western Australia. The effectiveness of these operations has sometimes been compromised by different environmental factors that affect prey and cat numbers. This study demonstrates that the ratio of cats to their preferred prey (small mammals) can be used to predict the most effective time to bait. The regular baiting of three conservation sites offered an opportunity to study the relationship between feral cat abundance, the abundance of their prey and ingestion of toxic baits. Peron Peninsula on the mid‐west coast, Lorna Glen station in the northern Goldfields and the central Gibson Desert of Western Australia are sites where cat control using toxic baits has been routinely applied over the last 15 years. We postulated that bait ingestion by cats was linked to the availability of live prey. Small mammal abundance (capture rates in pit‐fall traps) and relative cat abundance (based on daily track counts) were assessed at these sites and the data used to produce a predator‐prey ratio index (PPRI). We used generalised linear mixed models to test the effect of prey abundance, prebaiting cat abundance and PPRI on baiting efficacy (BE). The best model for predicting efficacy of baiting contained only PPRI. This simple model was able to predict baiting success over the entire range of outcomes, from highly successful ( >75% cat reduction) to unsuccessful (0% cat reduction). The ability to predict feral cat BE in advance of planned toxic baiting operations will provide a valuable tool for wildlife managers involved in cat control.     

Fourteen months of seed rain in three Australian semi‐arid communities

The abundance and composition of seed rain was measured over 14 months (February 2004 to March 2005) in Currawinya National Park, western Queensland. The experimental design included four measurement periods, three vegetation communities and two grazing regimes. A total of 12 586 seeds from 104 species were captured. There were significantly more seeds and species captured during the measurement period with the least rainfall, although no significant correlation was found between the amount of rainfall and the number of seeds or species captured. More seeds and species were captured where native and feral grazing pressure was removed, but this was only significant for the number of species. The above‐ground vegetation showed no significant difference between grazing treatments over the study period and exhibited far fewer species than the seed rain. However, the majority of species found in the above‐ground vegetation were represented in the seed rain. Hypotheses are explored as an attempt to understand the apparent lack of a relationship between seed rain and rainfall. The effects of grazing and seed movement and storage are also discussed.     

Spatial patterns of plant association in grazed and ungrazed shrublands in the semi‐arid Karoo,South Africa

Abstract. The investigation of vegetation pattern and plant association by spatial statistics has become increasingly popular among plant ecologists. Recently, Individual‐centered analysis (ICA) has been introduced as a new tool for analysis of multi‐species co‐occurrence patterns. We tested this new technique by applying it to spatial data from grazed and ungrazed shrub communities in the semi‐arid Great Karoo, South Africa. There were substantial but complex and scale‐dependent differences in pattern between grazed and ungrazed vegetation. Unpalatable species that increase in abundance in grazed vegetation possibly play a key role in the change of vegetation pattern. At small scales we found indications of aggregation (< 30 cm) at the ungrazed, but of repulsion (30 – 40 cm) at the grazed site. An additional non‐random pattern at 60 – 170 cm at the grazed site was probably due to the clumped distributions of some species on broader scales. We show that the interpretability of ICA results is improved when the actual observed and expected frequencies of species combinations are added to the program output. The main strength of ICA is that it has the potential to detect association patterns that involve more than two species.     

Effects of long‐term fire exclusion and frequent fire on plant community composition: A case study from semi‐arid shrublands

Time since last fire and fire frequency are strong determinants of plant community composition in fire‐prone landscapes. Our study aimed to establish the influence of time since last fire and fire frequency on plant community composition and diversity of a south‐west Australian semi‐arid shrubland. We employed a space‐for‐time approach using four fire age classes: ‘young’, 8–15 years since last fire; ‘medium’, 16–34; ‘old’, 35–50; and ‘very old’, 51–100; and three fire frequency classes: burnt once, twice and three times within the last 50 years. Species diversity was compared using one‐way ANOVA and species composition using PERMANOVA. Soil and climatic variables were included as covariables to partition underlying environmental drivers. We found that time since last fire influenced species richness, diversity and composition. Specifically, we recorded a late successional transition from woody seeders to long‐lived, arid‐zone, resprouting shrub species. Fire frequency did not influence species richness and diversity but did influence species composition via a reduction in cover of longer‐lived resprouter species – presumably because of a reduced ability to replenish epicormic buds and/or sufficient starch stores. The distinct floristic composition of old and very old habitat, and the vulnerability of these areas to wildfires, indicate that these areas are ecologically important and management should seek to preserve them.     

Positive climate feedbacks of soil microbial communities in a semi‐arid grassland

Soil microbial communities may be able to rapidly respond to changing environments in ways that change community structure and functioning, which could affect climate–carbon feedbacks. However, detecting microbial feedbacks to elevated CO₂ (eCO₂) or warming is hampered by concurrent changes in substrate availability and plant responses. Whether microbial communities can persistently feed back to climate change is still unknown. We overcame this problem by collecting microbial inocula at subfreezing conditions under eCO₂ and warming treatments in a semi‐arid grassland field experiment. The inoculant was incubated in a sterilised soil medium at constant conditions for 30 days. Microbes from eCO₂ exhibited an increased ability to decompose soil organic matter (SOM) compared with those from ambient CO₂ plots, and microbes from warmed plots exhibited increased thermal sensitivity for respiration. Microbes from the combined eCO₂ and warming plots had consistently enhanced microbial decomposition activity and thermal sensitivity. These persistent positive feedbacks of soil microbial communities to eCO₂ and warming may therefore stimulate soil C loss.     

Woody plant invasion at a semi‐arid/arid transition zone: importance of ecosystem type to colonization and patch expansion

Question: How do patterns in colonization and patch expansion of an invasive woody plant (Larrea tridentata, Zygo‐phyllaceae) differ between two grassland ecosystems at a biome transition zone? Location: Semi‐arid/arid transition zone in central New Mexico. Methods: Frequency of occurrence, height, and surface area of saplings (n= 134) and patches of adult plants (n= 247) of the invasive shrub, L. tridentata, were measured within a mosaic of ecosystems dominated either by the Chihuahuan Desert species, Bouteloua eriopoda (Poaceae), or the shortgrass steppe species, B. gracilis, located within 1 km of the L. tridentata‐dominated ecosystem. Distances between L. tridentata patches and patch area were used to estimate connectivity as a measure of propagule pressure. Sapling age (estimated from height using previously established relationships) and distance to the L. tridentata‐dominated ecosystem was used to evaluate patterns in dispersal. Cover by species or functional group inside each L. tridentata patch was compared with surrounding vegetation to estimate changes in species composition with patch expansion. Results: L. tridentata saplings (< 1%) and adult patches (15%) occurred less frequently in B. gracilis‐dominated ecosystems than expected based on areal extent of this ecosystem type. Propagule pressure did not differ with distance from the core ecosystem dominated by L. tridentata. Evidence for both local and long‐distance dispersal events was found. Similar relationships between number of plants and patch area in both grassland types indicate similar patterns in patch expansion. Cover of perennial forbs was higher and cover of dominant grasses was lower in L. tridentata patches compared with the surrounding vegetation for both ecosystem types. Conclusions Spatial variation in L. tridentata saplings and patches at this biome transition zone is related to the different susceptibilities to invasion by two grassland ecosystems. The persistence of grasslands at this site despite region‐wide expansion by L. tridentata may be related to the spatial distribution of B. gracilis‐dominated ecosystems that resist or deter invasion by this woody plant.     

Prediction of plant cover from seed bank analysis in a semi‐arid plant community on gypsum

Abstract. Question: Does the seed bank filter annual plant composition and determine cover at the species level? Location: 510 m a.s.l., central Spain. Methods: Seven transects and 136 quadrats were established in a semi‐arid gypsum system. Seed bank samples were collected in each quadrat in September. The community was sampled the following April. For each quadrat we measured slope, microslope, landform, elevation, perennial cover and crust cover. Seed bank was estimated using the direct emergence method in glasshouse. Relationship among seed bank and annual community was assessed by Mantel correlations. Above‐ground cover for the five most abundant species was modelled with GLMs. Results: Seed bank density was the best predictor for annual community cover; perennial cover and landform were also included in the model. Species composition between September seed bank and April annual community cover was also highly related according to the Mantel test. This relationship was constant, even when the effect due to other abiotic (landform, microslope) or biotic (perennial cover, crust cover) parameters were partialled out. Microslope, elevation and seed bank density were the best parameters to predict spring cover of the five most abundant species. Conclusions: Above‐ground and below‐ground community compartments are strongly related in terms of abundance and species composition. This relationship is filtered by several environmental factors (e.g. perennial cover, landform, microslope) that exert a strong control at community and individual levels. Our results support the hypothesis that annual community performance is affected by seed bank pattern.     

Soil as a mediator in plant‐plant interactions in a semi‐arid community

Abstract. Competition and facilitation may occur simultaneously in plant communities, and the prevalence of either process depends on abiotic conditions. Here we attempt a community‐wide approach in the analysis of plant interactions, exploring whether in a semi‐arid environment positive or negative interactions predominate and whether there are differences among co‐occurring shrub species. Most shrubs in our plot exerted significant effects on their understorey communities, ranging from negative to positive. We found a clear case of interference and another case where the effect was neutral, but facilitation predominated and the biomass of annuals under most shrubs in our community was larger than in gaps. Effects on soil water and fertility were revealed as the primary source of facilitation; the build‐up of soil organic matter changed soil physical properties and improved soil water relations. Facilitation by shrubs involved decoupling of soil temperature and moisture. Sheltering from direct radiation had an effect on productivity, but significant differences in understorey biomass did not parallel understorey light environment. A positive balance of the interaction among plants, essentially mediated by changes in soil properties, is the predominant outcome of plant interactions in this semi‐arid community.