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.     

Fine‐scale spatial distribution of murundus structures in the semi‐arid region of Brazil

Murundus are earth mounds widespread in most landscapes in the semi‐arid region of Brazil. Evidence obtained from predictive modelling has suggested a termite origin for these structures, opening up new opportunities for further research. Distribution of densely packed murundus at larger spatial scales is most related to climatic regime and soil nutrient availability. However, factors and processes underlying their distribution and density at smaller spatial scales are not yet fully understood. In this study, we adopted an approach based on mapping point data using high‐resolution satellite imagery, multi‐scale second‐order analysis and general linear models to examine the fine‐scale spatial distribution and density of murundus. Our results suggest that the distribution of those structures within densely packed areas is regulated by more than one process acting or interacting across multiple spatial scales. All densely packed murundus showed a significant regular distribution at the distance scale of up to 50 m radially and a completely random distribution across all other upper distance scales. We interpret the regular pattern as a result of competition for foraging territories between different termite colonies during the process formation of densely packed murundus. The random pattern at larger distance scales (above 50 m radially) can be attributed to habitat selection preferences by termite species builders of murundus mediated by local environmental resources and conditions (i.e. availability of food resources and nesting and open habitat), which would be randomly distributed in space. Thus, at finer spatial scales murundus distributions are associated with biotic interactions acting on an abiotic template. On the basis of significant linear correlations, we suggest that the density of murundus is strongly related to local temperature regime with soil‐type influencing its effect on the murundus densities. Our findings provide novel evidences that mound‐building termites are involved in the formation of murundus in the semi‐arid region of Brazil.     

Influence of fine‐scale habitat requirements and riparian degradation on the distribution of the purple‐crowned fairy‐wren (Malurus coronatus coronatus) in northern Australia

Species distributions are influenced by variation in environmental conditions across many scales. Knowledge of fine‐scale habitat requirements is important for predicting species occurrence and identifying suitable habitat for target species. Here we investigate the perplexing distribution of a riparian habitat specialist, the western subspecies of the purple‐crowned fairy‐wren (Malurus coronatus coronatus), in relation to fine‐scale habitat associations and patterns of riparian degradation. Surveys of vegetation attributes, river structure and disturbance indicators that are likely to be causal determinants of the species occurrence were undertaken at 635 sites across 14 catchments. Generalized Linear Mixed Modelling demonstrated that the probability of purple‐crowned fairy‐wren occurrence increased with Pandanus aquaticus crown cover, shrub density and height of emergent trees, while riparian structure and signs of cattle were indirect predictors of occurrence. As our study area predominantly contained Pandanus type habitat, we failed to identify river grass as an important component of habitat. Predictions from a cross‐validated model of purple‐crowned fairy‐wren occurrence suggested distribution is constrained by three factors: (i) low quality of local habitat within catchments where the species occurs; (ii) broad‐scale reduction in habitat quality that has resulted in extinction of the species from parts of its range; and (iii) unmeasured variables that limit the exploitation of suitable habitat. The reliance of the species on dense shrubby understorey suggests conservation efforts should aim to maintain the complexity of understorey structure by managing fire and grazing intensity. Efforts to halt the continuing decline of riparian condition and maintain connectivity between areas of quality habitat will help to ensure persistence of riparian habitat specialists in northern Australia.     

Landscape dysfunction and reduced spatial heterogeneity in soil resources and fertility in semi‐arid succulent thicket,South Africa

Abstract Succulent thicket in the Eastern Cape of South Africa is one of many rangeland ecosystems in the world that displays evidence of unsustainable grazing pressure. Widespread transformation of succulent thicket has resulted in the replacement of the typical two‐phase perennial vegetation patches with a structurally simple field layer of ephemeral and weakly perennial grasses and forbs. We hypothesized that (i) transformation of succulent thicket leads to a switch from a spatially heterogeneous landscape to a homogenous and relatively infertile state; and (ii) that this loss of fertility is associated with a breakdown in the processes that conserve resources and promote water use efficiency. We tested these hypotheses at five fenceline contrast sites in Sundays River Thicket, an arid form of succulent thicket. We compared soil fertility (organic carbon, available nitrogen and phosphorus), texture, matric potential, and surface microtopography in the two main microhabitats on either side of the fencelines using a nested anova. Our results show that intact Sundays River thicket has a distinct spatial pattern of soil fertility where nutrients and organic carbon are concentrated under the patches of perennial shrubs, compared with under canopy trees. Transformation results in a significant homogenization of this pattern and an overall reduction in the fertility of the landscape. The proportion of the landscape surface that promotes infiltration due to a distinct litter layer decreases from 60% to 0.6%. Soil moisture retention (matric potential) also decreases with transformation. We interpret these patterns within the framework of semi‐arid landscape functionality.     

Formation of nutrient‐poor soil patches in a semi‐arid woodland by the European rabbit (Oryctolagus cuniculus L.)

Abstract European rabbits (Oryctolagus cuniculus L.) occur over large areas of eastern Australia, where they disturb significant quantities of soil while constructing warrens. Although the effects of rabbits on vegetation are generally well understood, little is known about their effects on soil physical or chemical properties. We studied the effect of rabbit warrens on soil chemistry in a semi‐arid woodland supporting a high density of rabbit warrens. Within one large warren, we examined nutrient concentrations within three microsites (mound, inter‐mound and an intermediate disturbed area) representing a gradient of increasing rabbit disturbance, and supported this with a study of soil nutrients with depth within 1‐m‐deep trenches through five warrens. Landscape‐level changes in surface chemistry were also examined at an additional 23 warrens. At both patch and landscape scales, pH, electrical conductivity, soluble and exchangeable Ca²⁺ and K⁺, and total Al and Ca generally increased with increasing rabbit disturbance, while total C and S (LECO), total P and S (ICP), and soluble Na⁺ declined, and total N remained unchanged. Although chemical changes with depth were generally ill defined, surface soils tended to be more similar in their composition than deeper soils. Overall, our results reinforced the view that rabbits have a negative effect on surface soils in semi‐arid woodlands, and suggested that restoration of the original woodland vegetation may be hampered by changes in soil biogeochemistry associated with the warrens.     

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.     

高黎贡山自然保护区大型土壤动物群落特征

系统调查了高黎贡山国家自然保护区土壤性质与土壤动物的种类与分布.海拔高度决定土壤的性质,土壤温度、湿度、有机质含量和磷的含量与海拔有明显的相关.在不同海拔11个样地共采集到大型土壤动物标本1万余号,分属于2个门,5纲,18目,47科,13亚科.等翅目Isoptera、膜翅目Hymenoptera和弹尾目Collembola昆虫是主要类群,随着海拔高度的增加,土壤动物数量和种类,以及Simpson指数、Shannon-Wiener指数和均匀度指数都是先增加然后减少.在海拔1500～2500 m左右数量最多,然后递减,土壤动物的分布以中山部最多而山顶部和底部较少的变化趋势.土壤动物的分布受海拔和土壤性质的影响.     

Patch and soil characteristics of pastures with water‐spreading banks and woody encroachment in semi‐arid Australia

Water‐spreading banks are used in semi‐arid areas such as the Cobar pediplain in western New South Wales, Australia to encourage pasture growth, often after removal of woody encroachment. We studied the arrangement of bare inter‐patches and vegetated patches, and associated surface soil variables, in three pastures following installation of water‐spreading banks (2, 15, 38 years ago) and in an area of woody encroachment near Cobar. The aims of the study were as follows: (i) to determine the number and percent area of inter‐patches and vegetated patches, and associated surface soil variables at the three pasture sites and at the woody encroachment site and (ii) by inference, explore effects of establishing water‐spreading banks and pasture following removal of woody encroachment on these factors, to understand the role of water‐spreading banks as a management tool. The percent area of inter‐patches in pasture with 38‐year‐old water‐spreading banks was much lower, and the percent area of medium‐vegetated patches (but not of well‐vegetated patches) was substantially higher, than in the woody encroachment. Differences in soil carbon and nitrogen between the sites were related to their percent areas of inter‐patches and vegetated patches. The results suggest that the mosaic of bare inter‐patches and vegetated patches changes over time after clearing of woody encroachment and establishment of pasture with water‐spreading banks, from many large inter‐patches to a few small inter‐patches, and from small to large medium‐vegetated patches. Water‐spreading banks are a useful management tool in these landscapes because of their benefits for landscape function, that is, bare areas become less connected, the percent area of moderately vegetated patches increases, and soil carbon builds up with time following their installation.     

云南省三种典型气候带凋落物层弹尾类多样性分布格局

弹尾类是土壤动物中常见的优势类群,其作为土壤微食物网的重要组成部分,参与凋落物分解、土壤团聚体形成等重要生态过程。以往对弹尾类分布格局的研究通常关注其在不同海拔梯度或者不同生境类型下的分布情况,但在不同气候带下弹尾类多样性沿纬度分布格局仍不清楚。为讨论不同气候带下弹尾类沿纬度的分布格局及其潜在的环境影响机制,于2017年10月(雨季末期)在云南省同一经度(E 101°)分布的三种典型气候带设置海拔梯度样带:热带雨林(西双版纳,800 m、1000 m、1200 m、1400 m)、亚热带常绿阔叶林(哀牢山,2000 m、2200 m、2400 m、2600 m)、亚高山针叶林(丽江玉龙雪山,3200 m、3400 m、3600 m、3800 m),采集凋落物层弹尾类并调查分析土壤温度、土壤含水量、凋落物厚度、土壤pH、土壤容重及土壤孔隙度等环境因子。利用Berlese-Tullgren法收集土壤动物,共获得弹尾类19150只,隶属于10科29属,其中符■属(39.9%,等节■科)数量最多,其余优势属为棘■属(21.7%,棘■科)和球角■属(10.1%,球角■科),这3个属合计占总体的71.7%。凋落物层弹尾类的密度在亚高山针叶林明显高于热带雨林和亚热带常绿阔叶林,弹尾类的属数排序由多到少依次为热带雨林、亚高山针叶林、亚热带常绿阔叶林。通过最小二乘回归法对弹尾类多样性的海拔格局进行回归分析,得出弹尾类的丰富度指数(Margalef′s指数、Menhinick′s指数)、多样性指数(Shannon-Wiener多样性指数、Simpson多样性指数)和均匀度指数(Pielou均匀度指数)沿热带雨林、亚热带常绿阔叶林、亚高山针叶林呈单调递减格局。通过全模型子集回归筛选最佳环境模型表明,温度是影响弹尾类多样性沿不同气候带分布格局的主要环境因子。本研究为预测不同气候带下弹尾类多样性如何响应环境变化提供参考。     

Long‐term patterns of invertebrate abundance and relationships to environmental factors in arid Australia

Resource pulses are a key feature of semi‐arid and arid ecosystems and are generally triggered by rainfall. While rainfall is an acknowledged driver of the abundance and distribution of larger animals, little is known about how invertebrate communities respond to rain events or to vegetative productivity. Here we investigate Ordinal‐level patterns and drivers of ground‐dwelling invertebrate abundance across 6 years of sampling in the Simpson Desert, central Australia. Between February 1999 and February 2005, a total of 174 381 invertebrates were sampled from 32 Orders. Ants were the most abundant taxon, comprising 83% of all invertebrates captured, while Collembola at 10.3% of total captures were a distant second over this period. Temporal patterns of the six invertebrate taxa specifically analysed (Acarina, ants, Araneae, Coleoptera, Collembola and Thysanura) were dynamic over the sampling period, and patterns of abundance were taxon‐specific. Analyses indicate that all six taxa showed a positive relationship with the cover of non‐Triodia vegetation. Other indicators of vegetative productivity (seeding and flowering) also showed positive relationships with certain taxa. Although the influence of rainfall was taxon‐dependent, no taxon was affected by short‐term rainfall (up to 18 days prior to survey). The abundance of Acarina, ants, and Coleoptera increased with greater long‐term rainfall (up to 18 months prior to survey), whilst Araneae showed the opposite effect. Temperature and dune zone (dune crest vs. swale) also had taxon‐specific effects. These results show that invertebrates in arid ecosystems are influenced by a variety of abiotic factors, at multiple scales, and that responses to rainfall are not as strong or as predictable as those seen for other taxa. Our results highlight the diversity of invertebrates in our study region and emphasize the need for targeted long‐term sampling to enhance our understanding of the ecology of these taxa and the role they play in arid ecosystems.     

Relationship between group dynamics and spatial distribution of African elephants in a semi‐arid environment

Group dynamics related to distribution of African elephants in the semi‐arid environment of Tsavo East National Park, Kenya was studied between 2007 and 2010. I examined the seasonal distribution of lone bulls, bull groups, family units and mixed groups and group size. Lone bulls were widely distributed in the dry season and localized in the wet season, whereas bull groups were localized in both seasons. On average bull group size did not differ with seasons or areas, but larger groups were observed in preferred areas in the dry season. Family units were widely distributed in the dry season and localized in the wet season, whereas mixed groups were localized in both seasons. Although family units and mixed groups tended to be large in all areas in the wet season, large groups were also observed in the dry season. Large family units were widely distributed, whereas large mixed groups were localized. The relationship between group dynamics and distribution was attributed to the combination of social, ecological and poaching factors. This study provided insight into distribution patterns that can be applied to the security of the Tsavo elephant population.     

The effect of plant litter on ecosystem properties in a Mediterranean semi‐arid shrubland

Abstract. We studied the distribution of litter in a shrubland of the Negev with a semi‐arid Mediterranean climate of less than 200 mm of rainfall per year. Our focus was on the effects of litter on properties of landscape patches relevant to ecosystem processes (water runoff and soil erosion), annual plant community responses (seedling density, biomass production and species richness), and animal activity (soil disturbance by termites). Three 60‐m transects, extending across a pair of opposing north‐ and south‐facing slopes and their drainage channel, showed that litter accumulates not only under shrubs, but to a lesser extent also on the crusted inter‐shrub open areas. We used 35 experimental units (‘cells’, 0.5m × 1 m), each containing a crust and a shrub patch. Because runoff flows from crusted patches and is intercepted by shrub patches, the latter were in the lower third of the cells. Leaf litter was added in single and double amounts providing ca. 0.5 and 1.0 cm litter depth, to either, both, or none of the patches. Litter addition significantly decreased the amount of runoff, regardless of the location and amount of litter applied. Litter on the crust increased species number and seedling density of species with low abundance. Adding a double litter layer increased annual plant biomass production, while a single amount had no effect. Litter addition to the shrub patch affected neither biomass nor species richness. Litter addition to both patches at both quantities caused a large increase in termite activity. Termites caused disturbance by disrupting the crust, which may contribute to the reduction in runoff amounts. In the open, flat crust patches, annual plant communities are limited in their productivity and species richness, as there are few structures stopping the outflow of water, soil and seeds. Litter adds such structures, but affects the plant communities only when added to litter‐free crust. Litter accumulation and its patchy distribution have large impacts on landscape patch properties affecting resource distribution, plant productivity and diversity, and animal activity. Therefore, understanding litter distribution in relation to the patchy structure of the landscape of semi‐arid shrubland should be viewed as an important component of shrubland management.     

Vegetation and landscape connectivity control wildfire intervals in unmanaged semi‐arid shrublands and woodlands in Australia

Aim The aim of this study was to determine how spatial variation in vegetation type and landscape connectivity influence fire intervals in a semi‐arid landscape with low relief and complex mosaics of woodland and shrubland vegetation. Location Our study focused on a 15,500‐km² area of relatively undisturbed and unmanaged land in south‐western Australia, referred to as the Lake Johnston region. Methods We modelled fire‐interval data from a 67‐year (1940–2006) digital fire history database using a two‐parameter Weibull function, and tested for the effects of vegetation type and landscape connectivity on estimates of the length of fire intervals (Weibull parameter b) and the dependence of fire intervals on fuel age (Weibull parameter c). Results Vegetation type and landscape connectivity significantly influenced fire interval probability distributions. Fire intervals in shrublands (dense low shrub assemblage) were typically shorter (b = 46 years) and more dependent on fuel age (c = 2.33) than most other vegetation types, while fire intervals in open eucalypt woodlands were much longer (b = 405 years) and were less dependent on fuel age (c = 1.36) than in shrub‐dominated vegetation types. Areas adjacent to or surrounded by salt lakes burnt less frequently (b = 319 years) and fire intervals were less dependent on fuel age (c = 1.48) compared with more exposed areas (e.g. b < 101 years, c > 1.68). Fire intervals in thickets (dense tall shrub assemblage) were longer (b = 101 years) than would be expected from fuel loads, most likely because they were protected from fire by surrounding fuel‐limited woodlands. Main conclusions Fire intervals in south‐western Australia are strongly influenced by spatial variation in vegetation (fuel structure) and landscape connectivity. The importance of fuel structure as a control of fire intervals in south‐western Australia contrasts with other landscapes, where topographical gradients or climatic influences may override the effects of underlying vegetation. We found that, regardless of low relief, topographical features such as large salt lake systems limited the connectivity and spread of fire among landscape units in an analogous manner to lakes or mountainous features elsewhere.     

Interactive global change factors mitigate soil aggregation and carbon change in a semi‐arid grassland

The ongoing global change is multi‐faceted, but the interactive effects of multiple drivers on the persistence of soil carbon (C) are poorly understood. We examined the effects of warming, reactive nitrogen (N) inputs (12 g N m^?2 year^?1) and altered precipitation (+ or ? 30% ambient) on soil aggregates and mineral‐associated C in a 4 year manipulation experiment with a semi‐arid grassland on China's Loess Plateau. Our results showed that in the absence of N inputs, precipitation additions significantly enhanced soil aggregation and promoted the coupling between aggregation and both soil fungal biomass and exchangeable Mg²⁺. However, N inputs negated the promotional effects of increased precipitation, mainly through suppressing fungal growth and altering soil pH and clay‐Mg²⁺‐OC bridging. Warming increased C content in the mineral‐associated fraction, likely by increasing inputs of root‐derived C, and reducing turnover of existing mineral‐associated C due to suppression of fungal growth and soil respiration. Together, our results provide new insights into the potential mechanisms through which multiple global change factors control soil C persistence in arid and semi‐arid grasslands. These findings suggest that the interactive effects among global change factors should be incorporated to predict the soil C dynamics under future global change scenarios.     

Species distribution modelling in low‐interaction environments: Insights from a terrestrial Antarctic system

It is widely acknowledged that in the terrestrial Antarctic, interspecific interactions are typically unimportant in determining species distributions and community structure. Therefore, correlative models should prove useful for predicting current and future spatial variation in species abundance patterns. However, this idea has not been formally tested, and the utility of such models, which have shown value for understanding the distribution of diversity elsewhere, for investigating biodiversity patterns in Antarctica remains unclear. Here we make a start at such tests by using generalized linear and simultaneous autoregressive models to demonstrate that simple environmental variables and information about the spatial structure of the environment can explain more than 90% of the variation in the abundance of Maudheimia wilsoni (Oribatida; Maudheimiidae), a representative of one of the most significant groups of Antarctic terrestrial arthropods, the mites. We show that a single environmental variable, maximum soil moisture content, can account for as much as 80% of the variance in the abundance of the mite, and that linear models with only a few environmental and spatial terms can be used to forecast the species abundance at the landscape scale. Given ongoing calls for better understanding of the distribution of Antarctic diversity and its likely future change, this initial test indicates that such modelling procedures, and more sophisticated versions thereof, hold much promise for the region and should be tested for other taxa with different life forms and habitat requirements.     

Fire effects on the soil seed bank and post‐fire resilience of a semi‐arid shrubland in central Argentina

Soil seed bank is an important source of resilience of plant communities who suffered disturbances. We analysed the effect of an intense fire in the soil seed bank of a semi‐arid shrubland of Córdoba Argentina. We asked if the fire affected seed abundance, floristic and functional composition of the soil seed bank at two different layers (0–5 cm and 5–10 cm), and if fire could compromise the role of the soil seed bank as a source of resilience for the vegetation. We collected soil samples from a burned site and from a control site that had not burned. Samples were installed in a greenhouse under controlled conditions. During 12 months, we recorded all germinated seedlings. We compare soil seed bank with pre‐fire vegetation in terms of floristic and functional composition. The high‐intensity fire deeply affected the abundance of seeds in the soil, but it did not affect its floristic or functional composition. Floristic and functional composition of soil seed banks – at burned and unburned sites‐ differed markedly from that of the pre‐fire vegetation, although a previous study at the same site indicated high resilience after fire of this plant community. Our results indicate that resilience of this system is not strongly dependent on direct germination from seeds buried in the soil. Other sources of resilience, like colonization from neighbouring vegetation patches and resprouting from underground organs appear to gain relevance after an intense fire.