Ecological and hydrological processes can interact strongly in landscapes, yet these processes are often studied separately. One particularly important interaction between these processes in patchy semiarid lands is how vegetation patches serve to obstruct runoff and then how this retained water increases patch growth that, in turn, provides feedbacks to the system. Such ecohydrological interactions have been mostly demonstrated for semiarid landscapes with distinctly banded vegetation patterns. In this paper, we use data from our studies and from the literature to evaluate how strongly four ecohydrological interactions apply across other patchy semiarid vegetations, and how these interactions are affected by disturbances. We specifically address four questions concerning ecohydrological interactions: (1) if vegetation patches obstruct runoff flows during rainfall events, how much more soil water is stored in these patches compared to open interpatch areas; (2) if inputs of water are higher in patches, how much stronger is the pulse of plant growth compared to interpatches; (3) if more soil water in patches promotes greater biological activity by organisms such as earthworms that create macropores, how much does this improve soil infiltrability; and (4) if vegetation patches are damaged on a hillslope, how much does this increase runoff and erosion and decrease biomass production? We used the trigger–transfer–reserve–pulse framework developed for Australian semiarid woodlands to put these four questions into a landscape context. For a variety of patchy semiarid vegetation types in Australia, Europe, and North America, we found that patches significantly stored more soil water, produced more growth and had better infiltrability than interpatches, and that runoff and erosion can markedly increase on disturbed hillslopes. However, these differences varied greatly and appeared to depend on factors such as the intensity and amount of input events (rainstorms) and type of topography, soils, and vegetation. Experimental and modeling studies are needed to better quantify how these factors specifically affect ecohydrological interactions. Our current findings do support the conclusion that vegetation patches and runoff–erosion processes do strongly interact in many semiarid landscapes across the globe, not just banded landscapes.

Microbial activity in semiarid and arid environments is closely related to the timing, intensity, and amount of precipitation. The characteristics of the soil surface, especially the influence of biological soil crusts, can determine the amount, location, and timing of water infiltration into desert soils, which, in turn, determines the type and size of microbial response. Nutrients resulting from this pulse then create a positive feedback as increases in microbial and plant biomass enhance future resource capture or, alternatively, may be lost to the atmosphere, deeper soils, or downslope patches. When rainfall intensity overwhelms the water infiltration capacity of the plant interspace or the plant patch, overland water flow links otherwise separated patches at many different scales via the transport of nutrients in water, soil, and organic matter. For example, material carried from the plant interspace is often deposited under an adjacent plant. Alternatively, material from both of these patches may be carried to rills that feed ephemeral channels, thence to seasonally intermittent and, finally, perennial streams. These inputs can either be retained by the stream–riparian ecosystem or be exported in surface flow. However, in larger perennial streams, the fate of these material inputs is confounded by the impact of storm‐driven flows on the extant aquatic biota, as flash floods can also represent succession‐initiating disturbances to the stream–riparian ecosystem on a wide range of time scales. In contrast to uplands where precipitation initiates the microbial response, nutrient transfers can support a flush of plant uptake and microbial processing, triggered by high nutrient concentrations and changed nutrient form (e.g., nitrate or ammonium). The nature and strength of the linkages between the different ecosystem components define the structure and function of arid ecosystems. Losses of materials are natural processes, but it is problematic when “conserving” systems become “leaky” via anthropogenic disturbance and losses exceed gains.

The spatial organisation of three major landscape types within the semi-arid woodlands of eastern Australia was studied by a detailed analysis of gradient-oriented transects (gradsects). The aim was to characterise the spatial organisation of each landscape, and to account for that organisation in functional terms related to the differential concentration of scarce resources by identifiable processes. Terrain, vegetation and soils data were collected along each gradsect. Boundary analysis was used to identify the types of landscape units at a range of scales. Soil analyses were used to determine the degree of differential concentration of nutrients within these units, and to infer the role of fluvial and aeolian processes in maintaining them. All three major landscape systems were found to be highly organised systems with distinctive resource-rich units or patches separated by more open, resource-poor zones. At the largest scale, distinct groves of trees were separated by open intergroves. At smaller-scales, individual trees, large shrubs, clumps of shrubs, fallen logs and clumps of grasses constituted discrete patches dispersed across the landscape. Our soil analyses confirmed that these patches act as sinks by filtering and concentrating nutrients lost from source areas (e.g., intergroves). We suggest that fluvial runoff-runon and aeolian saltation-deposition are the physical processes involved in these concentration effects, and in building and maintaining patches; biological activities also maintain patches. This organisation of patches as dispersed resource filters (at different scales) has the overall function of conserving limited resources within semi-arid landscape systems. Understanding the role of landscape patchiness in conserving scarce resources has important implications for managing these landscapes for sustainable land use, and for the rehabilitation of landscapes already degraded.

Vegetation and soil patterns across a 200 fia semi-arid site 40 km north-west ofEouth. NSW. are described using plant cover data from line transects and soils data from points, sampled systematically (50 m intervals) across the site. A patterned sequence of alternating groves and intergroves with three vegetation types was identified: an Eragrostis eriopoda savanna occurring on runoff slopes (<0.5%) from low ridges, with a Monachather paradoxa savanna occurring at the toe of these runoff slopes (intergroves). followed by an Acacia anuera woodland occurring on runon areas (groves, either as discrete islands or more continuous along drainage lines). Data on landform. microtopography, and hydrological features indicate that thegrove-intergrove pattern is maintained by differential erosion-deposition processes similar to the dynamic erosion-transfer-sink geomorphic systems described for Central Australia.This vegetation grove-intergrove patterning in Eastern Australia is similar to, but differs in detail from, such patterns reported for arid and semi-arid Western and Central Australia. Groves or 'bands' of A. anuera in the Centre and the West tend to occur on the downslope side of 'risers' or on 'convex slope-breaks' where in the East such groves oecur in distinct 'steps' or 'flats' in the landscape: there is a drop into the grove and a sharp 'erosion-scarp' below the grove. A prominent 'grass band', identified by cluster analysis as the M. paradoxa community type, occurs immediately upslope of A. anuera groves in the East. The A. anuera groves in the East are also 'fertile' patches as soils data demonstrate that groves have mueh higher levels of organic and exehangeable nutrients (and plant cover) than soils in the intergroves. This paper demonstrates that patterning in mulga lands is more extensive geographieally, and has a wider climatic range, than previously reported.

Abstract:This paper highlights the relevant issues influencing the amount and arrangement of ground cover in savanna rangelands in Australia, and presents field measurements from hillslope scale flumes, which demonstrate how runoff and sediment loss vary with spatial patterns in ground cover. Hillslopes with relatively high mean cover, but with small patches bare of vegetation, are shown to have between 6 and 9 times more runoff, and up to 60 times more sediment loss than similar hillslopes that do not contain bare patches. The majority of sediment lost from the hillslopes is composed of fine (suspended) rather than coarse (bedload) material, although the absolute sediment loads are comparatively low. These low loads are considered to be the result of lower than average rainfall during the measurement period (2002)(2003)(2004)(2005) and the high and prolonged rates of historical hillslope erosion that have exhausted the erodible material from the A-horizon. The collected data also demonstrate that a large proportion of soil is lost during the initial 'flushing' period of runoff events.The results presented have important implications for the management of savanna grazing systems by highlighting (i) the significance of bare patches in contributing to runoff and soil loss from hillslopes; (ii) the importance of having medium to high cover patches at the bottom of hillslopes for trapping and storing sediment and therefore reducing its entry into the stream network; and (iii) how maintenance of ground cover during the dry season reduces sediment concentrations in runoff occurring early in the wet season.

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