David J. Tongway

VEGETATION PATCHES AND RUNOFF–EROSION AS INTERACTING ECOHYDROLOGICAL PROCESSES IN SEMIARID LANDSCAPES

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. Exper- imental 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.

Spatial organisation of landscapes and its function in semi-arid woodlands, Australia

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.

Stripes, strands or stipples: modelling the influence of three landscape banding patterns on resource capture and productivity in semi-arid woodlands, Australia

Abstract In the semi-arid open woodlands or savannas of eastern Australia banded vegetation is a common form of landscape patchiness. This banding can form relatively long strands or shorter stripes across the landscape, or small patches can occur in a stippled pattern. In degraded areas these patches can be completely removed from the landscape. This study addresses two related questions: does the type of patchiness (strands, stripes, or stipples) significantly influence how efficiently these semi-arid landscapes capture and store scarce soil resources; and how does this efficiency compare with landscapes that have lost all their patches? Results from a landscape simulation model, validated for a semi-arid woodland study site, demonstrated that the loss of landscape patchiness had the greatest influence on the capacity of the landscape to capture rainfall as soil water—reduced by about 25% compared to banded landscapes. This 25% loss of soil water reduced annual net primary productivity in these systems by about 40%. Banded patterns (stripes or strands) captured about 8% more rainfall as soil water than a stippled pattern; this increased their plant production by about 10%. However, these differences between banding patterns were relatively small compared to the impact of totally eliminating patchiness, which can occur with severe land degradation. This implies that preventing the loss of landscape patchiness is very important for managing savannas for production and conservation goals.

A Scaling Rule for Landscape Patches and How It Applies to Conserving Soil Resources in Savannas

ABSTRACT Scaling issues are complex, yet understanding issues such as scale dependencies in ecological patterns and processes is usually critical if we are to make sense of ecological data and if we want to predict how land management options, for example, are constrained by scale. In this article, we develop the beginnings of a way to approach the complexity of scaling issues. Our approach is rooted in scaling functions, which integrate the scale dependency of patterns and processes in landscapes with the ways that organisms scale their responses to these patterns and processes. We propose that such functions may have sufficient generality that we can develop scaling rules—statements that link scale with consequences for certain phenomena in certain systems. As an example, we propose that in savanna ecosystems, there is a consistent relationship between the size of vegetation patches in the landscape and the degree to which critical resources, such as soil nutrients or water, become concentrated in these patches. In this case, the features of the scaling functions that underlie this rule have to do with physical processes, such as surface water flow and material redistribution, and the ways that patches of plants physically “capture” such runoff and convert it into plant biomass, thereby concentrating resources and increasing patch size. To be operationally useful, such scaling rules must be expressed in ways that can generate predictions. We developed a scaling equation that can be used to evaluate the potential impacts of different disturbances on vegetation patches and on how soils and their nutrients are conserved within Australian savanna landscapes. We illustrate that for a 10-km2 paddock, given an equivalent area of impact, the thinning of large tree islands potentially can cause a far greater loss of soil nitrogen (21 metric tons) than grazing out small grass clumps (2 metric tons). Although our example is hypothetical, we believe that addressing scaling problems by first conceptualizing scaling functions, then proposing scaling rules, and then deriving scaling equations is a useful approach. Scaling equations can be used in simulation models, or (as we have done) in simple hypothetical scenarios, to collapse the complexity of scaling issues into a manageable framework.

Banded Vegetation Patterning in Arid and Semiarid Environments

Banded vegetation patterns and related structures * Theories on the origins, maintenance, dynamics and functioning of banded landscapes * Specific methods of study * Runoff and Erosion Processes * Soil Water balance * Soil Biota in Banded Landscapes * Vegetation dynamics: recruitment and regeneration in two-phase mosaics * Multiscale Modelling of Vegetation Bands * Landscape Models for Banded Vegetation Genesis * Productivity of patterned vegetation * Towards Improved Management of Arid and Semi-Arid Banded Landscapes * Banded landscapes: Ecological Developments and Management Consequences

Degradation and recovery processes in arid grazing lands of central Australia. Part 1: soil and land resources

The distribution and quality of soil and land resources in heterogeneous grazing lands of central Australia were changed by grazing. Sites located at increasing distances from livestock watering points showed greater degrees of landscape organization and soil productive potential. The depositional strata, where resources tended to accumulate, occupied a larger proportion of the landscape as distance increased. Physical and nutrient cycling soil properties improved. All soil chemistry variables except pH and electrical conductivity increased and the trend was most apparent in the top 1 cm of the soil. Increasing erosion closer to water was a key degrading process. We showed degradation to be a systematic decline in regulation of scarce resources, which had implications for potential productivity.

Degradation and recovery processes in arid grazing lands of central Australia. Part 2: vegetation

In a naturally heterogeneous landscape in arid central Australia, a previous study found that grazing changed the distribution of water and nutrients amongst different geomorphic strata of the landscape. In this concurrent study, we show that herbage biomass, cover and composition responded primarily to these geomorphic strata and not to grazing. The cover of palatable species as a group proved the exception, and decreased with increasing grazing. The quantity of shrubs responded to both strata and grazing, and was greatest under least grazing. We suggest several potential reasons for the failure of damaged sites and strata to recover.

Theories on the Origins, Maintenance, Dynamics, and Functioning of Banded Landscapes

In this chapter, we explore the whys and hows of banded landscapes: (1) why do they form? (2) how are they maintained? (3) how do they move? (4) how do they work to conserve resources? In addition, we discuss how understanding these processes contributes to advancing heterogeneity theory in landscape ecology generally. Where banded landscapes occur and what forms they take were described in chapter 1. Such landscapes occur in Africa, Australia, and the Americas, and perhaps elsewhere, and are often called two-phase mosaics because they form bands of bare soil alternating with bands of perennial vegetation. They tend to be situated on low planar slopes and are characterized by the long axis of the bands being oriented along the contours of these slopes.

Monitoring Soil Productive Potential

Desertification involves the loss of soil productive potential, but a means of assessing and monitoring the progress of desertification on the soil has been elusive. Soil is so varied and complex that methods of assessing condition are too slow, tedious, and expensive for routine use. Moreover, differences in soil type can be confused with soil condition. This paper presents a structured method of assessing soil condition. This method is based on recognizing and classifying soil surface features and examining soil properties that reflect the status of the processes of erosion, infiltration, and nutrient cycling. Published in the form of a user manual, the method has the following three stages: (1) defining the geomorphic setting of the site, (2) recognizing patch/interpatch associations and the mode of erosion at the landscape scale, and (3) assessing soil surface condition ratings in quadrats sited within the landscape pattern patches. Stage 3 is achieved by observing each of 11 features in the field and classifying their status according to detailed fieldnotes and photographs. The method applies to a wide range of soil types and biogeographical regimes and has proven to be repeatable among observers and quickly transferred to new observers.

Viewing rangelands as landscape systems

In this paper we take a fresh look at how to reach a predictable understanding of how rangelands work by viewing them as landscape systems3. What are the landscape structures and processes operating in rangelands that allow us to better understand how limited resources, such as water and soil nutrients, are naturally conserved within these systems? How can this understanding help us to better manage the use of rangelands so that degradation can be prevented? Can we distinguish between utilization and desertification? These kinds of questions can be addressed within a logical framework based on landscape ecology. This framework, labeled the trigger-transfer-reserve-pulse (TTRP) conceptual model, is built upon existing concepts applied to arid and semiarid, resource-limited rangelands. The TTRP framework focuses on how materials in short-supply such as water and soil nutrients (in sediments) are carried by runoff, or by winds, and are captured and concentrated within landscape patches (reserves). If the availability of these resources within a patch exceeds a critical threshold, a production ‘pulse’ occurs. The organic structures and materials produced in such pulses feedback to build on the integrity of landscape patches, and through them to the landscape as a whole, enabling them to capture more resources in the future. However, if patches are degraded by disturbances such as grazing, fewer scarce resources will be captured and landscapes can become dysfunctional. The relative ‘state’ of dysfunction for a rangeland unit can be measured and used to position the unit along a continuum of landscape functionality, from highly functional to highly dysfunctional. Whether this position on the continuum is acceptable for the current uses being made of the rangeland can then be evaluated. If an undesirable change in landscape functionality is detected by monitoring indicators on a unit of rangeland, then management recommendations and strategies can be developed to reverse this change by viewing the rangeland unit as a dynamic landscape system functioning to conserve resources.