The degree and extent of current dryland degradation as well as their impacts on economic and political stability are widely recognized. Accordingly, societal demand for ecosystem restoration is rapidly increasing and environmental policy is increasingly embracing restoration. Drylands may response in a gradual, continuous way to a gradual change in human-induced and/or climate pressures, but they may also exhibit non-linear dynamics and sudden shifts between alternative stable states, including shifts to degraded states in response to increasing pressure. The mere decrease or cessation of the degradation pressure may not result in the recovery of the degraded or undesired state, which could require much better initial conditions than those that resulted in the degradation of the system. Feedbacks between the degraded state and a variety of internal and external factors make the degraded state highly resilient, and act as potential barriers to restoration (Figure 1). Explicit consideration of these feedbacks in the design of restoration could help overcome these barriers and enhance restoration success.

Figure 1

The intrinsic spatial heterogeneity and patchiness of dryland vegetation are essential dryland features that control ecosystem functioning and dynamics. However, little previous research has focused on restoration of vegetation patches and investigated how diversity, size, and spatial arrangement of plant patches could affect dryland restoration success. Progressing in our capacity for reverting degradation and restoring degraded drylands requires a better understanding of the role played by the biotic and spatial structure of restored vegetation patches, as well as by the feedbacks that control the resilience of degraded drylands.

The general objective of the work reported in this section of CASCADiSis to determine degradation reversal dynamics and thresholds as a function of plant colonization pattern and diversity. We have addressed this objective at two levels: at the plant patch scale and at the ecosystem scale. Main research questions at the patch scale pursue identifying the spatial and biotic structure of plant patches that would optimize the recovery of degraded drylands. At the ecosystem scale, we investigated the potential for degradation reversal and restoration of dryland ecosystems as a function of the initial plant cover and the strength of the ecohydrological feedbacks that control dryland dynamics.

For more details see »Objectives, research questions and approach.

We built two large, unique experimental stations of 24 (2 x 1 m) plots and 56 (8 x 5 m) plots, respectively, for the assessment of plant diversity and spatial pattern effects on dryland restoration by means of mesocosm experiments at the scale of the plant community. We conducted a variety of manipulative experiments on both stations, which focused on different aspects of the questions addressed in this work, and for which patch cover, pattern, size and diversity were independently manipulated in order to test their independent and combined effects on restoration potential for a variety of plant communities created ad hoc. To analyze the main basic effects of global and local feedbacks on the recovery potential of degraded drylands, we followed a modelling approach, extending a well-known spatially explicit dryland model that exhibits catastrophic transitions with the incorporation of global and local feedbacks.

For more details see
»Manipulating dryland plant communities in mesocosms and
»Modelling the role of ecohydrological feedbacks on degradation reversal

The effect of patch diversity and size on plant performance depended on the plant functional types considered and the climatic conditions, yet some common pattern was found for a large variety of dryland species tested. Thus, at early stages of the restoration trajectory (first 1-2 years after planting), with all plant seedlings sharing similar rooting space, there was no evidence of complementarity between species that may have resulted in higher productivity in multispecies patches as compared with monospecific patches. However, there was no evidence either of detrimental effects of interspecific competition, as compared with intraspecific competition in monospecific patches. Big diverse patches benefited better from the higher capacity for trapping water and other resources from runoff than big monospecific patches. However, under stressful conditions, facing both intra-specific and interspecific competition within the plant patch is more challenging for the species than interacting only with conspecific individuals. Individual biomass was not significantly reduced by increasing the number of accompanying species in the same patch. Increasing patch size and diversity may reduce to some extent the probability of sapling survival in the restored patch. However, our results indicate a positive net outcome from the trade-off between a relatively low risk of decreasing survival and the potential benefits derived from increasing diversity.

For more details see
»The role of patch size, diversity and spatial pattern in dryland restoration

At the community scale, low initial plant cover did not constrain the potential for restoration success, which could be explained by the positive effect of water and sediment transfer from large bare soil areas to few existent plant patches. Our findings have demonstrated that ecohydrological feedbacks between resource redistribution and vegetation dynamics that are mediated by bare-soil connectivity exert an important role in modulating the restoration potential of dryland ecosystems. Larger bare-soil connectivity implies larger water and sediment losses from semiarid slopes, but it also implies larger inter-patch areas and associated larger runon inputs to existent plant patches, which is beneficial for the performance of the vegetation in the patch. This local feedback, if enough strong, increases the range of conditions (external stress, minimum initial cover) that allow the recovery of the system.

For more details see
»The role of ecohydrological feedbacks in dryland degradation reversal

From an applied perspective, in a context of dryland restoration, a number of recommendations can be derived from our results, including

1. using (creating) multispecies big patches, yet minimizing intraspecific competition by reducing the number of individuals per species within the same patch;
2. spatially arranging plant patches on slopes in a way that maximizes the capture of runoff water by plant patches;
3. combining species in the plant patches with plant traits that maximize the capture and deep infiltration of runoff water.

For more details see
»Conclusions and recommendations

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