Research conclusions

By performing a variety of manipulative experiments and modelling exercises, we have assessed the degradation reversal potential as a function of plant colonization pattern and diversity and the hypothesized ecohydrological feedbacks that modulate dryland dynamics. At the patch scale, we compared the performance of

1. multispecies versus monospecific patches, and
2. patches with single individuals versus patches with increasing number of individuals and or species.

At the patch scale, the effect of patch diversity and size on plant performance depended on the plant functional types considered and the environmental conditions, yet some common pattern was found for a large variety of dryland species tested.

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. 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.

Compared with patches with a single plant, 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, in general, the reduction in survival with increasing diversity is minor suggesting a positive net outcome from the trade-off between a relatively low risk of decreasing survival and the benefits derived from increasing diversity. Functional diversity did not appear to be more relevant than species diversity for plant patch performance at early stages of the restoration trajectory.

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.

Recommendations for dryland restoration

Dryland restoration faces important constraints and limitations, mostly derived from the limited and spatially and temporally heterogeneous resource availability (Whitford, 2002). Restoration approaches developed for mesic areas are seldom suitable for drylands, as ignoring dryland heterogeneity, the typical patchy nature of dryland landscapes, and the critical role played by plant-plant interactions in dryland vegetation communities could easily result in restoration failures.

Research on dryland restoration has mostly focused on improving nursery and field treatments to increase seedling survival and optimize plant growth (e.g., Chirino et al. 2009; Valdecantos et al., 2014). In the last two decades, the role of positive plant-plant interactions is being increasingly considered in restoration ecology (e.g., Maestre et al., 2001; Gómez-Aparicio et al., 2004). However, 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.

Ecological approaches to restoration emphasize process repair (i.e., reestablish rates or regimes of key processes that sustain the target ecosystems, such as fire or flooding regimes, erosion and sediment transport, etc.) over structural replacement (such as the construction of particular habitat or landscapes structures or the introduction of particular species) (Falk, 2006; Beechie et al., 2010). However, drylands are mostly controlled by ecohydrological processes that are tightly coupled to the biotic and spatial structure of their ecosystems and landscapes. For example, the transfer of resources from bare-soil interpatches to downslope vegetation patches contribute to plant productivity and overall ecosystem productivity (Aguiar and Sala 1999, Yu et al. 2008, Turnbull et al. 2012), with this transfer being modulated by the spatial pattern of the vegetation and the size of the upslope bare-soil areas (Bautista et al., 2007; Urgeghe et al., 2010; Urgeghe and Bautista, 2015) and the species functional group in the plant patch (Bochet et al., 2006; Mayor et al., 2009). Despite the importance of the spatial pattern and the patchy structure of vegetation for the overall functioning and dynamics of dryland ecosystems, to our knowledge few previous research has investigated the restoration of patches (Ludwig and Tongway, 1996) and no previous work has addressed how features such as the diversity, size, and spatial arrangement of plant patches could affect dryland restoration success.

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