|Contributing Authors:||Sonia Kéfi, Florian Schneider, Angeles G. Mayor, Alain Danet, Marina Rillo, Simon Benateau, Jacob Keizer, Ana Vasques, Susana Bautista, Paco Rodriguez, Alejandro Valdecantos, Jaime Baeza, Ramón Vallejo, Max Rietkerk, Mara Baudena, Mart Verwijmeren, Koen Siteur, Rubén Díaz-Sierra|
|Source document:||S. Kéfi, A. Vasques, F. Schneider, M. Rietkerk, A.G. Mayor, M. Verwijmeren, R. Diaz-Sierra and M. Baudena. 2016. Response of Mediterranean drylands to increasing pressures. CASCADE Project Deliverable 6.1, 54 pp
In drylands, the vegetation cover is typically composed of vegetation patches interspaced by bare ground . Both the total amount of vegetation cover and vegetation pattern greatly impact the potential of the ecosystem to conserve key resources such as water, soil, and nutrients [103–106]. Despite the existing consensus on the crucial role of such connectivity-mediated feedbacks in the behavior of dryland ecosystems, their impact on dryland response to land degradation drivers has been barely tested and assessed in experimental or modelling studies (but see ).
We extended the model of Kéfi et al.  by including a positive feedback between increased hydrological connectivity of bare-soil areas, global losses of water and nutrients from the system, and reduced plant productivity, which in turn then further increases hydrological connectivity (connectivity-mediated feedback, Figure 1). The loss of water and nutrients was estimated with Flowlength , a spatial metric designed to be used as a surrogate for the loss of resources on hillslopes where runoff is the main agent of sediment transport and deposition. Flowlength is based on the assumption that bare soil (i.e., either empty or degraded cells in the model) and vegetated patches behave, respectively as sources and sinks of runoff and sediments. Flowlength measures the connectivity of bare-soil areas by calculating the average of the runoff pathway lengths from all the cells in the system, with higher values representing higher hydrological connectivity of bare-soil areas.
The simulations suggested that the connectivity-mediated feedback decrease the amount of pressure required to cause a critical shift to a degraded state (ecosystem resilience). If environmental conditions improve, this feedback increases the pressure release needed to achieve the ecosystem recovery (restoration potential). Interestingly, the simulations also showed a higher sensitivity of the bare-soil connectivity index (Flowlength) to changes in the spatial organization of the vegetation during the transition to a degraded state, in comparison with bare-soil (or vegetation) cover, which shows a rather linear evolution during this transition.
Our results suggest that modelling studies on dryland vegetation dynamics not accounting for the connectivity-mediated feedbacks studied may overestimate the resistance, resilience and restoration potential of drylands in response to environmental and human pressures. Moreover, our results suggest that changes in both vegetation cover and pattern (and associated hydrological connectivity) along degradation trajectories may be more informative early-warning indicators of dryland degradation than changes in vegetation cover. Thus, the acceleration of bare-soil connectivity from spatially-explicit time-series data may provide an early warning of imminent shift. This bare-soil connectivity index could be of special interest for management, since it helps identifying the critical point at which measures should be adopted to prevent drastic changes in ecological conditions.
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