Recent theoretical studies have shown that spatial redistribution
of surface water may explain the occurrence of patterns
of alternating vegetated and degraded patches in semiarid grasslands.
These results implied, however, that spatial redistribution processes
cannot explain the collapse of production on coarser scales observed
in these systems. We present a spatially explicit vegetation model to
investigate possible mechanisms explaining irreversible vegetation
collapse on coarse spatial scales. The model results indicate that the
dynamics of vegetation on coarse scales are determined by the interaction
of two spatial feedback processes. Loss of plant cover in a
certain area results in increased availability of water in remaining
vegetated patches through run-on of surface water, promoting
within-patch plant production. Hence, spatial redistribution of surface
water creates negative feedback between reduced plant cover and
increased plant growth in remaining vegetation. Reduced plant cover,
however, results in focusing of herbivore grazing in the remaining
vegetation. Hence, redistribution of herbivores creates positive feedback
between reduced plant cover and increased losses due to grazing
in remaining vegetated patches, leading to collapse of the entire
vegetation. This may explain irreversible vegetation shifts in semiarid
grasslands on coarse spatial scales
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