Vegetation patches in drylands are localized structures of biomass and water. We study these structures using a mathematical modeling approach that captures biomass-water feedbacks. Biomass-water structures are found to differ in their spatial forms and ecological functions, depending on species type, soil conditions, precipitation range, and other environmental factors. Asymptotic spot structures can destabilize to form ring structures, expanding in the radial direction, or crescent structures, migrating uphill. Stable spot structures can differ in their soil-water distributions, forming water-enriched patches or water-deprived patches. The various biomass-water structures are expected to function differently in the context of a plant community, forming landscapes of varying species diversity.
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Drylands are water-limited systems comprising about 40% of the terrestrial earth surface.
Various factors contribute to this effect, including biological crusts that grow on bare soil and reduce the infiltration rate, but do not develop in vegetation patches due to shading and litter formation, and soil mounds, formed by litter accumulation and dust deposition, that intercept runoff.
We chose in this paper to view the closely related water-uptake and root-augmentation processes as two distinct feedbacks, one negative and one positive, rather than lumping them together under a single name, “uptake feedback,” as we did in earlier publications (Refs. 9 and 11).
The bifurcation is subcritical (supercritical) depending on whether the quantity is greater (lower) than unity.
Ecosystem engineers need not necessarily be plant species. They can be micro-organisms such as cyanobacteria in drylands which reduce the infiltration rate of surface water by forming soil crusts, thereby increasing the flow of surface water into lower places. Ecosystem engineers can also be animals. A well known example is the North American beaver which forms small ponds by building dams across rivers.
Positive (negative) engineering refers to situations in which the maximal soil-water density in the engineer’s patch is higher (lower) than the soil-water density in bare soil.
