Semi-arid climates are exquisitely balanced on a range of ecosystem properties. Most famously, these savanna ecosystems, which border the truly arid deserts, are susceptible to desertification. Ecologists say that they have a bimodal stability: they can exist in two very different states, either grassland or shrubland, and small changes in land management and climate can 'flip' them from one to the other. In Huxman, T.E. et al. Ecohydrological Implications of Woody Plant Encroachment. Ecology, 308-319 (2005) ecohydrology researchers hypothesize that the response of semi-arid ecosystems differs between wet semi-arid sites and dry semi-arid sites, with subtle, yet important differences between them if they flip from grassland to shrubland.:
"The relative contribution of T to ET will increase for systems dominated by woody plants as compared to those dominated by nonwoody plants in more mesic climate zones, with the opposite effect in semi arid systems. Four zones can be delimitated where changes in the T/E T are likely to occur. In Area 1, there is little change in T/ET because leaf area remains about the same and ET is dominated by E from large expanses of bare ground. In Area 2, E increases substantially in woody plant systems as a result of the loss of herbaceous cover in intercanopy spaces. Area 3 is a transition zone (no further changes in T are being caused b woody plants, and herbaceous vegetation still dominates intercanopy spaces). In Area 4, differences in T/ET are due to increased T by shrubs (shrubs are using ‘‘extra’’ water that, in a grassland system, would become groundwater recharge)."