Invasion of Semiarid Grasslands by Shrubs

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0003514SmallThe semiarid American Southwest experienced a dramatic vegetation change during the 20th century-perennial grasslands were invaded by shrubs. Similar transitions have occurred worldwide. As shrubs replace grasses, nutrients become increasingly concentrated in widely spaced "islands of fertility" around individual plants. In addition, the top layer of soil is eroded from between shrubs, reducing infiltration and enhancing runoff. Both processes may inhibit plant growth between shrubs, enhancing the initial vegetation change.Plant-water interactions may also play an important role in the shrub invasion process, so we will test the following hypothesis. The invasion of semiarid grasslands by shrubs enhances the convergence of overland flow and infiltration beneath plant canopies. The most dramatic effects exist during high-intensity rainfall events and on steep slopes. Compared to grasslands, infiltration is enhanced beneath shrub canopies because runon from upslope interspaces increases-interspaces are larger and the interspace infiltration capacity is reduced. The result is that soil moisture is elevated for longer periods of time beneath shrub canopies. Convergence of overland flow also yields net transport of soil from interspaces to beneath plant canopies. If this hypothesis is correct, then a positive feedback exists between shrub invasion and hydrologic changes. Other ecosystem links may be important. Enhanced convergence of surface water and soil beneath canopies will yield higher concentration of nitrogen and carbon. Deeper and longer-lived infiltration will enhance N mineralization. Both of these mechanisms will intensify the islands of fertility observed beneath shrub canopies.We will test this hypothesis via long-term monitoring and sprinkler experiments carried out in grass, shrub, and mixed environments on both gentle (2%) and steeper (7%) slopes (6 sites total). Runon and runoff from interspace and canopy areas will be collected. We will also measure surface flow convergence, infiltration, and soil moisture via a water balance approach. This requires observations of the effects of vegetation shading and interception. We will identify the causes of the observed changes in two ways: (1) mapping the interspace areas that contribute water to plant canopies, using tracers and photography; and (2) measuring variations in soil hydraulic properties in the field and in the lab. Transport of soil by surface water will be measured in the collected runon and runoff and grain size will be analyzed. We will use erosion bridges to further constrain source and sink areas for soil redistribution.The research proposed here will enhance our understanding of plant-water interactions in semiarid regions. Our results are important for quantifying historical changes in basin-scale water balance and recharge through desert vadose zones. Our data will be useful for ecologists and hydrologists working at the Sevilleta LTER and elsewhere, and therefore it will be distributed on the Sevilleta web site.