Invasion of Semiarid Grasslands by Shrubs

在20世纪，半干旱的美国西南部经历了戏剧性的植被变化--多年生草原被灌木入侵。世界各地也发生了类似的转变。随着灌木取代牧草，营养物质越来越集中在个体植物周围广泛分布的“肥沃孤岛”中。此外，表层土壤从灌木之间被侵蚀，减少了入渗，增加了径流。这两个过程都可能抑制灌丛之间的植物生长，促进初始植被的变化。植物-水分相互作用也可能在灌木入侵过程中发挥重要作用，因此我们将检验以下假设。灌木对半干旱草原的入侵促进了地表径流和植物冠层下入渗的汇聚。最显著的影响出现在高强度降雨事件和陡峭的斜坡上。与草地相比，灌木林冠下的入渗能力增强，这是由于上行空间径流的增加--空间较大，空间入渗能力降低。其结果是，灌木树冠下的土壤水分在较长时间内升高。陆上径流的汇聚也产生了土壤从空间到植物树冠下的净输送。如果这一假设是正确的，那么灌木入侵与水文变化之间存在正反馈。其他生态系统联系可能也很重要。加强地表水和树冠下土壤的汇聚将产生更高浓度的氮和碳。更深更持久的入渗将增强N的矿化作用。这两种机制都会加剧灌木树冠下观察到的肥力孤岛。我们将通过长期监测和在平缓(2%)和陡峭(7%)坡度(总共6个地点)的草、灌木和混合环境中进行的喷洒实验来验证这一假设。将收集空间间和树冠区域的径流和径流。我们还将通过水量平衡法测量地表径流汇聚、入渗和土壤湿度。这需要观察植被遮蔽和拦截的效果。我们将通过两种方法确定观察到的变化的原因：(1)使用示踪剂和摄影绘制为植物冠层提供水分的空间区域；以及(2)在田间和实验室测量土壤水力性质的变化。将在收集的径流中测量地表水对土壤的运移，并分析径流和颗粒大小。我们将利用侵蚀桥来进一步约束土壤再分配的源汇区域。本文提出的研究将加深我们对半干旱地区植物-水分相互作用的理解。我们的结果对于量化盆地尺度水平衡的历史变化和通过沙漠包气带进行补给具有重要意义。我们的数据将对在Sevilleta LTER和其他地方工作的生态学家和水文学家有用，因此将在Sevilleta网站上发布。

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.