Invasion of Semiarid Grasslands by Shrubs

灌木对半干旱草原的入侵

• 批准号: 0003514
• 负责人: Eric Small
• 金额: $ 17万
• 依托单位: New Mexico Institute of Mining and Technology
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2001
• 资助国家: 美国
• 起止时间: 2001-03-15 至 2002-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0003514&HistoricalAwards=false
• 关键词: Invasion; Semiarid; Grasslands; Shrubs

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.

0003514小半干旱的美国西南部经历了一个戏剧性的植被变化在20世纪的多年生草原被灌木入侵。 世界各地都发生了类似的转变。 随着灌木取代草，养分越来越集中在个体植物周围的广泛分布的“肥沃岛屿”。 此外，灌木丛之间的土壤表层被侵蚀，减少了渗透，增加了径流。 这两个过程都可能抑制灌木之间的植物生长，增强最初的植被变化。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，因此我们将测试以下假设。 灌木入侵半干旱草原增强了地表径流的汇聚和植物冠层下的渗透。 最显著的影响存在于高强度降雨事件和陡坡上。 与草地相比，灌木林冠下入渗增强，这是因为来自上坡空隙的径流增加，空隙变大，空隙入渗能力降低。 其结果是，土壤水分升高的灌木树冠下的时间更长。 地表径流的汇聚也产生了土壤从空间到植物冠层下的净迁移。 如果这一假设是正确的，那么灌木入侵和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。 地表水和土壤在林冠下的汇流增强将产生更高的氮和碳浓度。 更深、更长时间的入渗将促进氮矿化。 这两种机制将加强岛的肥力观察下灌木coveries.We将测试这一假设，通过长期监测和洒水实验中进行的草，灌木，和混合环境中的平缓（2%）和陡峭（7%）的斜坡（6个站点）。 将收集来自空间和树冠区域的径流和径流。 我们还将通过水平衡方法测量地表径流收敛、渗透和土壤湿度。 这需要观察植被遮蔽和拦截的影响。 我们将通过两种方式确定观察到的变化的原因：（1）使用示踪剂和摄影技术绘制为植物冠层提供水分的空间区域;（2）在田间和实验室中测量土壤水力特性的变化。 将在收集的径流中测量地表水对土壤的运输，并分析径流和粒度。 我们将利用侵蚀桥来进一步限制土壤重新分布的源汇区，这里提出的研究将提高我们对半干旱地区植物-水相互作用的理解。 我们的研究结果是重要的量化历史变化的流域规模的水平衡和补给通过沙漠包气带。 我们的数据将对在塞维利亚LTER和其他地方工作的生态学家和水文学家有用，因此将在塞维利亚网站上发布。