Department of Biological Sciences, Mail Stop 8007, Idaho State University, Pocatello, Idaho 83209, USA.
Ecol Appl. 2010 Oct;20(7):1890-902. doi: 10.1890/09-0750.1.
The invasion and spread of exotic plants following land disturbance threatens semiarid ecosystems. In sagebrush steppe, soil water is scarce and is partitioned between deep-rooted perennial shrubs and shallower-rooted native forbs and grasses. Disturbances commonly remove shrubs, leaving grass-dominated communities, and may allow for the exploitation of water resources by the many species of invasive, tap-rooted forbs that are increasingly successful in this habitat. We hypothesized that exotic forb populations would benefit from increased soil water made available by removal of sagebrush, a foundation species capable of deep-rooting, in semiarid shrub-steppe ecosystems. To test this hypothesis, we used periodic matrix models to examine effects of experimental manipulations of soil water on population growth of two exotic forb species, Tragopogon dubius and Lactuca serriola, in sagebrush steppe of southern Idaho, USA. We used elasticity analyses to examine which stages in the life cycle of T. dubius and L. serriola had the largest relative influence on population growth. We studied the demography of T. dubius and L. serriola in three treatments: (1) control, in which vegetation was not disturbed, (2) shrubs removed, or (3) shrubs removed but winter-spring recharge of deep-soil water blocked by rainout shelters. The short-term population growth rate (Lambda) of T. dubius in the shrub-removal treatment was more than double that of T. dubius in either sheltered or control treatments, both of which had limited soil water. All L. serriola individuals that emerged in undisturbed sagebrush plots died, whereas Lambda of L. serriola was high (Lambda > 2.5) in all shrub-removal plots, whether they had rainout shelters or not. Population growth of both forbs in all treatments was most responsive to flowering and seed production, which are life stages that should be particularly reliant on deep-soil water, as well as seedling establishment, which is important to most plant populations, especially during invasion. These data indicate the importance of native species, in this case the dominant shrub, in influencing soil resources and restricting population growth of exotic plants. These results argue that management of invasive plants should focus not only on removal of nonnatives, but also on reestablishment of important native species.
外来植物的入侵和扩散对半干旱生态系统构成威胁。在山艾灌丛草原，土壤水分稀缺，深根系的多年生灌木与浅根系的本地草本植物和草类共同瓜分这些水分。干扰通常会移除灌木，留下以草类为主的群落，并且可能会使许多入侵的、具主根的草本植物得以利用水资源，而这些植物在这种生境中越来越成功。我们假设，在半干旱灌丛草原生态系统中，外来草本植物种群将受益于山艾这种深根系基础物种被移除后可获得的更多土壤水分。为了验证这一假设，我们使用周期性矩阵模型，研究了实验性土壤水分处理对两种外来草本植物，毒蝇鹅膏菌（Tragopogon dubius）和皱叶莴苣（Lactuca serriola）在爱达荷州南部山艾灌丛草原种群增长的影响。我们使用弹性分析来研究毒蝇鹅膏菌和皱叶莴苣生活史的哪些阶段对种群增长有最大的相对影响。我们在三种处理方式下研究了毒蝇鹅膏菌和皱叶莴苣的种群动态：（1）对照，植被不受干扰；（2）移除灌木；（3）移除灌木，但用雨棚阻止深土水分在冬春两季补给。在移除灌木的处理中，毒蝇鹅膏菌的短期种群增长率（Lambda）是对照或雨棚处理的两倍多，后两种处理的土壤水分都有限。在未受干扰的山艾灌丛样地中，所有皱叶莴苣个体都死亡了，而在所有移除灌木的样地中，Lambda 都很高（Lambda > 2.5），无论是否有雨棚。在所有处理中，两种草本植物的种群增长对开花和种子生产的响应最为敏感，这是对深土水分特别依赖的生活阶段，而幼苗定植对大多数植物种群都很重要，特别是在入侵过程中。这些数据表明，本地物种（在这种情况下是优势灌木）在影响土壤资源和限制外来植物种群增长方面的重要性。这些结果表明，对入侵植物的管理不仅应集中于移除非本地物种，还应重新建立重要的本地物种。
