Yu Kailiang, D'Odorico Paolo
Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22904, USA.
National Socio-Environmental Synthesis Center, University of Maryland, Annapolis, MD 21401, USA.
New Phytol. 2015 Sep;207(4):1038-51. doi: 10.1111/nph.13431. Epub 2015 Apr 29.
The coexistence of woody plants and grasses in savannas is determined by a complex set of interacting factors that determine access to resources and demographic dynamics, under the control of external drivers and vegetation feedbacks with the physical environment. Existing theories explain coexistence mainly as an effect of competitive relations and/or disturbances. However, theoretical studies on the way facilitative interactions resulting from hydraulic lift affect tree-grass coexistence and the range of environmental conditions in which savannas are stable are still lacking. We investigated the role of hydraulic lift in the stability of tree-grass coexistence in savannas. To that end, we developed a new mechanistic model that accounts for both competition for soil water in the shallow soil and fire-induced disturbance. We found that hydraulic lift favors grasses, which scavenge the water lifted by woody plants. Thus, hydraulic lift expands (at the expenses of woodlands) the range of environmental conditions in which savannas are stable. These results indicate that hydraulic lift can be an important mechanism responsible for the coexistence of woody plants and grasses in savannas. Grass facilitation by trees through the process of hydraulic lift could allow savannas to persist stably in mesic regions that would otherwise exhibit a forest cover.
稀树草原中木本植物和草本植物的共存取决于一系列复杂的相互作用因素，这些因素在外部驱动因素以及植被与物理环境的反馈作用下，决定了对资源的获取和种群动态。现有理论主要将共存解释为竞争关系和/或干扰的结果。然而，关于水力提升产生的促进性相互作用如何影响树木 - 草本植物共存以及稀树草原稳定存在的环境条件范围的理论研究仍然缺乏。我们研究了水力提升在稀树草原树木 - 草本植物共存稳定性中的作用。为此，我们开发了一个新的机理模型，该模型考虑了浅层土壤中对土壤水分的竞争和火灾引起的干扰。我们发现水力提升有利于草本植物，它们会吸收木本植物提升的水分。因此，水力提升（以林地为代价）扩大了稀树草原稳定存在的环境条件范围。这些结果表明，水力提升可能是稀树草原中木本植物和草本植物共存的一个重要机制。通过水力提升过程，树木对草本植物的促进作用可能使稀树草原在原本会出现森林覆盖的湿润地区稳定存在。
