Divison of Biological Sciences, University of Missouri, Columbia, Missouri, USA.
PLoS One. 2013 Aug 12;8(8):e69625. doi: 10.1371/journal.pone.0069625. eCollection 2013.
The two-layer hypothesis of tree-grass coexistence posits that trees and grasses differ in rooting depth, with grasses exploiting soil moisture in shallow layers while trees have exclusive access to deep water. The lack of clear differences in maximum rooting depth between these two functional groups, however, has caused this model to fall out of favor. The alternative model, the demographic bottleneck hypothesis, suggests that trees and grasses occupy overlapping rooting niches, and that stochastic events such as fires and droughts result in episodic tree mortality at various life stages, thus preventing trees from otherwise displacing grasses, at least in mesic savannas. Two potential problems with this view are: 1) we lack data on functional rooting profiles in trees and grasses, and these profiles are not necessarily reflected by differences in maximum or physical rooting depth, and 2) subtle, difficult-to-detect differences in rooting profiles between the two functional groups may be sufficient to result in coexistence in many situations. To tackle this question, I coupled a plant uptake model with a soil moisture dynamics model to explore the environmental conditions under which functional rooting profiles with equal rooting depth but different depth distributions (i.e., shapes) can coexist when competing for water. I show that, as long as rainfall inputs are stochastic, coexistence based on rooting differences is viable under a wide range of conditions, even when these differences are subtle. The results also indicate that coexistence mechanisms based on rooting niche differentiation are more viable under some climatic and edaphic conditions than others. This suggests that the two-layer model is both viable and stochastic in nature, and that a full understanding of tree-grass coexistence and dynamics may require incorporating fine-scale rooting differences between these functional groups and realistic stochastic climate drivers into future models.
树木-草本共存的双层假说假设树木和草本在根系深度上存在差异，草本利用浅层土壤水分，而树木则独家获取深层水分。然而，这两个功能群在最大根系深度上没有明显差异，这使得该模型不再流行。替代模型，即人口瓶颈假说，表明树木和草本占据重叠的根系生态位，并且像火灾和干旱这样的随机事件会导致不同生命阶段的树木间歇性死亡，从而防止树木在至少在湿润的稀树草原中取代草本。这种观点有两个潜在问题：1）我们缺乏树木和草本的功能根系剖面数据，这些剖面不一定反映在最大或物理根系深度的差异上；2）两个功能群之间根系剖面的细微、难以察觉的差异可能足以在许多情况下导致共存。为了解决这个问题，我将植物吸收模型与土壤水分动态模型相结合，以探索在竞争水分的情况下，具有相同根系深度但根系分布（即形状）不同的功能根系剖面在何种环境条件下能够共存。我表明，只要降雨输入是随机的，基于根系差异的共存在广泛的条件下是可行的，即使这些差异很细微。结果还表明，基于根系生态位分化的共存机制在某些气候和土壤条件下比其他条件更可行。这表明双层模型本质上既是可行的也是随机的，并且要充分理解树木-草本共存和动态，可能需要将这些功能群之间的细微根系差异以及现实的随机气候驱动因素纳入未来的模型中。
