Interrelationships between plant functional types and soil moisture heterogeneity for semiarid landscapes within the grassland/forest continuum: a unified conceptual model
Interrelationships between plant functional types and soil moisture heterogeneity for semiarid landscapes within the grassland/forest continuum: a unified conceptual model
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DOI：
10.1023/a:1008040327508
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发表时间：
1999-10-01
影响因子：
5.2
通讯作者：
Barnes, FJ
中科院分区：
文献类型：
--
作者：
Breshears, DD;Barnes, FJ
在半干旱景观中，草本植物与木本植物生物量的比例是生态系统特性的主要决定因素。这一比例在很大程度上取决于植物可利用的土壤水分的数量和空间分布，而这些变量又主要由气候和土地利用决定。目前用于确定半干旱植物群落中草本植物与木本植物生物量之比的概念模型是基于土壤水分随深度的差异（垂直异质性）从一个地点到另一个地点（沃尔特的两层模型），或者基于同一地点冠层和冠层间斑块之间土壤水分的差异（水平异质性），这些差异是由与土地利用相关的干扰造成的（Schlesinger et al.的荒漠化模型）。我们开发了一个模型，通过放松沃尔特的两层模型的两个假设，统一了这两个观点。首先，我们的模型认识到，冠层和冠层间斑块的土壤水分水平变化，不仅由于土地利用干扰，一般假设的Schlesinger等。模型，但也由于冠层本身的物理性质。第二，虽然保持沃尔特的一般假设，木本植物获得水分从更深的土壤层比草本植物，我们的模型承认存在两种类型的木本植物：那些提取的大部分水分从更深的层和那些提取主要从浅层。通过将两层假设修改为包括四个土壤室，并区分浅根和深根木本植物，我们的模型集成了半干旱生态学中的三个关键概念：（1）木本覆盖比例随着深层土壤水分的增加而增加（沃尔特的草本植物和木本植物共存的两层假说）;（2）导致草本植被减少和冠层间土壤压实的土地利用做法导致木本植物比例的长期增加（Schlesinger et al.的概念，或更一般地，在给定地点，草本植物和木本植物生物量的比例的多种变化是可能的）;和（3）草本植物与木本植物生物量的比例的变化表现出复杂的行为（变化可能很快发生，并且在没有强化管理的情况下不直接可逆）。这种整合的概念的结果，而不是假设一个简单的，单向依赖的植物功能类型对土壤水分异质性，我们的模型假设两者之间的相互依存关系：土壤水分异质性约束的植物群落的组成，这反过来又修改土壤水分异质性。我们提出的四室模型，使，第一次，两个维度的土壤水分异质性-水平和垂直-和土壤水分异质性和植物功能类型的比例，构成一个给定的植物群落之间的相互依赖关系的综合图片。这个统一的概念模型可以应用于提供洞察个人和气候和土地利用对半干旱植物群落的草地/森林连续体，不同的冠层和冠层间斑块的比例的综合影响。
In semiarid landscapes, the ratio of herbaceous to woody plant biomass is a major determinant of ecosystem properties. This ratio depends to a large extent on the amount and spatial distribution of soil moisture that is available to plants, and these variables, in turn, are determined primarily by climate and land use. Current conceptual models for determining the ratio of herbaceous to woody plant biomass in semiarid plant communities are based either on differences in soil moisture with depth (vertical heterogeneity) from one site to another (Walter's two-layer model) or on differences in soil moisture between canopy and intercanopy patches at the same site (horizontal heterogeneity) that result from disturbances associated with land use (Schlesinger et al.'s model of desertification). We developed a model that unifies these two perspectives by relaxing two assumptions of Walter's two-layer model. First, our model recognizes that soil moisture varies horizontally between canopy and intercanopy patches, not only due to land-use disturbance, a general assumption of the Schlesinger et al. model, but also due to the physical nature of the canopy itself. Second, while retaining the general assumption of Walter that woody plants obtain moisture from deeper soil layers than do herbaceous plants, our model recognizes the existence of two types of woody plants: those that extract a substantial proportion of their moisture from deeper layers and those that extract mainly from shallower layers. By modifying the two-layer hypothesis to include four soil compartments and distinguishing between shallow- and deeper-rooted woody species, our model integrates three key concepts in semiarid ecology: (1) the proportion of woody cover increases as moisture in the deeper soil layers increases (Walter's two-layer hypothesis for coexistence of herbaceous and woody plants); (2) land use practices that cause a reduction in herbaceous vegetation and compaction of intercanopy soils lead to a long-term increase in the proportion of woody plants (Schlesinger et al.'s concept, or more generally, that at a given site multiple variations in the proportions of herbaceous and woody plant biomass are possible); and (3) changes in the ratios of herbaceous to woody plant biomass exhibit complex behavior (changes can happen quickly and are not directly reversible without intensive management). This integration of concepts results because rather than assuming a simple, one-way dependence of plant functional types on soil moisture heterogeneity, our model assumes an interdependence between the two: soil moisture heterogeneity constrains the composition of the plant community, which in turn modifies soil moisture heterogeneity. The four-compartment model that we propose enables, for the first time, an integrated picture of both dimensions of soil moisture heterogeneity - horizontal and vertical - and of the interdependence between soil moisture heterogeneity and the proportions of the plant functional types that make up a given plant community. This unified conceptual model can be applied to provide insight into the individual and the combined effects of climate and land use on semiarid plant communities within the grassland/forest continuum, which vary in the proportions of canopy and intercanopy patches.