Dodd M B, Lauenroth W K, Welker J M
Department of Rangeland Ecosystem Science, Colorado State University, Fort Collins, CO 80523, USA, , , , , , US.
Department of Rangeland Ecology and Watershed Management, University of Wyoming, Laramie, WY 82071, USA, , , , , , US.
Oecologia. 1998 Dec;117(4):504-512. doi: 10.1007/s004420050686.
We conducted a study to test the predictions of Walter's two-layer model in the shortgrass steppe of northeastern Colorado. The model suggests that grasses and woody plants use water resources from different layers of the soil profile. Four plant removal treatments were applied in the spring of 1996 within a plant community codominated by Atriplex canescens (a C shrub) and Bouteloua gracilis (a C grass). During the subsequent growing season, soil water content was monitored to a depth of 180 cm. In addition, stem and leaf tissue of Atriplex, Bouteloua and the streamside tree Populus sargentii were collected monthly during the growing seasons of 1995 and 1996 for analysis of the δO value of plant stem water (for comparison with potential water sources) and the δC value of leaves (as an indicator of plant water status). Selective removal of shrubs did not significantly increase water storage at any depth in the measured soil profile. Selective removal of the herbaceous understory (mainly grasses) increased water storage in the top 60 cm of the soil. Some of this water gradually percolated to lower layers, where it was utilized by the shrubs. Based on stem water δO values, grasses were exclusively using spring and summer rain extracted from the uppermost soil layers. In contrast, trees were exclusively using groundwater, and the consistent δC values of tree leaves over the course of the summer indicated no seasonal changes in gas exchange and therefore minimal water stress in this life-form. Based on anecdotal rooting-depth information and initial measurements of stem water δO, shrubs may have also had access to groundwater. However, their overall δO values indicated that they mainly used water from spring and summer precipitation events, extracted from subsurface soil layers. These findings indicate that the diversity of life-forms found in this shortgrass steppe community may be a function of the spatial partitioning of soil water resources, and their differential use by grasses, shrubs, and trees. Consequently, our findings support the two-layer model in a broad sense, but indicate a relatively flexible strategy of water acquisition by shrubs.
我们开展了一项研究，以验证沃尔特两层模型在科罗拉多州东北部短草草原的预测。该模型表明，草本植物和木本植物利用土壤剖面不同层的水资源。1996年春季，在一个由灰毛滨藜（一种C灌木）和细茎针茅（一种C草本植物）共同主导的植物群落中进行了四种植物去除处理。在随后的生长季节，对180厘米深度的土壤含水量进行了监测。此外，在1995年和1996年的生长季节每月收集灰毛滨藜、细茎针茅和溪边树木萨金特杨的茎和叶组织，用于分析植物茎水的δO值（与潜在水源进行比较）和叶片的δC值（作为植物水分状况的指标）。选择性去除灌木并没有显著增加所测土壤剖面任何深度的蓄水量。选择性去除草本下层植被（主要是草本植物）增加了土壤顶部60厘米的蓄水量。其中一些水逐渐渗透到下层，被灌木利用。根据茎水δO值，草本植物仅利用从最上层土壤层提取的春夏季降雨。相比之下，树木仅利用地下水，并且夏季树木叶片一致的δC值表明气体交换没有季节性变化，因此这种生命形式的水分胁迫最小。根据传闻的生根深度信息和茎水δO的初步测量，灌木可能也能获取地下水。然而，它们的总体δO值表明，它们主要利用从地下土壤层提取的春夏季降水事件中的水。这些发现表明，在这个短草草原群落中发现的生命形式多样性可能是土壤水资源空间分配以及草本植物、灌木和树木对其不同利用的函数。因此，我们的发现从广义上支持了两层模型，但表明灌木获取水分的策略相对灵活。
