ROW: A Field Study of the Role of Simulated Grazing and Time of Establishment on the Growth of a Woody Plant

本研究的目的是通过实验对草地向灌木林转化的机制进行评价。美国西南部的木本物种在受干扰的草原上建立起来，并作为其他木本物种招募的焦点。这种转变的原因被认为是过度放牧、草地竞争减少、缺乏火灾、气候变化、牲畜传播种子或多种因素的结合。虽然已经描述了许多这些连续序列，并假设了机制，但缺乏支持其机制的实验证据。历史上，木本植物被认为入侵了没有灌木的高产草地；但最近被认为是高度受干扰草原的殖民者。为了部分验证这一假设，将进行一系列田间试验，研究木本生长植物Prosopis glandullosa（蜜豆科植物）与北美西南部半干旱热带草原上的原生草之间的竞争。待检验的假设是木本植物建立后，木本植物和禾本科植物从不同的根区获取资源，两者之间的竞争明显减少。在不同深度的盆栽中，进行了甘露草与一种本地草的竞争试验。从统计学上看，H0表示所有处理（深度）的腺体藻生物量减少相等，而Ha表示减少不相等。如果glandullosa生物量的减少与深度呈负相关，则表明在木本植物的建立阶段，当草和木本植物的根都在同一区域时，竞争更加激烈。一旦木本植物的根延伸到草根区以下，竞争就会减少。虽然本实验受盆栽实验的限制很大，但如果只将结果外推到腺体假单胞菌的建立上，而不是在建立后的竞争相互作用上，则本实验可以提供有价值的信息。

The goal of the proposed research is to experimentally evaluate the mechanisms contributing to the conversion of grasslands to shrubland. Woody species in the southwestern USA become established in disturbed grassland and act as foci for the recruitment of other woody species. The reasons postulated for the conversion are overgrazing, reduced grass competition, lack of fires, changes in climate, spread of seeds by livestock, or a combination of factors. Although many of these successional sequences have been described, and mechanisms postulated, experimental evidence supporting ther mechanisms are lacking. Historically, woody plants were thought to invade shrub-free productive grasslands; but more recently have been thought to be colonizers of highly disturbed grasslands. To partially test this postulate, a series of field experiments will be conducted which deal with competition between a woody increaser, Prosopis glandulosa (honey mesquite) and native grasses of the semi-arid savanna of southwestern North America. The hypothesis to be tested is that after woody plant establishment, the woody plants and the grasses obtain resources from different root zones, and competition between the two is significantly reduced. A competion experiment between P. glandulosa and a native grass, grown in different depth pots will be conducted. Statistically, the H0 would be that the reduction of P. glandulosa biomass would be equal across all treatments (depths), while the Ha would be that the reduction is unequal. If the reduction in P. glandulosa biomass is inversely correlated to depth, it would suggest that competition is more intense during the establishment stage of the woody plant, when both the grasses and woody plants have roots in the same zone. Once the woody plant root extends below the grass root zone, competition then decreases. Although this experiment is very restricted due to pot experiments, if results are only extrapolated to P. glandulosa establishment rather than competitive interactions after establishment, the experiment could offer worthwhile information.