Barnes Paul W, Archer Steve
Department of Biology, Southwest Texas State University, 78666-4616, San Marcos, TX, USA.
Department of Rangeland Ecology and Management, Texas A&M University, 77843-2126, College Station, TX, USA.
Oecologia. 1996 Mar;105(4):493-500. doi: 10.1007/BF00330012.
The arborescent legume, honey mesquite (Prosopis glandulosa), appears to play a central role in patch dynamics of southern Texas savannas by modifying soils and microclimate and by facilitating the ingress, establishment and/or growth of shrubs in its understorey. As an indirect test for the occurrence and persistence of facilitation in mature shrub clusters (patches), we examined the gas exchange, water relations and production of associated shrubs growing in patches where a Prosopis overstorey was present and in patches where Prosopis had succumbed to natural mortality. Surface (0-10 cm) soils associated with shrub patches were enriched in total [N] and [C] compared to soils of neighboring herbaceous zones. However, there were no detectable differences in soil [N] or [C] in patches with and without Prosopis. Foliar [N] and biomass of various shrub species were also statistically comparable for patches with and without Prosopis. These results are in accordance with other studies that indicate the nutrient legacy associated with Prosopis occupation of a patch may persist for decades after its demise. In comparison to plants growing in the absence of Prosopis, leaf water potentials (predawn and midday), and net photosynthesis and water vapor conductance (morning and midday) of outer-canopy sunlit leaves over an annual growth cycle were comparable for two common evergreen shrubs, Zanthoxylum fagara and Berberis trifoliolata, growing in patches with a live Prosopis. These findings indicate that the presence of Prosopis was not enhancing the growth or activity of mature understorey shrubs; facilitation may, therefore, be important only during early stages of cluster development. In addition, we found no indication that the loss of Prosopis has initiated a downward phase in a cyclic succession of patch initiation, growth and death. Rather, the understorey shrubs appear to be able to maintain growth and productivity in the absence of a Prosopis overstorey, and may, therefore, represent persistent components of woody patches on these savanna landscapes.
树形豆科植物蜜牧豆（Prosopis glandulosa）似乎在德克萨斯州南部稀树草原的斑块动态中发挥着核心作用，它通过改变土壤和微气候，以及促进其下层灌木的进入、定植和/或生长来实现这一点。作为对成熟灌木群落（斑块）中促进作用的发生和持续性的间接测试，我们研究了生长在有牧豆树上层林冠的斑块中和牧豆树已自然死亡的斑块中的相关灌木的气体交换、水分关系和生产力。与相邻草本区域的土壤相比，与灌木斑块相关的表层（0 - 10厘米）土壤中的总氮和碳含量有所增加。然而，有和没有牧豆树的斑块中，土壤中的氮或碳没有可检测到的差异。有和没有牧豆树的斑块中，各种灌木物种的叶片氮含量和生物量在统计学上也具有可比性。这些结果与其他研究一致，这些研究表明，与牧豆树占据斑块相关的养分遗留效应在其死亡后可能会持续数十年。在一个年度生长周期中，对于生长在有活牧豆树的斑块中的两种常见常绿灌木花椒（Zanthoxylum fagara）和三叶小檗（Berberis trifoliolata），与没有牧豆树时生长的植物相比，其树冠外层受阳光照射叶片的叶水势（黎明前和中午）、净光合作用和水汽导度（早晨和中午）相当。这些发现表明，牧豆树的存在并没有促进成熟下层灌木的生长或活性；因此促进作用可能仅在群落发展的早期阶段才重要。此外，我们没有发现迹象表明牧豆树的消失引发了斑块形成、生长和死亡的循环演替中的下行阶段。相反，下层灌木似乎能够在没有牧豆树上层林冠的情况下维持生长和生产力，因此可能代表这些稀树草原景观上木本斑块的持久组成部分。
