Grazing and microhabitat interact to affect plant-plant interactions in subtropical seasonal wetlands, 2007-2008

The stress gradient hypothesis predicts that competition will be important in productive environments while facilitation will be common in environments with high stress or consumer pressure. However, abiotic stress and grazing may vary independently and even occur simultaneously. Here we examine the outcome of plant interactions in grazed wetlands where consumer pressure and abiotic stress occur concurrently. We hypothesized that cattle grazing and microhabitat would alter the outcome of plant interactions. Given that wetland edges are drier and less productive than wetland centers we expected that facilitation would be greatest in drier wetland edges due to greater abiotic stress regardless of cattle presence. We conducted an experiment for two growing seasons in ten wetlands, five exposed to cattle grazing and five fenced. Two wetland obligate plants were included (Panicum hemitomon and Alternanthera philoxeroides), and plots were assigned to three treatments 1) all neighbors removed, 2) all neighbors removed except Juncus effusus, a dominant, unpalatable plant, 3) all neighbors intact (control), in both wetland centers and edges. Differences in survival, change in height and number of leaves were assessed. In ungrazed wetlands, plant survival was higher in wetland edges vs. centers, while it did not differ in grazed wetlands. Survival in wetland edges was further increased by the presence of J. effusus. Positive interactions under grazed conditions were clear when plant height was assessed, but negative interactions affected leaf production in both ungrazed and grazed wetlands. Grazing interacts with wetland microhabitat to alter plant survival. Facilitative interactions on plant height were apparent in grazed wetlands. Understanding how plant interactions change under different biotic and abiotic contexts is important for informing ecosystem restoration and management.胁迫梯度假说（Stress Gradient Hypothesis）指出，种间竞争在高生产力生境中占据主导地位，而促进作用则在胁迫程度较高或存在消费者取食压力的生境中更为常见。然而，非生物胁迫与放牧压力可能独立变化，甚至同时出现。本研究聚焦于同时存在植食压力与非生物胁迫的放牧型湿地，探究其中的植物相互作用结果。 我们提出假说：牛群放牧与微生境条件会改变植物相互作用的结果。鉴于湿地边缘较湿地中心更为干燥、生产力更低，我们推测，无论是否存在牛群放牧，由于非生物胁迫更强，干燥的湿地边缘的促进作用会最为显著。 本研究在10处湿地中开展了两个生长季的野外实验，其中5处实施牛群放牧处理，另外5处设置围栏排除放牧。实验选取两种湿地专性植物（Panicum hemitomon与Alternanthera philoxeroides）作为研究对象，并在湿地中心与边缘两种微生境中，将样地划分为3种处理：1）移除所有邻近植物；2）仅保留优势且适口性差的灯心草（Juncus effusus），移除其余所有邻近植物；3）保留所有邻近植物（对照组，Control）。随后测定了植株存活率、株高变化以及叶片数量变化这三项指标。 在未放牧的湿地中，湿地边缘的植株存活率显著高于中心区域；而在放牧湿地中，二者存活率并无显著差异。灯心草（Juncus effusus）的存在进一步提升了湿地边缘的植株存活率。在放牧条件下，通过株高指标可清晰观测到正向相互作用（促进作用），但负向相互作用（种间竞争）则同时影响了未放牧与放牧湿地中的叶片生成量。 放牧与湿地微生境存在交互作用，共同改变植株存活率。在放牧湿地中，针对株高的促进作用表现显著。明确植物相互作用在不同生物与非生物环境下的变化规律，可为生态系统修复与管理提供科学依据。