Stability of C3 and C4 grass patches after fire and simulated grazing

As the woody legume, Prosopis glandulosa (honey mesquite) has encroached into grasslands and rangelands in the southern Great Plains, USA, two grass species, C4 shortgrass, Buchloe dactyloides (buffalograss), and C3 mid-grass, Nassella leucotricha (Texas wintergrass), have increased in dominance. Occurrence of more productive C4 mid-grasses and herbaceous diversity have declined. We measured effects of various combinations of spring clipping (to simulate cattle grazing) and summer and/or winter fire treatments on the stability of monoculture patches of these two grass species over an eight-year period, with the goal of reducing Nassella and increasing C4 mid-grass cover. All fire treatments top-killed most Prosopis trees that subsequently resprouted. Buchloe cover declined in the No Clip + No Fire treatment but remained intact with clipping and/or fire. Frequent clipping reduced Nassella cover across all fire treatments. Buchloe encroachment into Nassella patches was greatest in the Clip + Alternate Season fire treatment. C4 mid-grass cover increased to 15–25% in Nassella patches in several fire-only or Clip + Fire treatments; greatest gains were observed in treatments that included summer fire. In contrast, C4 mid-grass gains were lower in Buchloe patches. These results suggest that C4 mid-grass restoration was linked with treatments that reduced Nassella cover.

作为木本豆科植物，腺牧豆树（Prosopis glandulosa，俗称蜂蜜牧豆树）已侵入美国大平原南部的草原与牧场。该区域内的两种草本植物——C4短生草本野牛草（Buchloe dactyloides，buffalograss）与C3中生草本德克萨斯冬草（Nassella leucotricha，Texas wintergrass）的优势度均有所提升，而生产力更高的C4中生草本以及草本群落多样性则出现衰退。本研究开展了为期八年的定位试验，评估春季刈割（模拟牛群放牧）、夏季及/或冬季火烧的不同组合处理对这两种草本单优群落斑块稳定性的影响，试验目标为抑制德克萨斯冬草的扩散并提升C4中生草本的盖度。所有火烧处理均会杀死绝大多数腺牧豆树的地上部分，但后续这些树木会重新萌发。无刈割+无火烧处理组中，野牛草的盖度出现下降，但在刈割和/或火烧处理下其盖度得以保持稳定。频繁刈割可在所有火烧处理组中降低德克萨斯冬草的盖度。在刈割+轮季火烧处理组中，野牛草向德克萨斯冬草斑块的侵入程度最高。在仅火烧或刈割+火烧的多个处理组中，德克萨斯冬草斑块内的C4中生草本盖度提升至15%~25%，其中以包含夏季火烧的处理组增益最为显著。与之相对，野牛草斑块内的C4中生草本盖度提升幅度则较低。上述结果表明，C4中生草本的修复效果与德克萨斯冬草盖度的降低存在显著关联。