Schwinning Susanne, Meckel Heather, Reichmann Lara G, Polley H Wayne, Fay Philip A
Department of Biology, Texas State University, San Marcos, Texas, United States of America.
Department of Integrative Biology, University of Texas at Austin, Austin, Texas, United States of America.
PLoS One. 2017 May 3;12(5):e0176042. doi: 10.1371/journal.pone.0176042. eCollection 2017.
Invasive plant species often dominate native species in competition, augmenting other potential advantages such as release from natural enemies. Resource pre-emption may be a particularly important mechanism for establishing dominance over competitors of the same functional type. We hypothesized that competitive success of an exotic grass against native grasses is mediated by establishing an early size advantage. We tested this prediction among four perennial C4 warm-season grasses: the exotic weed Johnsongrass (Sorghum halepense), big bluestem (Andropogon gerardii), little bluestem (Schizachyrium scoparius) and switchgrass (Panicum virgatum). We predicted that a) the competitive effect of Johnsongrass on target species would be proportional to their initial biomass difference, b) competitive effect and response would be negatively correlated and c) soil fertility would have little effect on competitive relationships. In a greenhouse, plants of the four species were grown from seed either alone or with one Johnsongrass neighbor at two fertilizer levels and periodically harvested. The first two hypotheses were supported: The seedling biomass of single plants at first harvest (50 days after seeding) ranked the same way as the competitive effect of Johnsongrass on target species: Johnsongrass < big bluestem < little bluestem/switchgrass, while Johnsongrass responded more strongly to competition from Johnsongrass than from native species. At final harvest, native plants growing with Johnsongrass attained between 2-5% of their single-plant non-root biomass, while Johnsongrass growing with native species attained 89% of single-plant non-root biomass. Fertilization enhanced Johnsongrass' competitive effects on native species, but added little to the already severe competitive suppression. Accelerated early growth of Johnsongrass seedlings relative to native seedlings appeared to enable subsequent resource pre-emption. Size-asymmetric competition and resource-pre-emption may be a critical mechanism by which exotic invasive species displace functionally similar native species and alter the functional dynamics of native communities.
入侵植物物种在竞争中往往会占据本地物种的主导地位，同时增强其他潜在优势，例如摆脱天敌。资源抢占可能是对相同功能类型的竞争者建立优势的一个特别重要的机制。我们假设一种外来草对本地草的竞争成功是通过建立早期的大小优势来介导的。我们在四种多年生C4暖季草中检验了这一预测：外来杂草约翰逊草（高粱）、大须芒草（糙毛须芒草）、小须芒草（裂稃草）和柳枝稷（黍稷）。我们预测：a）约翰逊草对目标物种的竞争效应将与其初始生物量差异成正比，b）竞争效应和反应将呈负相关，c）土壤肥力对竞争关系影响不大。在温室中，这四种物种的植物从种子开始单独种植或在两种肥料水平下与一株约翰逊草邻居一起种植，并定期收获。前两个假设得到了支持：第一次收获（播种后50天）时单株植物的幼苗生物量与约翰逊草对目标物种的竞争效应排名相同：约翰逊草<大须芒草<小须芒草/柳枝稷，而约翰逊草对来自约翰逊草的竞争的反应比对本地物种的竞争反应更强。在最终收获时，与约翰逊草一起生长的本地植物达到其单株非根生物量的2%-5%，而与本地物种一起生长的约翰逊草达到单株非根生物量的89%。施肥增强了约翰逊草对本地物种的竞争效应，但对已经很严重的竞争抑制作用增加不多。相对于本地幼苗，约翰逊草幼苗的早期生长加速似乎能够实现后续的资源抢占。大小不对称竞争和资源抢占可能是外来入侵物种取代功能相似的本地物种并改变本地群落功能动态的关键机制。
