Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada.
PLoS One. 2012;7(8):e43703. doi: 10.1371/journal.pone.0043703. Epub 2012 Aug 21.
Ideas about how plant competition varies with productivity are rooted in classic theories that predict either increasing (Grime) or invariant (Tilman) competition with increasing productivity. Both predictions have received experimental support, although a decade-old meta-analysis supports neither. Attempts to reconcile the conflicting predictions and evidence include: expanding the theory to include other conditions (e.g. stress gradient hypothesis), development of indices to differentiate either the 'intensity' or 'importance' of competition, a focus on resource supply and demand, and explicit recognition that both growth and survival may exhibit different relationships with productivity. To determine which of these theories accurately predict how competition varies with productivity within a native grassland site, we estimated competitive intensity and relative competitive importance using 22 species across the range of productivity naturally occurring within that site. Plant performance was measured as survival and size with and without neighbours and the local environment was quantified according to variability in standing crop, gross water supply, and net water supply. On average, neighbours weakly facilitated seedling survival, but strongly reduced seedling growth. For both seedling survival and growth, relative competitive importance and competitive intensity declined with some measure of productivity; neighbour effects on survival declined with standing crop, while effects on growth declined with gross water supply. These results add to the growing evidence that plant-plant interactions vary among life history components with different life history components contingent upon separate environmental factors. Although the range of productivity measured in this study was not large, our results do not support the theories of Grime or Tilman. However, our results are consistent with the meta-analysis and parts of other theories, although no single theory is capable of explaining the entirety of these results. This suggests that, at least in moderately productive grasslands, new theory needs to be developed.
关于植物竞争随生产力变化的观点源于经典理论，这些理论预测竞争要么随着生产力的增加而增加（Grime），要么保持不变（Tilman）。这两种预测都得到了实验的支持，尽管一项十年前的荟萃分析并不支持这两种预测。为了调和相互矛盾的预测和证据，人们尝试了以下方法：将理论扩展到包括其他条件（例如，胁迫梯度假说），开发指标来区分竞争的“强度”或“重要性”，关注资源的供应和需求，以及明确认识到生长和生存都可能与生产力呈现不同的关系。为了确定这些理论中哪一个能更准确地预测竞争如何随生产力在原生草原点内变化，我们使用该地点自然发生的生产力范围内的 22 个物种来估计竞争强度和相对竞争重要性。植物表现通过有无邻体的生存和大小来衡量，当地环境则根据生物量的可变性、总水分供应和净水分供应来量化。平均而言，邻体微弱地促进幼苗的生存，但强烈地抑制幼苗的生长。对于幼苗的生存和生长，相对竞争重要性和竞争强度都随着某种生产力指标而下降；邻体对生存的影响随生物量的下降而下降，而对生长的影响随总水分供应的下降而下降。这些结果增加了越来越多的证据，表明植物-植物相互作用在不同的生活史成分之间存在差异，而不同的生活史成分则取决于不同的环境因素。尽管本研究中测量的生产力范围不大，但我们的结果并不支持 Grime 或 Tilman 的理论。然而，我们的结果与荟萃分析和其他部分理论一致，尽管没有一个理论能够解释这些结果的全部。这表明，至少在生产力适中的草原中，需要开发新的理论。
