性状关联跨尺度推演:高寒草甸植物种内及种间性状的协同与权衡

Scaling-up trait covariation: Coordination and trade-offs within and among plant species in alpine meadow communities

功能性状及其关联可以反映有机体响应环境的生态对策,是探究各生态层次生物与环境相互作用的关键.大量的研究报道了区域至全球尺度上植物种间性状的协同或权衡.基于此,研究者试图推演环境变化下种群、群落至植被的功能结构动态.然而,各生态层次性状关联的主导机理不尽相同.性状关系跨尺度推演的核心在于局域群落种内及种间的性状关联,而相关研究极其缺乏.整合性状关联的机理与群落构建理论,我们推测,与大尺度下生态位特化主导物种性状关联类似,局域群落中多物种共存且生态位分化明显时,种间也应有显著的性状协同与权衡;功能平衡和结构优化主导性状关联时,种内与种间存在一致的协同或权衡关系,可推演至群落水平.以环境差异较小而多物种共存的青藏高原高寒草甸植物群落为例,检验了这些推测.结果表明,在生态位分化明显的高寒草甸,局域群落物种间叶片氮磷含量与比叶面积协同变化(与叶片干物质含量权衡),但大多数种内这两类性状的关联并不显著.无论是种间还是大多数种内,功能平衡下叶片氮磷含量协同变化,结构优化使比叶面积与叶干物质含量呈权衡关系.性状的协同与权衡关系在多性状分析中更加明显.这些结果支持了以上预测,为性状关联的跨尺度推演提供了理论基础和实验依据.

Functional traits and their covariation could reflect specific ecological strategies of organisms in response and adaptation to changing environment as the basis of understanding organism-environment relations at the individual, population, and community levels. Many studies report trait coordination and trade-offs among species over broad scales, especially for the leaf economic spectrum at regional and global scales. An increasing number of studies have attempted to explain and predict the functional(trait) structure of populations, communities, and vegetation in response to environmental change,but the mechanisms underpinning trait coordination and trade-offs may differ across ecological levels. We know little about this trait coordination and trade-offs within and among species in local communities, lacking not only relevant theoretical frameworks but also experimental evidence. Merging trait covariation into theory for community assembly, we predict:(1)Significant trait coordination or trade-offs among coexisting species occur when coexisting species occupy distinct ecological niches, similar to the pattern induced by specialization of species niches on a broad scale and(2) if trait covariations are induced by functional balance and/or structural optima, then consistent trait coordination and trade-offs within and among species will occur and scale up to the community and ecosystem levels. In this study, we experimentally test these predictions in local communities and discuss the potential mechanisms underpinning trait coordination and tradeoffs from the scale of individual plants to vegetation. In Tibetan alpine meadows with high plant species richness but low environmental heterogeneity, we sampled two leaf stoichiometric traits(leaf nitrogen and phosphorus content), two leaf morphological traits(specific leaf area and leaf dry matter content) and mature plant height for three to five individuals for all coexisting species within each of 36 plant communities(plots) distributed along a gradient of environmental harshness.In each plot, we averaged trait values of individual samples of each species at the population-level, and used the mean of all populations of each species to indicate the species-level traits. We assessed pairwise trait correlation within and among 29 frequent species by standardized major axis regression and trait coordination and trade-offs among multiple traits by principal component analysis. Similar to the broad-scale pattern, we found that leaf nitrogen and phosphorus contents were significantly positively correlated with specific leaf area(negatively correlated with leaf dry matter content) among 29 coexisting species, but these trade-offs were significant at a population level in only a few species. Both among species and within most species,(1) leaf nitrogen content was significantly positively correlated with phosphorus content, likely induced by functional balance,(2) specific leaf area was negatively correlated with leaf dry matter content, indicating a structural optimum,(3) neither leaf stoichiometric nor morphological traits significantly correlated with plant height. Trait coordination and trade-offs were stronger for multiple trait relationships than for bivariate correlations. These results support our predictions and provide a basis and experimental evidence for scaling up trait covariations from individuals to communities.