Data from: Are leaf functional traits “invariant” with plant size, and what is “invariance” anyway?

Studies of size-related plant traits have established a suite of mathematical functions describing whole plant investment and allocation. In parallel, studies of plant “economic spectra” have measured the scaling and variance composition of traits related to the major dimensions of both structure and function. Here we explore the intersection of these two broad areas by exploring the notion that many leaf economic traits are invariant with species differences in adult plant size. Invariant traits are those that do not change with plant size and are invoked as a key simplifying assumption of prominent models that purport to explain the scaling of plant size and metabolism. Unfortunately, leaf trait invariance is neither well defined nor understood, and has never been critically evaluated. Using a global plant trait dataset, we evaluated whether nine key traits can be considered as effectively invariant as a function of the maximum height of plant species, within and across plant growth forms, and within and across broad taxonomic groups. We also examine the influence of habitat, biome and global spatial scales on the size-relative variance in plant functional traits. We suggest that while invariance is an intuitive concept, an objective statistical definition is elusive. Expanding on ideas drawn from the study of life history invariants, we propose five criteria to identify traits that are effectively invariant, depending on the research question. We show that all studied “leaf economic spectrum” (LES) traits approach invariance within and between herbaceous and woody plant groups; angiosperms and gymnosperms; and within most biome and habitat types. Individual leaf area, however, shows a modest increase with plant size, and there are significant shifts in the average LES trait values at a given plant maximum height, among the plant growth forms and taxonomic groups. Our results demonstrate that generally, LES traits show little interspecific variation with maximum plant height, which provides some support for attempts to model plants with “average” leaf properties. Our work also highlights the need for a better understanding of the drivers of leaf size variation within and across individuals, functional groups, clades, biomes and habitats.

针对植物体型相关性状的研究已确立了一系列描述整株植物资源投入与分配的数学函数。与此同时，植物「经济谱」相关研究则量化了与植物结构与功能核心维度相关的性状的缩放关系与变异组成。本研究聚焦上述两大研究领域的交叉点，提出假说：多数叶片经济性状（leaf economic traits）不会随物种成年植株体型的差异发生改变。所谓不变性状，指不随植物体型变化而改变的性状，这一概念是诸多主流模型的关键简化假设，这类模型旨在解释植物体型与代谢的缩放规律。遗憾的是，叶片性状的不变性既未得到明确定义与充分阐释，也从未经过严谨的批判性评估。本研究依托全球植物性状数据集，针对9个关键性状，分别在植物生长型内部、跨生长型，以及广义分类群内部、跨分类群的框架下，检验这些性状是否可被视为随物种最大株高变化的有效不变性状。此外，我们还探讨了生境、生物群区（biome）以及全球空间尺度对植物功能性状相对体型变异的影响。本研究认为，尽管不变性是一个直观的概念，但客观的统计学定义仍难以捉摸。本研究拓展了生命史不变性相关研究的思路，提出了5项标准，用于根据具体研究问题识别有效不变性状。本研究结果显示，所有被研究的「叶片经济谱（leaf economic spectrum, LES）」性状，在草本与木本植物类群之间、被子植物（angiosperms）与裸子植物（gymnosperms）之间，以及多数生物群区与生境类型内部，均趋近于不变性。不过，单叶面积会随植物体型增加呈现小幅增长，且在特定最大株高下，不同植物生长型与分类群的平均LES性状值存在显著差异。本研究结果表明，总体而言，叶片经济谱性状的种间差异随最大株高变化极小，这为采用「平均叶片属性」构建植物模型的尝试提供了一定支撑。同时，本研究也强调，我们仍需更深入地理解个体内、跨个体、功能群、演化支（clades）、生物群区以及生境间叶片大小变异的驱动因素。