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.