Learning from dynamic traits: Seasonal shifts yield insights into ecophysiological tradeoffs across scales from macroevolutionary to intra-individual

Premise of the Research. Phylogenetic comparative methods provide a
powerful approach for exploring the macroevolution of plant functional
traits. Such approaches can uncover trait-trait correlations through
evolutionary time, as well as provide evidence of the role of traits in
adaptation across environmental gradients. For continuous traits, most
phylogenetic comparative approaches to date employ a single trait value
per species, often a mean of sampled individuals, or alternatively
incorporate intraspecific variation as a distribution around such a mean.
It has been known for quite some time that many of the most
physiologically and ecologically important plant traits are actually
highly plastic, changing dynamically across a growing season, with
whole-plant development, or in response to environmental conditions. Here
we demonstrate one possible approach to assessing the evolution of such
dynamic traits, the use of function-valued phylogenetic comparative
methods. Methodology. Leaf traits were sampled across 25 taxa in the genus
Cornus at six time points throughout the growing season in a common garden
context, followed by contrasting sets of alternative analyses to
demonstrate the consequences of researcher decisions on study conclusions.
Pivotal Results. The vast majority of assessed traits exhibit substantial
seasonal shifts. These shifts cause traditional macroevolutionary
correlations assessed at different sampling dates to yield conflicting
results. Function-valued approaches indicate that seasonal shifts in many
traits are evolutionarily correlated, with implications for the origin of
trait-trait tradeoffs. Seasonal trait plasticity is also evolutionarily
correlated with native habitat environmental gradients across Cornus.
Conclusions. Because a very large number of plant functional traits are
not fixed, but vary dynamically over time or with environmental
conditions, stronger insights into the evolution of plant functional
traits can emerge when this dynamism is explicitly incorporated into
phylogenetic comparative approaches. We encourage the adoption of such
approaches, as well as the development of better tools for doing so.