Environmental drivers and phylogenetic constraints of growth phenologies across a large set of herbaceous species

Abstract

Because perennial herbs of temperate climates develop their above-ground parts every year anew, their success critically depends on the timing and speed of this growth (growth phenology). These parameters can play a role in species coexistence and may differ along environmental gradients. Still, we know little about them, as most phenological data come from observations of flowering and to a lesser degree leafing onset. We collected data on growth phenology of about 400 perennial herbs in a botanical garden to make the results independent of local differences in climatic drivers as much as possible. Using these data, we determined species-specific parameters of Day of peak growth, Day of maturity and two types of growth rates associated with the change in plant size. Environmental conditions in which these species occur in the field were assessed using Ellenberg indicator values, which express species’ optima along gradients of moisture, nutrients and temperature. Both timing and speed of growth estimated in the common garden were affected by light and moisture conditions of the habitats where the species typically occur. All parameters showed phylogenetic conservatism. We identified two relationships among these parameters of growth phenology: (1) species with early peak growth had high relative growth rates in contrast to late species; (2) tall species showed later peak growth than short species. The first relationship is associated with survival under forest canopy, where species are selected to grow early and fast before trees leaf out, which restricts their size. The latter is associated with (asymmetric) competition for light in open habitats, where the main selection factor is for tall stature, which cannot be attained early in the season. Synthesis. We show that large differences in size growth dynamics among herbaceous species are constrained by a few key trade-offs involving height at maturity, rate of growth and time when maximum height is attained. These trade-offs correspond to major selective forces acting on herbaceous plants in temperate climates.