Size isn't age: Decoupled and interacting effects of height and age on functional traits in grassland plants

Abstract In open biomes, plants tend to allocate most of their biomass below‐ground into coarse organs, which constitute the perennial portion of their body (roots or stems). As a result, in these environments, height and age can be decoupled, forming independent dimensions. These functional traits can shape plant fitness because (1) plant height influences above‐ground space occupancy, light capture, photosynthetic capacity and associated biomass production as well as dispersal of reproductive propagules, (2) plant age indicates individual on‐spot persistence defining the timespan a plant occupies a given space where it acquires, uses and stores resources in different above‐ and below‐ground organs. However, the extent to which plant height and age are decoupled in open biomes and the degree to which they may affect other plant functional traits remains largely untested using quantitative data. Here, we aim to tackle this gap, focusing on eight species (from five families) specialized and restricted to temperate dry grasslands in Central Europe. We examined how plant height and age affect other functional traits related to carbon allocation, resource use and conservation and transport efficiency—alone or in interaction, at the interspecific and intraspecific level. Our findings indicate that plant height and age are decoupled across and within species. Yet, these traits affect other plant functional traits when their effect is taken separately and in interaction—with highly species‐specific patterns. The interacting effect of plant height and age is especially strong for traits related to resource use and transport efficiency (annual radial growth, vessel size). We also reveal a widespread lack of trait coordination at the intraspecific level. Synthesis . While confined to a few species of one habitat type, our study provides detailed insights into a widely assumed yet poorly tested idea. Our results tend to align with the grow fast–die young versus grow slow–live long functional spectrum while also showing how plant height and age can affect other plant functions differentially. This, together with highly species‐specific responses, cautions against overly broad generalizations.