One of the greatest challenges in ecology is understanding species responses to environmental change. Functional traits are relatively easily-measured characteristics that relate to a species’ survival strategy and allow species with similar strategies to be grouped thus facilitating general predictions that do not rely on a species-by-species approach. A major limitation, however, with trait-based approaches is our current weak understanding of how traits vary within a single species. For example, we know very little about how developmental shifts in access to resources and growth strategy might be reflected in functional traits. Few recent studies suggest that functional traits of trees can change with size or age, yet we do not know whether this is consistent across species and life forms, including herbaceous plants. To determine whether perennial herbaceous plant functional traits change with plant age, we grew 8 species from four genera (4 congener pairs) native to Midwestern prairies in identical environmental conditions in a greenhouse, harvesting a subset of individuals every 2 weeks for measuring traits related to growth and biomass allocation.
Relative growth rates of species changed through time, increasing through the first half of the growth season, then decreasing through the second half for all species. The amount of change through time varied by species. Average specific leaf area was positively related to relative growth rate between species but specific leaf area did not change in a consistent way through time within a species. Relative investment in roots and stems tended to slightly increase with time in most species. There was considerable variation in these patterns from species to species but growth rates were more closely related to leaf characteristics than to phylogenetic relatedness. These results suggest that for some common functional traits like specific leaf area, plant age is not likely to have consistent effects that could bias functional characterizations of species. Changes in growth rate and associated characteristics were consistent between species, indicating that shifts in strategies as herbaceous perennials age could result in variation in other functional traits through time.