Leaf functional traits have commonly been used as a proxy for ecological strategy but usually those traits have only been measured on sun leaves of adult trees. Yet strong plasticity and intraspecific variability has been widely documented in many of these traits in studies of individual species. The regeneration niche theory of species coexistence suggests that juvenile and subadult traits may matter more than those of adults for processes of community assembly, yet few studies have considered the change in these traits over ontogeny.
This study considers variability of leaf traits within and among all species within a community of woody plants in a southeast Michigan forest. Specifically, we ask: 1) How do traits change from sapling to sub-adult to adult tree? 2) How much variability do we find in traits within these ontogenetic classes? 3) Is this variability related to a priori measures of shade tolerance of the species?
To test these questions, we sampled a total of 7195 leaves from 632 individuals representing 14 species at the E.S. George Reserve in Southeast Michigan. Specific leaf area, leaf laminar area, and leaf dry matter content were measured and recorded for each leaf; leaf nitrogen content samples were bulked by individual.
Traits varied between the three ontogenetic levels by a range of values from less than 1% to over 80%. Coefficients of variation within ontogenetic groups were highest in sun leaves of adults and subadults (0.22-0.31), while sapling leaves were less variable overall (CV < 0.19). Contrary to other previous work, we found little trend in ontogenetic change as a function of a priori shade tolerance levels. Nor was there greater intra-level variability as a result of less shade tolerance. However, trait mean values largely followed expectations, with shade intolerant species showing higher SLA and LNC values and lower LDMC.
The decreased variability within saplings of each species suggests these trees may be showing greater specialization on particular light strategies as saplings, while older trees are somewhat released from that pressure, providing at least some support for the regeneration niche hypothesis. A lack of trend in ontogenetic and within stage plasticity as a function of shade tolerance is surprising in light of the literature, and merits further consideration.