Effects of canopy structure and species diversity on primary production in upper Great Lakes forests

Canopy structure and tree species diversity, shaped by succession, disturbance, and community composition, are linked to numerous ecosystem functions, including net primary production (NPP). Understanding of how ecosystem structural metrics are interrelated and mechanistically link to NPP, however, is incomplete. We characterized leaf area index (LAI), Simpson's index of Diversity (D', a measure of species diversity), and canopy rugosity (R_c, a measure of canopy physical complexity) in 11 forest stands comprising two chronosequences varying in establishing disturbance, and in three late successional communities. We related LAI, D', and R_c to wood NPP (NPPw), and examined whether absorption of photosynthetically active radiation and light use-efficiency (LUE) link NPPw with ecosystem structure. We found that recovery of LAI and D' was delayed following more severe establishing disturbances, but that the development of R_c was strikingly conserved regardless of disturbance, converging on a common mean value in late-successional stands irrespective of differences in leaf area index and species diversity. LAI was significantly correlated with NPPw in each stage of ecosystem development, but NPPw was only correlated with R_c in early successional stages and with D' in late successional stages. Across all stands, NPPw was coupled with LAI and R_c, (but not D') through positive relationships with light absorption and LUE. We conclude by advocating for better integration of ecological disciplines investigating structure-function interactions, suggesting that improved understanding of such relationships will require ecologists to traverse disciplinary boundaries.