Background/Question/Methods   Early successional aspen and birch forests that emerged following disturbance in the upper Midwest, USA a century ago are approaching maturity and starting to senesce, giving way to structurally and biotically more diverse forests. Net primary productivity (NPP) of such forests is often considered past peak and declining, but recent studies observe surprising resiliency of NPP in some aging forests. The mechanisms sustaining productivity in older forests are unknown, but these studies implicate favorable changes in forest canopy structure. We are testing the hypothesis that aspen and birch mortality will increase leaf area of middle successional species and result in a multi-layered canopy with higher light use efficiency. Aspen and birch leaf area is confined to a shallow canopy band while the understory is more structurally heterogeneous, with leaf area distributed in a more complex arrangement. Mortality of aspen and birch may thus alter leaf area distribution in a way that increases canopy light use efficiency. We suggest that this transition from a mono- to multilayer canopy could improve NPP resiliency as middle successional species grow into the canopy.

Results/Conclusions   To examine the relationship between structural and biological complexity and NPP resiliency, we measured stand age, tree species diversity, leaf area, NPP of wood (NPPW), and stem mortality rates in 30 plots in Northern Lower Michigan from 1998-2008. In 2008, we used ground-based Portable Canopy LIDAR to generate vertical cross sections of canopy vegetation distribution from which we estimated rugosity, a metric of structural complexity summarizing variation in vertical vegetation distribution. We found that plots containing a diverse assemblage of mid-successional species showed an attenuated decline in NPPW over time. Leaf area production by mid-successional species correlated positively with stem mortality of early successional species, and 10-year mean of NPPW correlated strongly with increasing canopy rugosity. We found that canopy rugosity increased with stand age and that among similarly aged stands, those with more rugose canopies tended to have higher NPPW. These data suggest that even as mortality of early successional species accelerates, the forest is likely to accrete leaf area from mid-successional species in a spatial distribution that increases the efficiency of canopy C assimilation. We propose that both structural and biotic diversity contribute to NPP resiliency and may act to sustain forest productivity and ecosystem C storage as forests transition from early to middle succession.