Mechanisms maintaining productivity in old forests of the upper Great Lakes

Recent observations suggest that old forests continue to take up and store carbon (C) for centuries, but mechanisms underlying sustained C storage through successional time in forested ecosystems are poorly understood. In a chronosequence of forest plots in Northern Lower Michigan, we are testing the hypothesis that canopy structural complexity increases as forests age, enhancing light use efficiency, and thus C storage. We remotely sensed canopy structural complexity and quantified nitrogen (N) availability and productivity in forest stands spanning more than 150 years of development. We measured structural complexity throughout the canopy as rugosity, an integrated metric of the 3D arrangement of canopy leaf area. Canopy rugosity increased with stand age and correlated positively with net primary production of wood (r²=0.69, p<0.05), a major component of total forest C storage. Total canopy leaf area reaches an asymptote at ~50 years, suggesting that leaf area arrangement is more important than leaf area quantity in determining C storage of aging forests. The oldest stands we surveyed stored more C than expected after accounting for canopy structural complexity. Whole-canopy nitrogen content explained a significant fraction of residual variation in the relationship between C storage and canopy structural complexity. This suggests C storage in stands that have reached maximum canopy structural complexity is likely driven primarily by differences in site quality. We hypothesize that greater N availability and increasing canopy structural complexity together sustain higher rates of C uptake and storage by these mature forests than previously expected. These trends suggest that as Great Lakes forests age, their productive potential, and thus C storage capacity, may continue to be significant in the continental carbon budget. Our results further indicate that applications using remote sensing to predict forest C storage should characterize the complexity of canopy leaf area arrangement in addition to quantifying total leaf area. Management strategies that facilitate and promote development of old forests in the Great Lakes region will increase their C storage capacity.