At the University of Michigan Biological Station (UMBS) in northern lower Michigan, USA, I employed a combination of chronosequence observations and experimental forest manipulation to evaluate the potential of partial forest disturbances to facilitate resilience of carbon (C) storage over two centuries of succession.
In 2008, I assisted in initiation of the Forest Accelerated Succession ExperimenT (FASET). All aspen and birch trees in the treatment area were stem girdled to induce mortality. I used lysimeters to characterize nitrogen (N) leaching losses from soil in response to widespread tree mortality. N export was significantly greater in treated than control stands and corresponded to increased fine roots mortality, but total N export was insufficient to limit long-term C storage rates in affected stands.
From 2008 through 2011, I used a portable canopy LiDAR (PCL) system to monitor canopy structural changes following widespread mortality in FASET. I also measured leaf-level maximum C assimilation (Amax) rates in canopy co-dominant red oak, red maple, and white pine to evaluate physiological consequences of aspen and birch senescence. Canopies became shorter, more open, and more heterogeneous over time in canopies of treated but not control stands. Leaf Amax of successor canopy species did not change in response to canopy structural rearrangements or increased N availability.
I also used the PCL system to characterize canopy structure in stands spanning two centuries of succession. I define a novel metric of canopy structural complexity: rugosity. Rugosity increased with stand age in and explained more variation in aboveground C storage than other known drivers of forest C storage. Forests with more structurally complex canopies used light and nitrogen resources more efficiently than forests with structurally simpler canopies. I present evidence of a mechanistic linkage between canopy structure and forest function that allows aging forests to maintain higher than expected rates of C storage.
Results of these studies indicate that forests maintain high rates of C storage into maturity through a combination of efficient soil N retention and canopy structural reorganizations that boost resource use efficiency as forests age.