Print PageIn many forests of the upper Great Lakes region, early successional tree species are senescing and giving way to later successional canopy dominants. Will these aging forests continue to sequester carbon (C) at rates comparable to previous decades or will the region decline in its contribution to the North American C sink? This is an important question for stewards of our forest resources and others planning for climate change mitigation. Though some forests accumulate C for centuries following stand establishment, the mechanisms behind sustained rates of C storage in aging deciduous forests are unclear. At the University of Michigan Biological Station, in northern lower Michigan, we are combining observational and experimental C cycling studies to forecast how forest C storage responds to climate variation, disturbance, and successional shifts in species dominance. At this AmeriFlux site, the Forest Accelerated Succession ExperimenT (FASET), in which all early successional aspen and birch trees within a 39 ha stand were stem girdled in 2008, is testing the hypothesis that net ecosystem production will increase with forest age due to disturbance mediated changes in nitrogen (N) availability, N allocation to the canopy, and the development of a more biologically and structurally complex canopy.
Results/Conclusions
In control (undisturbed) stands, we found that ecosystem resilience to age-related declines in net primary production (NPP) is highest where a greater diversity of tree species is present, because later successional species rapidly compensate for declining growth of early successional species. Furthermore, canopy structural complexity, which increases with forest age, is positively correlated with decadal NPP in stands ranging in age from 12 to 186 years. In treatment stands, aspen and birch mortality reduced soil respiration, accelerated fine root turnover, and prompted the redistribution of N from the foliage of early to later successional species. Canopy gap formation following disturbance increased stand-level surface complexity, changed the subcanopy micro-climate, and thereby altered mass and energy exchanges with the atmosphere. Our data suggest that increased canopy complexity and N availability work in concert to support increases in light use efficiency and improve production as these forests age. Our results provide new insights into the mechanisms by which deciduous forests maintain high rates of C storage into late succession, and suggest that forests of the upper Great Lakes region can continue to be important C sinks for decades into the future.
