Changes to forest structure and productivity during an accelerated succession experiment in Northern Michigan

Mixed deciduous forests of the upper Midwest, USA are approaching an ecological threshold in which the colonizing species, aspen and birch, which continue to dominate the forest, are reaching maturity and beginning to die. Carbon uptake, which can be quantified as net ecosystem exchange (NEE), is predicted to decline in such maturing forests. The Forest Accelerated Succession ExperimenT (FASET), a large scale ecosystem manipulation, is testing this hypothesis. In 2008, all aspen and birch over 34 hectares of the FASET treatment plot were girdled to induce mortality. Aspen and birch mortality occurred within 2-3 years. This treatment simulates a natural disturbance that forces the forest into a later successional stage dominated by maples, oaks, and white pines. To quantify resulting changes in canopy structure we used ground-based and airborne LiDAR measurements of forest canopy structure. Canopy structural complexity and canopy height variability increased following onset of the treatment effect. Two micro-meteorology towers were used to observe changes inside the canopy and in the way the canopy interacts with the atmosphere. As the treatment begins to take effect, NEE rates in the treatment plot are declining relative to control plots. We hypothesize that this change in NEE can be attributed to the effect of the large scale disturbance that characterize this treatment. Continued measurements will determine whether NEE eventually recovers or even surpasses pre-treatment levels.