Aspen and birch dominate forests throughout the Upper Great Lakes region as a legacy of anthropogenic disturbance. These early successional species are approaching their maximum life expectancies and are beginning to senesce. As these species decline, forest canopy composition and structure will change with potential effects on regional NPP. Aspen and birch comprise ~35% of canopy leaf area at the University of Michigan Biological Station (UMBS) in an experimental stand wherein aspen and birch were girdled to accelerate succession. Our objective, as part of this Forest Accelerated Succession Experiment (FASET), is to understand how future changes in canopy structure will influence NPP. Since aspen and birch are even-aged, their leaf area is arranged in a relatively shallow band at the top of the canopy. The understory is more species-diverse and uneven-aged, with leaf area distributed in a more complex arrangement. Mortality of canopy aspen and birch will thus increase heterogeneity of vertical leaf distribution, potentially increasing whole-canopy light interception despite a net loss of leaf area. We measured canopy structure in 30 ungirdled plots at UMBS using ground-based Portable Canopy LIDAR (PCL). The system uses high frequency laser pulses to generate vertical cross sections of canopy vegetation distribution.

Results/Conclusions

From PCL data we calculated rugosity, a measure of canopy structural complexity that summarizes plot-level variability of vertical vegetation distribution. Comparison of rugosity with wood NPP indicates that vegetation distribution within the canopy was a stronger predictor (R² =0.46) of plot-level NPP than was LAI or canopy species diversity. Stepwise multiple regression analysis using multiple canopy structural variables, LAI, and canopy species diversity yields a model that includes only rugosity and LAI but which explains >60% of the variability in wood NPP (p<0.01).  The greater explanatory power of rugosity indicates that total photosynthetic surface area within the canopy has less influence on wood NPP than its arrangement within the canopy. This observation likely results from increased light transmission through the canopy in a greater range of incident angles with increased rugosity. Increased canopy rugosity will also affect the spatial pattern of turbulence inside and above the canopy, which would affect the mean rates and spatial patterns of water, energy, and carbon fluxes between forest and atmosphere. We are currently measuring these fluxes in tandem with canopy rugosity in ungirdled and girdled FASET plots and expect that accelerated changes in canopy structure in treated plots will have significant consequences for NPP.