Forest stand dynamics and accumulating down dead wood under chronic simulated nitrogen deposition in Michigan

Background/Question/Methods

Coarse woody debris, which encompasses both down dead wood (DDW) and standing dead wood (snags), can play an important role in ecosystem C storage and may be directly impacted by factors that alter stand dynamics. Little is known about the effects of elevated atmospheric nitrogen (N) deposition on forest stand dynamics and rates of mortality.  We hypothesized that long term simulated N deposition would increase DDW production through enhanced tree growth, leading to accelerated stand dynamics and greater mortality. We further hypothesized that DDW biomass would increase due to greater inputs and reduced decomposition. To test these hypotheses, we examined tree mortality and assessed DDW (volume and biomass) at four northern hardwood sites in Michigan receiving elevated simulated N deposition. Three 30 m x 30 m plots at each site have been amended with 3 g N ha^-1 y^-1 and three receive only ambient atmospheric deposition. Tree mortality was assessed annually at the sites, and the volume of DDW existing at the sites was quantified in 2012. Subsamples of the DDW were taken for determination of wood density.   

 Results/Conclusions

 Since treatments began in 1994, tree mortality has not significantly increased for the N deposition treatment. DDW biomass has accrued for both the control and N deposition treatment, with DDW biomass in N amended plots being 16% greater than that for the ambient plots. In contrast, DDW volume was actually 6% lower on N amended plots. Linear regression demonstrates that wood density in ambient plots significantly decreases with time since tree death. Plots receiving chronic experimental N deposition did not exhibit the same strong negative trend in density over time. In the later stages of decay, wood from the N deposition plots exhibited higher densities than wood from ambient plots, indicating reduced rates of decomposition for the N deposition treatment. DDW volume was highly variable across and within sites; therefore we were unable to detect statistical differences between treatments in DDW biomass. However, given the response of DDW density to the N deposition treatment, biomass will likely continue to accrue to a greater degree than volume and therefore may accumulate under N deposition in the future.