Will more nitrogen enhance the growth of young forest stands in central Appalachia?

Background/Question/Methods Increased forest growth in response to N deposition may account for 10% of the terrestrial C sink globally but the magnitude and longevity of any N-fertilization effect is uncertain. Part of our uncertainty arises from constraints on forest growth that can result from excessive acid deposition. This study examined how the growth of aggrading forests would respond to additional N in a region of elevated N inputs. To accomplish this task we measured the basal areas of 3,278 live trees (≥ 2.54 cm DBH) and the mass of fine roots in 180 soil cores in the Long Term Soil Productivity (LTSP) plots of the Fernow Experimental Forest, West Virginia. The LTSP experiment includes 12 plots (0.2 ha in size) that have received one of three treatments: (1) whole-tree harvesting (removal of all aboveground biomass); (2) whole-tree harvesting + ammonium-sulfate fertilization; and (3) whole-tree harvesting +  ammonium-sulfate fertilization + dolomitic lime additions.  Ammonium-sulfate is added at a rate of 35 kg N/ha. To separate direct effects of N from the indirect effects of acidification, dolomitic lime is applied every other year at a rate of 22.5 kg Ca/ha*yr to four of the experimental plots that also receive ammonium sulfate additions. 

Results/Conclusions After 13 years we found that ammonium-sulfate additions – either with or without the addition of dolomitic lime – increased the average basal area per tree by about 39% (p = 0.01). However, there was no detectable difference (p = 0.54) between any of the three treatments for the total basal area per plot or the mass of fine roots in the upper 10 cm of soil (p = 0.46). Our results are consistent with faster self-thinning taking place in response to greater site fertility - the “Sukatschew effect”. Since the N-stimulation of tree growth appears to be compensated for by a lower stand density, our results suggest that the response of C storage to more N will be constrained by N-induced changes in the stand dynamics of young forests.