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Post-clearcutting forest floor nitrogen dynamics and regeneration response in the Coastal Western Hemlock wet subzone Martin, Wayne Lloyd


The objective of this study was to describe the dynamics of post-clearcutting forest floor N on mesic sites in the wet subzone of the Coastal Western Hemlock biogeoclimatic zone. Post-clearcutting release of N was examined by measuring the following parameters for a chronosequence of five sites ranging in age from an old-growth forest to a 26-yr-old stand: forest floor N capital; rate of in situ mineralization of forest floor; rate of decomposition of cellulose; concentrations of inorganic-N and total-N in soil solution; and adsorption of inorganic-N by ion exchange resin bags. Effects of the post-clearcutting release of N on tree growth were assessed by measuring foliar N levels and height growth of Abies amabilis (Dougl.) Forbes advanced regeneration. The following sinks for the mineralized forest floor N were investigated: accumulation in plant biomass; solution transfer of inorganic-N and total-N from the forest floor to storage in the upper mineral soil. Based on the difference between the old-growth and the 6- year-old forest floor N capitals, post-clearcutting mobilization of forest floor N was estimated to be 950 kg.N.ha⁻¹ . Although not significant (P=.05), rates of cellulose decomposition were about 3 times higher in the young clearcuts than in the old-growth stand. Indicators of N availability, including in situ rates of mineralization, soil water concentrations and ion exchange resin bags, revealed higher levels of nitrate in the forest floor and/or mineral soil in the 3- and 6-year-old clearcuts than in the other sites. Nitrate concentrations declined to pre-clearcut levels by 8-10 years after clearcutting. %N and N content of current needles, and 1983 height increment of amabilis fir regeneration reflected the pattern of N availability. Of the observed change in forest floor N (kg.N.ha⁻¹ ) over the 10 year post-clearcutting period, plant biomass accumulation accounted for 105 and solution transfer from the forest floor for 187. Together, this amounts to 1/3 of the observed decline of 950 kg.N.ha⁻¹ Potential rates of denitrification in the 5- and 12-year-old clearcut forest floors were 2-5 times greater than in the old-growth stand. The potential rates were sufficiently high to provide a plausible explanation for the post-clearcutting decline in forest floor N.

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