Post-Clearcutting Chronosequence in the B.C. Coastal Western Hemlock Zone

Abstract

We estimated the magnitude of various sinks for forest floor N released by decomposition in a 400-year-old stand and along a 26-year post-clearcutting chronosequence in a Pacific silver fir (Abies amabilis [Dougl.] Forbes) and western hemlock (Tsuga heterophylla [Raf.] Sarg.) ecosystem on Vancouver Island. The total biomass and N-content of various components of the overstory and understory vegetation were measured, as well as the upper mineral-soil N-storage, and the fluxes of dissolved forms of N in forest floor and mineral-soil leachates. The overstory vegetation was not an important sink for N until at least 10 years after clearcutting; uptake by trees was not a significant sink for the post-clearcutting assart flush. In contrast, the early-seral understory vegetation played a small but probably significant role during the assart period. Competition for N between the understory and overstory vegetation during the first 10 years was probably not significant because the amount of N released from the forest floor far exceeded the amount of N taken up by either of these two components. Total leaching losses of dissolved-N between 3 and 12 years after clearcutting did not appear to vary significantly from total leaching losses in the old forest. Based on the lysimeter data, the upper mineral soil appeared to have the biological and/or chemical capacity to prevent most of the N released from the forest floor after clearcutting from moving down to below the rooting zone. However, we do not conclude from this that the majority of the N released is retained within the ecosystem. In a subsequent paper we develop a forest floor N-bud-get model to evaluate whether or not this is the case.