EFFECTS OF CLEARCUTTING AND ALTERNATIVE SILVICULTURAL SYSTEMS ON RATES OF DECOMPOSITION AND NITROGEN MINERALIZATION IN A COASTAL MONTANE CONIFEROUS FOREST

Rates of litter decomposition and N mineralization were measured in an old growth forest and in adjacent areas harvested by clearcut, patch cut, shelterwood and green tree retention systems. The site was a mont ane forest of western hemlock (Tsuga heterophylla (Raf.) Sarg.) and am abilis fir (Abies amabilis Dougl.) on Vancouver Island, in British Col umbia, Canada. During the first two years after harvesting, weight los s of needle litter was fastest in the old growth forest, possibly owin g to higher moisture in surface layers in the uncut forest during the summer. Forest floor material lost about 10% of its initial weight dur ing the two years in all systems. In-situ rates of net N mineralizatio n in the forest floor were greatest in the clearcut and least in the o ld growth. Concentrations of nitrate were greater in the clearcut than in the other systems or the old growth. The results indicated that al ternative silvicultural systems affected N mineralization less than cl earcutting, and that the increase in N mineralization and nitrificatio n after clearcutting was not the result of faster decomposition of org anic matter. Reduced input of fresh litter and the resulting decline i n C availability and immobilization of N into microbial biomass may be tter explain the increase in N availability alter clearcutting in this ecosystem. (C) 1997 Elsevier Science B.V.