Dead bole mass and nutrients remaining 23 years after clear-felling of a northern hardwood forest

Abstract

Dead bole mass and nutrients were measured in a northern hardwood forest watershed at the Hubbard Brook Experimental Forest, New Hampshire, 23 years after all of the trees were felled and left in place. The experimental treatment of this watershed provided a unique opportunity to study large woody decay in a northern hardwood forest ecosystem with a well-documented disturbance history. We classified dead boles along line transects into four decay classes based on objective criteria, including bole shape and degree of fragmentation. Subsamples were returned to the laboratory for measurements of wood density and root ingrowth, species identification, and nutrient analysis. In 23 years, mass of dead boles (not including stumps) declined by approximately 90%, from 116.5 to 12.7 Mg•ha−1, and had an exponential decay rate constant, k, of 0.096 year-1. Significant differences in deadwood density were found among decay classes. The density of hardwood species declined approximately 50% by decay class 1, and softwood density declined 17% by decay class 1. Rates of decay appeared to be different among species, with maple and beech decomposing more quickly than birch and ash. Differences in concentrations of some nutrients were highly significant among decay classes (P and Ca) and among species (K, P, and Mg). Net release of nutrients ranged from 31% (N) to 93% (K). Root proliferation was highly correlated with decay class of dead boles. Highly decayed boles had much higher root biomass, but the total fine-root mass associated with deadwood was very low (2.1 g•m−2) compared with fine-root mass measured in forest floor (314 g•m−2). Our study demonstrates that decay of boles in this northern hardwood forest was rapid relative to conifer forests, and that decaying boles were not a site of significant nutrient accumulation or an important medium for tree roots.