Biomass and nutrients in Nova Scotian forests, and implications of intensive harvesting for future site productivity

Abstract

Among four conifer and four hardwood stands in central Nova Scotia, Canada, average potential whole-tree (above-ground) harvest removals of biomass, N, P, K, Ca, and Mg were 136 000, 310, 38, 144, 360, and 39 kg/ha, respectively. These represented average increases over bole-only harvesting of 50% for biomass, 170% for N, 200% for P, 160% for K, 100% for Ca, and 120% for Mg. Thus, a moderate increase in potential biomass yield via the whole-tree harvest, was obtained at the expense of much larger increases in removals of major nutrients. On average, the whole-tree contents of biomass and nutrients were smaller than quantities present in the forest floor plus mineral soil. For biomass, the whole-tree averaged 35% the organic matter (d.w.) of the forest floor plus mineral soil, 6% of total nitrogen, 4% of total phosphorus, 1% of total potassium, 29% of total calcium, and 0.5% of total magnesium. Within this data set, only the calcium comparisons indicated a short-term cause for concern with respect to impoverishment of site nutrient capital by whole-tree harvesting. Bivariate statistical analyses of data for 12 stands and 20 variables revealed that: (i) soil organic matter and nutrient quantities were only occasionally significantly intercorrelated; (ii) quantities of biomass and nutrients in the whole-tree compartment were strongly inter-correlated with each other, but were independent of site class; and (iii) only soil K and Mg were significantly ( P<0.05) but weakly ( r 2=−0.35) correlated with site class. Factor analysis of the same data set revealed that four multivariate factors accounted for 93% of total variance: (i) factor 1 (45% of total variance) loaded heavily on tree variables; (ii) factor 2 (an additional 18% of total variance) loaded heavily on bole and soil variables; (iii) factor 3 (an additional 16%) loaded heavily on soil variables; and (iv) factor 4 (an additional 14%) loaded heavily on soil variables, especially Ca, Mg, and K, and site class. It seems unlikely that one or several whole-tree harvests of these natural stands, if done on rotations of >ca. 50 years, would result in important depletions of site nutrient capital. However, calcium removals as a percentage of total site capital were large. This may be a cause for concern, and warrants further investigation.