Effects of soil disturbance on vegetation recovery and nutrient accumulation following whole-tree harvest of a northern hardwood ecosystem.
Abstract
Accumulation of recovering vegetation is often the principal mechanism limiting nutrient loss following large-scale disturbance of forest ecosystems. The relationships were examined between soil disturbance associated with whole-tree harvest, and the rates and patterns of biomass and nutrient accumulation in regrowing vegetation, at Hubbard Brook Experimental Forest (HBEF), New Hampshire. Recovering vegetation was dominated initially by pin cherry (Prunus pensylvanica) and Rubus spp., but the importance of the tree species that dominate the mature forest (beech, Fagus grandifolia; maple, Acer saccharum and A. pensylvanicum; and yellow birch, Betula alleghaniensis) increased through 6 years of succession. In general, vegetation recovery was comparable to that following previous clear cutting experiments at HBEF. Very high spatial variation was observed in the composition and density of the recovering vegetation. The principal causes of this variation were disturbance to the soil during the harvest operation and availability of reproductive propagules. Significant differences in composition and growth of the plant community were observed among three soil disturbance classes. On severely disturbed sites, where mineral soil was exposed, pre-existing seedings were mostly eliminated and early rates of biomass and nutrient accumulation were low, partly because of slow early growth of colonizing yellow birch and probably also partly because of low site quality (water and nutrient availability). Later, the root system of trees growing on severely disturbed sites grew laterally into adjacent, more fertile locations and growth rates increased. Scarified sites often supported very dense stands of pin cherry that grew rapidly for a few years, but in years 5-6 suffered from severe intraspecific competition that apparently limited biomass and nutrient accumulation. Within this scarified class the nature and intensity of soil disturbance were quite variable so that high within-class variation in vegetation recovery was observed. The highest average rates of biomass and nutrient accumulation were observed on undisturbed sites, but very high spatial variability in tree density was also observed in this class. Very high nutrient concentrations (especially nitrogen) in pin cherry and Rubus spp. in the first 2 years resulted in non-linear relationships between biomass and nutrient accumulation in vegetation across sites and years. Also, relatively low calcium concentrations in the wood and bark of pin cherry led to relatively low rates of calcium accumulation in vegetation.