In the northeastern US, both hardwood and conifer forests have developed on sites with contrasting parent materials, allowing us to examine the effect of site and forest type on ecosystem nutrient cycling. We measured biomass production and nutrient fluxes in four northern hardwood and three softwood stands differing in soil acidity at Sleepers River, VT; Hubbard Brook, NH; and Cone Pond, NH. Nutrient turnover was calculated as the sum of nutrient fluxes in litterfall, net throughfall (throughfall minus precipitation flux), and root turnover. We described the efficiency of nutrient use by forests by comparing growth to nutrient turnover in ephemeral tissues and foliar leaching. Conventionally, nutrient use efficiency is reported as the biomass produced divided by the nutrient content or nutrient use. We used the ratio of biomass accumulation to nutrient uptake as one measure of nutrient use efficiency. Biomass accumulation was calculated as the increase in biomass based on increment growth. We also used the nutrient content of biomass accumulation in the numerator, to produce efficiencies in units that could be compared across nutrients (with both numerator and denominator in units of nutrient flux per unit area per unit time).
Results/Conclusions
Tissue concentrations of Ca, Mg, and K were sensitive to the gradient in soil base cation availability, while concentrations of N and P were not. Nutrient turnover in litterfall, throughfall, and foliage was generally an order of magnitude higher than nutrient retention in perennial tissues. Phosphorus cycling efficiency was lower than that of other elements, with efficiency defined as the nutrient content of perennial tissue production divided by nutrient turnover in ephemeral tissues and leaching losses. Softwoods and hardwoods had similar nutrient cycling efficiency, although softwoods were less productive. We did not find the efficiency of base cation use to relate to the cation availability gradient across sites, indicating that plasticity in base cation concentrations in tree tissues is not an important mechanism of nutrient conservation. However, P cycling efficiency was greatest where N availability was highest, which could be a mechanism of maintaining N and P co-limitation.