Sampling Intensity and Uncertainty in Leaf Litterfall Mass and Nutrient Flux in Northern Hardwoods

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Designs for litterfall monitoring can be improved by quantifying uncertainty in litterfall mass and nutrient concentration. We compared the coefficient of variation of litterfall mass and chemistry (N, P, Ca, Mg and K) at different spatial scales and across years for six northern hardwood species from 23 stands in the White Mountains of New Hampshire. Stands with steeper slopes (P= 0.01), higher elevations (P= 0.05) and more westerly aspect (P = 0.002) had higher interannual variation in litter mass. The spatial variation of nutrient concentrations varied more across stands than within stands for all elements (P < 0.001).  Phosphorus was the most spatially variable of all nutrients across stands (P < 0.001).  Litter chemistry varied less from year to year than litter mass, but the magnitude of difference depended on the element and tree species. We compared the relative importance of these sources of variation to estimates of nutrient flux by simulating different sampling schemes of one while holding the other constant.  In this data set, interannual variability of leaf litter mass contributes more to uncertainty in litterfall flux calculations than interannual variation in nutrient concentrations. Optimal sampling schemes will depend on the elements of interest and local factors affecting spatial and temporal variability.