Foliar resorption ratios suggest greater phosphorus limitation with stand age in White Mountain National Forest

Background/Question/Methods

Trees mobilize and resorb nutrients from leaves prior to senescence, which is an important mechanism of nutrient conservation.  High nitrogen deposition in the northeastern United States may be increasing the limitation of phosphorus in the region’s forests.  Based on Multiple Element Limitation (MEL) model simulations, we predicted that phosphorus limitation would be greatest in older stands.  Nitrogen and phosphorus resorption can indicate the effort allocated to the acquisition of each nutrient.  We studied a total of ten stands in three different age classes: 21-26, 33-36, and >100 years.  In 2009 and 2010, we sampled fresh foliage in August and leaf litter in October across three experimental forests in northern New Hampshire.  The species sampled included American beech, sugar maple, yellow birch, white birch, and red maple.  For both sampling years, resorption efficiencies were calculated for nitrogen and phosphorus as the percentage nutrient retained in leaf tissue between live foliage and freshly fallen leaves.

Results/Conclusions

Phosphorus resorption efficiencies tended to be higher in older stands than younger stands.  We compared the ratio of nitrogen to phosphorus resorption to control for variation in mass loss during senescence.  At Bartlett Experimental Forest, our most intensively sampled site, phosphorus resorption relative to nitrogen was greater in older stands for white birch (p= .004), red maple (p= 0.01), and American beech (p= 0.06).  The fact that old stands are resorbing higher proportions of phosphorus than nitrogen suggests greater limitation by P in older stands, which is consistent with MEL model simulations.