Scaling nutrient limitation from trees to forests

Background/Question/Methods

Understanding the response of vegetation to nutrient limitation over long time scales is essential for estimating terrestrial carbon storage. Recent work has highlighted carbon balance throughout forest succession at two opposing scales. The ecosystem scale emphasizes declines in productivity with increasing stand age. At the organism scale, the ability of trees to continuously increase their carbon storage rates with growth is not resolved with nutrient uptake capacity of trees, nor is it clear how tree-level observations may emerge at ecosystem scales. We lack empirical and theoretical understanding of how root uptake efficiencies change with increasing plant size and ecosystem age. Our research links the scales of individual trees responding to nutrient limitation via fine roots and uptake efficiencies, with larger ecosystem scales of productivity and nutrient retention. We hypothesize that increases in tree and stand age will alter the scaling relationships between nutrient uptake efficiency and aboveground biomass, indicating that the uptake capacity of stands and trees cannot solely be predicted by fine root allocation. Fine root biomass, morphology, aboveground biomass, and nutrient availability were measured across a 350-year chronosequence of secondary forest succession. A nutrient uptake model enables the calculation of uptake rate constants as a measure of efficiency.

Results/Conclusions

Across the chronosequence, consistently low inorganic N concentrations in soils and soil N:P ratios point to strong nitrogen limitation. Initial calculations of ecosystem level nutrient uptake rate constants for inorganic N are consistent with strong limitation by, and retention of mineral N. We will present empirical evaluation of these theoretical findings and how tree-level uptake rate constants scale with individual tree size. Allometric functions used to estimate aboveground biomass show increases in biomass that follow increases in stand age, indicating increasing nutrient requirement with increasing ecosystem and tree age. Allometric scaling relationships between uptake rate constants and aboveground biomass define the relative contributions of fine root biomass, morphology, and uptake affinity toward meeting individual tree and stand-level demand for nutrients. Changes with tree age or stand development in the scaling relationship between uptake rate constants and biomass indicate changes in uptake efficiency as it relates to tree ontogeny, ecosystem age, or nutrient limitation.