Relative growth rates of trees decline with tree size, suggesting that there are also size-related shifts in nutrient acquisition and usage by trees. However, correlations between tree growth and nutrient acquisition remain poorly quantified. We use repeated tree censuses from the US Forest Inventory & Analysis (FIA) database for trees with dbh > 2.5 cm along with published allometric equations on allocation to tissue types and carbon to nitrogen ratios to determine how relative nitrogen demand (nitrogen usage per aboveground biomass) changes with tree biomass. We estimate nitrogen access by excavating root systems of 60 trees of two tree species, Betula nigra and Robinia pseudoacacia, ranging in size from 0.02 to 65 kilograms, and quantify the horizontal spatial distribution of their roots. We then characterize the effect of tree size on relative nitrogen access (soil nitrogen access per aboveground biomass) by simulating root systems of various sizes encountering nitrogen from a mosaic of soil patches.
Results/Conclusions
Between small trees (1-10 kg aboveground biomass) and large trees (1000-5000 kg aboveground biomass) in the FIA database, relative nitrogen demand declines nearly five-fold from 2.6 to 0.6 g nitrogen per kg biomass per year. This trend parallels changes in relative growth rates, which decline from an average of 13.5% per year for small trees to 2.1% per year for large trees. We estimate that large trees use 83% of newly acquired nitrogen each year to replace senescent tissues, while small trees use only 68%, leaving more nitrogen available to produce new tissues. In terms of nutrient availability, relative nitrogen access declines at least four-fold by the time a tree reaches 3 kg aboveground biomass. Variation in relative nitrogen access is very high for seedlings and decreases with tree size as larger root systems integrate over larger volumes of soil. Our simulations show that the variance and spatial autocorrelation that characterize soil nitrogen distributions in nature, when compared with homogenous distributions of soil nitrogen, can increase variance in tree nitrogen access by more than five-fold for seedlings during their establishment phase. Our results suggest that spatial heterogeneity of soil nitrogen limits variation in relative nitrogen access as trees grow, which drives a reduction in maximum relative growth rates. As trees continue to grow, further decreases in relative growth are driven predominantly by increasing allocation to wood, while turnover of leaves and fine roots becomes an increasingly important component of a tree’s nitrogen budget.