COS 227-4 - Calculating nitrogen uptake rates in forests: which components can be omitted, simplified, or taken from trait databases and which must be measured in situ?

Background/Question/Methods

Because nitrogen commonly limits forest productivity and carbon sequestration and because nitrogen uptake depends on a wide range of fundamental ecological processes, measuring nitrogen uptake rates in forests is an important research objective. However, it is also methodologically challenging and time-consuming. Here, we present a case study that explores ways to simplify the methodology with a minimum sacrifice in accuracy. We estimated nitrogen uptake rates in 18 mature monodominant forest stands comprising 13 species of diverse taxonomy at the Morton Arboretum in Lisle, IL USA. We measured all nitrogen concentrations, foliage allocation, and fine root biomass in situ. We estimated wood biomass increments by in situ stem diameter and stem core measurements combined with allometric equations. We estimated fine root turnover rates from database values. In addition, we analyze similar published data from forest FACE sites.

Results/Conclusions

Simplifications involving fine root productivity had the largest impact on the accuracy of nitrogen uptake estimates at sites with intermediate overall nitrogen uptake (including Morton Arboretum), whereas simplifications involving foliage or wood estimates had the largest impact at sites with low and high overall nitrogen uptake, respectively. However, simplifications involving foliage or wood (but not fine root) estimates generally preserved the relative ranking of plots’ nitrogen uptake at a given site. Perhaps surprisingly, trait database nitrogen concentrations at higher taxonomic levels (e.g. plant functional type) better reflected in situ estimates than those at lower taxonomic levels (e.g. species). Careful sorting of fine root and foliage samples to target species did not dramatically affect estimates. Accurate estimates of forest nitrogen uptake rates appear to require in situ measurements of fine root productivity and are appreciably better paired with in situ measurements of foliage productivity. Wood productivity and tissue nitrogen concentrations may be taken from trait databases at higher taxonomic levels. The clear lack of correlation between total nitrogen uptake rate and aboveground nitrogen uptake rate without accounting for belowground productivity calls into question published inferences made using only aboveground nitrogen uptake rates.