Nitrogen use efficiency in relation to forest type, N expenditure, and climatic gradients in North America

Forest nitrogen use efficiency (NUE) values are often compared along N-availability gradients or between forest types, without adjustment for climate. In this study, NUE (leaf-fall mass/leaf-fall N) was examined with concurrent data on forest type, N expenditure (leaf-fall N), foliar life-span, and major climatic gradients. The hypotheses were that (1) NUE is negatively correlated with N expenditure, (2) NUE is positively correlated with climatic factors such as temperature and light availability, and (3) NUE differs between deciduous and evergreen forests. The data set included 76 deciduous broadleaf forests, 52 evergreen coniferous forests, and 6 mixed forests in North America. All three hypotheses are supported by best-fit models. NUE decreases by about 30% for each doubling of N expenditure for both deciduous and evergreen forests. It increases over 50% in deciduous forests and nearly triples in evergreen forests across the climate data range. Evergreen forests tend to have higher NUEs than deciduous forests only in areas with relatively high temperatures and light availability. This climate–forest type interaction is attributed to contrasts between the forest types in terms of growth period, and regional patterns of foliar N concentration and N resorption.