Evidence for species-specific nutrient limitation of growth efficiency in northern hardwoods

Nutrient co-limitation of NPP has been shown in some temperate forests by the additive or synergistic productivity response to nitrogen and phosphorus. Co-limitation could arise from different mechanisms, one of which operates at the community level with some species responding primarily to N and others to P. This mechanism is possible if differences in species traits cause species-specific responses to nutrient availability. This study examines the possibility of community level co-limitation in northern hardwood forests by comparing growth efficiencies of different tree species in relation to nutrient use.  We estimated growth efficiency as productivity per unit leaf mass.  We compared this index to leaf tissue nutrient concentrations and soil nutrient availability, expecting biomass produced, per unit leaf nutrient concentration, to increase with availability of the limiting nutrient. The growth efficiency of beech was positively related to foliar P, and that of yellow birch was positively related to foliar N. Similar relationships were seen between growth efficiencies and metrics of soil N and P, suggesting that beech growth is limited by P and yellow birch growth is limited by N. The difference between these species reveals potential for community co-limitation.  These forest stands are now receiving multi-year nutrient additions (N, P and N+P), to test whether species respond differentially to the addition of each of these nutrients, and whether ecosystem scale production will respond non- additively to both N and P.