Nutrient limitation of primary productivity arises, at the ecosystem level, from the different nutrient limitation status among tree species. Interspecific differences in nutrient limitation have been shown for aboveground productivity, whereas less is known for belowground fine roots. Sensitivity to nutrient availability of the fine root community could be driven by variation in nutrient demand and plasticity of allocation among tree species, and have important implications for belowground competition and aboveground responses to nutrient availability. Studying nutrient limitation of fine root density by individual species can be quite challenging in mixed forests, owing to the difficulty in separating fine roots by species in root mixtures. We refined a simple method to quantitatively describe root community composition by amplifying the trnL intron region of plant plastid DNA. We then examined the relative abundance of individual tree species in root mixture in response to elevated N and P in a long-term factorial N×P fertilization experiment in northern hardwood forests in central New Hampshire, USA. After 6-year’s fertilization with N, P, N + P, or nothing, soil cores were collected and separated into Oe, Oa, and mineral horizons. Roots were isolated from the cores by hand and species composition was analyzed.
Results/Conclusions
We found that in northern hardwood forests, elevated N led to higher relative abundance of red maple and sugar maple in the fine root community, elevated N+P caused lower relative abundance of American beech, and the relative abundance of birch did not change under any fertilization treatment. Most fine root responses occurred in mid-aged and mature rather than in young stands. We suggest that low N availability constrains belowground resource competition by maple, whereas low N and P availability improves belowground resource competition by beech roots.