It is convenient to interpret foliar N:P ratios as indicating the relative limitation of plant growth by N versus P. However, tests of these relationships in temperate forests are few, in part because N has been presumed to be limiting, and thus P fertilization trials are rare. Multiple Element Limitation in Northern Hardwood Ecosystems (MELNHE) is a unique full-factorial N by P fertilization study—perhaps the longest-running in a temperate forest anywhere in the world. MELNHE involves 13 forest stands of three age classes distributed across three study sites in the White Mountains of New Hampshire, USA: Hubbard Brook, Jeffers Brook, and Bartlett Experimental Forests. Since 2011, plots measuring 0.09 to 0.25 ha have been treated with N (30 kg N/ha/yr as NH4NO3), P (10 kg P/ha/yr as NaH2PO4), N+P (at the same rates), or left as an untreated control. The goal of this study was to test whether pre-treatment foliar concentrations of N or P or N:P ratios could predict growth responses to additions of N vs. P, measured in the six dominant hardwood species.
Results/Conclusions
Pre-treatment foliar N:P ratios of the dominant species, from low to high, were 16.5 for red maple (Acer rubrum), 17.4 for sugar maple (A. saccharum), 19.1 for pin cherry (Prunus pensylvanica), 19.5 for yellow birch (Betula alleghaniensis), 21.7 for American beech (Fagus grandifolia), and 21.8 for white birch (B. papyrifera). These values would predict P limitation, based on thresholds reported in the literature for other forest and vegetation types: 13.8 in a montane forest in Hawaii (Herbert & Fownes, 1995), 14.7 in a beech forest in Europe (Ljungstrom & Nihlgard, 1995), and 17.7 in understory vegetation in Catskill, NY (Tessier & Raynal, 2003). Indeed, after 4 years of treatment, at the stand level, increases in relative diameter growth of trees were observed in response to P addition (P = 0.02) but not to N addition (P = 0.37). Average plot-level foliar N:P ratios by species did not predict growth responses to N vs. P, but we explored the use of aggregated community-level averages and also individual tree responses to identify thresholds more relevant to this forest type than those previously reported. Evaluation of growth measured in 2019 will improve these estimates.