Foliar N and P concentrations of six northern hardwood species in three age classes in a factorial N and P addition experiment in the White Mountains of New Hampshire

Background/Question/Methods

Responses to nutrient perturbations in ecosystem processes may depend on forest age and species composition. In the White Mountains of central New Hampshire, plots in ten forest stands of three age classes have been fertilized annually since 2011 with nitrogen (30 kg N ha^-1 y^-1 as NH₄NO₃) and phosphorus (10 kg P ha^-1 y^-1 as NaH₂PO₄) in a full factorial design. Between 2014 and 2016, spanning the 4th to 6th years of fertilization, we collected green leaves of American beech (Fagus grandifolia), pin cherry (Prunus pensylvanica), red maple (Acer rubrum), sugar maple (A. saccharum), white birch (Betula papyrifera), and yellow birch (B. alleghaniensis) in stands where each species was found. Two young stands (last cut between 1988-1990) were sampled in 2014 and 2016; four mid-aged (last cut between 1971-1985) and four mature (last cut between 1883-1910) stands were sampled in 2015 and 2016.

Results/Conclusions

In all species and all stands, treatment effects were significant after 4 - 6 years of treatment: we found that foliar N was 10% higher with N addition and foliar P was 31% higher with P addition. The greater response of P might reflect the very low N:P ratio of our nutrient additions (3:1) and relatively high pretreatment foliar N:P ratios. More intriguing was the fact that P addition reduced foliar N concentrations by 4%; this response may be related to the observation that tree diameter growth was increased by P addition after 4 years of treatment (Goswami et al. 2018). Similarly, N addition reduced foliar P concentrations by 7% overall, but treatment effects differed by age class: while N addition reduced foliar P concentrations in the mid-aged (p=0.10) and mature (p=0.01) stands, N addition increased foliar P concentrations in the youngest stands (p=0.12). Average N:P ratios were 13% higher with N addition and 43% lower with P addition. Treatment effects were generally the same despite different species composition in each age class. Our results show a greater community response to P addition than to N addition across age classes.