Sugar maple (Acer saccharum) and American beech (Fagus grandifolia) are archetypical climax species in northern hardwoods that have been experiencing changes in growth and recruitment associated with changes in soil nutrient availability. Sugar maple decline in areas of low soil calcium and magnesium has been a concern for timber and maple sugar industries. Beech is threatened by beech bark disease, which is sensitive to both nitrogen (N) and phosphorus (P) availability. Dying beech often produce root sprouts, resulting in dense populations of understory beech, which can interfere with recruitment and growth of other species. The purpose of this study is to investigate the effects of nutrient availability on the recruitment of six northern hardwood species across three study sites in the White Mountains of New Hampshire, USA: Hubbard Brook, Jeffers Brook, and Bartlett Experimental Forests. Since 2011, plots received N (30 kg N/ha/yr as NH4NO3), P (10 kg P/ha/yr as NaH2PO4), N+P (at the same rates), a one-time application of calcium (1150 kg Ca/ha as CaSiO2) in 2011, or left as an untreated control. Saplings (2-10 cm DBH) of American beech, sugar maple, pin cherry (Prunus pensylvanica), red maple (A. rubrum), white birch (Betula papyrifera), and yellow birch (B. alleghaniensis) were inventoried in 2011, 2015 and 2019.
Results/Conclusions
Between 2011 and 2015, plots receiving N had a greater recruitment of beech saplings (p < 0.01) but lower recruitment of sugar maple saplings (p < 0.02) while adding P did not have a significant effect on either species. Surprisingly, adding N and P together resulted in a lower recruitment of beech saplings than the predicted main effects of N and P addition (p < 0.02). Bartlett had the highest recruitment of beech saplings (p < 0.01), followed by Hubbard Brook and Jeffers Brook. As expected, young stands had the highest recruitment of beech and sugar maple saplings (p < 0.01), followed by mid-aged and mature stands. The effect of N on beech recruitment may reflect an exacerbation of beech bark disease by high N:P ratios (Cale et al. 2017). We explored multivariate analysis to include additional species important in this forest type. Understanding these nutritional influences could lead to a better management of beech and the species that compete with it in the context of continuing environmental change.