Foliar nutrient resorption, in which nutrients are translocated to other parts of plants during senescence, is an important mechanism of nutrient conservation, allowing plants to be less dependent on nutrient uptake from soil. The timing of senescence influences foliar resorption and reflects a tradeoff between continuing photosynthesis and risking winter damage. This study looked at the effects of nutrient additions (N, P, N+P, and Ca) on foliar concentrations, resorption, and the timing of resorption of six northern hardwood species in Bartlett Experimental Forest, Hubbard Brook Experimental Forest, and Jeffers Brook within the White Mountains of New Hampshire, USA. Nutrient addition involved 5 years of application of 30 kg N/ha/yr as NH4NO3, 10 kg P/ha/yr as NaH2PO4, a combined N+P treatment at the same rates, and a one-time application of 1150 kg/ha Ca as CaSiO2 in 2011. Green leaves were collected in nine stands in the beginning of August and freshly fallen leaves from the same trees were collected in late October. Litter was periodically collected in traps in fall 2016 and sorted by species. Phosphorus was characterized by acid digestion and inductively coupled plasma-optical emission spectrometry and N was determined using a CN elemental analyzer.
Results/Conclusions
Foliar N and P concentrations, measured in four species in nine stands, had higher N concentrations, on average, under N addition (p < 0.01) and higher P concentrations under P addition (p < 0.01), which was not surprising. Element interactions were more interesting: foliar P concentrations were significantly lower with N addition (p < 0.01), which could reflect dilution with addition of a limiting nutrient or immobilization by soil microbes. Conversely, N concentrations were higher with P addition (p = 0.19). Pin cherry (Prunus pensylvanica) had the lowest resorption proficiency for N and P, indicted by high N and P concentrations in litter (p < 0.01 for both), followed by American beech (Fagus grandifolia). Yellow birch (Betula alleghensis) had the highest resorption proficiency, followed by white birch (B. papyrifera); sugar maple (Acer saccharum) and red maple (A. rubrum) were intermediate. For most species, resorption of N occurred earlier in the season than resorption of P. This means that comparisons of N vs. P resorption will be sensitive to the timing of sample collection, which has implications for interpretation of nutrient limitation.