Effect of nitrogen and light on nutrient concentrations and associated physiological responses in birch and fir seedlings

We grew seedlings of two co-occurring high elevation tree species in contro lled light and nitrogen (N) environments to examine the effect on foliar N and P concentrations and the resulting correlation with photosynthesis and growth. Foliar N concentrations in both heart-leaf paper birch (Betula cord ifolia) and balsam fir (Abies balsamea) seedlings were greater in low light treatments than in high light treatments. P concentrations, however, were lower in birch and fir foliage grown in low light than in high light. N-ava ilability had no effect on foliar N in birch but tended to increase N conce ntration in fir needles at all but 100% ambient light. N-availability had n o effect on P concentration in fir seedlings, but high N decreased foliar P in birch. There was a positive relationship between foliar N-concentration (mg g(-1)) and mass-based maximum photosynthetic rate (Asat) in birch seed lings and a corresponding growth response to increased N-availability (sugg esting N-limitation). Fir photosynthesis exhibited a positive correlation u p to 22 mg g(-1) - N and a negative correlation above that point, suggestin g that high N concentrations may be detrimental to photosynthesis in the fi r seedlings. There was no significant effect of N-treatment on growth.