Profiles of photosynthetically active radiation, nitrogen and photosynthetic capacity in the boreal forest: Implications for scaling from leaf to canopy

Profiles of photosynthetically active radiation (PAR), leaf nitrogen per unit leaf area (N-area), and photosynthetic capacity (A(max)) were measured in an aspen, two jack pine, and two black spruce stands in the BOREAS northern study area. N-area decreased with decreasing %PAR in each stand, in all conifer stands combined (r = 0.52) and in all stands combined (r = 0.46). Understory alder had higher N-area for similar %PAR than did aspen early in the growing season. A(max) decreased with decreasing N-area except for the negative correlation between N-area and A(max) during shoot flush for jack pine. For the middle and late growing season data, N-area and A(max) had r values of 0.51 for all stands combined and 0.60 for all conifer stands combined. For similar N-area the aspen stand had higher A(max) than did the conifer stands. Photosynthetic capacity expressed as a percentage of A(max) at the top of the canopy (%A(max0)) decreased with %PAR similarly in all stands, but %A(max0) decreased at a much slower rate than did %PAR. To demonstrate the implications of the vertical distribution of A(max), three different assumptions were used to scale leaf A(max) to the canopy (A(can-max)): (1) constant A(max) with canopy depth, (2) A(max) scaled proportionally to %PAR, and (3) a linear relationship between A(max) and cumulative leaf area index derived from our data. The first and third methods resulted in similar A(can-max); the second was much lower. All methods resulted in linear correlations between normalized difference vegetation indices measured from a helicopter and A(can-max) (r = 0.97, 0.93, and 0.97, respectively), but the slope was strongly influenced by the scaling method.