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 p er unit leaf area (N-area), and photosynthetic capacity (A(max)) were measured in an aspen, two jack pine, and two black spruce stands in th e BOREAS northern study area. N-area decreased with decreasing %PAR in each stand, in all conifer stands combined (r = 0.52) and in all stan ds combined (r = 0.46). Understory alder had higher N-area for similar %PAR than did aspen early in the growing season. A(max) decreased wit h 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 st ands 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. Ph otosynthetic capacity expressed as a percentage of A(max) at the top o f the canopy (%A(max0)) decreased with %PAR similarly in all stands, b ut %A(max0) decreased at a much slower rate than did %PAR. To demonstr ate the implications of the vertical distribution of A(max), three dif ferent 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 propo rtionally to %PAR, and (3) a linear relationship between A(max) and cu mulative leaf area index derived from our data. The first and third me thods resulted in similar A(can-max); the second was much lower. All m ethods resulted in linear correlations between normalized difference v egetation 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.