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

Abstract

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.