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 (Narea), and photosynthetic capacity (Amax) were measured in an aspen, two jack pine, and two black spruce stands in the BOREAS northern study area. Nareadecreased 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 Nareafor similar %PAR than did aspen early in the growing season.Amaxdecreased with decreasing Narea, except for the negative correlation between NareaandAmaxduring shoot flush for jack pine. For the middle and late growing season data, NareaandAmaxhadrvalues of 0.51 for all stands combined and 0.60 for all conifer stands combined. For similar Nareathe aspen stand had higherAmaxthan did the conifer stands. Photosynthetic capacity expressed as a percentage ofAmaxat the top of the canopy (%Amax0) decreased with %PAR similarly in all stands, but %Amax0decreased at a much slower rate than did %PAR. To demonstrate the implications of the vertical distribution ofAmax, three different assumptions were used to scale leafAmaxto the canopy (Acan‐max): (1) constantAmaxwith canopy depth, (2)Amaxscaled proportionally to %PAR, and (3) a linear relationship betweenAmaxand cumulative leaf area index derived from our data. The first and third methods resulted in similarAcan‐max; the second was much lower. All methods resulted in linear correlations between normalized difference vegetation indices measured from a helicopter and Acan‐max(r=0.97, 0.93, and 0.97, respectively), but the slope was strongly influenced by the scaling method.