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 (Narea), and photosynthetic capacity (Amax) were measured in an aspen, two jack pine, and two black spruce stands in the BOREAS northern study area. Narea 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 Narea for similar %PAR than did aspen early in the growing season. Amax decreased with decreasing Narea, except for the negative correlation between Narea and Amax during shoot flush for jack pine. For the middle and late growing season data, Narea and Amax had r values of 0.51 for all stands combined and 0.60 for all conifer stands combined. For similar Narea the aspen stand had higher Amax than did the conifer stands. Photosynthetic capacity expressed as a percentage of Amax at the top of the canopy (%Amax0) decreased with %PAR similarly in all stands, but %Amax0 decreased at a much slower rate than did %PAR. To demonstrate the implications of the vertical distribution of Amax, three different assumptions were used to scale leaf Amax to the canopy (4can-max): (1) constant Amax with canopy depth, (2) Amax scaled proportionally to %PAR, and (3) a linear relationship between Amax and cumulative leaf area index derived from our data. The first and third methods resulted in similar Acan-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.