Submitted to: Journal of Geophysical Research
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/30/1998
Publication Date: N/A
Citation: Interpretive Summary: There is a need to extend our knowledge of photosynthesis of boreal forest trees from individual leaves or branches to landscape, regional, and continental scales. Measurements of elements to assess photosynthesis (light absorption, leaf nitrogen, and a measure of photosynthetic capacity) within aspen, jack pine, and black spruce were made as a function of height. Light absorption was found to be the same on sunny and overcast days. Minimum understory light absorption was correlated with spectral reflectances measured from a helicopter system. Leaf nitrogen decreased with decreases in light absorption in all three species through the growing season. Photosynthetic capacity decreased as nitrogen decreased. Stands with greater light absorption with depth into the canopy had decreased photosynthetic capacity as depth increased. Three procedures for scaling leaf to canopy photosynthetic capacity were tested to demonstrate the effects of different assumptions: constant capacity with depth, capacity scaled proportionally to absorbed light, and integrating capacity versus cumulative foliage density. Methods one and three gave similar results. Method two gave a lower value. All the methods resulted in canopy photosynthetic capacities that were significantly correlated with canopy spectral reflectances. This study provides information necessary for accurate forest growth simulations and assessments, and for monitoring forest growth using remotely sensed data. Ecologists and forest managers will find this study especially useful for monitoring ecosystem primary production and addressing the effects of global climate change.
Technical Abstract: Profiles of photosynthetically active radiation (PAR), leaf nitrogen per unit leaf area (Narea), and photosynthetic capacity (Amax) were measured in aspen, jack pine, and black spruce to understand photosynthesis of boreal forests from leaves to the landscape. The understory %PAR was stable on cloudy days and variable on sunny days; daily average %PAR and its vertical distribution did not differ. Minimum understory %PAR was correlated with canopy simple ratio and normalized difference vegetation index (NDVI). The Narea decreased with decreasing %PAR. The Amax decreased with decreasing Narea except for jack pine which showed a negative correlation between Narea and Amax early in the season. The aspen had a higher Amax/Narea than the conifer, probably due to a smaller proportion of foliar nitrogen allocated to photosynthesis. When expressed as a percentage of Amax at canopy top, relative photosynthetic capacity (%Amax0) decreased with %PAR at similar rates. Thus, stands with a greater extinction of %PAR with depth had a greater rate of %Amax0 decrease with depth. The %Amax0 did not scale proportionally to %PAR. Vertical distribution assumptions of Amax were tested using three methods for scaling leaf Amax to canopy (Acan-max): assuming constant Amax with depth; assuming Amax scaled with %PAR; and integrating Amax versus cumulative leaf area index. Methods one and three gave similar Acan-max estimates; the second method gave lower estimates. All three had significant correlations between NDVI and Acan-max. This study provides information for forest growth simulations, for monitoring forest growth using remote sensing, and studying global climate change.