Goetz, S. J. and S. D. Prince. 1998. Canadian Journal of Forest Research 28(3):375-389.
Variability in net primary production (NPP) and light use efficiency were explored for a range boreal forest stands in northeastern Minnesota using an ecophysiological model. The model was parameterized with extensive field measurements that included above-ground NPP for Populus tremuloides and Picea mariana stands. The results showed that the proportion of total CO2 assimilation used for respiratory processes (i.e., the respiration : assimilation ratio, R:A) was significantly different for the two tree species and this explained much of the variability in the amount of net production per unit absorbed photosynthetically active radiation (APAR), referred to as PAR utilization (En). This is the first known study to directly link variability in respiratory costs to E(n). Attempts to link the R:A ratio with standing above-ground biomass in order to simplify estimation of E(n) were complicated by covariation in the amount of actively respiring biomass (i.e., foliage and sapwood) with both CO2 assimilation and respiration.
Total assimilation per unit APAR (Eg) was much less variable than E(n), and unlike E(n) and net CO2 exchange, was not significantly different between species. Greater stomatal control on some moisture stressed sites, however, introduced variability in the relationship. The unstressed value of E(g) was very close to the quantum yield of photosynthesis. The lack of a simple relationship between light harvesting and net carbon gain indicates that estimation of boreal forest stand NPP from light harvesting (e.g., with satellite remote sensing) requires additional information on both respiration and stomatal control, however unstressed E(g) can be modeled with the relatively invariant physiological parameter quantum yield.