Factors controlling the decline of net primary production with stand age for balsam fir in Newfoundland assessed using an ecosystem simulation model

Net primary production (NPP) and growth efficiency (wood growth per leaf area) decline with stand age for forest ecosystems. Three hypotheses have been suggested: high respiration to photosynthesis ratios, hydraulic limitations on water transport in trees, and nutrient limitations of photosynthesis. Recent work indicates that hydraulic or nutrient limitations probably are the mechanism underlying the decline of growth efficiency in Douglas fir (Pseudotsuga menziesii) and lodgepole pine (Pinus contorta), both of which are native to western North America. However, balsam fir (Abies balsamea) grows in the cool-humid boreal forest and has very high stem respiration rates, suggesting that the mechanism may be different. We predicted measured aboveground NPP of 24 out of 25 balsam fir stands reasonably well. These stands were selected for having large differences in leaf area index and wood mass. Simulations indicate that increased wood respiration is the major factor causing the decline of NPP and growth efficiency with stand age; a secondary factor for mature stands is nitrogen limitation. For simulations in which balsam fir was fertilized, final growth efficiency was less than that of unfertilized forest stands because of increased stem respiration. From values in the literature, we calculated the potential difference conductance to water flow in the xylem from the soil to the leaves for a mean tree in each stand. Twigs had the smallest conductance; however, because the twigs conductances are added in parallel, the bole was the smallest conductance in the series: bole, branch and twigs. The overall drop of water potential with very high transpiration flux densities was generally less than 2.5 MPa. Therefore, we conclude that, for balsam fir, the decline in NPP and growth efficiency with stand age may be caused by the high respiration to photosynthesis ratios. This conclusion differs from recent work, so there is not a single unifying mechanism of age-related decline for all forest ecosystems.