Maximizing productivity and economic value of selection-managed stands using a cohort-based canopy competition model

Background/Question/Methods

A better understanding of the relationship between stand structure and productivity is required to develop harvest regimes that maximize timber yield and economic return. Toward this end, we used inventory data to parameterize a cohort-based canopy competition model for simulating the growth and yield dynamics of uneven-aged tolerant hardwood stands in central Ontario, Canada. Tree growth, mortality and recruitment were quantified as functions of diameter and asymmetric competition, using a competition index (CAI_h) defined as the total projected area of tree crowns at a given tree’s mid-crown height. To explore the relationship between stand structure and productivity, we simulated stand growth, mortality, and yield under different selection harvest regimes. The harvest regimes differed in harvest intensity, leaving a residual basal area (BA) between 10-35 m²/ha, and in residual size structure: the steepness of the negative exponential diameter distributions increased with q (1.3-1.9), the ratio of stem densities between adjacent diameter classes. To identify the optimal harvest regime, we used a simulated annealing algorithm to search for the values of BA and q (within the bounds specified above) which maximized both the yield (ΔVol, m³/ha/year) and the net present value (NPV, $/ha), which is the value of harvests discounted over time.

Results/Conclusions

For a given tree size distribution, stand growth increased asymptotically with stand volume, whereas mortality increased monotonically. This resulted in net productivity peaking at an intermediate stand volume of 100-150 m³/ha, and approaching zero at 250 m³/ha. For a given volume, mortality due to senescence decreased as the proportion of large trees decreased. Thus, yield was optimized by increasing q and decreasing BA, because the senescence of large trees was minimized and competition for light was reduced. When optimized for yield, stands produced a negative value of NPV, a net loss. By contrast, when optimized for maximum NPV, the optimal value of q was lower, and BA was higher. This is because, while small trees are more productive, they are worth less and cost the same to process. Retaining larger trees was therefore advantageous in maximizing NPV, when compared to the optimal harvest regime for maximized productivity.