Production efficiency and leaf area estimation in multi-cohort stands of pure Pinus ponderosa var. scopulorum in South Dakota's Black Hills National Forest

Background/Question/Methods

Contemporary North American forestry has moved away from management primarily for fiber production toward management for a suite of priorities, including aesthetics, forest health, wildlife habitat, and restoration of pre-settlement conditions. Multi-cohort forest stands are better suited to contemporary management priorities than single-cohort stands in many instances, largely because stand density can be held in check and regeneration initiated without wholesale removal of the overstory. However, potential growth efficiency differences between small and large trees make it unclear that multi-cohort stands produce biomass as efficiently as single-cohort stands, which has implications both for fiber production and carbon sequestration. We used stemwood volume production per unit leaf area as a metric of production efficiency to compare both individual cohorts within multi-cohort stands, and multi-cohort to single-cohort stands. Leaf area is closely related to light capture, thus silvicultural systems that manipulate leaf area in multi-cohort stands through density management in effect allocate sunlight among cohorts. An explicit connection between leaf area and a standard forestry metric would allow managers to consciously regulate light capture by cohort in multi-cohort stands. A widely-used forest stocking tool called stand density index (SDI) is highly correlated with leaf area and has been suggested as a suitable tool for allocating leaf area in multi-cohort stands. However, it is unclear that the relationship between SDI and leaf area is unbiased by tree size.

Results/Conclusions

This work sampled 1,824 trees in 21 three-cohort and 10 single-cohort stands of pure Pinus ponderosa var. scopulorum in South Dakota’s Black Hills National Forest. We found trees in the smallest cohort in three-cohort stands produced significantly less stemwood volume per unit of leaf area than trees in larger cohorts. Despite this, there was no statistical difference in production efficiency between three-cohort and single-cohort stands. SDI explained almost 90% of the variation in leaf area, with no statistical difference in the relationship across cohorts in multi-cohort stands. Results suggested no penalty in terms of production efficiency for the multi-cohort stands we measured compared to their single-cohort counterparts. Furthermore, SDI provided an unbiased estimate of leaf area in multi-cohort stands, supporting its use as a stocking tool for management of complex stand structures.