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-aged forest stands are better suited to contemporary management priorities than even-aged stands in many instances, largely because stand density can be held in check and regeneration initiated without wholesale removal of the overstory. However, competitive interaction between trees of varying size and inherent physiological differences between small and large trees make it unclear that multi-aged stands produce stemwood volume as efficiently as even-aged stands. In South Dakota’s Black Hills National Forest, fiber production remains an important management objective, which raises questions regarding potential impacts to wood production associated with creating multi-aged structures.

We used stemwood volume production per unit leaf area as a metric of production efficiency to compare productivity of different sized trees and cohorts of trees within multi-aged stands, as well as to compare productivity of multi-aged to even-aged stands of pure Pinus ponderosa var. scopulorum . Leaf area is a good measure of resource acquisition for productivity analysis both because it is closely related to light capture, and because multi-aged silvicultural systems can use leaf area per unit ground area (leaf area index) as a stocking tool to regulate density of individual cohorts within a stand. Direct measurement of leaf area is currently unfeasible in the context of daily forestry operations. Consequently, an explicit relationship between leaf area and a standard forestry metric is needed to allow managers to allocate leaf area among cohorts within multi-aged stands using available inventory data. A widely-used stocking tool called stand density index (SDI) is highly correlated with leaf area and has been suggested for this purpose. Yet, it is unclear that the relationship between SDI and leaf area is unbiased across cohorts within multi-aged stands.

This work sampled 1,824 trees in 21 multi-aged and 10 even-aged stands to address questions of production efficiency and implementation of multi-aged silviculture. We found trees in the smallest cohort in multi-aged stands produced stemwood on average 20% less efficiently than trees in larger cohorts. Growth dominance analysis showed efficiency increased with increasing size for the smallest trees in multi-aged stands, but this relationship was inverted for larger trees. Despite size related efficiency differences between trees in multi-aged stands, there was no statistical difference in production efficiency between stand structures. SDI explained almost 90% of leaf area variation in multi-aged stands, with no statistical difference in the relationship across cohorts. Results suggested no penalty in terms of production efficiency for multi-aged stands compared to their even-aged counterparts. Furthermore, SDI provided an unbiased estimate of leaf area in multi-aged stands, supporting its use as a stocking tool for management of complex stand structures.