Structural-complexity effects on forest demographics and self-thinning in ponderosa pine and Douglas-fir on U.S. public lands

Abstract

Western U.S. public forest managers are applying structurally complex treatments not only to restore established mature stands, but to juvenile and reforested stands through practices such as cluster planting. Although studies suggest that structural complexity may negatively impact productivity and ecosystem services throughout stand development, findings are inconsistent. The inconsistencies may reflect a) the structural complexity attributes investigated, b) focusing on tree growth without recruitment or mortality, and/or c) unidentified site quality and stand factors that modulate structural complexity-productivity effects. We used forest inventory data to investigate how structural complexity influences productivity in western U.S. ponderosa pine (Pinus ponderosa) and Douglas-fir (Pseudotsuga menziesii) forest types. To evaluate the responses of survivor-tree growth, recruitment, mortality, and the size-density relationship to diameter-distribution complexity (tree-size variation) vs. horizontal-structural complexity (patchiness in stocking) across gradients of stand density, mean tree size, and site quality we used multi-model inference with generalized linear models. For both forest types, structural complexity reduced survivor basal area increment on high-quality sites, with the effect decreasing as site quality declined. Structural complexity interacted with mean stand diameter to increase recruitment rates in small-diameter stands while reducing recruitment in large-diameter stands. Diameter-distribution complexity exacerbated mortality in small-diameter Douglas-fir stands while reducing mortality in large-diameter stands. Higher horizontal structural complexity lowered maximum stocking. Our findings suggest that site quality and stand structure influence structural-complexity effects and that positive complexity effects on recruitment could counter survivor growth losses. These findings have implications for management of western U.S. and potentially other forest types.

Citation

Looney, Christopher E.; Moser, W. Keith. 2026. Structural-complexity effects on forest demographics and self-thinning in ponderosa pine and Douglas-fir on U.S. public lands. Forest Ecology and Management. 617: 123941. https://doi.org/10.1016/j.foreco.2026.123941.