Fuels reduction treatments are common in ponderosa pine ecosystems of the interior western United States, but the long-term effects on many key ecosystem attributes remain poorly understood, including: tree growth and mortality; forest fuel loads; understory vegetation diversity and composition; production and distribution of aboveground biomass; and physiological response of trees to drought stress. A 1992 experiment at the Lick Creek Demonstration / Research Forest in western Montana was analyzed to evaluate tradeoffs among alternative cutting and burning strategies in ponderosa pine – Douglas-fir stands. One portion of the experiment tested a commercial thinning strategy, while a second tested a retention shelterwood strategy. Units were burned one-to-two years after harvesting, using different broadcast prescribed fire treatments to simulate a range of burning conditions.

All treatments led to a growth release that persisted to the time of re-sampling (23 years; 2015). Changes in stable-carbon isotope relationships were pronounced. Reduction in competition enabled trees to fix carbon and incorporate it into new stem growth when the climate became sufficiently stressful to drive slower-growing trees in uncut stands to either cease new assimilation or become more dependent on stored carbohydrates. During the second post-treatment plot census interval (2005–2010), when mountain pine beetle activity increased locally, tree mortality rates under each control more than doubled compared to the respective treatments. Treatments maintained substantially lower canopy fuel loads (and lower canopy bulk density in thinning units), and tended to lower 1-hr fuels, litter, and duff. The heavier cutting associated with the shelterwood accelerated ladder fuel development, reducing canopy base height. Understory cover (except forbs) was initially reduced by treatments, but then returned to pre-treatment levels within 15 years. Species richness increased, and then declined, but remained slightly higher than pretreatment. Forbs responded most strongly to treatments. Understory cover was negatively related to overstory basal area (but species richness and composition were unrelated to basal area). Across all treatments, tree biomass recovered to pre-harvest levels by 2015 (after 23 years). In the thinning, the control exhibited greatest total aboveground and live-tree biomass, but those did not differ among the three cut fuel treatments. In the shelterwood, total aboveground and live-tree biomass were both greater in the unburned treatments relative to the burned treatments. Forest floor and snag biomass tended to be lower in the burned treatments. Seedling, vegetation, and stump biomass were similar across treatments.

Fuel treatment longevity was strongly influenced by the initial silvicultural prescription, which produced divergent fuel loads and fuel structures. Stand density reduction was key to improving the ability of residual trees to tolerate climatic stress and associated biotic disturbances. Faster growth and enhanced ability to assimilate carbon under more stressful climate following treatments became evident as tree survivorship increased. Understory vegetation was resilient to the treatments in the long term. However, deviation in species composition and non-native invasion occurred by treatment, indicating that the more severe the treatment the greater the deviation from pre-treatment and greater non-native understory vegetation. Treated stands recovered tree biomass to pre-harvest levels in less than 23 years, while yet exhibiting stand densities and fuel loads that foster resilience and advance forest restoration objectives.