Stewardship of western larch forests in an era of uncertainty

Western larch (Larix occidentalis) is a conifer species with deciduous needles, a long life span, and high fire resistance and tolerance that grows in the interior Northwest U.S. and British Columbia. Extensive western larch-mixed conifer forests in the Northern Rockies provide valuable timber, habitat for high risk wildlife, and many other ecosystem benefits. However, 20th century fire exclusion and widespread logging have caused shade-tolerant species to succeed larch, and projected climate-related changes (drought and fire) also pose a serious risk. Land managers need the best available science to manage dense, at-risk stands while mitigating the effects of climate change to perpetuate western larch forests.
The Rocky Mountain Research Station and Flathead National Forest are collaborating in a nation-wide study called Adaptive Silviculture for Climate Change (ASCC).  The goal of this project is to test different silvicultural approaches to climate change adaptation that will serve as useful examples across the country.

In developing treatments for western larch forests, the project team considered direct and indirect regional climate predictions: drier summer and wetter winter/spring; earlier snowpack melts; longer fire seasons with more frequent fire; and more frequent insect and disease outbreaks. Four silvicultural treatments were designed to address these climate-driven concerns: no-action Control, Resistance, Resilience, and Transition. These treatments are meant to forestall, allow, or encourage change in forest structure and composition given changing climate and associated disturbances.

The Northern Rockies’ ASCC project is a long-term study. Because climate changes slowly and trees take long to develop, this study may not realize gains until years in the future. However, responsible stewardship of western larch forests includes preparing for them for the future, even if it takes time.

Treatment approach

Four replicates of each treatment were established on the Flathead National Forest and Coram Experimental Forest, with logging in 2019-2022. Planting is scheduled for spring of 2024.

• The Resistance treatment slightly modifies the forest for improved structure and composition over time by using commercial thinning from below.
• The Resilience treatment aims to change current stand structure so that a similar structure and composition re-develops in time. This treatment created 2–4-acre openings with scattered reserve tree clumps in the openings. Planting will supplement natural regeneration with shade-intolerant species.
• The Transition treatment aims to actively facilitate change toward the most future-adapted composition. This treatment largely removed the overstory, retaining about seven reserve tree clumps per acre as a seed source and for long-lasting structure. Planting will supplement natural regeneration with both on-site and off-site species and genotypes that are best suited for future climate.
• The relative performance of these three treatments (in contrast to the Control) over time will help managers identify the values and tradeoffs associated with managing for climate adaptation, whether managing to reduce future impacts on established structure and composition or to facilitate adaptive responses more suited for expected climate and disturbance.

Monitoring

Pre-treatment monitoring (vegetation and fuels) was completed before harvest, and subsequent monitoring will track forest responses to treatment. Continued monitoring is intended to:

• Assess overstory responses to treatment
• Assess natural tree and understory regeneration
• Assess planted tree survival and growth
• Assess forest fuel development
• Assess the effects of complex forest structure on microenvironment and soil moisture, which will in turn be related to vegetation growth and survival

Tracking these forest responses to treatments will be used to inform forest managers of the tradeoffs of successively more intensive climate mitigation strategies, the feasibility of assisted range expansion (a variant of assisted migration) in the northern Rockies, and develop process-based understanding of how creating complex forest structure drives subsequent forest change.

Other 

Other forest types and sites in the national ASCC project network include:

• red pine on the Cutfoot Experimental Forest, Minnesota
• mixed-conifer on the San Juan National Forest, Colorado
• longleaf pine/hardwood on the Jones Ecological Research Center, Georgia
• northern hardwoods on the Second College Grant Forest, New Hampshire

Past Contributing Personnel

Elaine K. Sutherland

Terrie B. Jain

Robert E. Keane II

Melissa M. Jenkins - Flathead National Forest

Christopher Keyes - University of Montana