Balancing timber production and deer forage in young-growth silviculture in southeast Alaska

Project Description

An unthinned, 27-year-old control stand on Prince of Wales Island, Alaska.

The temperate rainforest of the Pacific Northwest extends from northern California to south-central Alaska and delivers a wide range of ecosystem goods and services, including quality fiber, biological diversity, subsistence lifestyle support, clean air and water, prolific fisheries, and stunning scenery. Much of the temperate rainforest lies within the Tongass National Forest, which manages 16.9 million acres in southeast Alaska. The Tongass has assigned strategic land use designations to optimize the delivery of ecosystem goods and services across the forest, in line with the Tongass Land and Resource Management Plan. The subset of forestland specifically designated for timber development provides globally valuable products and jobs to rural economies and includes both old-growth and young-growth stands. In recent years and per the Tongass Advisory Council’s recommendation, the Tongass has been transitioning away from a predominantly old-growth management approach to primarily young-growth management.

Ten years after precommercial thinning a 48-year-old stand at 23-foot spacing on Chichagof Island, Alaska.

While managing young-growth forests has become a priority, questions remain about intermediate silvicultural treatments (treatments that occur in between regeneration and harvesting, like thinning). Silviculturists on the Tongass aim to manage within the bounds of existing land use designations (e.g., manage for timber in area designated for timber), but managers are also keenly interested in improving wildlife habitat, especially forage for the Sitka black-tailed deer. These small deer are endemic to the Alexander Archipelago and are valuable prey to local bear and wolves. The deer are also a staple of the subsistence lifestyle for local people, including Alaska Natives. Managers need to know what management techniques are appropriate to balance immediate and long-term needs (i.e., deer forage and timber, respectively) in young-growth stands.

A series of projects were co-developed by the Pacific Northwest Research Station and the Tongass National Forest between 2001 and 2010 to address this question. Research frameworks were set up to provide a scientifically rigorous basis for adaptive management of these young-growth forests. These include the geographically extensive Tongass-Wide Young-Growth Studies (TWYGS), and Prince of Wales Commercial Thinning Study (POW-CT), and a study of subcanopy snow and slash patterns by treatment age. Together, these valuable long-term studies are demonstrating the effects and tradeoffs of silvicultural techniques such as alder enrichment planting, precommercial thinning with wide spacing, pruning, slash treatment, and commercial thinning methods on understory response, timber growth, and wildlife habitat.

Approach

Immediately following commercial thinning from below on Prince of Wales Island.

These studies have been implemented in operational stand settings to provide relevance for future management and meaningful forage areas for Sitka black-tailed deer. TWYGS and POW-CT studies use repeat measurements of vegetation structure and composition to demonstrate ecosystem response over time. Vegetation data are input into the Food and Resource Evaluation System for Habitat (FRESH) – Deer calculator to model combined forage nutrition and availability across treatments. These measurements of vegetation structure, composition, and forage provide a valuable means of comparing the long-term effects of treatment options. These research studies have been hallmarks of research-management collaboration thanks to active engagement by the Rocky Mountain Research Station, Pacific Northwest Research Station, and Tongass National Forest.

Timeline Tongass Young Growth Studies (TWYGS). Concerns about large-scale timber harvest and its effects on forest and wildlife date back to the 1970s. The current Tongass National Forest Plan updated in 2015 emphasizes a shift from old growth harvest to young growth over a 15-year timeline.

Key Findings

Response of understory biomass 8 to 10 years after precommercial thinning in units catagorized by stand age. Panel A shows the biomass (kg ha-1) in thinned units. Panel B shows the ratio of biomass in the thinned unit to the mean biomass of control units in that year.

Findings are still being developed. Current findings include:

• When considering treatment in all age classes, precommercial thinning increases understory biomass and forage through ten years post-treatment.
• Simultaneously meeting resource objectives for improved deer forage and timber production is possible. This is especially true in younger stands where understory species composition provides valuable deer nutrition.
• Year-to-year variability in precipitation, especially, can impact vegetation production and assessment. While weather variability affects absolute vegetation biomass, the benefits of thinning over not thinning persist.  
  16 years after treatment (currently only analyzed in the 15-25 year age class), thinning still effectively maintains 3 to 6 times more understory biomass than the control.
• Lack of major difference in deer forage production between the 4.3-m and 5.5-m spacing treatments after 16 years suggests managers can select whichever of these spacings better meet other goals, including timber production.