Maintaining the recovery of the formerly federally endangered Kirtland’s warbler (Setophaga kirtlandii) requires continual creation of early-successional jack pine (Pinus banksiana) habitat every ...year, which, in turn, requires a steady supply of commercial timber sales. However, dedicated Kirtland’s warbler habitat plantations are established at high stem densities (ca. 3600 trees ha−1) which can delay the production of marketable timber compared to a conventional forestry planting density (ca. 1900 trees ha−1). We used a retrospective approach to assess the impacts of tree density on Kirtland’s warbler occupancy, and an experimental approach to understand benefits and costs of pre-commercial thinning of Kirtland’s warbler habitat plantations. We observed an unexpected negative relationship between realized plantation density and maximum occupancy by Kirtland’s warbler singing males. We found that thinning of high-density jack pine planted for Kirtland’s warbler habitat resulted in increased diameter growth, increased live crown ratio, and a trend towards increased intrinsic water-use efficiency in the older age classes. These findings provide strong support for the importance of using adaptive management approaches to more rigorously evaluate the impacts of plantation spacing on Kirtland’s warbler productivity. Although increases in volume growth and timber prices following thinning are unlikely to ever recoup the costs of implementation, we argue that thinning could still be an important tool to accelerate the rate at which these stands attain marketability for the timber sales that are necessary to create a continuous supply of Kirtland’s warbler breeding habitat.
•Kirtland’s warbler occupancy did not increase with increasing stem density.•Kirtland’s warbler occupancy was influenced by stand age and habitat connectivity.•Plantations that had aged out of breeding habitat showed no evidence of stagnation.•Financial returns are unlikely to ever recoup the costs of pre-commercial thinning.
Multiple land-management agencies have implemented a management program in which ca. 1,550 hectares of high-density jack pine (Pinus banksiana) plantations are established annually to provide a ...continuous supply of breeding habitat for the conservation-reliant Kirtland’s warbler (KW; Setophaga kirtlandii). These KW habitat plantations are established at a much higher density than a traditional forestry plantation, resulting in increased competition that results in slower individual tree growth and delayed attainment of merchantable size. Pre-commercial thinning (PCT) after stands age out of KW breeding habitat is an option to reduce densities and increase individual tree vigor and growth for the remaining years of the rotation. I initiated a small plot-level thinning experiment to assess the growth response to PCT of high-density jack pine plantations planted for KW habitat. In addition, two stands of KW jack pine plantations were put out for commercial bid to assess the costs and residual stand characteristics of operational scale PCT. The increase in diameter growth response was 16% for the 11-14 year age class, 30% for the 19-26 year age class, and 55% for the 27-35 year age class. After 25 years of age, there was a sharp drop in live crown ratio (LCR), indicating that increased competition for light results in lifting of the live crowns. Therefore, thinning as soon as possible after KW occupancy ends, and before 25 years when LCRs drop rapidly, would have the greatest impact on growth of the residual trees. Under my most optimistic scenario (2% rate of return), PCT would need to increase final harvest volume by 139% for mechanical thinning and 233% for hand thinning to break-even financially. I conclude that PCT does not appear to be financially feasible at any rate of return. Even though PCT of KW plantations may never break-even financially, the practice may be justified on public lands if it can accelerate attainment of marketability while increasing resilience to climatic warming.
