Decision support for managing mixed species riparian forests in southeast Alaska

Abstract

Riparian zones are important ecotones, integrating terrestrial and aquatic functions that support ecosystem productivity. A key ecological role of riparian zones is the production of large conifers that eventually enter the stream and provide important habitat complexity within the freshwater ecosystem. Many riparian zones in south-east Alaska were heavily disturbed during logging operations from 1950 to 1980, which depleted the large wood supply to streams. Abundant conifer and red alder regeneration was common after harvest, replenishing a young population of trees, but the presence of red alder has been perceived to inhibit conifer stand development and large wood recruitment to streams. Therefore, alder thinning has been used to accelerate conifer growth and address the large wood deficit in streams. However, the trajectory of stand growth in these mixed species riparian stands is complex, and the response to alder or conifer thinning has not been thoroughly documented since riparian restoration projects have been implemented throughout southeast Alaska. Further, few protocols exist for evaluating the progress and success of restoration actions. To address this need, we measured species abundance, tree density, light, and growth history on dominant landforms associated with varying stream geomorphological classes in 13 postharvest riparian stands. The presence of alder did not negatively affect conifer growth across a range of stand and landform conditions. Thinning conifers on terrace landforms increased the annual stand growth increment where conifers dominated. In the absence of thinning, equal stocking of alder and coni-fer did not affect conifer growth increment on terraces in mixed stands. We found that the density to tree size ratio (trees per hectare to quadratic mean diameter) is a useful metric to monitor the progress of stands toward desired late-seral conditions. These results indicate that mixed species stands do not impede conifer growth and provide valuable riparian nutrient inputs, improved light, and biological diversity.

Citation

D’Amore, David V.; Nay, S. Mark; Edwards, Rick; Harris, Scott H. 2026. Decision support for managing mixed species riparian forests in southeast Alaska. Gen. Tech. Rep. PNW-GTR-1040. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 25 p. https://doi.org/10.2737/pnw-gtr-1040.