Density effects on growth and dominance in pure and mixed stands of red alder and western hemlock in western Oregon

Abstract

Maintaining diversity of conifer and hardwood species in riparian and upland areas is of increasing interest to forest managers. For example, having conifer and hardwood inputs into streams increases structural diversity and influences assemblages of aquatic and terrestrial organisms, and the presence of both components in stands may be beneficial in resiliency after disturbances. To examine the impacts of density and weeding on the establishment of red alder (Alnus rubra Bong.) and western hemlock (Tsuga heterophylla (Raf.) Sarg.), combination Nelder and replacement series plots were established on three sites in western Oregon. Species ratios of hemlock:alder were 100:0, 75:25, 50:50, 25:75, and 0:100, with densities ranging from 988 to 85,400 tph. Measurements were available for two sites after 24 years, and one site for 14 years. Weeding increased heights and diameters at lower densities, especially in the early years. As crowns closed, the impacts of weeding decreased through time, but were still evident in some plots after 24 years. Increasing density of red alder resulted in lower survival and growth of both species. Hemlock density did not impact red alder growth through 24 years. Although mixed stands resulted in greater volume in early years, that did not continue in later years. Greater juvenile growth rates of red alder compared to western hemlock resulted in dominance of alder over hemlock in early years, especially at higher densities and ratios of alder. Hemlock growing with lower alder density have maintained co-dominance with alder and will become dominant as alder growth rates decline. Alder density needs to be low enough for hemlock to survive and grow. Density, site, species ratio, animal damage, and presence of other competing vegetation resulted in different scenarios of hemlock and alder dominance, making long-term outcomes difficult to predict with precision.