Impact of Gap Size on Environmental Conditions, Survival, and Early Growth of 5 Pacific Northwest Species

Abstract

The formation of forest gaps and their resulting size and shape is highly variable across forest types and can produce a wide range of patterns in seedling establishment. The impact of gap size on microclimate and seedling growth and survival after planting, specifically in mixed-species stands is poorly studied. This study examines the influence of gap size on the light environment, soil moisture distribution, and understory vegetation dynamics and in turn, how these environmental characteristics effect seedling survival, health, and growth of 5 Pacific Northwest native tree species with varying shade and drought tolerances. More than 5000 seedlings were planted in groups of 5 individuals, both monoculture and mixed-species pentads, across 16 clear-cut plots of 4 sizes (0.05, 0.1, 0.25, and 0.5 ha). Larger gaps were found to have higher light and lower vegetation cover, while gap size had no influence on soil moisture. Initial mortality of conifer species was largely determined by high soil moisture in dense, clay soils, specifically in the most downslope row of gaps. Douglas-fir (26.8%) and western hemlock (27.9%) mortality was significantly higher than western redcedar (5.0%), red alder (6.9%), and grand fir (8.1%) mortality. Chlorophyll fluorescence tracked over summer drought months suggested that as soils became drier, seedling health improved for all species. Environmental predictors of growth differed by species but were consistently influenced by abiotic factors soil moisture and light as well as biotic factors vegetation and ungulate browse. Increasing soil moisture and light led to positive growth for all species except western hemlock which was not significantly impacted. Increased height of surrounding understory vegetation led to reductions in basal area increment in western redcedar (Thuja plicata), western hemlock (Tsuga heterophylla), Douglas-fir (Pseudotsuga menziesii), and grand fir (Abies grandis). Western hemlock and western redcedar saw increased growth as vegetation cover increased until full coverage where growth then decreased. Red alder (Alnus rubra), western redcedar, and western hemlock were significantly impacted by ungulate browse which occurred in half of western redcedar and red alder seedlings. Western redcedar saw an increase in basal area increment in response to browse while red alder and western hemlock saw reductions in height. No influence of surrounding seedling competition was found two years post planting. Overall, gap size impacts both abiotic and biotic environmental characteristics and these differing microclimates resulted in varying seedling growth and survival among species. Further research in this long-term experimental site is necessary to determine when mixed-species plantings begin interacting and how these interactions are influenced by gap size and shifts in microclimate.