NEIGHBORHOOD APPROACH FOR QUANTIFYING INTERSPECIFIC COMPETITION IN COASTAL OREGON FORESTS

Abstract

We examine use of the neighborhood approach for quantifying interspecific competition around Douglas-fir seedlings that were planted in early-successional forest vegetation. We manipulated woody and herbaceous vegetation in the neighborhoods of >685 trees on four sites in the Oregon Coast Range for five years. We sought the best neighborhood expression of interspecific competition by systematically examining the influence of various measures of woody-neighbor abundance, height, distance, and spatial arrangement in regression models that predict the annual basal area and height growth of Douglas-fir. We identified the optimum neighborhood height and radius for each year. Visual estimates of shrub species cover provided the best assessment of neighbor abundance. Stratifying the neighborhood by the height of shrubs relative to the height of the Douglas-fir improved regression models and quantified patterns of competitive asymmetry. Differences in the response of Douglas-fir height and basal area to neighborhood height suggested that neighbor influences on height growth were asymmetric, while effects on basal area growth were symmetric. Shrub neighbors whose crowns intermingled with that of the Douglas-fir had the greatest influence on Douglas-fir growth and defined the optimum neighborhood radius. The competitive influence of neighboring shrubs decreased rapidly as their distance increased beyond the optimum neighborhood radius. Accounting for neighbor spatial arrangement using two measures of angular dispersion did not improve the models. The best neighborhood dimensions for Douglas-fir seedlings changed with time, apparently with the dynamics of vertical structure in the plant community. The neighborhood approach was effective for (1) evaluating various approaches forest managers might use to quantify competing vegetation, (2) examining the dynamics of competitive effects, (3) quantifying asymmeteric competition, (4) determining the relative competitive effects of woody and herbaceous vegetation, and (5) documenting the differing response of tree height and basal area growth to neighboring plants.