Globally, grasslands are threatened by the invasion of woody plants. In many grasslands, vigorous growth of mature trees suggests that invasion is regulated not by constraints on adult growth, but by factors that limit establishment. Predicting and managing invasion thus requires an understanding of the factors that influence tree seedling establishment and growth. While top-down effects such as fire, climate, and grazing are often used to explain the dynamics of woody plant invasions, biotic interactions can also be important. Yet, few studies have considered the potential for complex interactions among multiple factors, thus limiting our ability to predict the conditions under which invasions will occur.
We examined the role of established vegetation in regulating the invasion of the conifer Pseudotsuga menziesii in western Washington prairies. Using a fully factorial randomized block design we tested the influence of aboveground competition (yes/no, i.e. with or without 80% shade cloth), belowground competition by herbaceous vegetation (yes/no, achieved via application of glyphosate), and soil origin (forest vs. prairie, achieved using reciprocal soil transfers) on the germination, survival, and first-year growth of sown seeds (n = 5,850) and 1-yr-old planted seedlings (n = 585) of Pseudotsuga menziesii (Douglas-fir) at three prairie sites in western Washington.
Results/Conclusions
We found strong, hierarchical effects among the factors tested. Shading was the primary influence, resulting in strong positive effects on germination (5.5% shaded vs. 0.5% unshaded) and survival (70% shaded vs. 12% unshaded). Belowground competition and forest soil origin had significant negative secondary and tertiary influences. Among surviving seedlings, belowground competition had a significant negative effect on diameter and height growth. In contrast to effects on germination and survival, shading had a negative influence on stem diameter but a positive influence on height. Positive effects of forest (vs. prairie) soil were small; nevertheless, of the top 10% of seedlings by diameter growth, most (66%) were grown in forest soil.
Our results demonstrate that tree establishment may be regulated by complex biotic interactions that vary in sign and magnitude, reflecting a combination of resource competition, amelioration of environmental stress, and differences in the biological or chemical properties of soils. These interactions are critical to understanding and predicting future invasions.
