Competition vs. Facilitation of Tree Seedling Growth and Survival in Early Successional Communities

Abstract

Competition has been widely assumed to be one of the principal mechanisms underlying the resistance of shrub and herbaceous communities to invasion by trees. However, there are potential mechanisms by which low—growing species, particularly in physically stressful sites, could enhance growth or survival of tree seedlings (facilitation). The balance of inhibition and facilitation will determine the net effects of a community on tree seedling growth and survival. We conducted a large—scale field experiment to quantify the net effects of four major physiognomic types (shrub tickets, shrubby grass meadows, grass meadows, and herbaceous meadows) commonly found both in old fields and along utility rights—of—way on the growth and survival of three common tree species: sugar maple (Acer saccharum), red maple (Acer rubrum), and gray birch (Betula populifolia). The eight community types were distributed in 23 sites representative of the range of upland environments present in the Hudson Valley of New York. Growth of planted seedlings of all three tree species was uniformly slow in all of the community types and environments. In all cases, the net effect of intact vegetation was to inhibit the growth of the tree seedlings. The intensity of competition varied substantially among the 23 sites but was not consistently related either to community type or to the biomass of the intact community. Instead, variation in the intensity of competition was related to the underlying favorability of the site for the growth of a particular target tree species. On the physically unfavorable sites, tree seedlings grew slowly because of physical stress rather than competition with the intact vegetation. As site quality increased, the intensity of competition increased. Thus, competition and physical stress traded off along a site–quality gradient, with the result that tree seedling growth was uniformly slow. In contrast to the results for growth, there were cases in which the survival of these young tree seedlings was enhanced (facilitated) by the presence of intact vegetation. In particular, survival of the highly shade tolerant but drought intolerant sugar maple seedlings was facilitated by intact vegetation at many sites, especially for the 1988 cohort, which experienced a drought during its first growing season. Since we detected no consistent differences among communities in the intensity of competitive effects on seedling growth, the apparent differences among early successional communities in resistance to tree seedling establishment may be caused by variation in the duration of competition resulting from differences in height and canopy structure of the low—growing communities.

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