COMPETITION VS FACILITATION OF TREE SEEDLING GROWTH AND SURVIVAL IN EARLY SUCCESSIONAL COMMUNITIES

Competition has been widely assumed to be one of the principal mechani sms underlying the resistance of shrub and herbaceous communities to i nvasion by trees. However, there are potential mechanisms by which low -growing species, particularly in physically stressful sites, could en hance growth or survival of tree seedlings (facilitation). The balance of inhibition and facilitation will determine the net effects of a co mmunity on tree seedling growth and survival. We conducted a large-sca le field experiment to quantify the net effects of four major physiogn omic types (shrub thickets, shrubby grass meadows, grass meadows, and herbaceous meadows) commonly found both in old fields and along utilit y rights-of-way on the growth and survival of three common tree specie s: sugar maple (Acer saccharum), red maple (Acer rubrum), and gray bir ch (Betula populifolia). The eight community types were distributed in 23 sites representative of the range of upland environments present i n the Hudson Valley of New York. Growth of planted seedlings of all th ree 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 competitio n varied substantially among the 23 sites but was not consistently rel ated either to community type or to the biomass of the intact communit y. Instead, variation in the intensity of competition was related to t he underlying favorability of the site for the growth of a particular target tree species. On physically unfavorable sites, tree seedlings g rew slowly because of physical stress rather than competition with the intact vegetation. As site quality increased, the intensity of compet ition increased. Thus, competition and physical stress traded off alon g a site-quality gradient, with the result that tree seedling growth w as uniformly slow. In contrast to the results for growth, there were c ases in which the survival of these young tree seedlings was enhanced (facilitated) by the presence of intact vegetation. In particular, sur vival of the highly shade tolerant but drought intolerant sugar maple seedlings was facilitated by intact vegetation at many sites, especial ly for the 1988 cohort, which experienced a drought during its first g rowing season. Since we detected no consistent differences among commu nities in the intensity of competitive effects on seedling growth, the apparent differences among early successional communities in resistan ce to tree seedling establishment may be caused by Variation in the du ration of competition resulting from differences in height and canopy structure of the low-growing communities.