One of the most prevalent ecological interactions is competition for limited resources. I recently completed an extensive field experiment in which I tested the hypothesis that on a per unit size basis, species may be competitively equivalent (in terms of competitive effect). This idea was most clearly presented as a hypothesis in a 1981 American Journal of Botany paper by Goldberg & Werner. My experiment established plots of neighbor plants around a target plant, and used growth of the target plants as a measure of competitive effect of the neighbors. Each plot had neighbor densities of one of the following levels: 0, 8, 16, 24, or 32, and neighbors (all of the same species in a given plot) of either Betula lenta or Fagus grandifolia. Target plants were either B. lenta, F. grandifolia, or Acer pensylvanicum. Preliminary analyses show that all three species of target plants experienced statistically significant reductions in growth with increases in density of neighbors. However, intraspecific competition was NOT necessarily more intense than inter-specific, and effects of the two neighbor species on growth of Fagus grandifolia were statistically equivalent, on a per-unit biomass basis.
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This figure shows the effect of increasing biomass of neighbor plants on the growth (diameter increase) of Fagus target plants. The different colors of lines and points represent different species of neighbors: Fagus or Betula. The key point is that the slopes of the two lines do not differ, indicating that the per-unit-biomass effect of these two neighbor species on the growth of Fagus is not different, i.e. the neighbors have competitively equivalent effects. |
Moreover, the reduction in growth with increasing neighbor density did not differ statistically among the three species of target plants. The results of this experiment are still in the final stages of analysis, and a manuscript will be submitted for publication later this year.
In a separate study, I examined the growth of selected trees in an aspen-white pine forest in northern Michigan, as influenced by size and distance of neighboring trees. This forest was successional, with initially-dominant aspen (Populus tremuloides and Populus grandidentata) being gradually replaced by white pine (Pinus strobus) that has colonized beneath the aspen canopy. Pine saplings showed pronounced (and significant) spatial segregation from the aspen canopy trees, but the growth of those few pines nearest the aspens was unexpectedly greater than the pines clustered in areas furthest from the aspens. Strong intraspecific competition among the pines apparently caused the greater reduction in growth than did interspecific competition with aspens. The asymmetry of this competition effect was less than that reported in the few other studies that have attempted to quantify asymmetry of competitive effects. Perhaps the most intriguing finding was that the apparent asymmetry decreased with increasing distance between trees, suggesting that competition with immediate neighbors is more asymmetric than that with more distant neighbors. These findings are presented in Peterson & Squiers 1995a (abstract here).