RESPONSES OF EARLY SUCCESSIONAL NORTHERN HARDWOOD FORESTS TO CHANGES IN NUTRIENT AVAILABILITY

In many mesic forests the dominant trees are limited concurrently by l ight and soil resources, and understanding the mechanisms of competiti on and predicting outcomes of competition are especially difficult whe n co-limitation exists. We altered soil resource availability during t he early stages of stand development after clearcutting of northern ha rdwood forests to examine the mechanism of competition. Specifically, we sought empirical evidence about the role of various physiological, morphological, allocational, and architectural responses in regulating plant competition. We expected the competitive ability of the extreme pioneer species, Prunus pensylvanica (pin cherry), to be enhanced by increased nutrient supply, with consequent effects at the community an d ecosystem levels of organization. Nutrient availability was increase d by about three-fold by monthly fertilization for 6 yr in nine even-a ged northern hardwood stands dominated by pin cherry, three each of th ree ages (6, 12, and 18 yr at initiation of the experiment). Measureme nts in the control plots indicated that the interval of stand developm ent from age 6 to 23 yr was marked by a peak in basal area and leaf ar ea of pin cherry at about age 17 yr, followed by a steady decline in P . pensylvanica dominance thereafter. Fertilization increased and prolo nged the dominance of P. pensylvanica, indicating that nutrient limita tion accelerates the demise of this species during the second and thir d decades of stand development. All species in the plots responded to fertilization with increased foliar nutrient (N, P, and K) concentrati ons and often higher specific leaf area (area:mass ratio), and these r esponses were most pronounced for P. pensylvanica. Although the light response curve for photosynthesis of P. pensylvanica was altered by fe rtilization, with higher rates at low light levels, photosynthesis of its principal competitor, Betula papyrifera, was not affected. The mar ked growth response of P. pensylvanica was accompanied by changes in i ts canopy architecture, as the trees had more leaf area per unit stem basal area, and proportionally more of this leaf area was in the upper canopy. In contrast, height and leaf area of B. papyrifera were simil ar in the control and fertilized plots. Seed deposition of P. pensylva nica also increased in the fertilized plots during one year of high se ed production. Thus, the performance in competition of P. pensylvanica was improved by the removal of apparent nutrient limitations on its p hysiological performance, canopy growth, and ability to compete for li ght.Leaf area index of the fertilized plots was only slightly higher t han the control plots, and the same was true for stand basal area. The removal of nutrient limitation increased the intensity of one-sided c ompetition for light by concentrating the dominance among the largest trees; consequently, very high mortality of suppressed stems of all sp ecies occurred. The increased dominance of the fast-growing P. pensylv anica contributed to increases in aboveground net primary productivity (ANPP) in the fertilized plots. Some of this ANPP response was probab ly associated with reduced C allocation to roots in some of the fertil ized plots, but this pattern was not consistent across all the stands. The results indicate that the outcome of interspecific competition in mesic forests, where co-limitation by light and soil resources prevai ls, depends upon the effect of site quality upon the relative intensit y of one-sided competition (for light).