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

Abstract

In many mesic forests the dominant trees are limited concurrently by light and soil resources, and understanding the mechanisms of competition and predicting outcomes of competition are especially difficult when co-limitation exists. We altered soil resource availability during the early stages of stand development after clearcutting of northern hardwood 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 and ecosystem levels of organization. Nutrient availability was increased by about three-fold by monthly fertilization for 6 yr in nine even-aged northern hardwood stands dominated by pin cherry, three each of three ages (6, 12, and 18 yr at initiation of the experiment). Measurements in the control plots indicated that the interval of stand development from age 6 to 23 yr was marked by a peak in basal area and leaf area of pin cherry at about age 17 yr, followed by a steady decline in P. pensylvanica dominance thereafter. Fertilization increased and prolonged the dominance of P. pensylvanica, indicating that nutrient limitation accelerates the demise of this species during the second and third decades of stand development. All species in the plots responded to fertilization with increased foliar nutrient (N, P, and K) concentrations and often higher specific leaf area (area:mass ratio), and these responses were most pronounced for P. pensylvanica. Although the light-response curve for photosynthesis of P. pensylvanica was altered by fertilization, with higher rates at low light levels, photosynthesis of its principal competitor, Betula papyrifera, was not affected. The marked growth response of P. pensylvanica was accompanied by changes in its 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 similar in the control and fertilized plots. Seed deposition of P. pensylvanica also increased in the fertilized plots during one year of high seed production. Thus, the performance in competition of P. pensylvanica was improved by the removal of apparent nutrient limitations on its physiological performance, canopy growth, and ability to compete for light. Leaf area index of the fertilized plots was only slightly higher than the control plots, and the same was true for stand basal area. The removal of nutrient limitation increased the intensity of one-sided competition for light by concentrating the dominance among the largest trees; consequently, very high mortality of suppressed stems of all species occurred. The increased dominance of the fast-growing P. pensylvanica contributed to increases in aboveground net primary productivity (ANPP) in the fertilized plots. Some of this ANPP response was probably associated with reduced C allocation to roots in some of the fertilized 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 prevails, depends upon the effect of site quality upon the relative intensity of one-sided competition (for light).