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...

Nitrogen inputs, fluxes, internal generation and consumption, and outputs were monitored in a subalpine spruce-fir forest at approximately 1000-m elevation on Whiteface Mountain in the Adirondacks of New York, USA. Nitrogen in precipitation, cloudwater and dry deposition was collected on an event basis and quantified as an input. Throughfall, stemflow, litterfall and soil water were measured to determine fluxes within the forest. Nitrogen mineralization in the forest floor was estimated to determine internal sources of available N. Lower mineral horizon soil water was used to estimate output from the ecosystem. Vegetation and soil N pools were determined.During four years of continuous monitoring, an average of 16 kg N ha-' yr-' was delivered to the forest canopy as precipitation, cloudwater and dry deposition from the atmosphere. Approximately 30% of the input was retained by the canopy. Canopy retention is likely the result of both foliar uptake and immobilization by bark, foliage and microorganisms. Approximately 40 kg of N was made available within the forest floor from mineralization of organic matter. Virtually all the available ammonium (mineralized plus input from throughfall) is utilized in the forest floor, either by microorganisms or through uptake by vegetation. The most abundant N component of soil water solutions leaving the system was nitrate. Net ecosystem fluxes indicate accumulation of both ammonium and nitrate. There is a small net loss of organic N from the ecosystem. Some nitrate leaves the bottom of the B horizon throughout the year. Comparisons with other temperate coniferous sites and examination of the ecosystem N mass balance indicate that N use efficiency is less at our site, which suggests that the site is not severely limited by N. * One error in the original equation was corrected (T. G. Siccama, Yale Univ., pers. comm.).

We mapped the occurrence of canopy gaps periodically between 1978 and 1990 in a 1.5 ha study plot within a 70-year-old (in 1978) Neotropical forest on Barro Colorado Island (BCI), Republic of Panama. The total area of the forest under canopy gaps in the plot averaged 4.3% (3.1% to 5.7%, 95% CI). There was high year-to-year variability in the rate of new gap formation. On the basis of repeated observations for four yearly intervals, the annual rate of new gap formation ranged from 0.45% y−1 to 6.5% y−1. Most gaps were small. The mean size of individual gaps originally was 79 m2 (range: 8-604 m2). However, large gaps (≥150 m2) occurred more frequently than expected for a secondary forest on BCI. Gaps closed rapidly the first year after formation but the rate of closure slowed thereafter. Despite the absence of any obvious environmental gradients, gaps were spatially clustered. Even in this relatively small plot, there seemed to be distinct gap-prone and gap-free areas.

Range shifts of infectious plant disease are expected under climate change. As plant diseases move, emergent abiotic-biotic interactions are predicted to modify their distributions, leading to unexpected changes in disease risk. Evidence of these complex range shifts due to climate change, however, remains largely speculative. Here, we combine a long-term study of the infectious tree disease, white pine blister rust, with a six-year field assessment of drought-disease interactions in the southern Sierra Nevada. We find that climate change between 1996 and 2016 moved the climate optimum of the disease into higher elevations. The nonlinear climate change-disease relationship contributed to an estimated 5.5 (4.4–6.6) percentage points (p.p.) decline in disease prevalence in arid regions and an estimated 6.8 (5.8–7.9) p.p. increase in colder regions. Though climate change likely expanded the suitable area for blister rust by 777.9 (1.0–1392.9) km2 into previously inhospitable regions, the combination of host-pathogen and drought-disease interactions contributed to a substantial decrease (32.79%) in mean disease prevalence between surveys. Specifically, declining alternate host abundance suppressed infection probabilities at high elevations, even as climatic conditions became more suitable. Further, drought-disease interactions varied in strength and direction across an aridity gradient—likely decreasing infection risk at low elevations while simultaneously increasing infection risk at high elevations. These results highlight the critical role of aridity in modifying host-pathogen-drought interactions. Variation in aridity across topographic gradients can strongly mediate plant disease range shifts in response to climate change.