Title: Consequences of elevated levels of amospheric CO2 and O3 for growth of Poplus tremuloides clones: the role of competition
Author: McDonald, Evan P.; Kruger, E. L.; Riemenschneider, Don E.; Isebrands, J. G.
Source: Air Pollution, Global Change and Forest in the New Millennium. p. 58. (2001)
Description: Concomitant increases in atmospheric CO2 and 03 are expected to impact temperate forest ecosystems. Free air CO2 and 03 enrichment (FACE) was used to examine the interactive effects of these gases on shoot growth responses of quaking aspen grown in an experimental system (FACTS-II, Aspen FACE) in northern Wisconsin. A central goal was to examine how the interactions of atmospheric treatments and genotype impact competitive relations among genotypes. Several aspen clones with differing sensitivities to CO2 and 03 were planted in randomized mixtures within half of each 30 m diameter FACE ring, with 3 replications of each combination of low and high levels of CO2 (ambient and 56 Pa) and 03 (ambient and 1.5xambient). We hypothesized that several factors, apart from atmospheric treatments, will have strong effects on individual tree growth in this system, such as genotypic potential, initial size, and competitive status. We non-destructively measured tree size and estimated annual biomass increments. A competitive index for each individual tree at the beginning of the growth interval was calculated using the ratio of its size to the average size of adjacent neighbors. We tested treatment effects using initial size as a covariate and competitive index divided into representative categories was evaluated as a class variable nested within clone. In 1998, effects of CO2,03 and their interaction were not significant, but O3xclone and O3xcompetitive index interactions were significant. In 1999, CO2xO3xcompetitive index interactions were highly significant, so we tested treatment effects within competitive index categories where individuals are competitively disadvantaged, neutral or advantaged in terms of size relative to thek neighbors. Main effects and interactions of CO2 and O3 became larger and more significant as competitive index increase. At high competitive indices, elevated CO2 increased net growth in 1999 by 29% (p<0.01), compared with an 11% (ns) increase at low competitive indices. Elevated CO2 exacerbated growth responses to elevated 03 exposure. Overall, elevated O3 decreased growth 34% in elevated CO2 compared with 24% in ambient CO2 (CO2xO3 p=0.003). Significant clonextreatment interactions were limited to competitive indices reflecting moderately suppressed conditions. Competition effects in these mixed clonal stands have emerged before canopy closure, and long-term competitive outcomes may depend on the interactive effects of atmospheric CO2 and 03 levels.
Keywords: Quakingaspen, elevated CO2, ozone, growth, competition
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McDonald, Evan P.; Kruger, E. L.; Riemenschneider, Don E.; Isebrands, J. G. 2001. Consequences of elevated levels of amospheric CO2 and O3 for growth of Poplus tremuloides clones: the role of competition. Air Pollution, Global Change and Forest in the New Millennium. p. 58. (2001)
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