Summary• Concentrations of atmospheric CO 2 and tropospheric ozone (O 3 ) are rising concurrently in the atmosphere, with potentially antagonistic effects on forest net primary production (NPP) and implications for terrestrial carbon sequestration.• Using free-air CO 2 enrichment (FACE) technology, we exposed north-temperate forest communities to concentrations of CO 2 and O 3 predicted for the year 2050 for the first 7 yr of stand development. Site-specific allometric equations were applied to annual nondestructive growth measurements to estimate above-and below-ground biomass and NPP for each year of the experiment.• Relative to the control, elevated CO 2 increased total biomass 25, 45 and 60% in the aspen, aspen-birch and aspen-maple communities, respectively. Tropospheric O 3 caused 23, 13 and 14% reductions in total biomass relative to the control in the respective communities. Combined fumigation resulted in total biomass response of − 7.8, +8.4 and +24.3% relative to the control in the aspen, aspen-birch and aspensugar maple communities, respectively.• These results indicate that exposure to even moderate levels of O 3 significantly reduce the capacity of NPP to respond to elevated CO 2 in some forests.

Three model communities of trembling aspen (monoculture, and mixed with either paper birch or sugar maple) were grown for seven years in elevated atmospheric CO 2 and O 3 using Free Air CO 2 Enrichment (FACE) technology. We utilized trends in species' importance, calculated as an index of volume growth and survival, as indications of shifting community composition. For the pure aspen communities, different clones emerged as having the highest change in relative importance values depending on the pollutant exposure. In the control and elevated CO 2 treatments, clone 42E was rapidly becoming the most successful clone while under elevated O 3 , clone 8 L emerged as the dominant clone. In fact, growth of clone 8 L was greater in the elevated O 3 treatment compared to controls. For the mixed aspen-birch community, importance of aspen and birch changed by -16% and + 62%, respectively, in the controls. In the treatments, however, importance of aspen and birch changed by -27% and + 87%, respectively, in elevated O 3 , and by -10% and + 45%, respectively, in elevated CO 2 . Thus, the presence of elevated O 3 hastened conversion of stands to paper birch, whereas the presence of elevated CO 2 delayed it. Relative importance of aspen and maple changed by -2% and + 3%, respectively, after seven years in the control treatments. But in elevated O 3 , relative importance of aspen and maple changed by -2% and + 5%, respectively, and in elevated CO 2 by + 9 and -20%, respectively. Thus, elevated O 3 slightly increases the rate of conversion of aspen stands to sugar maple, but maple is placed at a competitive disadvantage to aspen under elevated CO 2 .

We analyzed growth data from model aspen (Populus tremuloides Michx.) forest ecosystems grown in elevated atmospheric carbon dioxide (

No abstract

Dispersal determines the flux of individuals, energy and information and is therefore a key determinant of ecological and evolutionary dynamics. Yet, it remains difficult to quantify its importance relative to other factors. This is particularly true in cyclic populations in which demography, drift and dispersal contribute to spatio-temporal variability in genetic structure. Improved understanding of how dispersal influences spatial genetic structure is needed to disentangle the multiple processes that give rise to spatial synchrony in irruptive species. In this study, we examined spatial genetic structure in an economically important irruptive forest insect, the spruce budworm (Choristoneura fumiferana) to better characterize how dispersal, demography and ecological context interact to influence spatial synchrony in a localized outbreak. We characterized spatial variation in microsatellite allele frequencies using 231 individuals and seven geographic locations. We show that (i) gene flow among populations is likely very high (Fst  ≈ 0); (ii) despite an overall low level of genetic structure, important differences exist between adult (moth) and juvenile (larvae) life stages; and (iii) the localized outbreak is the likely source of moths captured elsewhere in our study area. This study demonstrates the potential of using molecular methods to distinguish residents from migrants and for understanding how dispersal contributes to spatial synchronization. In irruptive populations, the strength of genetic structure depends on the timing of data collection (e.g. trough vs. peak), location and dispersal. Taking into account this ecological context allows us to make more general characterizations of how dispersal can affect spatial synchrony in irruptive populations.