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Mycorrhizal associations are plant-fungal mutualisms that are fairly ubiquitous and likely evolved multiple times in phylogenic history; however, some plant families have consistently been identified as non-mycorrhizal, including the Brassicaceae. In this paper, we reviewed the literature and DNA databases for potential mechanisms that preclude mycorrhizal symbioses in the Brassicaceae and for exceptions to the general observation of non-mycorrhizal status within this plant family. In instances of association between members of the Brassicaceae and arbuscular mycorrhizal fungi we posed hypotheses for why these interactions occur in the species and sites observed. Instances of inconsistent association with mycorrhizal fungi were attributed to inter- and intraspecific variations in plant biology, disagreements in vernacular, methodology contradicting historical mycorrhizal surveys, and association being a facultative, variable trait that is determined by species-site interactions. We propose further research on a) the extent of mycorrhizal association in the Brassicaceae, b) the molecular mechanisms dictating association, and c) whether Brassicaceae-mycorrhizal fungal interactions result in nutrient transfer, and their particular roles in the family's distribution across heterogeneous and harsh environments. [ABSTRACT FROM AUTHOR]

Climate models project that by 2100, the northeastern US and eastern Canada will warm by approximately 35 °C, with increased winter precipitation. These changes will affect trees directly and also indirectly through effects on 'nuisance' species, such as insect pests, pathogens, and invasive plants. We review how basic ecological principles can be used to predict nuisance species' responses to climate change and how this is likely to impact northeastern forests. We then examine in detail the potential responses of two pest species (hemlock woolly adelgid (Adelges tsugae Annand) and forest tent caterpillar (Malacosoma disstria Hubner)), two pathogens (armillaria root rot (Armillaria spp.) and beech bark disease (Cryptococcus fagisuga Lind. + Neonectria spp.)), and two invasive plant species (glossy buckthorn (Frangula alms Mill.) and oriental bittersweet (Celastrus orbiculatus Thunb.)). Several of these species are likely to have stronger or more widespread effects on forest composition and structure under the projected climate. However, uncertainty pervades our predictions because we lack adequate data on the species and because some species depend on complex, incompletely understood, unstable relationships. While targeted research will increase our confidence in making predictions, some certainty will always persist. Therefore, we encourage policies that allow for this uncertainty by considering a avide range of possible scenarios. Les modeles climatiques prevoient des temperatures 3-5°C plus elevees et des precipitations hivernales plus abondantes vers 2100 dans le nord-est des Etats-Unis et l'est du Canada. Ces changements affecteront les arbres directement et indirectement via leurs effets sur des especes nuisibles telles que les insectes ravageurs, les agents pathogenes et les plantes invasives. Nous passons en revue la facon dont les principes ecologiques peuvent etre utilises pour predire la reaction des especes nuisibles aux changements climatiques et la facon dont cela pourrait affecter les forets du nordest. Nous etudions ensuite en detail les reactions potentielles de deux insectes ravageurs : le puceron lanigere de la pruche (Adelges tsugae Armand) et la livree des forets (Malacosoma disstria Hunner), deux agents pathogenes : le pourridie agaric (Armillaria spp.) et la maladie corticale du hetre (Cryptococcus fagisuga Lind. + Neonectria spp.) et deux especes de plantes invasives : le nerprun bourdaine (Frangula alnus Mill.) et la celastre asiatique (Celastrus orbiculatus Thunb.). Plusieurs de ces especes auront probablement des effets plus prononces ou plus etendus sur la composition et la structure de la foret dans les conditions climatiques anticipees. Cependant, nos predictions demeurent incertaines a cause du manque de donnees adequates sur ces especes et parce que certaines especes dependent de relations complexes, peu connues et instables. La recherche orientee nous permettra de faire des predictions avec une plus grande confiance, mais il restera toujours une certaine incertitude. Par consequent, nous encourageons les politiques qui tiennent compte de cette incertitude en considerant une vaste gamme de scenarios potentiels. [Traduit par la Redaction], Introduction In northeastern North America, forests host a variety of native and introduced insect pests, invasive plant species, and pathogens (which we collectively term 'nuisance species') that currently affect forest [...]

PREMISE OF THE STUDY: Many plants increase reproduction in response to rising levels of atmospheric CO2. However, environmental and genetic variation across heterogeneous landscapes can lead to intraspecific differences in the partitioning of CO2-induced carbon gains to reproductive tissue relative to growth. METHODS: We measured the effects of rising atmospheric CO2 on biomass allocation in the allergenic plant Ambrosia artemisiifolia (common ragweed) across a geographic climate gradient. We grew plants from three latitudes at 400, 600, and 800 µL·L−1 CO2 and analyzed biomass allocation and natural selection on flowering phenology and growth. KEY RESULTS: Both the latitude of origin and CO2 treatment had significant effects on allocation and on estimates of selection. Northern plants were under stronger selection than southern plants to flower quickly, and they produced larger seeds and more reproductive mass per unit of growth. Northern plants were under stronger selection than southern plants to flower quickly, and they produced larger seeds and more reproductive mass per unit of growth. While all plants grew larger and produced heavier seeds at higher CO2, only northern plants increased male flower production. Both size and time to flowering were under selection, with a relaxation of the size–fitness function in northern ecotypes at high CO2. CONCLUSIONS: Northern ecotypes allocate more CO2-induced carbon gains to reproduction than do southern plants, pointing to a geographic gradient in future pollen and seed production by this species arising from local adaptation. Relaxed selection on size at elevated CO2 could amplify reproductive enhancements to northern ecotypes, although more growth and seed provisioning can be expected overall. Our results demonstrate potential for ecotypic divergence in ragweed responses to climate change. [ABSTRACT FROM AUTHOR]

Garlic mustard (Alliaria petiolata) is a nonnative, shade-tolerant forb that was introduced into North America in the mid-1800s. Currently, garlic mustard is spreading across the landscape at a rate of 6400 square kilometers per year. In this article, we synthesize the current state of knowledge on the mechanisms underlying garlic mustard's widespread success and the ecological impacts of its invasion. Although no single mechanism appears to explain the success of garlic mustard, a combination of plant traits--all slightly different from those of native plants--seems to confer garlic mustard with tremendous success in the new habitats it invades. The domination of this new species in eastern forests is clearly changing the ecology of these systems. The consequences of garlic mustard invasion include the loss of biological diversity, ripple effects through higher trophic levels, and changes in the function of soil microbial communities. [ABSTRACT FROM AUTHOR]

Testing whether and how subordinate individuals differ from dominants in the utilization of enriched CO2 atmospheres is important for understanding future stand and community structure. We hypothesized that subordinate and dominant Ambrosia artemisiifolia L. (Asteraceae) (common ragweed) plants growing in dense stands would not equally acquire or utilize carbon gains from CO2-enrichment, and that the resulting disproportionate growth gains to subordinates would reduce size inequalities in competing stands. We grew experimental stands of A. artemisiifolia in either ambient (360 μL L−1) or twice ambient (720 μL L−1) levels of atmospheric CO2. We compared the relative growth, photosynthetic capacity, and architecture of subordinate and dominant plants in each treatment, and assessed size inequalities using the stand-level coefficient of variation (CV). In elevated CO2, plants grew larger, but subordinate plants shifted more mass to upper stem allocation than dominants. Dominant plants demonstrated reduced leaf-level photosynthetic gains in elevated CO2 compared with subordinate plants. Reduced CVs in plant size reflected smaller proportional growth gains by dominants over subordinates in elevated vs. ambient stands. We conclude that differences in the architectural and physiological responses of subordinate and dominant ragweed plants reduce competition and allow subordinate plants to catch up to dominants in elevated CO2 conditions. [ABSTRACT FROM AUTHOR]

Sap-feeding insects such as aphids are the only insect herbivores that show positive responses to elevated CO2. Recent models predict that increased nitrogen will increase aphid population size under elevated CO2, but few experiments have tested this idea empirically. To determine whether soil nitrogen (N) availability modifies aphid responses to elevated CO2, we tested the performance of Macrosiphum euphorbiae feeding on two host plants; a C3 plant ( Solanum dulcamara), and a C4 plant ( Amaranthus viridis). We expected aphid population size to increase on plants in elevated CO2, with the degree of increase depending on the N availability. We found a significant CO2× N interaction for the response of population size for M. euphorbiae feeding on S. dulcamara: aphids feeding on plants grown in ambient CO2, low N conditions increased in response to either high N availability or elevated CO2. No population size responses were observed for aphids infesting A. viridis. Elevated CO2 increased plant biomass, specific leaf weight, and C : N ratios of the C3 plant, S. dulcamara but did not affect the C4 plant, A. viridis. Increased N fertilization significantly increased plant biomass, leaf area, and the weight : height ratio in both experiments. Elevated CO2 decreased leaf N in S. dulcamara and had no effect on A. viridis, while higher N availability increased leaf N in A. viridis and had no effect in S. dulcamara. Aphid infestation only affected the weight : height ratio of S. dulcamara. We only observed an increase in aphid population size in response to elevated CO2 or increased N availability for aphids feeding on S. dulcamara grown under low N conditions. There appears to be a maximum population growth rate that M. euphorbiae aphids can attain, and we suggest that this response is because of intrinsic limits on development time and fecundity. [ABSTRACT FROM AUTHOR]

The impact of exotic species on native organisms is widely acknowledged, but poorly understood. Very few studies have empirically investigated how invading plants may alter delicate ecological interactions among resident species in the invaded range. We present novel evidence that antifungal phytochemistry of the invasive plant, Alliaria petiolata, a European invader of North American forests, suppresses native plant growth by disrupting mutualistic associations between native canopy tree seedlings and belowground arbuscular mycorrhizal fungi. Our results elucidate an indirect mechanism by which invasive plants can impact native flora, and may help explain how this plant successfully invades relatively undisturbed forest habitat., Other Research Unit