Kristina A. Stinson

Loss of foundation species: consequences for the structure and dynamics of forested ecosystems

In many forested ecosystems, the architecture and functional ecology of certain tree species define forest structure and their species-specific traits control ecosystem dynamics. Such foundation tree species are declining throughout the world due to introductions and outbreaks of pests and pathogens, selective removal of individual taxa, and over-harvesting. Through a series of case studies, we show that the loss of foundation tree species changes the local environment on which a variety of other species depend; how this disrupts fundamental ecosystem processes, including rates of decomposition, nutrient fluxes, carbon sequestration, and energy flow; and dramatically alters the dynamics of associated aquatic ecosystems. Forests in which dynamics are controlled by one or a few foundation species appear to be dominated by a small number of strong interactions and may be highly susceptible to alternating between stable states following even small perturbations. The ongoing decline of many foundation species provides a set of important, albeit unfortunate, opportunities to develop the research tools, models, and metrics needed to identify foundation species, anticipate the cascade of immediate, short- and long-term changes in ecosystem structure and function that will follow from their loss, and provide options for remedial conservation and management.

Invasive Plant Suppresses the Growth of Native Tree Seedlings by Disrupting Belowground Mutualisms

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.

NOVEL WEAPONS: INVASIVE PLANT SUPPRESSES FUNGAL MUTUALISTS IN AMERICA BUT NOT IN ITS NATIVE EUROPE

Why some invasive plant species transmogrify from weak competitors at home to strong competitors abroad remains one of the most elusive questions in ecology. Some evidence suggests that disproportionately high densities of some invaders are due to the release of biochemicals that are novel, and therefore harmful, to naive organisms in their new range. So far, such evidence has been restricted to the direct phytotoxic effects of plants on other plants. Here we found that one of North Americas most aggressive invaders of undisturbed forest understories, Alliaria petiolata (garlic mustard) and a plant that inhibits mycorrhizal fungal mutualists of North American native plants, has far stronger inhibitory effects on mycorrhizas in invaded North American soils than on mycorrhizas in European soils where A. petiolata is native. This antifungal effect appears to be due to specific flavonoid fractions in A. petiolata extracts. Furthermore, we found that suppression of North American mycorrhizal fungi by A. petiolata corresponds with severe inhibition of North American plant species that rely on these fungi, whereas congeneric European plants are weakly affected. These results indicate that phytochemicals, benign to resistant mycorrhizal symbionts in the home range, may be lethal to naïve native mutualists in the introduced range and indirectly suppress the plants that rely on them.

Ready or Not, Garlic Mustard Is Moving In: Alliaria petiolata as a Member of Eastern North American Forests

ABSTRACT 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 mustards 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.

Impacts of Garlic Mustard Invasion on a Forest Understory Community

Abstract To assess the community-level responses of a New England forest to invasion by the Eurasian biennial Alliaria petiolata (garlic mustard), we conducted a vegetation census at twenty-four plots ranging from low to high invasive cover, and experimentally removed 0, 50, or 100% of garlic mustard from adjacent highly invaded plots at the same study site. Species richness did not respond to natural or experimental levels of invasion, but the Shannon diversity and equitability indices declined with increasing in situ densities of garlic mustard, and increased in response to removal of garlic mustard at the experimental plots. Individual species demonstrated variable responses to high-, intermediate-, and low-level invasion. Of all plant functional groups, tree seedlings declined most notably with increasing in situ levels of invasion. This functional group, and seedlings of three key canopy tree species within the group, increased in response to partial, but not full eradication of garlic mustard. Our results demonstrate that the effectiveness of full or partial removal depends on management priorities for promoting overall diversity, species richness, native species composition, and/or individual species performance within native communities.

Differences in arbuscular mycorrhizal fungal communities associated with sugar maple seedlings in and outside of invaded garlic mustard forest patches

Garlic mustard (Alliaria petiolata) is a Eurasian native that has become invasive in North America. The invasive success of A. petiolata has been partly attributed to its production of allelopathic compounds that can limit the growth of arbuscular mycorrhizal fungi (AMF). Although such effects are well known, specific effects on the richness and community composition of AMF associated with woody species have not been explored. We collected sugar maple (Acer saccharum) seedlings from eight natural forest sites in Ohio and Massachusetts, containing areas either invaded or uninvaded by A. petiolata. We measured AMF root colonization of seedlings, isolated DNA from the roots and performed PCR-TRFLP analysis to assess the richness and community composition of AMF. As expected, we found reduced AMF colonization in A. petiolata invaded patches. A. petiolata did not alter the detected TRF richness, but was associated with significant changes in the composition of AMF communities in half of the sites monitored in each region. Our results suggest that although AMF colonization was reduced in A. petiolata patches, many indigenous AMF communities include AMF that are tolerant to allelopathic effects of A. petiolata.

Catching up on global change: new ragweed genotypes emerge in elevated CO2 conditions

Resource uptake by neighboring plants can be an important driver of natural selection in a changing environment. As climate and resource conditions are altered, genotypes that dominate within mixed populations today may differ markedly from those in future landscapes. We tested whether and how the dominance of different genotypes of the allergenic plant, common ragweed, may change in response to projected atmospheric CO2 conditions. We grew twelve maternal lines in experimental stands at either ambient or twice-ambient levels of CO2. We then constructed a model that combines classical quantitative genetics theory with a set of a priori predictions about the relative performance of genotypes in the two treatments. Our findings show a complete reversal in the genotypic size hierarchy of ragweed plants in response to projected atmospheric CO2 conditions. Genotypes that are competitively suppressed in size at ambient levels become dominant under experimental doubling of CO2. Subordinated plants, in turn, boost their reproductive allocation to that of dominants, shrinking the fitness gap among all genotypes in high CO2. Extending our model to a contextual analysis framework, we further show that natural selection on size is reduced at elevated CO2, because an individuals position within the size hierarchy becomes less important for reproduction than it is in ambient conditions. Our work points to potential future ecological and evolutionary changes in this widespread allergenic plant.

Physiological constraints on the spread of Alliaria petiolata populations in Massachusetts

The expansion of plant species into new sites is limited by a combination of environmental conditions and the capacity for adaptive variability in trait expression. Here, we investigated whether and how adaptation to forest edge conditions might be limiting the spread of the invasive plant Alliaria petiolata (garlic mustard) into the forest interior in eastern Massachusetts. We conducted a common garden experiment to test whether plants from forest edge vs. forest interior microhabitats differ in their plasticity and physiological responses to experimental shading. All plants in the experiment responded to shading with reductions in growth, photosynthetic activity, and reproduction regardless of the source environment, indicating a high degree of plasticity and a strong likelihood that most seeds in our study populations are produced at the forest edge. Effects of the source habitat on physiological function were detectable, but small compared to the magnitude in growth and reproductive responses to light. Plants from the forest edge physiologically outperformed those from the interior when grown in the shade, but had equally low reproductive success. Plants originating in the forest interior, in contrast, demonstrated greater allocation to growth in relation to photosynthesis and reproduction compared to plants originating at the forest edge. We compare our findings to earlier work on the importance of plasticity for invasive spread in this species, and conclude that failure by garlic mustard to invade some forest interior sites is due in part to overwhelming reproductive and physiological disadvantages in low light. We further suggest that in some cases shade tolerance in this species is constrained in favor of plastic responses that optimize fitness in high light conditions. The implications for geographic variation in the spread and management of this species are discussed.

Elevated CO 2 boosts reproduction and alters selection in northern but not southern ecotypes of allergenic ragweed

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.