Progress 01/01/08 to 12/31/11
Outputs
OUTPUTS: Over the past four years our research has made significant contributions to understanding the impact of forest plant invasions on forest processes and associated performance of amphibians that use forest habitats. Field and experimental studies consisted of captively rearing and releasing post-metamorphic amphibians into invaded and non-invaded stilt-grass habitats across 8 sites in the Georgia Piedmont. Every year results have been presented at regional and international meetings, and peer academic institutions. A noteworthy presentation was at the 2010 national summit on the impacts and management of stilt-grass invasions on forest ecosystems, titled "Influences of M. vimineum invasion on forest floor consumers: Bottom-up and top-down processes" (DeVore, J.L., J.C. Maerz, M.S. Strickland, and M.A. Bradford.; Carbondale, IL.). Jayna DeVore, a doctoral student who conducted the majority of the field work, presented results at the Southeastern Partners in Amphibian and Reptile Conservation and at the Joint Meeting of Ichthyologists and Herpetologists. At the Joint meeting, Jayna was awarded the Stoye Award for most outstanding student paper and she received a Graduate Student Association presentation grant from Warnell School of Forestry and Natural Resources. In November 2011, Jayna defended her dissertation titled "An exercise in complexity: indirect influences of invasion by an exotic grass (M. vimineum) on forest floor food weds." Our research on stilt-grass has resulted in an expanded research initiative across the GA piedmont and into the southern Appalachian Mountains through collaboration with the Coweeta Long-Term Ecological Research program. The second half of our proposal focused on how forest responses to elevated CO2 will effect survival to and size at metamorphosis of larval amphibians captively reared with leaf litter produced under ambient or elevated CO2 and the growth and survival of post-metamorphic amphibians in terrestrial pens containing the leaf litter produced under ambient or elevated CO2. Preliminary work was presented at the Smithsonian's Salamander Conservation Workshop. This event brought in experts who presented on current threats to eastern U.S. amphibian diversity, and I (Dr. John Maerz) was invited to present on issues related to the impact of rising CO2 and climate change on salamander diversity. Due to sources of elevated CO2 litter becoming limited and student/employee turnover, development of this portion of the project was limited and is now suspended. We still have materials for this project and plan to resume the work in the future when more resources and personnel have been secured. PARTICIPANTS: Dr. John C. Maerz, Associate Professor, Warnell School of Forestry and Natural Resources; Dr. Jayna DeVore, PostDoc, Odum School of Ecology, University of Georgia, Athens, Georgia; Dr. Mark Bradford, Assistant Professor, Yale School of Forestry & Environmental Studies, New Haven, CT; Dr. Michael Strickland, Postdoc, Yale School of Forestry & Environmental Studies, New Haven, CT. Although our study of elevated CO2 effects on forest leaf litter quality and larval amphibians was not able to be completed at this time, the study was supported by Dr. Richard Norby at the Oak Ridge National Laboratory (ORNL). Dr. Norby provided access to ambient and elevated leaf litter sources from ORNL's Free Air Carbon Exchange. TARGET AUDIENCES: The targets of our research are forest ecologists and managers, invasive species biologists, and wildlife conservation biologists. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
We found that stilt-grass invasions are associated with increased soil microbial activity and reduced short-term and moderate-term soil carbon pools. Results suggest that inputs of carbohydrates from invasive plant roots are priming decomposition, resulting in faster decomposition of all litter resources and faster cycling and loss of soil carbon stocks. This is significant, as there is little empirical support for the theory that plants have priming effects on decomposition. It suggests that stilt-grass invasions result in reduced carbon fertility and potentially reduced c storage in forest soils and may affect carbon flow to above ground consumer food webs, which has bottom up effects on some amphibian species. However, effects of stilt-grass invasion on higher level consumers are more complex. We found that increased mortality of amphibians in stilt-grass invaded forest is likely the result of increased predation by ground foraging spiders. The effects of stilt-grass invasion on the structural complexity of forest understory results in increased abundance of the spiders. We found that the effects of grass invasions differ between species, as results showed an increased mortality rate of post-metamorphic American toads and no effect on post-metamorphic mortality rate of leopard frogs. However, leopard frogs did show reduced growth rates. We speculate that because leopard frogs are larger at metamorphosis, they are less vulnerable to predation by spiders; however, the lower abundance of prey in invaded habitats results in slower growth of those frogs that do survive. We found that amphibian performance in forests can be highly naturally variable. The grass invasions reduce the quality of forest habitats such that it homogenizes variable forests to all poor-quality amphibian habitats. These outcomes are among the first demonstrations that plant invasions have complex indirect effects on native fauna and the impacts of can differ for closely related species. We discovered native herbivores feeding on stilt-grass and many were deriving 60-100% of their carbon from the plant. Recent publications report no known herbivores on the plant, and it was assumed that the plant was disconnected from native food webs accept through decomposer pathways. These animals are acquiring stilt-grass carbon by feeding on the plant, through root exudates and through rapid decomposition of stilt-grass litter within the detrital food web. These results demonstrate that invasive plants are connected as a basal resource to native fauna through multiple pathways, which may affect management decisions that were originally premised on the assumption that invasive plants contribute little to supporting native species populations. Our preliminary work on elevated CO2 effects on forest litter quality and larval amphibians confirms hypotheses that forest fertilization reduces litter quality, thereby reducing the biomass of amphibians that emerge from experimental aquariums. Limited availability of CO2 fertilized leaf litter resources has limited replication of these results; we plan to continue this work when more resources are secured
Publications
- DeVore, J. L. 2011. An exercise in complexity: Indirect influences of invasion by an exotic grass (Microstegium vimineum) on forest floor food webs. Ph.D. Dissertation. University of Georgia, Athens, GA. 186 pp.
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Progress 01/01/10 to 12/31/10
Outputs
OUTPUTS: During this reporting period, we completed analaysis of soil chemistry, microarthropod, macroarthropod, and amphibian growth and survival data from a two-year study across 8 forest sites under invasion by Japanese stilt-grass (Microstegium vimineum). The results of that research were presented at several regional meetings, international meetings, and peer academic institutions. Particularly relevant among those events was 2010 national summit on the impacts and management of stilt-grass invasions on forest ecosystems (DeVore, J.L., J.C. Maerz, M.S. Strickland, and M.A. Bradford. "Influences of Microstegium vimineum invasion on forest floor consumers: Bottom-up and top-down processes" 2010 Stiltgrass Summit, Carbondale, IL.). The culmination of our current work on stilt-grass invasions of forest ecosystems has resulted in an expanded research program across the GA piedmont and into the southern Appalachian Mountains through work will collaborators on the Coweeta Long-Term Ecological Research program. That work will focus broadly on the effects of stilt-grass invasions on forest fertility. PARTICIPANTS: John C. Maerz, Associate Professor of Vertebrate Ecology, Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA. Mark Bradford, Assistant Professor, Yale School of Forestry and Environmental Science, Yale University. Jayna DeVore, Ph.D. Candidate, Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA. Michael Strickland, Postdoctoral Research Associate, Yale School of Forestry and Environmental Science, Yale University. TARGET AUDIENCES: Academic, federal agencies, state agencies, land managers PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
The results of our research show that stilt-grass invasions have a consistent effect across forest sites in reducing soil carbon pools. The mechanism appears to be a priming effect on soil microbial activity from the root exudates and inputs of highly labile detritus sources. The result of reduced soil carbon appears to be a reduction in soil microarthropod biomass, which has bottom up effects on the growth rates of some amphibian species; however, the effects of stilt-grass invasion on higher level consumers such as amphibians was more complex than simple bottom-up effects. Our research shows that the effects of stilt-grass invasion on the structural complexity of forest understories results in increased abundance of carnivorous macroinvertebrates, namely ground foraging spiders. The high density of spiders results in higher mortality rates from some amphibian species in forests invaded by stilt-grass. Another important result from our research was that we desmonstrated that amphibian performance in forests can be highly naturally variable. Therefore, the effect of stilt-grass is context dependent. At sites where amphibian performance was naturally low, stilt-grass has (intuitively) a negligible effect on amphibian performance; however, at sites of amphibian performance, stilt-grass dramatically reduces amphibian performance. The result is stilt-grass invasions reduce the quality of forest habitats for amphibians such that it homogenizes variable forests to all poor-quality amphibian habitats.
Publications
- Bradford, M. A., J. L. DeVore, J. C. Maerz, J. V. McHugh, C. L. Smith, M. S. Strickland. 2010. Native, insect herbivore communities derive a significant proportion of their carbon from a widespread invader of forest understories. Biological Invasions 12:721-724.
- Strickland, M. S., J. S. DeVore, J. C. Maerz, and M. A. Bradford. 2010. Grass invasion of a hardwood forest is associated with declines in belowground carbon pools. Global Change Biology 16:1338-1350.
- Strickland, M. J., M. A. Bradford, J. L. DeVore, and J. C. Maerz. 2011. Loss of faster-cycling soil carbon pools following grass invasion across multiple forest sites. Soil Biology and Biogeochemistry: in press.
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Progress 01/01/09 to 12/31/09
Outputs
OUTPUTS: My program has made significant progress towards our understanding of forest plant invasions on forest processes and associated performance of amphibians that use forest habitats. We have completed several research projects including measuring the impacts of Japanese stilt-grass (Microstegium vimineum) invasion on soil carbon dynamics and invertebrate abundance, and grass invasion on the survival and growth of post metamorphic amphibians. Some of our findings have been published in peer-reviewed journals, and nearly all results have been presented at international research meetings and in seminars to state and federal agencies and peer academic institutions. We have made limited progress on our research into elevated CO2 impacts on amphibians. Sources of elevated CO2 litter have become limited, and student/employee turnover on this portion of the project have limited development of this research program. We still have materials and plans to continue this work in the future. PARTICIPANTS: Ms. Jayna DeVore, Ph.D. Candidate, Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA Dr. Mark Bradford, Assistant Professor, Yale School of Forestry & Environmental Studies, New Haven, CT Dr. Michael Strickland, Postdoc, Yale School of Forestry & Environmental Studies, New Haven, CT TARGET AUDIENCES: The targets of our research are forest ecologists and managers, invasive species biologists, and wildlife conservation biologists. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
Our key outcomes are: 1. Grass invasions are associated with a reduction in faster cycling soil carbon pools. Our results suggest that the addition of a detrital resource and outputs of root exudates from living stilt-grass prime microbial activity in invaded soils, resulting in faster cycling of carbon and accelerated decomposition of native litter. These results suggest that grass invasions of forests may result in long-term reductions in soil fertility and reduce the c storage potential of forest soils. The latter result is relevant to the interest in forests as carbon sinks to reduce CO2 levels in the atmosphere. 2. A suite of native herbivores feed significantly on living stilt-grass. It was assumed previously that few native species feed on stilt-grass, so the plant was largely disconnected from supporting fauna. These results suggest that stilt-grass is more integrated into forest food webs than previously assumed. Animals are acquiring stilt-grass carbon by feeding on the plant, through root exudates that are rapidly incorporated into the soil food web, and through rapid decomposition of stilt-grass litter within the detrital food web. These results demonstrate that invasive plants are connected as a basal resource to native fauna through multiple pathways, which may also affect management decisions that were originally premised on the assumption that invasive plants contribute little to supporting native species populations. 3. Soil invertebrate abundances are lower in stilt-grass invaded habitats. Soil invertebrates are influential in affecting rates of decomposition and forest nutrient cycling, and are also important prey for larger forest fauna, most notably amphibians. These results suggest that grass invasions do affect prey resources for some wildlife. 4. Stilt-grass invasions have different effects on different amphibian species. Grass invasion increased mortality rate of recently metamorphosed American toads. The mechanism for this mortality is increased predation by ground foraging spiders, which are more abundant in stilt-grass invaded habitats. In contrast, grass invasion had no effect on metamorphic leopard frog survival rate. Leopard frogs in grass-invaded habitats did show reduced growth rates. We speculate that because leopard frogs are larger than American toads at metamorphosis, they are less vulnerable to predation by spiders; however, the lower abundance of prey in stilt-grass invaded habitats results in slower growth of those frogs that do survive. These outcomes are significant for several reasons. First, they are among the first demonstrations that plant invasions have complex indirect effects on native fauna. Second, the impacts of an invasive plant can be significantly different even for closely related species. These findings present a challenging reality for management. TIt may difficult to manage for the effects of an invader, as those effects may be complex, and impacts documented for one species may not be transferable [generalizable] to other species.
Publications
- No publications reported this period
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Progress 01/01/08 to 12/31/08
Outputs
OUTPUTS: In 2008 [and continuing into 2009] we continued our field and experimental studies of plant invasion impacts on forest habitat quality for amphibians. We raised >1600 amphibians to metamorphosis and released them in invaded and non-invaded habitats across 8 sites in the Georgia Piedmont. We tracked survival and growth of amphibians. In addition, we have processed soil microbial and biogeochemistry samples and litter and soil invertebrate samples from all sites. We are in the process of analyzing those data. In 2008, we presented the results of our research at several venues. First, we presented our preliminary work on the potential impacts of CO2 fertilization on forest litter quality and larval amphibian performance at the Smithsonian's Salamander Conservation Workshop. This event brought in experts to present current information on threats to eastern U.S. salamander [and more broadly amphibian] diversity and to discuss potential management options. I was asked to present on issues related to rising CO2 and associated climate change. In February and July 2008, we presented the results of our studies on Microtegium vimineum invasions on the performance of metamorphic amphibians in hardwood forests to the Southeastern Partners in Amphibian and Reptile Conservation and the Joint Meeting of Ichthyologists and Herpetologists respectively. The work was presented by Doctoral Candidate, Jayna DeVore. She was awarded the Stoye Award for most outstanding student paper at the Joint Meeting. We maintain an up to date description of our research activities at: http://www.uga.edu/maerzlab/Site/Research.html PARTICIPANTS: Our work on elevated CO2 effects on forest leaf litter quality and larval amphibians was supported by Dr. Richard Norby at the Oak Ridge National Laboratory (ORNL) (contact information: One Bethel Valley Road, Bldg. 1062, Oak Ridge TN 37831-6422; Phone: (865) 576-5261; norbyrj@ornl.gov). Dr. Norby provided access to ambient and elevated leaf litter sources from ORNL's Free Air Carbon Exchange (FACE; http://face.ornl.gov/). Our work on the impact Microstegium invasions on forest soils and amphibian habitat quality has been a joint collaboration between my lab and the lab of Dr. Mark Bradford formerly of the Odum School of Ecology at the University of Georgia and now in the Yale School of Forestry and Environmental Studies (contact information: Email: mark.bradford@yale.edu; Tel: 203-436-9148, Yale School of Forestry & Environmental Studies, 195 Prospect Street, New Haven, CT 06511). Two Ph.D. students, Michael Strickland and Jayan DeVore, have been responsible for much of the execution of our research on Microstegium invasions. Michael Strickland is a Ph.D. candidate in the University of Ph.D. candidate in the Unversity of Georgia's D. B. Warnell School of Forestry and Natural Resources (devorej@warnell.uga.edu). In addition to primary responsible over research execution, Mr. Strickland and Ms. DeVore have been trained in various research techniques, presented research findings at international meetings, and taken the lead on the publication of research. TARGET AUDIENCES: Nothing significant to report during this reporting period. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
Our preliminary work on elevated CO2 effects on forest litter quality and larval amphibians confirms preliminary hypotheses that forest fertilization reduces litter quality, thereby reducing the biomass of amphibians that emerge from experimental aquatic communities. Limited availability of CO2 fertilized leaf litter resources has limited replication of these results. Our work on plant invasion impacts on amphibian forest habitat has focused on the effects of Microstegium vimineum invaions of hardwood forests. Microstegium is a grass, and grass invasions of mature forests are rare and poorly studied. We found that grass invasions are associated with increased soil microbial activity and reduced short-term and moderate-term soil carbon pools. Pulse-chase studies suggest that litter inputs from the grass or inputs of carbohydrates from invasive plant roots are stimulating ["priming"] decomposition in invaded forests, resulting in faster decomposition of all litter resources and faster cycling and loss of soil carbon stocks. This is a significant find, as there is little empirical support for the theory that plants have priming effects on decomposition. This result is also significant because it suggests that Microstegium invasions will result in reduced carbon fertility in forest soils, and potentially will reduce carbon flow to above ground consumer food webs that support wildlife. In addition to the potential for "bottom-up" effects of Microstegium invasions on energy flow in forest food webs, our results show that increased rapid mortality of amphibians in forests invaded by Microstegium is likely the result of increased predation by ground foraging spiders. This result is important because it is generally assumed that plant invasions will affect native ecosystems through "bottom-up" processes, and there has been generally neglect of the potential "top-down" effects of plant invasions through altered predator-prey dynamics. In other systems, the effects of plant structure on increased predator densities and predation pressure on prey species are well-documented, but our work is the first to apply this fundamental ecological concept to understand the impact of a plant invasion. Finally, during the course of our research, we discovered a suite of native herbivores feeding on Microstegium. All these insects were native North American species, and many were deriving 60-100% of their carbon [energy] from Microstegium. This was significant because recent publications report no know herbivores on the plant in its native or introduced ranges, and it was assumed that the plant was disconnected from native food webs accept through decomposer pathways. This also demonstrates that some native species have shifted to using Microstegium nearly exclusively in invaded habitats.
Publications
- Strickland, M. S., J. S. DeVore, J. C. Maerz, and M. A. Bradford. 2009. Grass invasion of a hardwood forest is associated with declines in belowground carbon pools. Global Change Biology: accepted.
- Bradford, M. A., J. L. DeVore, J. C. Maerz, J. V. McHugh, C. L. Smith, M. S. Strickland. 2009. Native, insect herbivore communities derive a significant proportion of their carbon from a widespread invader of forest understories. Biological Invasions: in press.
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