1. Effects of Elevated Atmospheric CO2 on Aquatic Ecosystems
- We grew trembling aspen (Populus tremuloides) trees under ambient (360 ppm) and elevated (720 ppm) CO2 at the University of Michigan Biological Station. Autumn leaf litter collected from trees grown at 720 ppm CO2 had lower C:N and higher concentrations of secondary compounds, such as condensed tannins and phenolics.
- Since leaf litter enters adjacent aquatic ecosystems, changes in litter quality due to elevated atmospheric CO2 may affect aquatic food webs and ecosystem function. Leaf litter grown at 720 ppm CO2 decomposed more slowly in streams and supported less microbial biomass than leaf litter grown at 320 ppm CO2. Changes in litter chemistry inhibited invertebrate growth and assimilation efficiencies, and this further inhibited fish growth rates.
- Dissolved organic carbon (DOC) is rapidly leached from leaf litter entering streams, and the majority of DOC in headwater streams has a leaf-litter-origin. We tested the effects of elevated atmospheric CO2 on DOC concentrations and subsequent effects on periphyton (algae and microbes). DOC concentrations were higher from leaf litter grown at 720 than 360 ppm CO2, and this resulted in a shift in dominant algal taxa within stream periphyton. We further tested the indirect effects of elevated atmospheric CO2 on crayfish feeding preferences as mediated through changes in dominant algal taxa within stream periphyton due to differences in DOC concentrations. During experimental trials, crayfish were able to detect differences in periphyton amended with DOC from leaf litter grown under ambient and elevated CO2 and perferred periphyton from the ambient treatment.
- Elevated atmospheric CO2 can have indirect effects on aquatic ecosystem function and food web dynamics through altered leaf litter quality.
2. Effects of Resource and Consumer Diversity on Ecosystem Function in a Detritus-Based Watershed
- Climate change, invasive plant species (e.g., salt cedar), pathogens, and pests (e.g., hemlock woolly adelgid) are impacting riparian plant communities. We tested the effects on nonrandom tree species loss on leaf litter breakdown dynamics (mass loss, litter chemistry, microbial and invertebrate communities) in coupled stream and riparian ecosystems at Coweeta Hydrologic Laboratory (LTER).
- Using a full-factorial design of single- and mixed-species leaf litter, we tested the effects of leaf litter species diversity (richness and composition) on breakdown dynamics within and among stream and riparian ecosystems. Leaf litter diversity had nonadditive effects on in-stream mass loss and additive effects on litter mass loss in riparian plots. Changes in litter chemistry during in-stream breakdown were explained by nonadditive effects of litter species diversity as well as additive effects of litter species identity. Stream invertebrate communities appear to be affected by litter species richness, and communities colonizing persistent (i.e., slow decomposing) and heterogeneous litter mixtures have higher species richness, abundance, and biomass.
- We are exploring the importance of environmental context in explaining different functional responses of aquatic and terrestrial ecosystems to species diversity.
- Microbial (e.g., bacteria and fungi) communities are drivers of organic matter processing and biogeochemical cycling in ecosystems. Effects of nonrandom tree species loss may differentially impact bacteria and fungi (which can compete for resources and have antagonistic effects on one another). In the southern Appalachians, rhododendron (Rhododendron maximum) is a dominant riparian shrub that is predicted to increase in abundance due to declines in eastern hemlock (Tsuga canadensis). In-stream litter mixtures containing rhododendron have overall lower bacterial and fungal biomass. We tested the effects of rhododendron presence/absence in litter mixtures on bacterial and fungal community structure and function associated with individual litter species.
3. Predicting effects of nutrient enrichment and riparian plant biodiversity loss on stream ecosystem function
- We are investigated the effects of stream water nutrient enrichment and leaf litter species diversity on breakdown dynamics in watersheds at Coweeta Hydrologic Laboratory (LTER). We manipulated litter species diversity in two headwater streams (an artificially enriched and reference stream) at Coweeta LTER to determine if effects of litter mixing on breakdown dynamics were altered by the presence of elevated streamwater nutrients. Nonadditive effects of litter species diversity were suppressed by nutrient enrichment, which suggests the potential for world-wide mobilization of nutrients to suppress expressions of diversity, further dampening that expression beyond what is currently predicted based on species loss.
1. Ecological implications of riparian management and forest succession on stream and riparian ecosystem function
- Cross-ecosystem energy flows link streams and riparian forests. Forest
harvesting alters the composition of riparian tree species, which can
affect the structure and function of stream ecosystems through changes
in terrestrial resource subsidies. We examined how variation in the
ratio of deciduous to coniferous forest composition may affect stream
invertebrate and microbial consumers and subsequent leaf litter
breakdown rates of red alder (deciduous tree) and western hemlock
(conifer) in 10 small streams. Breakdown rates of alder litter were
faster in streams containing a greater proportion of deciduous than
coniferous canopy; whereas breakdown rates of hemlock litter were
independent of canopy composition. When invertebrates were excluded
using fine mesh to isolate microbe-specific processing dynamics,
breakdown rates of both species were an order of magnitude less and did
not vary with canopy composition. Benthic invertebrates, and not
microbes, appear to explain variation in organic matter processing
dynamics attributed to forest canopy composition. Dominant
invertebrates vary among streams with different forest canopies,
whereas invertebrate community diversity is linked to litter type.
Analyses of microbial communities may reveal trophic variation in
responses of stream food webs to terrestrial subsidies. Our findings
provide further evidence linking terrestrial energy resources with
stream consumers and ecosystem function.