Leaf litter decomposition plays an important role in nutrient cycling in both terrestrial and aquatic systems. Decay rates vary based on species, habitat, climate, and local environmental conditions. Invasive plants alter decomposition processes; however, there is a lack of research exploring patterns at regional and continental scales. In this study we examined the decomposition of both native and nonnative, invasive woody plant leaf litter and mixtures of the two, in both terrestrial and aquatic habitats at nine locations in the eastern and midwestern U.S.A. There was significant variation among locations, which was not clearly related to either average air temperature or precipitation. Unexpectedly, in locations with multiple years of data, there were higher rates of decomposition in years with lower temperatures and precipitation in both terrestrial and aquatic habitats. We found decay rates were generally higher in aquatic than terrestrial habitats and leaf litter from nonnative invasive species generally decayed faster than that of native species in both terrestrial and aquatic systems. Differences in litter decay rates among invasive species were significant in both terrestrial and aquatic habitats; whereas no differences were found among native species in either habitat. In mixed litter bags, decay rates were lower than what was predicted based on the relative amounts of native and invasive litter in each bag, possibly indicating the presence of native leaf litter slows the decomposition of invasive leaf litter. Additionally, there may have been threshold effects in the mixed litter bags, especially in aquatic systems. While this study supported several generalizations about leaf decomposition rates (invasive > native, aquatic > terrestrial), the variability in the decay rates from different locations and habitats indicates combinations of different species and local conditions may overshadow other general trends related to litter decomposition.

Wetland creation is a form of ecological engineering that often relies on establishing a plant community to increase ecosystem services. However, we still lack understanding about how species composition and diversity affect certain wetland ecosystem functions associated with these ecosystem services. Our objective was to investigate the link between plant community (diversity and composition) and plant carbon storage, root surface area and methane (CH4) emission. We hypothesized that more diverse mixtures would 1) store more carbon in above and belowground biomass and 2) emit less CH4 due to increased root surface area and aeration. Three plant groups that represented different growth forms (i.e., "functional groups": ferns, reeds and tussocks) were planted in mesocosms to represent four levels of functional diversity and every combination of functional groups at each diversity level. Unplanted mesocosms served as the zero diversity treatment. At peak biomass in 2014 and 2015, we harvested aboveground biomass and took soil cores from each mesocosm at 0-10 cm to determine the root biomass and surface area. Methane fluxes were measured in situ using a closed chamber system and a Picarro G2301 CRDS analyzer. Fluxes did not vary by functional group richness level or composition. Presence of the reed and tussock functional groups decreased fluxes; however, root surface area was not correlated with CH4 emission. Plant carbon storage was greater in all treatments containing tussocks (expect for the reed tussock mixture) compared with ferns grown alone. Functional group richness did not have a significant effect on plant carbon storage, however there was a positive trend. Root surface area increased 87% from the first to second year; however, there were no differences among functional group richness levels or mixtures. Our results indicate that planting reed and tussock functional groups in dense clumps during restoration could maintain a high level of carbon storage and relatively low CH4 emissions. (C) 2017 Elsevier B.V. All rights reserved.

Invasive aquatic plants in U.S. lakes and reservoirs frequently require managers to implement plant control, yet little is known about how these management efforts alter habitat available to adult fish. We used a before-after, control-impact (BACI) design to study four Minnesota lakes (two treated lakes and two untreated control lakes) to evaluate the influence of plant management using herbicides (i.e., endothall/2,4-D) on diets of adult bluegill (Lepomis macrochirus Rafinesque) over the course of 4 yr. We hypothesized that removing plants would result in an immediate increase of prey items available to bluegill reflected in an increase in total items in the bluegill diet, and that an increase in prey items would affect diet breadth. Invasive Eurasian watermilfoil, Myriophyllum spicatum L., was eliminated following herbicide treatment; however, native plants immediately expanded and plant overall abundance in the littoral zone was not reduced. We found no significant treatment effects on number of prey items per stomach, stomach content mass, diet composition, or abundance of major diet groups. However, we found that diet breadth increased posttreatment in the fall season, as evidenced by a more even distribution of bluegill diet items in stomach contents. Bluegill diet composition varied across years and lakes (primarily due to changes in Cladocera), but not due to treatment. We concluded that early seasonal application of herbicides resulted in an immediate shift from invasive aquatic plants to a diverse native community, which had minor effects on diets of bluegill.

Nitrogen (N) deposition from anthropogenic sources can facilitate the encroachment of plant species with high-N demands into nutrient-poor ecosystems such as sphagnum bogs. Prior research has demonstrated that altered leaf morphology of the carnivorous pitcher plant Sarracenia purpurea L. can serve as a biological indicator of increased bog nitrification. Our objective was to assess the effect of N addition on the root morphology of S. purpurea. To make this assessment, nine S. purpurea plants were grown in microcosms with their roots positioned on transparent acrylic tubes so that root growth could be monitored. Three replicate microcosms received either a high-N treatment (1.0 mg NH4-N·L−1), low-N treatment (0.1 mg NH4-N·L−1), or no additional N. After 7 weeks, we scanned the roots with WinRhizo Pro software, recorded leaf dimensions, and measured the dry mass of the roots and leaves. The high-N treatment had significantly greater root length, surface area, and dry biomass than the controls. In contrast, we found no difference in leaf dimensions or aboveground biomass among treatments. The results of this study support our hypothesis that S. purpurea increases root growth to uptake nutrients from the soil under conditions of increased N deposition.

Wetlands are the largest natural source of methane to the atmosphere, but factors controlling methane emissions from wetlands are a major source of uncertainty in greenhouse gas budgets and projections of future climate change. We conducted a controlled outdoor mesocosm experiment to assess the effects of plant community structure (functional group richness and composition) on potential methane production and potential iron reduction in freshwater emergent marshes. Four plant functional groups (facultative annuals, obligate annuals, reeds, and tussocks) were arranged in a full-factorial design and additional mesocosms were assigned as no-plant controls. Soil samples from the top 10 cm were collected three times during the growing season to determine potential methane production and potential iron reduction (in unamended soils and in soils amended with 200 mM formate). These data were compared to soil organic matter, soil pH, and previously published data on above and belowground plant biomass. We found that functional group richness was less important than the presence of specific functional groups (reeds or tussocks) in mediating potential iron reduction. In our mesocosms, where oxidized iron was abundant and electron donors were limiting, iron reducing bacteria outcompeted methanogens, keeping methane production barely detectable in unamended lab incubations. When the possibility of re-oxidizing iron was eliminated via anaerobic incubations and the electron donor limitation was removed by adding formate, potential methane production increased and followed the same patterns as potential iron reduction. Our findings suggest that in the absence of abundant oxidized iron and/or the presence of abundant electron donors, wetlands dominated by either reeds or tussocks may have increased methane production compared to wetlands dominated by annuals. Depending on functional traits such as plant transport and rhizospheric oxygenation capacities, this could potentially lead to increased methane emissions in some wetlands. Additional research examining the role these plant functional groups play in other aspects of methane dynamics will be useful given the importance of methane as a greenhouse gas.

Biodiversity and ecosystem functioning experiments have demonstrated that plant biomass of species grown in mixtures is often greater than plant biomass of monocultures (i.e., mixtures over yield). While we understand that plant species utilize resources differently, how a combination of species increases resource use and productivity is not well known, especially in wetland ecosystems. Here, we used a mesocosm experiment to explore diversity effects on plant biomass production and to examine the role of N partitioning as a mechanism for overyielding in wetland ecosystems. Plant functional groups (FGs) represented the unit of diversity, and we included five levels of diversity (0-4 FGs). To test for N partitioning, we used a stable isotope technique to determine niche breadth and proportion similarity of inorganic N use (NO3- and NH4-) for individual FGs as well as mixtures containing 3 and 4 FGs. We found that total plant biomass increased in the first season from an average of 290 +/- 60 SE g ash-free dry mass (AFDM) m(-2) at the 1 FG level to 490 +/- 70 g AFDM m(-2) at the 4 FG level and in the second season from an average of 560 +/- 80 g AFDM m(-2) at the 1 FG level to 1000 +/- 90 g AFDM m(-2) at the 4 FG level indicating overyielding. Plant species comprising the majority of mesocosm biomass demonstrated preferential uptake of (NO3-)-N-15, while species with relatively less biomass (e.g., Acorus calamus and Carex crinita) preferred (NH4+)-N-15. Concentrations of N-15 in biomass increased with FG richness, but only in the 15NO(3)(-) treatment. Niche breadth did not vary among levels of FG richness. We observed a greater niche overlap with an increase of FGs, with species taking up greater proportion of (NO3-)-N-15 than (NH4+)-N-15. Our results indicate that plant overyielding in wetland mesocosms is not the result of niche partitioning of N chemical forms, but is associated with greater uptake of NO3. (C) 2011 Elsevier B.V. All rights reserved.

Biological invasions of aquatic plants (i.e., macrophytes) are a worldwide phenomenon, and within the last 15 years researchers have started to focus on the influence of these species on aquatic communities and ecosystem dynamics. We reviewed current literature to identify how invasive macrophyte species impact fishes and macroinvertebrates, explore how these mechanisms deviate (or not) from the accepted model of plant–fish interactions, and assess how traits that enable macrophytes to invade are linked to effects on fish and macroinvertebrate communities. We found that in certain instances, invasive macrophytes increased habitat complexity, hypoxia, allelopathic chemicals, facilitation of other exotic species, and inferior food quality leading to a decrease in abundance of native fish and macroinvertebrate species. However, mechanisms underlying invasive macrophyte impacts on fish and macroinvertebrate communities (i.e., biomass production, photosynthesis, decomposition, and substrate stabilization) were not fundamentally different than those of native macrophytes. We identified three invasive traits largely responsible for negative effects on fish and macroinvertebrate communities: increased growth rate, allelopathic chemical production, and phenotypic plasticity allowing for greater adaptation to environmental conditions than native species. We suggest that information on invasive macrophytes (including invasive traits) along with environmental data could be used to create models to better predict impacts of macrophyte invasion. However, effects of invasive macrophytes on trophic dynamics are less well-known and more research is essential to define system level processes.

Changes in the world’s species composition and the loss of biodiversity have prompted a closer investigation of the importance of biodiversity and community composition to ecosystem functioning. However, few studies have explored this relationship outside of controlled experiments. Here, we examined the relationship between plant diversity, primary production, and methane efflux in freshwater wetlands in an across-site field study and assessed the applicability of experimental findings to natural wetlands. Four wetland sites in central Ohio (USA) were divided into two plant communities, one dominated by clonal species and one dominated by non-clonal species. We found that plant diversity was negatively correlated with aboveground biomass in both the clonal and non-clonal communities. Overall, plant community composition was a stronger predictor than diversity of the response variables and in certain instances a stronger predictor than environmental factors such as soil organic matter content, moisture content, and pH. Thus, plant community composition is an important driver of ecosystem functioning in depressional wetlands beyond the well-known environmental factors. Additionally, our work indicates that results from experimental wetland studies of the relationship among diversity, biomass and methane emission are not applicable to the wetland ecosystems included in our study.

Background/Question/Methods The loss of biodiversity worldwide has prompted a close investigation of the link between diversity and ecosystem functions. Most experimental studies have looked at the relationship between grassland plant diversity and aboveground productivity. Less is known about other ecosystems or how diversity affects belowground processes. Our objective was to investigate the link between plant community (diversity and composition) and key belowground processes such as root biomass production and CH4 dynamics in wetland ecosystems. We hypothesized that 1) root biomass would increase with functional group diversity due to complementarity and 2) the sediment pool of CH4 would decrease with diversity due to increased CH4 oxidation facilitated by root biomass. Four plant functional groups (facultative annuals, obligate annuals, reeds and tussocks) were planted in controlled mesocosms to represent five levels of functional diversity and every combination of functional groups at each diversity level. Unplanted mesocosms served as the zero diversity treatment. At peak biomass in 2007, porewater samplers were used to extract water at 5, 15, and 25 cm. The samples were frozen prior to headspace analysis of CH4 and CO2. Porewater was also analyzed for DOC. Afterward, we took soil cores from each mesocosm at 0-10 cm, 10-20 cm and 20-30 cm to determine the root biomass in each depth. Results/Conclusions Root biomass increased with functional diversity (F 3, 71 = 2.78, P < 0.05), however only the lowest diversity treatment had significantly lower root biomass than the highest diversity level; 150.6 ± 24.94 (SE) g DW m-2 and 307.2 ± 49.9 g DW m-2, respectively. At each depth, root biomass increased with diversity (P < 0.01), and root biomass at 0-10 cm >10-20 cm >20-30 cm. The facultative annual and obligate annual functional groups had significantly less root biomass than the other functional groups and combinations (P < 0.001). Porewater concentrations of CH4 did not significantly differ between functional group combinations or diversity levels (P > 0.05). However, CH4 was positively correlated with depth (F 1, 194 =19.75, P < 0.001), CO2 concentration (F 1, 194 = 42.94, P < 0.001) and DOC (F 1,194 = 4.98, P < 0.001) and was not correlated with root biomass (P > 0.05). Since this data is from the first year of sampling following mesocosm establishment, the insignificant relationship between root biomass and CH4 in the sediment pool may indicate that the microbial processes were influenced more by starting soil conditions than plant-mediated conditions.