Tracking the effects of invasive Asian earthworms in Wingra and Gallistel woods

Because Upper Midwest temperate forests lack native earthworms, the invasions of European and Asian earthworms can significantly alter soils and understory vegetation. Earthworms’ ability to increase leaf litter decay, alter nutrient cycling by mixing the organic layer with mineral soil, and decrease plant species richness leads to concern about the ecological impact of Asian “jumping worms” (Amynthas agrestis and A. tokioensis).

While leaf litter, plant species richness, and tree and shrub seedling abundance were generally reduced in areas with European earthworms, they were typically slightly increased in areas with A. agrestis and A. tokioensis versus those without. Although our results do not show substantial impacts of A. agrestis and A. tokioensison vegetation in the initial years of invasion, the rapid replacement of European earthworms by A. agrestis and A. tokioensis suggests continued monitoring of these new invasive species is important to better understand their potential to change the Upper Midwest’s forests.

Laushman, K.M., Hotchkiss, S.C. & Herrick, B.M. Tracking an invasion: community changes in hardwood forests following the arrival of Amynthas agrestis and Amynthas tokioensis in Wisconsin. Biological Invasions (2017). https://doi.org/10.1007/s10530-017-1653-4

Effects of Common Buckthorn (Rhamnus cathartica) on small mammal post-dispersal seed predation

Invasive plant species can drastically affect ecosystem functions as well as plant and animal communities. These changes in habitat structure might generate novel indirect effects in ecological communities, such as affecting activities of foraging animals. Bartowitz is studying whether exotic shrubs might alter rates of seed predation by providing a structural refuge for small mammals or altering the litter layer. She completed three seed removal studies in July, September, and November 2014, in an Arboretum forest invaded by common buckthorn (R. cathartica). Half the research plots were cleared of R. cathartica. Seed depots were used to track predation by small mammals. The depots has R. cathartica seeds as well as native maple, cherry, and oak seeds (Acer rubrumAcer saccharumPrunus serotina, and Quercus rubra).

Bartowitz found that total seed removal was significantly higher in buckthorn plots than in cleared plots, and leaf litter depth was significantly higher in cleared plots. Removal rates differences suggest that buckthorn invasion affects seed removal rates. These differences could influence the survival of native seeds and the way plant communities are established. Understanding these changes can assist with restoration efforts through management and seed additions.

Bartowitz, K.J., and J.L. Orrock. 2016. Invasive exotic shrub (Rhamnus cathartica) alters the timing and magnitude of post-dispersal seed predation of native and exotic species. Journal of Vegetation Science 27(4): 789–799. doi: 10.1111/jvs.12397

How do invasive Asian earthworms affect soil and litter properties?

Jiangxiao Qiu and Monica Turner investigated the effects of a newly arrived exotic, invasive Asian earthworm on soil and litter properties at the Arboretum. Commonly referred to as “jumping worms” these earthworms were first documented in Wisconsin in 2013 at the Arboretum. Qiu and Turner found that jumping worms significantly reduced leaf litter and increased total carbon, total nitrogen and total phosphorus in the upper 5 cm of soil. Soil inorganic nitrogen and dissolved organic carbon both increased. This rapid mineralization of soil nutrients may make ecosystems susceptible to nutrient losses which may affect plants and soil biota.

Qiu, J., and M.G. Turner. 2016. Effects of non-native Asian earthworm invasion on temperate forest and prairie soils in the Midwestern US. Biological Invasions. 16 pp. doi:10.1007/s10530-016-1264-5

Hedge bets by adding heterogeneous topography to restoration sites that are subject to highly variable rainfall

A field experiment that began in the wettest June on record with high mortality to tussock sedge plantings was followed by the 2nd driest June, again with high planting mortality. However, James Doherty benefited from the extreme conditions, which showed that mounds were needed in wet years and depressions in dry years.

Doherty, J. M., and J. B. Zedler. 2015. Increasing substrate heterogeneity as a bet-hedging strategy for restoring wetland vegetation. Restoration Ecology 23(1):15–25.

Do invasive plants really reduce native plant diversity?

Data from 7 sedge meadows sampled at 4 plot scales say yes, consistently, by 50 percent. The invader is reed canary grass (Phalaris arundinacea), Wisconsin’s worst wetland weed. These new data show just how much damage one invasive plant can do to wetland vegetation.

Rojas, I. M., and J. B. Zedler. 2015. An invasive exotic grass reduced sedge meadow species richness by half. Wetlands Ecology and Management. Jan 0923–4861. doi: 10.1007/s11273-015-9409-3

Do native grass occurrences track soil moisture?

Bob Wernerehl tested John Curtis’s hypothesis that prairie vegetation responds primarily to a moisture gradient. He used an innovative indicator of moisture availability (the stable carbon isotopic signature of C3 plants) along with features of prairie structure (leaf area index and average leaf height). Rather than supporting Curtis’s hypothesis, the “continuum index” appeared to reflect soil fertility and mechanical impedance more than moisture….except for 5 dominant native grasses: Three grasses were sensitive to flooding–side-oats grama, Bouteloua curtipendula, little blue stem, Schizachyrium scoparium; and Indian grass, Sorghastrum nutans. In contrast, big blue stem, Andropogon gerardii, and prairie cordgrass, Spartina pectinata grew best in wetter conditions and Andropogon had high flood tolerance. Prairie grasses respond to the moisture gradient, even though the pattern for the prairie vegetation as a whole is less clear.

Wernerehl, Robert W. 2015. Causes of differential distribution of dominant prairie grasses in the Upper Midwest. PhD Dissertation, University of Wisconsin–Madison.

Ecosystem services responded strongly to small differences in hydroperiod (water regime)

Three constructed wetlands that differed only slightly in subsoil texture developed unique vegetation and ecosystem services. Our interdisciplinary research team found that plant productivity, which was highest in the cattail pond, did not indicate high levels of five other services. Instead, the pond had the lowest plant diversity; it failed to reduce flooding; and it exported the most nutrients and sediments. This study demonstrates that services should be measured, not just assumed to relate to some indicator.

Doherty, J. M., J. F. Miller, S. Prellwitz, A. M. Thompson, S. Loheide, and J. B. Zedler. 2014. Hydrologic Regimes Revealed Bundles and Tradeoffs Among Six Wetland Services. Ecosystems 17(6):1026–1039.

Does species richness increase wetland functioning, so that planting more species restores both diversity and productivity?

James Doherty’s study of Arboretum wetlands showed no such pattern in for urban, nutrient-rich wetlands or experimental mesocosms. In Curtis Prairie wetlands, tall productive grasses and sedges increased shoot growth but had few co-occurring species. In contrast to diversity-function theory, urban wetlands tend to be overtaken by productive dominants that suppress diversity. A suitable restoration approach is thus to plant a matrix dominant that can co-exist with other species (e.g., tussock sedge, Carex stricta) and a cover crop (an annual, such as beggar’s tick, Bidens cernua) to compete with aggressive graminoids, then add forbs as the cover crop dies back.

Doherty, J. M., and J. B. Zedler. 2014. Dominant graminoids support restoration of productivity but not diversity in urban wetlands. Ecological Engineering 65:101–111.

Mosses excelled in soil stabilization in three constructed wetlands

Using a Cohesive Strength Meter to assess the ability of mosses to hold sediment in place, researchers found that epibenthic moss mats were highly resistant to erosion, but they only developed dense cover in the well-drained wetlands. The role of moss mats was disproportionate to that of larger, more productive vascular plants.

Prellwitz, S. G., and A. M. Thompson. 2014. Biota and hydrology influence soil stability in constructed wetlands. Ecological Engineering 64:360-366.