Meredith Thomsen

Ecological Resistance in Theory and Practice

Abstract Ecological resistance refers to the biotic and abiotic factors in a recipient ecosystem that limit the population growth of an invading species. Although there is interest in applying this concept to the management and restoration of habitats influenced by damaging, invasive species, practical difficulties in restoring resistance have inhibited its broad-scale incorporation. Also, some ecologists have argued that resistance is unimportant in generating landscape pattern casting doubt on its potential usefulness in large-scale management. In this study, we argue that despite temporal and spatial fluctuations in resistance being the norm, the concept provides a valuable foundation for a more sustainable approach to long-term weed management. This goal should be achievable through identification and manipulation of successional processes in natural communities. Additional index words: Biotic resistance, community invasibility, ecological restoration, weed control.

Control of Reed Canarygrass Promotes Wetland Herb and Tree Seedling Establishment in an Upper Mississippi River Floodplain Forest

Phalaris arundinacea (reed canarygrass) is recognized as a problematic invader of North American marshes, decreasing biodiversity and persisting in the face of control efforts. Less is known about its ecology or management in forested wetlands, providing an opportunity to apply information about factors critical to an invader’s control in one wetland type to another. In a potted plant experiment and in the field, we documented strong competitive effects of reed canarygrass on the establishment and early growth of tree seedlings. In the field, we demonstrated the effectiveness of a novel restoration strategy, combining site scarification with late fall applications of pre-emergent herbicides. Treatments delayed reed canarygrass emergence the following spring, creating a window of opportunity for the early growth of native plants in the absence of competition from the grass. They also allowed for follow-up herbicide treatments during the growing season. We documented greater establishment of wetland herbs and tree seedlings in treated areas. Data from small exclosures suggest, however, that deer browsing can limit tree seedling height growth in floodplain restorations. Slower tree growth will delay canopy closure, potentially allowing reed canarygrass re-invasion. Thus, it may be necessary to protect tree seedlings from herbivory to assure forest regeneration.

Climate change and grassland restoration in California: lessons from six years of rainfall manipulation in a north coast grassland.

ABSTRACT Native perennial bunchgrasses have undergone steep declines across much of California but persist in sizable populations along the northern coast. The longer rainy season and less severe summer drought in this region are thought to facilitate bunchgrass persistence in the face of extensive invasion by exotic annual species. Changes in the seasonality and intensity of precipitation that accompany global climate change could critically influence efforts to conserve and restore these plants in California grasslands. We established a large-scale manipulation of rainfall in a protected Mendocino County grassland to investigate how predicted shifts in precipitation affect the performance of three native perennial bunchgrass species in exotic-dominated stands. We added seeds, plugs, and mature tussocks of Danthonia californica, Elymus glaucus, and Elymus multisetus into replicate plots of exotic annual grassland and subjected the plots to one of three experimental precipitation regimes: increased winter rainfall, increased spring rainfall, and ambient rainfall. Responses to rainfall addition varied widely by age class and species and depended heavily on seasonal timing of the increase. Establishment from seed was rare for all three species and showed little response to water addition, likely due to concomitant changes in the surrounding communities. Production of exotic annual grasses rose markedly following repeated extensions of the rainy season, and while established bunchgrasses benefited despite this change, new plants could not establish into thickening stands of exotic vegetation. In contrast, survival was high for transplanted plugs and tussocks of all three species across all three rainfall treatments, suggesting that plugs and tussocks can survive a wide range of climatic conditions and high local densities of exotic annual grasses. Restoration approaches focused on these life stages may be most robust to changing climate. Transplanted individuals can provide a continual source of propagules to surrounding areas that then recruit during years in which conditions in the physical and biological environment are amenable to seedling establishment.

Effects of Flooding on Ion Exchange Rates in an Upper Mississippi River Floodplain Forest Impacted by Herbivory, Invasion, and Restoration

We examined effects of flooding on supply rates of 14 nutrients in floodplain areas invaded by Phalaris arundinacea (reed canarygrass), areas restored to young successional forests (browsed by white-tailed deer and unbrowsed), and remnant mature forests in the Upper Mississippi River floodplain. Plant Root Simulator ion-exchange probes were deployed for four separate 28-day periods. The first deployment occurred during flooded conditions, while the three subsequent deployments were conducted during progressively drier periods. Time after flooding corresponded with increases in NO3−-N, K+ and Zn+2, decreases in H2PO4−-P, Fe+3, Mn+2, and B(OH)4-B, a decrease followed by an increase in NH4+-N, Ca+2, Mg+2 and Al+3, and an increase followed by a decrease for SO4−2-S. Plant community type had weak to no effects on nutrient supply rates compared to the stronger effects of flooding duration. Our results suggest that seasonal dynamics in floodplain nutrient availability are similarly driven by flood pulses in different community types. However, reed canarygrass invasion has potential to increase availability of some nutrients, while restoration of forest cover may promote recovery of nutrient availability to that observed in reference mature forests.

Winter Browse Selection by White-Tailed Deer and Implications for Bottomland Forest Restoration in the Upper Mississippi River Valley, USA

ABSTRACT: White-tailed deer (Odocoileus virginianus) forage selectively, modifying upland forest species composition and in some cases shifting ecosystems to alternative stable states. Few studies, however, have investigated plant selection by deer in bottomland forests. Herbaceous invasive species are common in wetlands and their expansion could be promoted if deer avoid them and preferentially feed on native woody species. We surveyed plant species composition and winter deer browsing in 14 floodplain forest restoration sites along the Upper Mississippi River and tributaries. Tree seedling density declined rapidly with increasing cover of invasive Phalaris arundinacea, averaging less than 1 per m2 in all sites in which the grass was present. Deer browsed ∼46% of available tree seedling stems (branches) at mainland restorations, compared to ∼3% at island sites. Across all tree species, the number of browsed stems increased linearly with the number available and responded unimodally to tree height. Maximum browsing rates were observed on trees with high stem abundances (>10 per plant) and of heights between 50 and 150 cm. Deer preferred Ulmus americana and Acer saccharinum, and avoided Fraxinus pennsylvanica, Acer negundo, and Quercus spp. at mainland sites, and did not browse Phalaris arundinacea if present. Depending on plant growth responses to herbivory and the competitive effects of unbrowsed species, our results suggest that selective foraging could promote the expansion of invasive species and/or alter tree species composition in bottomland forest restorations. Islands may, however, serve as refuges from browsing on a regional scale.

Maximizing Stake Performance: Storage, Soaking, Scoring, and Auxin as Pre-Planting Techniques for Sandbar Willow (Salix exigua) Stakes

ABSTRACT: Willow (Salix spp.) stakes are used in riparian restoration due to their wetland adaptations, large root systems, and production of adventitious roots. Best practices for maximizing stake performance need to be developed. We tested the effects of storage, scoring, and auxin treatments on the performance of sandbar willow (Salix exigua) stakes. Storage significantly increased height, longest stem length, and biomass. Height and aboveground biomass increased when stakes were stored/soaked. There were no significant effects of scoring or auxin application. Results show storing and soaking significantly improve willow stake performance, which could lead to faster canopy closure and better invasive species control in the field, increasing restoration success.

How Does Simulated Gopher Disturbance Affect the Establishment of Holcus lanatus L. (Poaceae) in Calfornia Coastal Prairie

Abstract Disturbance often has the net effect of promoting invasive plant establishment, but the precise nature of the relationship between disturbance and invasion can depend on community context. We used simulated gopher mounds in bare and monoculture plots of three California native grasses (Bromus carinatus var. maritimus [Piper] C. L. Hitch., Festuca rubra L., and Calamagrostis nutkaensis [J. Presl] Steudel) to test the effects of mounds on seedling establishment and survival of the European perennial grass Holcus lanatus L. Soil disturbance treatments were crossed with manipulations of the plant canopy (shade cages in bare and Bromus plots vs. pinning back grass leaves in Festuca and Calamagrostis plots) to separate some of the positive and negative effects of natural mounds. Mean PAR measured in February at the soil surface varied from 11 to 969 µm/m2/s. As predicted, shade structures that decreased light availability but also increased soil moisture generally increased Holcus seedling establishment and survival in bare and Bromus plots. In contrast, Holcus seedling establishment increased in response to increased light availability and soil disturbance in Festuca and Calamagrostis plots, even where water availability was lower. Thus, the relative importance of light and water availability across plot types appeared to determine the effect of disturbance on invasive plant establishment. Ultimately, Holcus survival was low on mounds in bare plots and on unshaded mounds in Bromus plots, and similarly low numbers of Holcus seedlings survived across all treatment combinations in Festuca and Calamagrostis plots. Therefore, our results do not support the hypothesis that gopher mounds explain the invasion of Holcus in native-dominated coastal prairie sites.