Laura G. Perry

Immobilizing nitrogen to control plant invasion

Increased soil N availability may often facilitate plant invasions. Therefore, lowering N availability might reduce these invasions and favor desired species. Here, we review the potential efficacy of several commonly proposed management approaches for lowering N availability to control invasion, including soil C addition, burning, grazing, topsoil removal, and biomass removal, as well as a less frequently proposed management approach for lowering N availability, establishment of plant species adapted to low N availability. We conclude that many of these approaches may be promising for lowering N availability by stimulating N immobilization, even though most are generally ineffective for removing N from ecosystems (excepting topsoil removal). C addition and topsoil removal are the most reliable approaches for lowering N availability, and often favor desired species over invasive species, but are too expensive or destructive, respectively, for most management applications. Less intensive approaches, such as establishing low-N plant species, burning, grazing and biomass removal, are less expensive than C addition and may lower N availability if they favor plant species that are adapted to low N availability, produce high C:N tissue, and thus stimulate N immobilization. Regardless of the method used, lowering N availability sufficiently to reduce invasion will be difficult, particularly in sites with high atmospheric N deposition or agricultural runoff. Therefore, where feasible, the disturbances that result in high N availability should be limited in order to reduce invasions by nitrophilic weeds.

Elevated CO2 does not offset greater water stress predicted under climate change for native and exotic riparian plants

In semiarid western North American riparian ecosystems, increased drought and lower streamflows under climate change may reduce plant growth and recruitment, and favor drought-tolerant exotic species over mesic native species. We tested whether elevated atmospheric CO₂ might ameliorate these effects by improving plant water-use efficiency. We examined the effects of CO₂ and water availability on seedlings of two native (Populus deltoides spp. monilifera, Salix exigua) and three exotic (Elaeagnus angustifolia, Tamarix spp., Ulmus pumila) western North American riparian species in a CO₂-controlled glasshouse, using 1-m-deep pots with different water-table decline rates. Low water availability reduced seedling biomass by 70-97%, and hindered the native species more than the exotics. Elevated CO₂ increased biomass by 15%, with similar effects on natives and exotics. Elevated CO₂ increased intrinsic water-use efficiency (Δ¹³C(leaf) ), but did not increase biomass more in drier treatments than wetter treatments. The moderate positive effects of elevated CO₂ on riparian seedlings are unlikely to counteract the large negative effects of increased aridity projected under climate change. Our results suggest that increased aridity will reduce riparian seedling growth despite elevated CO₂, and will reduce growth more for native Salix and Populus than for drought-tolerant exotic species.

The influence of light availability on competition between Phalaris arundinacea and a native wetland sedge

Invasions by Phalaris arundinacea (reed canarygrass) preclude establishment of sedge meadow vegetation in restored wetlands in the midwest USA. To evaluate cover crops as a potential method of P. arundinacea control, we examined the effects of lowering light availability (from 600 to 200 and 10 μmol m−2 s−1) on competition between P. arundinacea and a common wetland sedge, Carex hystericina (porcupine sedge), in a greenhouse experiment. Lowering light availability substantially reduced P. arundinacea total biomass, by 52% at 200 μmol m−2 s−1 and by 99% at 10 μmol m−2 s−1. However, shade also reduced C. hystericina total biomass, by 62% at 200 μmol m−2 s−1 and by 99% at 10 μmol m−2 s−1. Further, shade did not favor C. hystericina in competition with P. arundinacea. Instead, C. hystericina biomass was reduced significantly more by competition with P. arundinacea in partial shade than in full sun (by 69% compared to 58%), while P. arundinacea biomass was reduced significantly less by competition with C. hystericina in partial shade than in full sun (by 42% compared to 66%). Lowering light availability with a cover crop in restored prairie pothole wetlands might slow P. arundinacea invasion, but is unlikely to improve sedge meadow establishment.

Founder control and coexistence in a simple model of asymmetric competition for light.

Size asymmetry in plant light acquisition complicates predictions of competitive outcomes in light-limited communities. We present a mathematically tractable model of asymmetric competition for light and discuss its implications for predicting outcomes of competition during establishment in two-, three-, and many-species communities. In contrast to the resource-reduction model of symmetric competition for a single resource, the model we present predicts that outcomes of asymmetric competition for light will sometimes depend on the timing of establishment and the consequent hierarchy among species in canopy position. Competitive outcomes in the model depend on the minimum light requirements (L(c)) and self-shading of species lower in the canopy compared to the light available (L(out)(*)) beneath species higher in the canopy. Succession progresses towards species with decreasing values for L(c), but arrested successions occur when initial dominants have relatively high values for L(c) but low values for L(out)(*), leading to founder control. A theoretically limitless number of species may coexist in competition for light when dominance is founder controlled. These model predictions have implications for an array of applied ecological questions, including methods to control invasive species in light-limited restored ecosystems.

Native cover crops suppress exotic annuals and favor native perennials in a greenhouse competition experiment

In a greenhouse experiment, we examined the effectiveness of four native cover crops for controlling four exotic, invasive species and increasing success of four western North American grassland species. Planting the annual cover crops, annual ragweed (Ambrosia artemisiifolia) and common sunflower (Helianthus annuus), reduced the biomass of the exotic species cheatgrass (Bromus tectorum), Japanese brome (Bromus japonicus), Canada thistle (Cirsium arvense), and whitetop (Cardaria draba). The annual cover crops also reduced the desired species biomass in competition with the perennial exotics, but either increased or did not affect the desired species biomass in competition with the annual exotics. Planting the perennial cover crops, Canada goldenrod (Solidago canadensis) and littleleaf pussytoes (Antennaria microphylla), rarely inhibited exotic species, but did increase the desired species biomass. Field experiments are needed to test the cover crops under more ecologically relevant conditions, but our results suggested that the annual cover crops may be effective for controlling invasive annuals and for facilitating native perennials.

Phytotoxic Allelochemicals From Roots and Root Exudates of Leafy Spurge (Euphorbia esula L.)

Invasive plants are a widespread problem but the mechanisms used by these plants to become invasive are often unknown. The production of phytotoxic natural products by invasive weeds is one mechanism by which these species may become successful competitors. Here, we conducted a bioactivity-driven fractionation of root extracts and exudates from the invasive plant leafy spurge (Euphorbia esula L.), and structurally characterized jatrophane diterpenes and ellagic acid derivatives. Ellagic acid derivatives and one of the jatrophane diterpenes, esulone A, have been previously reported from leafy spurge, but another of the jatrophane diterpenes, kasuinine B, has not. We show that these compounds are phytotoxic but affect plants in different ways, either inducing overall plant necrosis or reducing root branching and elongation.

Light competition for invasive species control: A model of cover crop–weed competition and implications for Phalaris arundinacea control in sedge meadow wetlands

SummarySince resource competition plays a critical role in many plant invasions, controlling invasive vegetation may require managing the supply of limiting resources. For example, lowering light availability with a cover crop might prevent invasions during community establishment in light-limited restored ecosystems. However, most cover crops evaluated for invasive species control either do not adequately suppress invasives or equally suppress desired species. To improve our ability to predict cover crop effectiveness, we use a theoretical model of plant competition to identify potential mechanisms by which cover crops might favor desired species over invasives. In addition, we consider the models implications for controlling an invasive forage, Phalarisarundinacea, in restored sedge meadows. The model suggests that cover crops will improve the outcome of competition between desired and invasive species only when (1) desired species have lower minimum light requirements than invasive species and (2) invasive species dominance results from rapid establishment and resource preemption. Cover crops in the model favor desired species over faster-growing invasives because faster-growing invasives are positioned higher in the canopy. Invasive species higher in the canopy shade desired species more than desired species shade invasives. Consequently, by reducing invasive species biomass, cover crops give desired species a competitive advantage. The simple requirements for cover crop success in the model suggest that cover crops may be effective for invasive species control in light-limited restored ecosystems. The available information on P. arundinacea responses to shade suggests, however, that cover crops are unlikely to favor sedge meadow species over P. arundinacea.

Implications of climate change for water management of an arid inland lake in Northwest China

Abstract To evaluate potential effects of climate change on the water budget of Bosten Lake, the largest inland freshwater lake in China, we evaluated trends and step change points between 1980 and 2011 in evaporation, air temperature, and precipitation. Significant increases in air temperature accelerated glacier melt, increasing flow in the Kaidu River, which ultimately discharges into Bosten Lake. The increased inflow resulted in increased lake water depth and 10% greater water surface area. Increased surface area along with higher temperatures led to increased lake evapotranspiration. If glaciers continue to recede and snowpacks continue to decline with projected warmer temperatures under climate change, inflows to Bosten Lake will decrease substantially in the future, with critical implications for long-term water resource management. Our calculations suggest that the water surface area of Bosten Lake should be maintained >963.14 km2 to maintain low lake water salinity (total dissolved solids: TDS ≤ 1.5 g/L), 3-year and 5-year means for annual net inflows to Bosten Lake must be approximately 21.80 × 108 m3 and 23.50 × 108 m3, respectively, and Kaidu River mean runoff must be approximately 35.83 × 108 m3 to maintain that surface area while supplying 14.42 × 108 m3/yr of water to meet public water demand in the Kongqi Basin. Water should only be diverted from Bosten Lake to the lower Tarim River for ecological restoration in years when Kaidu River annual runoff is >35.83 × 108 m3. To minimize evaporation loss, water diversions from Bosten Lake should occur between May and September.

Root Exudation and Rhizosphere Biology: Multiple Functions of a Plant Secondary Metabolite

Plant root exudates have important effects on plant and soil communities, mediating positive and antagonistic plant-plant and plant-microbe interactions, and altering soil processes. Here, we discuss the potential for a single plant secondary metabolite to serve multiple functions in the rhizosphere. Secondary metabolites that serve multiple functions may provide plants with multiple benefits at relatively low metabolic cost, with consequences for competitive ability, disease resistance, and resource availability. Specifically, we describe recent research on (±)-catechin, a secondary metabolite exuded from the roots of the invasive weed Centaurea maculosa. Depending on concentration and the species that are present, (±)-catechin has the potential to act as an allelochemical, autoinhibitor, and antimicrobial agent, and to increase soil nutrient availability.

Riparian soil development linked to forest succession above and below dams along the Elwha River, Washington, USA

Riparian forest soils can be highly dynamic, due to frequent fluvial disturbance, erosion, and sediment deposition, but effects of dams on riparian soils are poorly understood. We examined soils along toposequences within three river segments located upstream, between, and downstream of two dams on the Elwha River to evaluate relationships between riparian soil development and forest age, succession, and channel proximity, explore dam effects on riparian soils, and provide a baseline for the largest dam removal in history. We found that older, later-successional forests and geomorphic surfaces contained soils with finer texture and greater depth to cobble, supporting greater forest floor mass, mineral soil nutrient levels, and cation exchange. Forest stand age was a better predictor than channel proximity for many soil characteristics, though elevation and distance from the channel were often also important, highlighting how complex interactions between fluvial disturbance, sediment deposition, and biotic retention regulate soil development in this ecosystem. Soils between the dams, and to a lesser extent below the lower dam, had finer textures and higher mineral soil carbon, nitrogen, and cation exchange than above the dams. These results suggested that decreased fluvial disturbance below the dams, due to reduced sediment supply and channel stabilization, accelerated soil development. In addition, reduced sediment supply below the dams may have decreased soil phosphorus. Soil δ15N suggested that salmon exclusion by the dams had no discernable effect on nitrogen inputs to upstream soils. Recent dam removal may alter riparian soils further, with ongoing implications for riparian ecosystems.