When Invasive Plants Disappear: Transformative Restoration Possibilities in the Western United States Resulting from Climate Change

Most ecologists believe that climate change poses a significant threat to the persistence of native species. However, in some areas climate change may reduce or eliminate non-native invasive species, creating opportunities for restoration. If invasive species are no longer suited to novel climate conditions, the native communities that they replaced may not be viable either. If neither invasive nor native species are climatically viable, a type of "transformative" restoration will be required, involving the translocation of novel species that can survive and reproduce under new climate conditions. Here, we illustrate one approach for restoration planning by using bioclimatic envelope modeling to identify restoration opportunities in the western United States, where the invasive plant cheatgrass ( Bromus tectorum ) is no longer climatically viable under 2100 conditions projected by the Geophysical Fluid Dynamics Laboratory (GFDL2.1) coupled atmosphere-ocean general circulation model. We then select one example of a restoration target area and identify novel plant species that could become viable at the site in the wake of climate change. We do so by identifying the closest sites that currently have climate conditions similar to those projected at the restoration target area in 2100. This approach is a first step toward identifying appropriate species for transformative restoration.     

Does prescribed burning mean a threat to the rare satyrine butterfly Hipparchia fagi? Larval-habitat preferences give the answer

The ecological effects of fire management, especially regarding arthropods are poorly investigated. Burning in winter was assumed to pose a threat to butterfly species hibernating as larvae. To assess the impact of prescribed burning on population viability, we analysed larval-habitat preferences of the highly endangered, xero-thermophilous butterfly Hipparchia fagi in vineyards of the Kaiserstuhl region (southern Germany). Microhabitat preference analyses for mature larvae and egg-laying females revealed a preference of H. fagi for Bromus erectus-dominated communities with sparse vegetation coverage and a distinct tuft growth of the host plant B. erectus on microclimatically benefited slopes. We explain the preference of B. erectus by a preference of vegetation structure. The grass tufts offer a suitable climatically buffered living space for larvae. Egg deposition took place on dry substrate at positions of high solar radiation, thus adapted to hot and dry microclimate. As the larval habitat was sparsely vegetated as well as generally legally protected, fire management was not applicable and therefore not affecting the populations. We think it is conceivable that H. fagi, occurring here at its northern range limit, might expand its larval habitat into denser, combustible B. erectus stands in the course of global warming. A change in habitat preferences would necessitate a re-evaluation of management options.     

Plant–Soil Feedbacks Contribute to the Persistence of Bromus inermis in Tallgrass Prairie

As invasive plants become a greater threat to native ecosystems, we need to improve our understanding of the factors underlying their success and persistence. Over the past 30 years, the C₃ nonnative plant Bromus inermis (smooth brome) has been spreading throughout the central grasslands in North America. Invasion by this grass has resulted in the local displacement of natives, including the tallgrass species Panicum virgatum (switchgrass). To determine if factors related to resource availability and plant–soil interactions were conferring a competitive advantage on smooth brome, field plots were set up under varying nitrogen (N) levels. Plots composed of a 1:1 ratio of smooth brome and switchgrass were located in a restored tallgrass prairie and were randomly assigned one of the following three N levels: (a) NH₄NO₃ added to increase available N, (b) sucrose added to reduce available N, and (c) no additions to serve as control. In addition, soil N status, soil respiration rates, plant growth, and litter decomposition rates were monitored. Results indicate that by the 2nd year, the addition of sucrose significantly reduced available soil N and additions of NH₄NO₃ increased it. Further, smooth brome had greater tiller density, mass, and canopy interception of light on N-enriched soils, whereas none of these characteristics were stimulated by added N in the case of switchgrass. This suggests that smooth brome may have a competitive advantage on higher-N soils. Smooth-brome plant tissue also had a lower carbon–nitrogen (C:N) ratio and a higher decomposition rate than switchgrass and thus may cycle N more rapidly in the plant–soil system. These differences suggest a possible mechanism for the persistence of smooth brome in the tallgrass prairie: Efficient recycling of nutrient-rich litter under patches of smooth brome may confer a competitive advantage that enables it to persist in remnant or restored prairies. Increased N deposition associated with human activity and changing land use may play a critical role in the persistence of smooth brome and other N-philic exotic species.     

Predation of annual grass weed seeds in arable field margins

Seeds of three species of annual grass weeds (Alopecurus myosuroides, Avena fatua and Bromus sterilis) were placed in field margins around arable fields in a randomised block split-plot design experiment. The field margin vegetation was either sown or naturally regenerated and either cut or uncut. The seeds were either placed in cages designed to exclude small mammals and birds or were uncaged. The proportion of seeds removed was monitored on 10 occasions and mean seed loss was analysed. In general, a greater proportion of weed seeds was removed from uncaged trays in uncut swards, suggesting predation by small mammals, which inhabit tall grass. This effect was mainly due to removal of seeds of the two large-seeded species (A. fatua and B. sterilis), with A. fatua being especially favoured. It is therefore likely that small mammals play a role in the population dynamics of major crop weeds by feeding on their seeds in field margins, especially when these are dense and uncut.     

Introduced annual grass increases regional fire activity across the arid western USA (1980–2009)

Non‐native, invasive grasses have been linked to altered grass‐fire cycles worldwide. Although a few studies have quantified resulting changes in fire activity at local scales, and many have speculated about larger scales, regional alterations to fire regimes remain poorly documented. We assessed the influence of large‐scale Bromus tectorum (hereafter cheatgrass) invasion on fire size, duration, spread rate, and interannual variability in comparison to other prominent land cover classes across the Great Basin, USA. We compared regional land cover maps to burned area measured using the Moderate Resolution Imaging Spectroradiometer (MODIS) for 2000–2009 and to fire extents recorded by the USGS registry of fires from 1980 to 2009. Cheatgrass dominates at least 6% of the central Great Basin (650 000 km²). MODIS records show that 13% of these cheatgrass‐dominated lands burned, resulting in a fire return interval of 78 years for any given location within cheatgrass. This proportion was more than double the amount burned across all other vegetation types (range: 0.5–6% burned). During the 1990s, this difference was even more extreme, with cheatgrass burning nearly four times more frequently than any native vegetation type (16% of cheatgrass burned compared to 1–5% of native vegetation). Cheatgrass was also disproportionately represented in the largest fires, comprising 24% of the land area of the 50 largest fires recorded by MODIS during the 2000s. Furthermore, multi‐date fires that burned across multiple vegetation types were significantly more likely to have started in cheatgrass. Finally, cheatgrass fires showed a strong interannual response to wet years, a trend only weakly observed in native vegetation types. These results demonstrate that cheatgrass invasion has substantially altered the regional fire regime. Although this result has been suspected by managers for decades, this study is the first to document recent cheatgrass‐driven fire regimes at a regional scale.     

A comparative analysis of the temperature response of leaf elongation in Bromus stamineus and Lolium perenne plants in the field: intrinsic and size-mediated effects

BACKGROUND AND AIMS: Growth of grass species in temperate-humid regions is restricted by low temperatures. This study analyses the origin (intrinsic or size-mediated) and mechanisms (activity of individual meristems vs. number of active meristems) of differences between Bromus stamineus and Lolium perenne in the response of leaf elongation to moderately low temperatures. METHODS: Field experiments were conducted at Balcarce, Argentina over 2 years (2003 and 2004) using four cultivars, two of B. stamineus and two of L. perenne. Leaf elongation rate (LER) per tiller and of each growing leaf, number of growing leaves and total leaf length per tiller were measured on 15-20 tillers per cultivar, for 12 (2003) or 10 weeks (2004) during autumn and winter. KEY RESULTS: LER was faster in B. stamineus than in L. perenne. In part, this was related to size-mediated effects, as total leaf length per tiller correlated with LER and B. stamineus tillers were 71% larger than L. perenne tillers. However, accounting for size effects revealed intrinsic differences between species in their temperature response. These were based on the number of leaf meristems simultaneously active and not on the (maximum) rate at which individual leaves elongated. Species differences were greater at higher temperatures, being barely notable below 5 degrees C (air temperature). CONCLUSIONS: Bromus stamineus can sustain a higher LER per tiller than L. perenne at air temperatures > 6 degrees C. In the field, this effect would be compounded with time as higher elongation rates lead to greater tiller sizes.     

Enemy release from the effects of generalist granivores can facilitate Bromus tectorum invasion in the Great Basin Desert

The enemy release hypothesis (ERH) of plant invasion asserts that natural enemies limit populations of invasive plants more strongly in native ranges than in non‐native ranges. Despite considerable empirical attention, few studies have directly tested this idea, especially with respect to generalist herbivores. This knowledge gap is important because escaping the effects of generalists is a critical aspect of the ERH that may help explain successful plant invasions. Here, we used consumer exclosures and seed addition experiments to contrast the effects of granivorous rodents (an important guild of generalists) on the establishment of cheatgrass (Bromus tectorum) in western Asia, where cheatgrass is native, versus the Great Basin Desert, USA, where cheatgrass is exotic and highly invasive. Consistent with the ERH, rodent foraging reduced cheatgrass establishment by nearly 60% in western Asia but had no effect in the Great Basin. This main result corresponded with a region‐specific foraging pattern: rodents in the Great Basin but not western Asia generally avoided seeds from cheatgrass relative to seeds from native competitors. Our results suggest that enemy release from the effects of an important guild of generalists may contribute to the explosive success of cheatgrass in the Great Basin. These findings corroborate classic theory on enemy release and expand our understanding of how generalists can influence the trajectory of exotic plant invasions.     

Effect of strain and cultural conditions on the production of cytochalasin B by the potential mycoherbicide Pyrenophora semeniperda (Pleosporaceae,Pleosporales)

The seed pathogen Pyrenophora semeniperda has demonstrated potential as a mycoherbicidal biocontrol for eliminating persistent seed banks of annual bromes on western North American rangelands. This pathogen exhibits variation in virulence that is related to mycelial growth rate, but direct laboratory tests of virulence on seeds often have low repeatability. We developed a rapid and sensitive high pressure liquid chromatography method for quantification of the phytotoxin cytochalasin B in complex mixtures in order to evaluate its use in virulence screening. All 10 strains tested produced large quantities of this metabolite in solid wheat seed culture, with production varying over a fourfold range (535–2256 mg kg^?1). No cytochalasin B was produced in liquid potato dextrose broth culture, showing that its synthesis is strongly dependent on cultural conditions. In a Bromus tectorum coleoptile bioassay, solid culture extracts showed mild toxicity similar to the cytochalasin B standard at a concentration equivalent to 10^?4 M cytochalasin B (72–95% of control), whereas at 10^?3 M equivalent, the extracts exhibited significantly higher toxicity (8–18% of control) than the cytochalasin B standard (34% of control). This suggests the possible presence of other phytotoxic metabolites. Cytochalasin B production in solid wheat seed culture exhibited the predicted significant negative correlation with mycelial growth rate on potato dextrose agar, but the correlation was not very strong, possibly because cytochalasin B production and growth rate were measured under different cultural conditions.