Non-native Species Cover Research Articles

Thinning and chipping small-diameter ponderosa pine changes understory plant communities on the Colorado Front Range

Novel fire mitigation treatments that chip harvested biomass on site are increasingly prescribed to reduce the density of small-diameter trees, yet the ecological effects of these treatments are unknown. Our objective was to investigate the impacts of mechanical thinning and whole tree chipping on Pinus ponderosa (ponderosa pine) regeneration and understory plant communities to guide applications of these new fuel disposal methods. We sampled in three treatments: (1) unthinned forests (control), (2) thinned forests with harvested biomass removed (thin-only), and (3) thinned forests with harvested biomass chipped and broadcast on site (thin + chip). Plots were located in a ponderosa pine forest of Colorado and vegetation was sampled three to five growing seasons following treatment. Forest litter depth, augmented with chipped biomass, had a negative relationship with cover of understory plant species. In situ chipping often produces a mosaic of chipped patches tens of meters in size, creating a range of woodchip depths including areas lacking woodchip cover within thinned and chipped forest stands. Thin-only and thin + chip treatments had similar overall abundance and species richness of understory plants at the stand scale, but at smaller spatial scales, areas within thin + chip treatments that were free of woodchip cover had an increased abundance of understory vegetation compared to all other areas sampled. Relative cover of non-native plant species was significantly higher in the thin-only treatments compared to control and thin + chip areas. Thin + chip treated forests also had a significantly different understory plant community composition compared to control or thin-only treatments, including an increased richness of rhizomatous plant species. We suggest that thinning followed by either chipping or removing the harvested biomass could alter understory plant species composition in ponderosa pine forests of Colorado. When considering post-treatment responses, managers should be particularly aware of both the depth and the distribution of chipped biomass that is left in forested landscapes.

Feral horses dung piles as potential invasion windows for alien plant species in natural grasslands

Small scale disturbances could act as patches that provide sites for the colonization of competitively inferior species, promoting the establishment of non-native species in some cases. We analyzed the vegetation associated with feral horse dung piles in montane pampas grasslands in Mid-East Argentina and described the changes following their abandonment, evaluating whether dung piles act as invasion windows, allowing the entrance of alien plant species. We estimated the portion of the study area directly covered by horse manure and dung height was used to estimate the time elapsed after the abandonment of each pile. Vegetation replacement on dung piles of different ages was assessed and compared with grassland controls using discriminant analysis. We used regression analysis to look for changes in vegetation cover, species richness, species diversity and evenness in response to height (age) of the dung piles, and principal component analyses (PCA) to identify groups of plants associated with different successional stages. We compared cover of alien plant species on dung piles with grassland controls using one-way ANOVA. On average, 2.5% of the study area was covered by horse dung. Total vegetation cover, species richness, diversity and evenness increased after the piles were abandoned. Characteristic plant groups were associated with initial, middle and last phases of the studied succession. Vegetation on the dung piles significantly differed from that in grassland controls and two species were consistently associated with dung piles: the invasive Red Star Thistle, Centaurea calcitrapa, and a native grazing-intolerant grass, Nassella clarazii. Non-native species cover was also higher in dung piles than in control plots. Dung piles cover a significant portion of grassland area in our study site, produce significant changes in the vegetation and are associated with some invasive alien plants that could eventually colonize more pristine areas in the vicinity. On the other hand, they might represent refuges for palatable species, since horses seem to avoid them for grazing.

Ecological restoration needs derived from reference conditions for a semi-arid landscape in Western Colorado, USA

Semi-arid ecosystems cover tens of millions of hectares in the Intermountain West of the United States, and most have altered plant communities due to land use, especially livestock grazing. Thus, relatively unaltered “reference” plant community information is needed to guide restoration. Plant communities were sampled over a large (∼600 000 ha) semi-arid landscape in western Colorado, within piñon–juniper woodlands, sagebrush shrublands, and grasslands, and over conditions ranging from relict areas without livestock grazing to heavily utilized areas. Ordination was used to group samples into ranked categories of ecological condition within 18 communities, and means–tests and other techniques were used to identify ecological differences among ranks across vegetation types. With few exceptions, grass and forb cover, biological soil crust cover, and species diversity declined concomitant with ranks representing increasingly degraded conditions, while non-native species cover was variable among ranks. Landscape-level species abundance patterns also differed significantly among ranks, with herbaceous species generally exhibiting less constancy and cover within degraded samples compared to samples of reference quality. These semi-arid ecosystems will require both active (e.g., reseeding) and passive (grazing management) restoration approaches, at local- and landscape-levels, respectively, if the goal is to restore native plant composition and abundance.

Reduced soil compaction enhances establishment of non-native plant species

Many studies have shown that soil disturbance facilitates establishment of invasive, non-native plant species, and a number of mechanisms have been isolated that contribute to the process. To our knowledge no studies have isolated the role of altered soil compaction, a likely correlate of many types of soil disturbance, in facilitating invasion. To address this, we measured the response of seeded non-native and native plant species to four levels of soil compaction in mesocosms placed in an abandoned agricultural field in the Methow Valley, Washington, USA. Soil compaction levels reflected the range of resistance to penetration (0.1–3.0 kg cm−2) measured on disturbed soils throughout the study system prior to the experiment. Percent cover of non-native species, namely Bromus tectorum and Centaurea diffusa, decreased by 34% from the least to the most compacted treatments, whereas percent cover of native species, mostly Pseudoroegneria spicata and Lupinus spp., did not respond to compaction treatments. Experimental results were supported by a survey of soil penetration resistance and percent cover by species in 18 abandoned agricultural fields. Percent cover of B. tectorum was negatively related to soil compaction levels, whereas none of the native species showed any response to soil compaction. These results highlight a potentially important, though overlooked, aspect of soil disturbance that may contribute to subsequent non-native plant establishment.

Historical influences on the vegetation and soils of the Martha’s Vineyard, Massachusetts coastal sandplain: Implications for conservation and restoration

Both disturbance history and previous land use influence present-day vegetation and soils. These influences can have important implications for conservation of plant communities if former disturbance and land use change species abundances, increase colonization of nonnative plant species or if they alter soil characteristics in ways that make them less suitable for species of conservation interest. We compared the plant species composition, the proportion of native and nonnative plant species, and soil biogeochemical characteristics across seven dominant land use and vegetation cover types on the outwash sandplain of Martha’s Vineyard that differed in previous soil tillage, dominant overstory vegetation and history of recent prescribed fire. The outwash sandplain supports many native plant species adapted to dry, low nutrient conditions and maintenance of native species is a management concern. There was broad overlap in the plant species composition among pine ( Pinus resinosa, P. strobus) plantations on untilled soils, pine plantations on formerly tilled soils, scrub oak ( Quercus ilicifolia) shrublands, tree oak ( Q. velutina, Q. alba) woodlands, burned tree oak woodlands, and sandplain grasslands. All of these land cover categories contained few nonnative species. In contrast, agricultural grasslands had high richness and cover of nonnative plants. Soil characteristics were also similar among all of the woodland, shrubland and grassland land cover categories, but soils in agricultural grasslands had higher pH, extractable Ca 2+ and Mg 2+ in mineral soils and higher rates of net nitrification. The similarity of soils and significant overlap in vegetation across pine plantations, scrub oak shrublands, oak woodlands and sandplain grasslands suggests that the history of land use, current vegetation and soil characteristics do not pose a major barrier to management strategies that would involve conversion among any of these vegetation types. The current presence of high cover of nonnative species and nutrient-enriched soils in agricultural grasslands, however, may pose a barrier to expansion of sandplain grasslands or shrublands on these former agricultural lands if native species are not able to outcompete nonnative species in these anthropogenically-enriched sites.

Conservation Value of Clustered Housing Developments

Traditionally, exurban lands in Colorado have been subdivided into a grid of parcels ranging from 2 to 16 ha. From an ecological perspective, this dispersed pattern of development effectively maximizes the individual influence of each home on the land. Clustered housing developments, designed to maximize open space, are assumed to benefit plant and wildlife communities of conservation interest. They have become a popular alternative for rural development despite the lack of empirical evidence demonstrating their conservation benefits. To better inform rural land-use planning, we evaluated clustered housing developments by comparing their spatial pattern with that of dispersed housing developments and by comparing their conservation value with that of both dispersed housing developments and undeveloped areas in Boulder County, Colorado. We used four indicators to assess conservation value: (1) densities of songbirds, (2) nest density and survival of ground-nesting birds, (3) presence of mammals, and (4) percent cover and proportion of native and non-native plant species. Clustered and dispersed housing developments did not differ on the majority of variables we examined. Both types of housing development had significantly higher densities of non-native and human-commensal species and significantly lower densities of native and human-sensitive species than undeveloped areas. More rigorous ecological guidelines and planning on a regional scale may help create clustered developments with higher conservation value.

EVALUATING PLANT INVASIONS FROM BOTH HABITAT AND SPECIES PERSPECTIVES

We present an approach to quantitatively assess nonnative plant invasions at landscape scales from both habitat and species perspectives. Our case study included 34 nonnative species found in 142 plots (0.1 ha) in 14 vegeta- tion types within the Grand Staircase-Escalante National Monument, Utah. A plot invasion index, based on nonnative species richness and cover, showed that only 16 of 142 plots were heavily invaded. A species invasive index, based on frequency, cover, and number of vegetation types invaded, showed that only 7 of 34 plant species were highly invasive. Multiple regressions using habitat characteristics (moisture index, elevation, soil P, native species richness, maximum crust development class, bare ground, and rock) explained 60% of variation in nonnative species richness and 46% of variation in nonnative species cover. Three mesic habitats (aspen, wet meadow, and perennial riparian types) were par- ticularly invaded (31 of 34 nonnative species studied were found in these types). Species-specific logistic regression models for the 7 most invasive species correctly predicted occurrence 89% of the time on average (from 80% for Bromus tectorum, a habitat generalist, to 93% for Tamarix spp., a habitat specialist). Even with such a modest sampling intensity (<0.1% of the landscape), this multiscale sampling scheme was effective at evaluating habitat vulnerability to invasion and the occurrence of the 7 most invasive nonnative species. This approach could be applied in other natural areas to develop strategies to document invasive species and invaded habitats.

Establishment of non-native plant species after wildfires: effects of fuel treatments, abiotic and biotic factors, and post-fire grass seeding treatments

Establishment and spread of non-native species following wildfires can pose threats to long-term native plant recovery. Factors such as disturbance severity, resource availability, and propagule pressure may influence where non-native species establish in burned areas. In addition, pre- and post-fire management activities may influence the likelihood of non-native species establishment. In the present study we examine the establishment of non-native species after wildfires in relation to native species richness, fire severity, dominant native plant cover, resource availability, and pre- and post-fire management actions (fuel treatments and post-fire rehabilitation treatments). We used an information-theoretic approach to compare alternative hypotheses. We analysed post-fire effects at multiple scales at three wildfires in Colorado and New Mexico. For large and small spatial scales at all fires, fire severity was the most consistent predictor of non-native species cover. Non-native species cover was also correlated with high native species richness, low native dominant species cover, and high seeded grass cover. There was a positive, but non-significant, association of non-native species with fuel-treated areas at one wildfire. While there may be some potential for fuels treatments to promote non-native species establishment, wildfire and post-fire seeding treatments seem to have a larger impact on non-native species.

Life-history Habitat Matching in Invading Non-native Plant Species

We briefly reviewed the literature on habitat matching in invading non-native plant species. Then we hypothesized that the richness and cover of native annual and perennial plant species integrate complex local information of vegetation and soils that would help to predict invasion success by similarly adapted non-native plant species. We tested these ‘life-history habitat matching’ relationships in 603 0.1-ha plots, including 294 plots in Colorado, which were relatively high for the cover of native perennial plant species, and for 309 0.1-ha plots in southern Utah, which were relatively high in the cover of native annual plant species. We found strong positive relationships between the richness and foliar cover for both native and non-native species, whether they were annual or perennial species (0.34 > r 2 < 0.53; P < 0.0001). We also found significant positive relationships between the cover of native annual species at a site and the richness (r 2 = 0.13; P < 0.0001) and the foliar cover (r 2 = 0.06; P < 0.0001) of non-native annual species. The proportion of non-native annual species in the flora of a plot also increased significantly with the foliar cover of native annual species. Conversely, the richness and cover of non-native annual species were significantly negatively associated with the foliar cover of native perennial species (r 2 = 0.05 and 0.06, respectively; P < 0.0001). The cover of non-native annual or perennial species was not significantly correlated with soil texture variables, %N, or %C. We conclude that there may be a high degree of life-history habitat matching by non-native annual species in these study sites. Information on native annual and perennial species richness and cover may help characterize the complex soils, climate, and disturbance environment in which similarly adapted non-native plant species establish and gain foliar cover.

SKI SLOPE VEGETATION OF LEE CANYON, NEVADA, USA

Ski-slope plant communities on Lee Canyon in the Spring Mountains of southern Nevada were surveyed to assess effects of disturbance and environmental factors important in structuring vegetation. Ski runs extended from 2,620 to 2,890 m on the north face of Mount Charleston. Eleven plant communities were identified, including 4 types only on slopes, 1 type only in adjacent forest, and 6 types that occurred on both slopes and in forest. Distance to gravel service roads on the ski runs, soil compaction, ski run width, distance to the forest edge, and gullies were the most important variables in structuring vegetation. Nonnative species were found at all elevations and were frequently dominant. Nonnative species cover and richness were negatively correlated with percent bare area, because the nonnative species vegetated the slopes more densely than the native species. Relative to ski runs in other areas, those on Mount Charleston had high nonnative species invasion and few bare areas; vegetation rarely resembled that of surrounding forests and meadows, except in areas of heavy tree invasion.

Assessing Vulnerability to Invasion by Nonnative Plant Species at Multiple Spatial Scales

Basic information on where nonnative plant species have successfully invaded is lacking. We assessed the vulnerability of 22 vegetation types (25 sets of four plots in nine study areas) to nonnative plant invasions in the north-central United States. In general, habitats with high native species richness were more heavily invaded than species-poor habitats, low-elevation areas were more invaded than high-elevation areas, and riparian zones were more invaded than nearby upland sites. For the 100 1000-m2 plots (across all vegetation types), 50% of the variation in nonnative species richness was explained by longitude, latitude, native plant species richness, soil total percentage nitrogen, and mean maximum July temperature (n = 100 plots; P < 0.001). At the vegetation-type scale (n = 25 sets of four 1000-m2 plots/type), 64% of the variation in nonnative species richness was explained by native plant species richness, elevation, and October to June precipitation (P < 0.001). The foliar cover of nonnative species (log) was strongly positively correlated with the nonnative species richness at the plot scale (r = 0.77, P < 0.001) and vegetation-type scale (r = 0.83, P < 0.001). We concluded that, at the vegetation-type and regional scales in the north-central United States, (1) vegetation types rich in native species are often highly vulnerable to invasion by nonnative plant species; (2) where several nonnative species become established, nonnative species cover can substantially increase; (3) the attributes that maintain high native plant species richness (high light, water, nitrogen, and temperatures) also help maintain nonnative plant species richness; and (4) more care must be taken to preserve native species diversity in highly vulnerable habitats.

Fire, mowing, and hand-removal of woody species in restoring a native wetland prairie in the willamette valley of oregon

The invasion of prairies by woody species is a worldwide conservation concern. Fire is frequently used to inhibit this invasion. However, there is little documentation of the effect of fire in wetland prairies, which are also threatened with encroachment of woody species. The present study investigated wetland species responses to experimental burning, hand-removal of woody species, and mowing with removal of cut material. The possible ecological mechanisms responsible for individualistic responses of species, including direct mortality, ability to resprout, and release from competition are considered. We also evaluated these treatments as tools for meeting restoration objectives of reducing the abundance of woody species, reducing or preventing spread of non-native pest species, and increasing or at least maintaining native species’ abundance. After two years of treatments (1994 and 1996) three patterns emerged. 1) Woody species: Burning and hand-removal caused the greatest reductions in cover of woody species. Mowing with removal of cut material, however, did not reduce the cover of woody species compared to controls. As woody plant cover decreased, plant mortality increased, indicating that treatments influenced woody plant cover at least partially through mortality. 2) Native herbaceous species: Burning significantly decreased inflorescence production of Deschampsia cespitosa, the dominant wetland prairie grass. In contrast, burning, along with mowing, significantly increased flowering of Juncus tenuis. Flowering and cover of all native graminoids combined, however, showed no significant responses to treatments. Burning and hand-removal significantly promoted the cover of native forbs as a group, with Lotus purshiana and Veronica scutellata showing the greatest increases. 3) Non-native herbaceous species: Burning and hand-removal significantly reduced the cover of non-native forbs as a group and particularly reduced the cover of Hypericum perforatum. The number of inflorescences of non-native grasses (Holcus lanatus and Anthoxanthum odoratum) increased with hand-removal and mowing. Overall, no treatment was clearly superior in fulfilling the restoration objectives. Burning was effective in reducing woody cover and did not promote abundance of non-native herbaceous species. Burning, however, reduced the flowering of the key native grass, Deschampsia cespitosa. Hand-removal of woody species was also effective at reducing woody cover and promoted the abundance of some native species, but it sometimes increased the cover of non-native herbaceous species. Because mowing with removal of cut material was ineffective in reducing woody cover and tended to promote non-native herbaceous species, this treatment is not recommended as a management tool.

Ski Slope Vegetation of Mount Hood, Oregon, U.S.A.

Ski slope vegetation on Mount Hood, Oregon, U.S.A. was surveyed to assess vegetation that has developed under a constant disturbance regime and the environmental factors that are important in structuring the vegetation. Ski runs extend from 1200 to 2200 m on the south face of Mount Hood. TWINSPAN distinguished 17 plant communities including 4 above treeline and 13 below treeline; 3 of the latter also occur in the forest adjacent to the ski runs. Elevation, which is correlated to temperature, precipitation, depth of snowpack, and timing of snow melt, is the most important variable structuring the vegetation. Soil texture is also important. Distance to the forest boundary influences the vegetation only at lower elevations. Most of the slopes we examined were thickly vegetated, except above treeline where vegetation is typically sparse. Non-native species were detected only at the lowest elevations and were infrequently dominant. Non-native species cover and richness were correlated with percent bare area. Relative to ski runs in other areas, those on Mount Hood have little non-native species invasion, probably due to the harsh conditions prevalent on the ski runs and to a relative lack of bare areas, which occupy 17% of the runs below treeline. The relative scarcity of bare areas may be a result of the relatively great age of the ski runs (64 yr). Above treeline there was no detectable difference in vegetative composition on and off the ski run, probably due to extensive trampling of the vegetation in the summer.

Ski Slope Vegetation of Mount Hood, Oregon, U.S.A.

Ski slope vegetation on Mount Hood, Oregon, U.S.A. was surveyed to assess vegetation that has developed under a constant disturbance regime and the environmental factors that are important in structuring the vegetation. Ski runs extend from 1200 to 2200 m on the south face of Mount Hood. TWINSPAN distinguished 17 plant communities including 4 above treeline and 13 below treeline; 3 of the latter also occur in the forest adjacent to the ski runs. Elevation, which is correlated to temperature, precipitation, depth of snowpack, and timing of snow melt, is the most important variable structuring the vegetation. Soil texture is also important. Distance to the forest boundary influences the vegetation only at lower elevations. Most of the slopes we examined were thickly vegetated, except above treeline where vegetation is typically sparse. Non-native species were detected only at the lowest elevations and were infrequently dominant. Non-native species cover and richness were correlated with percent bare area. Relative to ski runs in other areas, those on Mount Hood have little non-native species invasion, probably due to the harsh conditions prevalent on the ski runs and to a relative lack of bare areas, which occupy 17% of the runs below treeline. The relative scarcity of bare areas may be a result of the relatively great age of the ski runs (64 yr). Above treeline there was no detectable difference in vegetative composition on and off the ski run, probably due to extensive trampling of the vegetation in the summer.