Exotic Plant Species Richness Research Articles

Land‐use intensity and relatedness to native plants promote exotic plant invasion in a tropical biodiversity hotspot

Abstract Exotic plant invasions threaten biodiversity and are costly to farmers. Land use is a major pathway promoting the spread of exotic plant species; however, little is known about the processes underlying the success of exotic plants in tropical agricultural landscapes. Focussing on the heterogeneous smallholder landscapes of north‐eastern Madagascar, we studied exotic plants of understorey communities across a land‐use intensity gradient from unburned lands (old‐growth forests, forest fragments and forest‐derived vanilla agroforests) to burned ones (fallow‐derived vanilla agroforests, woody fallows and herbaceous fallows). We quantified the absolute species richness, abundance and cover of exotic plants across land‐use types and their proportional contribution to community richness, abundance and cover as indicators of exotic plant invasion. We tested for the effects of land‐use parameters, namely land‐use history, canopy closure and landscape‐level forest cover, on exotic plants. Additionally, we tested whether the phylogenetic relatedness between exotic and native species in the same plot affected invasion success, testing Darwin's naturalisation and pre‐adaptation hypotheses. All indicators of exotic plant invasion were lowest in old‐growth forests and forest fragments and highest in fallow‐derived vanilla agroforests, woody fallows and herbaceous fallows. Absolute and proportional exotic richness were negatively affected by canopy closure, and landscapes with high forest cover had lower proportions of exotic plant richness. High phylogenetic relatedness between exotics and natives was associated with lower proportional richness but higher proportions of exotics in abundance and cover. However, individual exotic species showed contrasting responses to land‐use parameters and relatedness to natives. Synthesis and applications: Our results indicate that maintaining unburned lands, land‐use types with dense canopies and landscapes with high forest cover prevents the spread of exotic plants within the agricultural landscapes of north‐eastern Madagascar. Supporting Darwin's pre‐adaptation hypothesis, exotic plants that are phylogenetically closely related to native plants are more likely to become successful invaders in terms of abundance and cover. Nevertheless, individual species show different responses to land‐use changes and phylogenetic relatedness. Therefore, land‐use decisions and management choices can be tailored to limit the spread of exotic species and to preserve native plants in this global biodiversity hotspot.

Floristic homogenization as a result of the introduction of exotic species in China

AbstractAimThe introduction of exotic plants can both increase (homogenize) and decrease (differentiate) floristic similarity between areas. We have a poor understanding of the degree to which plant species introductions tend to homogenize or differentiate floras, and relevant studies covering large spatial extent are scarce. China has been heavily invaded by exotic plants. Here, we analyse a comprehensive dataset of vascular plants to determine whether the introduction of exotic plant species has homogenized or differentiated species composition in regions across China.LocationChina.MethodsWe calculated the Jaccard index and Simpson index of similarity for each pair of province‐level regions for native and exotic species separately and jointly, and calculated a homogenization index for each pair of regional floras. We correlated species richness of native and exotic plants to climatic factors, and correlated the Jaccard index and Simpson index to geographic and climatic distances. We used variation partitioning analysis to determine the relative importance of geographic and climatic distances on species turnover. We also examined the effect of human population density on florisitic similarity of exotic species.ResultsWe found that the geographic range of each species was, on average, larger for exotics than for natives; floristic similarity between regions was greater for exotics than for natives; the vast majority of pairwise regional floras have been homogenized; the introduction of exotic species has caused stronger biotic homogenization for pairwise floras with greater dissimilarity in their species composition; geographic distributions of exotic and native species were determined by different sets of climatic factors; and distributions of exotic species were determined by climatic factors more strongly, compared to those of native species. Human population density had a moderate effect on florisitic similarity of exotic species.Main conclusionsThe introduction of exotic plant species has homogenized regional floras across China. Because strong international trades between China and other countries and dramatic development of transportation systems are continuing in China, which help spread of exotic species, we predict that exotic species will continue to spread and will strengthen biotic homogenization in China.

Different levels of disturbance influence the distributional patterns of native but not exotic plant species on New Zealand small islands

Disturbances of oceanic origin can severely affect plant communities on islands, but it is unclear whether they promote or deter biological invasions. Here, I collected floristic data from 97 small islands subject to different levels of ocean-borne disturbances (i.e. inside and outside Wellington Harbour, New Zealand). First, I tested how relationships between the richness of native and exotic species and island characteristics (e.g. area, isolation, height, distance from nearest dwelling) changed depending on island location. Next, I assessed compositional differences on inner and outer islands for both native and exotic species, and how they vary with geographic distance between islands (i.e. distance-decay). Results show that the richness of both native and exotic plant species was similarly related to island characteristics regardless of island location. Both native and exotic species richness consistently increased with area and nearest dwelling. However, only exotics richness always declined with isolation, while natives richness alone consistently increased with height (elevation). Natives on outer, more exposed islands were floristically more homogenous, and compositional differences changed less strongly with the distance between islands than inside Wellington harbour. In contrast, exotics exhibited similar distributional patterns regardless of island location. Different levels of ocean-borne disturbances might explain distinct distributional patterns in native species. Conversely, results for exotic species might reflect a lack of coastal specialists in the species pool. Perhaps time-lags in the invasion process and non-equilibrium dynamics play a role as well. Conservation bodies should similarly manage islands sustaining different levels of ocean-borne disturbances.

Patterns and potential drivers of exotic plant diversity in managed northern temperate and boreal forests

Although exotic (i.e., non-native) plant species are among the main factors contributing to ecosystem change globally, investigations of northern temperate and boreal forest ecosystems in North America in the context of invasion science are scarce. Multiple hypotheses, including the ‘biotic resistance’, ‘intermediate disturbance’ and ‘disturbance-mediated’ hypotheses, can serve as a framework to assess invasibility. Here we used the NEBIE (natural disturbances and extensive, basic, intensive, and elite silviculture intensities) plot network, consisting of field sites across different forest types that originated from a range of silviculture systems to assess the role of silviculture intensity on exotic plant diversity. Botanical surveys were conducted pre- and two-, five-, and ten years post-forest harvesting. Thirty exotic plant species were observed across all studied forests, of which 20 belonged to the Asteraceae, Fabaceae, and Poaceae families. Exotic species richness was highly correlated with native species richness and peaked as a result of intermediate linked-press (i.e., climate, soils, historic disturbance regimes, and silviculture systems) stresses/disturbances and compounded pulse (i.e., harvest intensity, site preparation, and herbicide use) disturbances for all post-harvesting measurement years. Observed exotic species are considered to have traits consistent with those of ruderal species. In this context, our results support the intermediate disturbance and disturbance-mediated hypotheses, but not the biotic resistance hypothesis. To reduce the richness of exotic plant species in northern temperate and boreal forests we recommend minimizing the use of intermediate compounded pulse disturbances.

Species richness, phylogenetic diversity and phylogenetic structure patterns of exotic and native plants along an elevational gradient in the Himalaya

BackgroundSo far, macroecological studies in the Himalaya have mostly concentrated on spatial variation of overall species richness along the elevational gradient. Very few studies have attempted to document the difference in elevational richness patterns of native and exotic species. In this study, this knowledge gap is addressed by integrating data on phylogeny and elevational distribution of species to identify the variation in species richness, phylogenetic diversity and phylogenetic structure of exotic and native plant species along an elevational gradient in the Himalaya.ResultsSpecies distribution patterns for exotic and native species differed; exotics tended to show maximum species richness at low elevations while natives tended to predominate at mid-elevations. Native species assemblages showed higher phylogenetic diversity than the exotic species assemblages over the entire elevational gradient in the Himalaya. In terms of phylogenetic structure, exotic species assemblages showed majorly phylogenetic clustering while native species assemblages were characterized by phylogenetic overdispersion over the entire gradient.ConclusionsThe findings of this study indicate that areas with high native species richness and phylogenetic diversity are less receptive to exotic species and vice versa in the Himalaya. Species assemblages with high native phylogenetic overdispersion are less receptive to exotic species than the phylogenetically clustered assemblages. Different ecological processes (ecological filtering in case of exotics and resource and niche competition in case of natives) may govern the distribution of exotic and native species along the elevational gradient in the Himalaya.

Road verges facilitate exotic species’ expansion into undisturbed natural montane grasslands

AbstractQuestionsHow has exotic plant species richness and composition changed in and adjacent to a montane road verge over a 10‐year period? Are montane road verges conduits of exotic species’ dispersal into adjacent, undisturbed hinterland?LocationMontane grassland, Maloti‐Drakensberg Transfrontier Conservation Area, South Africa.MethodsWe surveyed road verge and adjacent hinterland transects (25 × 2 m; N = 80) across an elevational gradient of 1,500–2,874 m a.s.l. in 2007 and 2017. In each transect, we estimated the cover of each exotic plant species and the total cover of indigenous species. Generalised Linear Models were fitted to test if exotic species’ richness and cover had changed over time. A Canonical Correspondence Analysis was used to determine if exotic species’ composition had changed over time.ResultsTen years onwards, exotic species richness had increased significantly across the entire elevational gradient, predominantly in the low‐ to mid‐elevational range. Road verge and adjacent hinterland transects differed in species composition in 2007, but no longer in 2017.ConclusionExotic species richness and composition displayed a high spatiotemporal rate of change. Over time, exotic species increasingly moved into the hinterland from the verges as a result of human‐mediated colonisation pressure. These results indicate that montane road verges are far better conduits for the dispersal of exotic species than previously assumed, and that further colonisation of the hinterland by exotics can be expected.

Similar yet distinct distributional patterns characterize native and exotic plant species richness across northern New Zealand islands

AbstractAimA better understanding of plant invasions on islands can be gained from comparing patterns of exotic and native species richness. We asked four questions: (1) Is exotic species richness on islands related to native species richness? (2) If they are related, does this result from similar responses of native and exotic species to specific island characteristics? (3) Is residual variation in native‐exotic richness relationships associated with distinctive island characteristics? (4) Are relationships between species richness and island characteristics scale‐dependent, and do they differ between native and exotic species?LocationNorthern New Zealand.TaxonVascular plants.MethodsWe conducted field surveys and augmented our field data with previously published surveys to quantify the number of native and exotic plant species on 264 islands. We then explored the relationship of species richness and several island characteristics (e.g. area and isolation) using multiple and iterative regression techniques.ResultsSeventy‐two percent of among‐island variation in exotic species richness was positively related to native species richness. Both native and exotic richness increased with island area and declined with isolation and exposure to ocean‐borne disturbances (a proxy for salt spray, wave action, etc). However, exotic species responded more strongly to these three variables. Exotic richness also decreased with latitude and the distance from the nearest urban area, but native species did not. Island area was a better predictor of species richness on larger islands, whereas isolation and exposure were better predictors on smaller islands. Scale‐dependent relationships between species richness and island characteristics were stronger for exotic species.Main ConclusionsInsular distribution patterns of native and exotic plant species richness are governed by similar biogeographic principles. However, in New Zealand, exotic species exhibited subtle, yet distinctive, invasion patterns preferring larger, less isolated, less exposed islands that were located at higher latitudes and closer to urban areas.

Invasion shadows in soil system overshadow the restoration of invaded ecosystems: Implications for invasive plant management

Recognizing the global urgency of restoring degraded ecosystems, the United Nations has proclaimed 2021–2030 as the Decade on Ecosystem Restoration. Among various global drivers of ecosystem degradation, the long-persisting impact of invasive species in the form of invasion shadow (or legacy effect) within belowground soil system, even after physical removal of plant invaders aboveground, is one of the major impediments to successful restoration. Using field manipulation experiment, we investigated the invasion shadow effects by evaluating ecological changes both in above- and belowground systems, following removal of Leucanthemum vulgare – a global plant invader – across multiple sites selected along an elevational gradient in Kashmir Himalaya. At each site, three types of plots were set up: plots invaded by L. vulgare – referred to as invaded reference, invader removal plots within the invaded area – referred to as removal plots, and spatially separated uninvaded plots outside the invaded area – referred to as uninvaded reference. The removal plots were significantly different from both invaded and uninvaded reference plots in terms of multiple biodiversity metrics. Both species richness and cover of exotic plants increased significantly in the removal plots when compared with the reference plots. However, in terms of integrated community composition metric, the removal plots differed significantly from the invaded reference plots only. Further, in terms of standard soil abiotic metrics, the removal plots were significantly different from both invaded and uninvaded reference plots, thus clearly suggesting invasion shadows. Our results reveal that, even after removal of invasive species aboveground, there are persistent invader-induced changes in soil chemistry, which impedes successful restoration of invaded ecosystem. We conclude that physical removal of invasive species aboveground alone is inadequate for restoration of invaded ecosystems, and call for additional management actions to anticipate invasion shadow effects in soil system. Building on the lessons learnt from this study, we propose a way-forward for effective management and policy interventions to guide ecological restoration of invaded ecosystems.

Disturbance reinforces community assembly processes differentially across spatial scales.

There is a paucity of empirical research and a lack of predictive models concerning the interplay between spatial scale and disturbance as they affect the structure and assembly of plant communities. We proposed and tested a trait dispersion-based conceptual model hypothesizing that disturbance reinforces assembly processes differentially across spatial scales. Disturbance would reinforce functional divergence at the small scale (neighbourhood), would not affect functional dispersion at the intermediate scale (patch) and would reinforce functional convergence at the large scale (site). We also evaluated functional and species richness of native and exotic plants to infer underlying processes. Native and exotic species richness were expected to increase and decrease with disturbance, respectively, at the neighbourhood scale, and to show similar associations with disturbance at the patch (concave) and site (negative) scales. In an arid shrubland, we estimated species richness and functional dispersion and richness within 1 m2 quadrats (neighbourhood) nested within 100 m2 plots (patch) along a small-scale natural disturbance gradient caused by an endemic fossorial rodent. Data for the site scale (2500 m2 plots) were taken from a previous study. We also tested the conceptual model through a quantitative literature review and a meta-analysis. As spatial scale increased, disturbance sequentially promoted functional divergence, random trait dispersion and functional convergence. Functional richness was unaffected by disturbance across spatial scales. Disturbance favoured natives over exotics at the neighbourhood scale, while both decreased under high disturbance at the patch and site scales. The results supported the hypothesis that disturbance reinforces assembly processes differentially across scales and hampers plant invasion. The quantitative literature review and the meta-analysis supported most of the model predictions.

Establishment dynamics of native and exotic plants after disturbance along roadsides

AbstractQuestionsHow does small‐scale soil disturbance influence establishment dynamics of native and exotic species along roadsides? Does soil disturbance provide potential opportunities to exotic invasions along elevational gradients? Does the established plant community after disturbance reflect the surrounding vegetation?LocationPrealps, Italy.MethodsTo disentangle the role of elevation and soil disturbance in promoting exotic invasions, we performed a manipulative experiment along 12 roads spanning an elevational gradient of ca. 1,200 m. Additionally, we sampled species richness of native and exotic plants in the surroundings of the experimental plots.ResultsSoil disturbance reduced species richness of native plants, total plant biomass and vegetation cover compared to the undisturbed plots. The decrease in species richness of natives and plant biomass was stronger at higher than lower elevations, suggesting higher colonization opportunities for exotics. However, exotic species richness did not increase with disturbance, probably due to a low propagule and colonization pressure. We observed strong species replacement between control and disturbed plots, indicating that plant communities established after disturbance were not a subset of species communities already occurring in the surrounding vegetation.ConclusionsWith increasing elevation, disturbance had a stronger negative effect on native than on exotic species. Higher elevations, hitherto considered to have low invasibility, may provide new potential colonization opportunities for exotics but only if disturbance is coupled with an increased propagule pressure. Disturbance along roadsides promoted the establishment of species that did not occur in the surrounding vegetation.

Decoupled recovery of ecological communities after reclamation.

Grassland restoration is largely focused on creating plant communities that match reference conditions. However, these communities reflect only a subset of the biodiversity of grassland systems. We conducted a multi-trophic study to assess ecosystem recovery following energy development for oil and gas extraction in northern US Great Plains rangelands. We compared soil factors, plant species composition and cover, and nematode trophic structuring between reclaimed oil and gas well sites (“reclaims”) that comprise a chronosequence of two—33 years since reclamation and adjacent, undeveloped rangeland at distances of 50 m and 150 m from reclaim edges. Soils and plant communities in reclaims did not match those on undeveloped rangeland even after 33 years. Reclaimed soils had higher salt concentrations and pH than undeveloped soils. Reclaims had lower overall plant cover, a greater proportion of exotic and ruderal plant cover and lower native plant species richness than undeveloped rangeland. However, nematode communities appear to have recovered following reclamation. Although total and omni-carnivorous nematode abundances differed between reclaimed well sites and undeveloped rangeland, community composition and structure did not. These findings suggest that current reclamation practices recover the functional composition of nematode communities, but not soil conditions or plant communities. Our results show that plant communities have failed to recover through reclamation: high soil salinity may create a persistent impediment to native plant growth and ecosystem recovery.

Management of invasive Pinaceae is imperiled by the lack of invasive ungulate control: successful restoration requires multiple-species management

The magnitude and direction of an invasive species’ impact may be determined by its co-existence and interaction with other species. In Argentina, wild boar, red deer, and fallow deer are introduced mammals that have important negative impacts on ecosystems. In Patagonia, nonnative Pinaceae removal programs have been promoted since the 1980s to control exotic conifers and restore native vegetation. The consequences of removing exotic trees without continued management intervention may be unexpected due novel interactions between nonnative species. We evaluate habitat use of introduced ungulates, interactions with native and exotic plants, and their implications for restoration objectives in areas where pines have been removed. Exotic plant species richness and abundance was greater in clear-cuts than native forest or pine plantations. In contrast, richness and abundance of native plants was highest in native forests. Clear-cuts had the highest plant species diversity, primarily driven by high numbers of exotics, relative to native forests. Boar and deer were recorded in clear-cuts, native forests, and pine plantations, but deer were more frequently observed in clear-cuts. Removal of pines created a suitable habitat for deer. The herbivory pressure of deer, coupled with the presence and abundance of invasive plants, can prevent or hinder the establishment and development of native shrubs and trees that are required for the structure and function of the native ecosystem. Although the conditions in clear-cuts present complex challenges for natural regeneration, these sites could be an excellent opportunity for active restoration strategies, by excluding ungulates and controlling exotic plants to promote native species recovery.

Coverage of Native Plants Is Key Factor Influencing the Invasibility of Freshwater Ecosystems by Exotic Plants in China.

Understanding the biotic and abiotic factors that influence the susceptibility of a community to invasion is beneficial for the prediction and management of invasive species and the conservation of native biodiversity. However, the relationships between factors and invasibility of a community have not been fully confirmed, and the factors most associated with the susceptibility of a community to invasion have rarely been identified. In this study, we investigated the species richness patterns in aquatic exotic and native plants and the relationships of exotic species richness with habitat and water environment factors in 262 aquatic plant communities in China. A total of 11 exotic plant species were recorded in our field survey, and we found neither a negative nor a positive relationship between aquatic exotic and native plant species richness. The aquatic exotic plant species richness is negatively correlated with the relative coverage and biomass of native plants but positively correlated with the total nitrogen (TN), total phosphorus (TP), and chemical oxygen demand (COD) concentrations in the water. The native plant species richness, native species’ relative coverage, and native species’ biomass were positively related to each other, whereas the TP, TN, and COD were also positively related to each other. The native plant species richness, native species’ relative coverage, and native species biomass were each negatively correlated with the TP, TN, and COD. In addition, biotic rather than abiotic predictors accounted for most of the variation in exotic plant richness. Our results suggest that improving the vegetation coverage and the biodiversity of native plants is the most effective approach for preventing alien plant invasions and minimizing their impacts on freshwater ecosystems.

Livestock activity increases exotic plant richness, but wildlife increases native richness, with stronger effects under low productivity

Abstract Grazing by domestic livestock is one of the most widespread land uses world‐wide, particularly in rangelands, where it co‐occurs with grazing by wild herbivores. Grazing effects on plant diversity are likely to depend on intensity of grazing, herbivore type, co‐evolution with plants and prevailing environmental conditions. We collected data on climate, plant productivity, soil properties, grazing intensity and herbivore type, and we measured their effects on plant species richness from 451 sites across 0.4 M km2 of semi‐arid rangelands in eastern Australia. We used structural equation modelling to examine the direct and indirect effects of increasing grazing intensity by different herbivores (cattle, sheep, kangaroos, rabbits) on native and exotic plant species richness across all sites, and in subsets focusing on three woodland communities spanning a gradient in productivity. Direct effects of grazing by all herbivores were strongest under low productivity but waned with increasing productivity. Increases in the intensity of recent and historic livestock grazing corresponded with greater exotic plant richness under low productivity and less native plant richness under both low and moderate productivity. Rabbit effects were greatest under moderate productivity. Overall, the effects of kangaroos were benign. Grazing indirectly affected native and exotic plant richness by increasing soil phosphorus and reducing soil health (i.e. nutrient cycling). Synthesis and applications. Our study shows that livestock grazing increases exotic species richness but reduces native richness, while kangaroo grazing increases native richness in environments with low productivity. The results provide clear messages for land managers and policy makers: (1) the coexistence of livestock grazing and plant diversity is only possible within more productive environments and (2) grazing under low or moderate productivity will impact upon native and exotic plant richness.

Disturbance maintains native and exotic plant species richness in invaded grassy woodlands

AbstractQuestionCommunity invasion and dominance by exotic species can be abetted by disturbance and elevated nutrient levels. However, impacts of exotics on native species are often manifest through competition, suggesting disturbance or nutrient management could benefit native species. I therefore asked: does nutrient reduction through carbon addition, or disturbance to reduce biomass, contribute to restoration by increasing occurrence and richness of native plant species?LocationSE Australia.MethodsUsing a field experiment spanning 4 yrs, this study assessed the potential for carbon addition, three types of disturbance (burn, crop, slash) and their interaction with native herbivore grazing to reduce exotic, and increase native, plant species in a critically endangered grassy woodland in SE Australia.ResultsAdding carbon reduced occurrence of one native and three exotic species, while increasing one exotic and one native species. The value of adding carbon for restoration was therefore ambiguous. Fencing to exclude the high‐density kangaroo population had temporary beneficial effects, suggesting that grazing reduced the number of native and exotic plant species by up to 56% and 21%, respectively. Species richness declined by year 3 in fenced areas, likely through competitive exclusion. Burn, slash and crop treatments also provided disturbance that maintained species richness and occurrence of many species, both native and exotic. Most grazing interactions reflected the benefit of alternative disturbance in the absence of grazing. However, there was also evidence that two species declined when crop and grazing acted in combination.ConclusionsVery high grazing by native herbivores reduced plant species richness in this study. However, grazing or other forms of disturbance appeared critical for preventing a small number of exotic species from driving down occurrence and richness of many other native and exotic plant species. Although increased exotic species richness is undesirable, disturbance led to a better outcome for conservation than the low‐disturbance alternatives.

A century of ecosystem change: human and seabird impacts on plant species extirpation and invasion on islands

We used 116 years of floral and faunal records from Mandarte Island, British Columbia, Canada, to estimate the indirect effects of humans on plant communities via their effects on the population size of a surface-nesting, colonial seabird, the Glaucous-winged gull (Larus glaucescens). Comparing current to historical records revealed 18 extirpations of native plant species (32% of species historically present), 31 exotic species introductions, and one case of exotic introduction followed by extirpation. Contemporary surveys indicated that native species cover declined dramatically from 1986 to 2006, coincident with the extirpation of ‘old-growth’ conifers. Because vegetation change co-occurred with an increasing gull population locally and regionally, we tested several predictions from the hypothesis that the presence and activities of seabirds help to explain those changes. Specifically, we predicted that on Mandarte and nearby islands with gull colonies, we should observe higher nutrient loading and exotic plant species richness and cover than on nearby islands without gull colonies, as a consequence of competitive dominance in species adapted to high soil nitrogen and trampling. As predicted, we found that native plant species cover and richness were lower, and exotic species cover and richness higher, on islands with versus without gull colonies. In addition, we found that soil carbon and nitrogen on islands with nesting gulls were positively related to soil depth and exotic species richness and cover across plots and islands. Our results support earlier suggestions that nesting seabirds can drive rapid change in insular plant communities by increasing nutrients and disturbing vegetation, and that human activities that affect seabird abundance may therefore indirectly affect plant community composition on islands with seabird colonies.

Shrub removal in reforested post-fire areas increases native plant species richness

Large, high severity fires are becoming more prevalent in Sierra Nevada mixed-conifer forests, largely due to heavy fuel loading and forest densification caused by past and current management practices. In post-fire areas distant from seed trees, conifers are often planted to re-establish a forest and to prevent a potential type-conversion to shrub fields. Typical reforestation efforts promote conifer survival and growth by reducing competing shrub cover, yet the effects of these practices on plant species richness and composition are not well understood. We compared the effects of treatment and time since fire on (1) native and exotic plant species richness, (2) plant community composition, and (3) stand structure. Plots were installed throughout three different aged but proximate fires located in the canyon of the South Fork of the American River in California, 10, 22, and 41years after fire. All three fires included large patches of stand-replacing fire that had been reforested with conifers as well as unplanted areas. Native plant species richness was significantly higher in planted areas where shrub cover was lower and planted trees successfully established. Native species richness decreased as time since fire increased, but the relationship between shrub control and richness persisted. Exotic species richness was higher on treated sites in the more recent fires, while the opposite was true in the oldest fire. As time since fire increased, understory species composition shifted from a community dominated by annuals and perennials to one dominated by shrubs and shade-tolerant trees. Shrub cover and July soil moisture were the top two factors influencing understory richness levels. Natural regeneration was low in the youngest fire and high in the oldest fire but highly heterogeneous in all three fires. Our study suggests that while retaining some shrub cover for post-fire habitat may be desirable, some level of shrub reduction does favor native plant richness and overall herbaceous cover.

Post-fire logging produces minimal persistent impacts on understory vegetation in northeastern Oregon, USA

Post-fire forest management commonly requires accepting some negative ecological impacts from management activities in order to achieve management objectives. Managers need to know, however, whether ecological impacts from post-fire management activities are transient or cause long-term ecosystem degradation. We studied the long-term response of understory vegetation to two post-fire logging treatments – commercial salvage logging with and without additional fuel reduction logging – on a long-term post-fire logging experiment in northeastern Oregon, USA. We sampled understory plant cover and species diversity on 10–11 sampling plots within each of nine experimental treatment units to see if post-fire logging treatments produced any lasting effects on understory plant cover, species diversity, community composition, or exotic species cover. Post-fire logging treatments produced no significant effects on understory vegetation cover, diversity, or community composition 15years after treatment. We found no significant treatment effects on graminoid, forb, woody plant, or exotic plant cover and species richness, and differences among treatment means were generally small. Differences in treatment means were larger at the individual species level, but were only statistically significant for one native grass, California brome (Bromus carinatus), which responded differently to the two logging treatments. Multivariate analysis of understory plant communities across 91 sample plots found two major gradients in understory plant community composition, one correlated with regenerating forest (sapling) density and one correlated with residual overstory tree density, suggesting that initial fire severity (tree mortality) and post-fire regeneration may have greater long-term impacts on post-fire understory vegetation than post-fire logging. This study demonstrates that understory vegetation can be resilient to post-fire logging, particularly when best management practices, like logging over snow, are used to limit damage to soils and understory vegetation. Further research is needed to establish the generality of our results and to identify sources of variability in understory plant community responses to wildfire and post-fire logging. If further research confirms our findings, post-fire logging debates will be able to focus more on how to mitigate short-term disturbance impacts and manage fire-killed trees to meet wildlife habitat, fuel reduction, and economic objectives, and less on concerns over long-term ecosystem degradation.

Response of native versus exotic plant guilds to cattle and elk herbivory in forested rangeland

AbstractQuestionsAre exotic plant species favoured by non‐native ungulate herbivores and disadvantaged by native herbivores in forested rangelands? Do the impacts of ungulates on exotic vs native plants depend on forest management activities such as prescribed fire and stand thinning?LocationNortheastern Oregon, USA.MethodsWe recorded changes in richness and cover of different exotic and native plant life forms in experimental plots that were grazed only by cattle (a non‐native herbivore), only by elk (a native herbivore) or not grazed by any ungulate over a 7‐yr period at both managed (recently burned and thinned) and unmanaged (where no fire and thinning has occurred in >40 yr) forest stands.ResultsThere was a general decrease in exotic plant species richness and cover across all treatments. However, the decrease in exotic richness, particularly of exotic annual forbs, was slightly lower in plots grazed by elk than in ungrazed plots at managed stands. Managed stands also displayed a larger increase in native annual forb richness and exotic graminoid richness, and a larger decrease in native perennial graminoid cover with cattle grazing than elk grazing. At unmanaged stands, cover of woody native plants such as shrubs, sub‐shrubs and trees as well as native perennial forbs decreased or remained relatively constant with elk grazing while increasing strongly at plots that were ungrazed or grazed by cattle.ConclusionsCattle and elk have variable effects on different plant guilds at managed vs unmanaged forest stands. Overall, cattle grazing tends to have a larger impact on herbaceous plant guilds at managed stands, while elk grazing tends to have a larger impact on woody plant guilds at unmanaged stands. However, in contrast to findings from other ecosystems, grazing only has a minor impact on exotic plant dynamics in our study area, and cattle grazing does not favour exotic plants any more than grazing by elk or ungulate exclusion.

A Theory of Island Biogeography for Exotic Species.

The theory of island biogeography has played a pivotal role in the way ecologists view communities. However, it does not account for exotic species explicitly, which limits its use as a conservation tool. Here, I present the results of a long-term study of plant communities inhabiting an archipelago of small islands off the coast of New Zealand and derive a modified version of the theory of island biogeography to predict differences in the turnover and diversity of native and exotic species. Empirical results showed that, although species richness of both native and exotic plant species increased with island area, native species consistently outnumbered exotic species. Species turnover increased with species richness in both groups. However, opposite to species-area patterns, turnover increased more rapidly with species richness in exotic species. Empirical results were consistent with the modified version of the theory of island biogeography, which distinguishes exotic species from native species by decoupling extinction rates of exotic species from island area, because they are represented by only small populations at the initial stages of invasion. Overall results illustrate how the theory of island biogeography can be modified to reflect the dynamics of exotic species as they invade archipelagos, expanding its use as a conservation tool.