Riparian Butterfly (Papilionoidea and Hesperioidea) Assemblages Associated with Tamarix-Dominated, Native Vegetation–Dominated, and Tamarix Removal Sites along the Arkansas River, Colorado, U.S.A.

We studied butterfly assemblages at eight riparian sites over five years. Sites included Tamarix spp.–dominated riparian areas; sites where mechanical means or biological control agents (Diorhabda elongata deserticola) were used to limit Tamarix; sites that were mixtures of native woody vegetation (e.g., Populus and Salix) and Tamarix; and native vegetation sites. We identified a gradient in butterfly community composition that changed from treated Tamarix sites, through mixed vegetation, to native vegetation sites. Tamarix sites had lower butterfly metric (riparian butterfly index [RBI]) values than did native vegetation sites. The RBI is based on a combination of richness measures and the presence of specific species and groups of butterflies. There was no significant change in the RBI over sampling periods at any sites, including both Tamarix eradication sites. The RBI at sites where Tamarix control took place did not approach restoration goals based on values at unimpacted sites. Positive effects on riparian butterfly assemblages were not linked to any Tamarix control efforts, nor did we detect a decline from initial butterfly metrics at Tamarix sites. Direct ordination provided information on environmental variables, such as amount of nectar and herbaceous plant richness, which may be important in riparian restoration efforts.     

An unfortunate alliance: Native shrubs increase the abundance,performance, and apparent impacts of Bromus tectorum across a regional aridity gradient

Interspecific facilitation contributes to the assembly of desert plant communities. However, we know little of how desert communities invaded by exotic species respond to facilitation along regional-scale aridity gradients. These measures are essential for predicting how desert plant communities might respond to concomitant plant invasion and environmental change. Here, we evaluated the potential for Bromus tectorum (a dominant invasive plant species) and the broader herbaceous plant community to form positive associations with native shrubs along a substantial aridity gradient across the Great Basin, Mojave, and San Joaquin Deserts in North America. Along this gradient, we sampled metrics of abundance and performance for B. tectorum, all native herbaceous species combined, all exotic herbaceous species combined, and the total herbaceous community using 180 pairs of shrub and open microsites. Across the gradient, B. tectorum formed strong positive associations with native shrubs, achieving 1.6–2.2 times greater abundance, biomass, and reproductive output under native shrubs than away from shrubs, regardless of relative aridity. In contrast, the broader herbaceous community was not positively associated with native shrubs. Interestingly, increasing B. tectorum abundance corresponded to decreasing native abundance, native species richness, exotic species richness, and total species richness under but not away from shrubs. Taken together, these findings suggest that native shrubs have considerable potential to directly (by increasing abundance and performance) and indirectly (by increasing competitive effects on neighbors) facilitate B. tectorum invasion across a large portion of the non-native range.     

Do riparian plant community characteristics differ between <Emphasis Type="Italic">Tamarix</Emphasis> (L.) invaded and non-invaded sites on the upper Verde River,Arizona?

Invasion by Tamarix (L.) can severely alter riparian areas of the western U.S., which are globally rare ecosystems. The upper Verde River, Arizona, is a relatively free-flowing river and has abundant native riparian vegetation. Tamarix is present on the upper Verde but is a minor component of the vegetation (8% of stems). This study sought to determine whether riparian vegetation characteristics differed between sites where Tamarix was present and sites where Tamarix was absent during the invasion of the upper Verde. We hypothesized that herbaceous understory and woody plant communities would differ between Tamarix present and absent sites. Our hypothesis was generally confirmed, the two types of sites were different. Tamarix present sites had greater abundance of all vegetation, native understory species, graminoids, and native trees, and a positive association with perennial native wetland plant species. Tamarix absent sites had greater abundance of exotic plants and upland adapted plants and an association with greater abiotic cover and litter. These results are contrary to other reports of Tamarix association with depauperate riparian plant communities, and suggest that Tamarix invasion of a watershed with a relatively natural flow regime and a robust native plant community follows similar establishment patterns as the native riparian plant community.     

Shifts in dominance of native and invasive plants in experimental patches of vegetation

While it is widely accepted that disturbance often promotes the process of plant invasion, few data are available on specific relationships between different types of disturbance and success of plant invaders. Nor is much known about variation in invader behaviour in response to changing composition of the dominant native species in the recipient habitats. In the middle Main valley near Würzburg (Germany), two invasive Brassicaceae forbs, Bunias orientalis and Rorippa austriaca, are frequently associated with a small number of dominant native species in the herbaceous vegetation of productive, disturbed sites. Variation in the relative dominance of these species suggests high variability between sites in the factors influencing the local invasion process. In our study, we examined variation in regeneration mode, type and frequency of disturbance, and native species composition, and considered life history differences between the two invasive species as factors contributing to the distinct dominance patterns. These factors were tested in a controlled field experiment in which individuals of the two invasive species and four native species were grown in mixed stands for almost 3 years. The development of the plants was monitored by cover values and all plants were harvested at the end of the study. All factors investigated contributed considerably to the development of distinct dominance patterns in the species mixtures. Relative to the natives, both invasive species were promoted by regeneration from vegetative fragments as compared to development from sexual offspring. While both invasive species were generally promoted by disturbance relative to the natives, they differed in their response to distinct types of disturbance, and these differences were strongly affected by species composition. Our results indicate that invasion success of R. austriaca may depend more on soil disturbance, soil transport and deposition, while B. orientalis is expected to particularly expand at mown sites that do not have dense cover by meadow grasses. Whether the native resident vegetation is mainly composed of successional competitors that suffer from disturbance or of ruderal competitors that compete early and strongly with the invasive species may be of critical importance for invasion by the two species.     

Disturbance effects on herbaceous layer vegetation and soil nutrients in Populus forests of northern lower Michigan

Abstract. Recent disturbance models have identified changes in resource availability as factors that control plant community response. Soil nutrient resources typically are assumed to change following forest disturbance, usually with nutrient availablity increasing initially and subsequently decreasing through later stages of succession. We examined the effects of disturbance (clearcut harvesting with a brief recovery period) on soil organic matter, pH and extractable soil nutrients in successional aspen forests of northern lower Michigan to determine relationships of these variables to changes in herbaceous layer vegetation. Two site types were identified: drymesic (glacial outwash sands, low in organic matter) and mesic (calcareous clay till, high in organic matter). Extractable nutrient concentrations were 1.5 to 3 times higher in the A₁ horizon of mesic sites than those of dry-mesic sites. Soil pH and cations increased after disturbance on mesic sites, but not on dry-mesic sites. Patterns of change with disturbance were less pronounced in lower horizons on both site types. Herblayer species diversity increased after disturbance on mesic sites, but with decreases in the importance of shade-tolerant tree species and Maianthemum canadense. Species characteristic of open habitats (e.g. Pteridium aquilinum, Rubus spp., Fragaria virginiana, and Diervilla lonicera), increased in importance. Soil factors, species composition and diversity on dry-mesic sites changed little after disturbance, with Pteridium aquilinum and ericaceous species remaining dominant in both mature (55–82 yr) and disturbed (≤ 15 yr) stands. These results suggest that soil nutrient resources do not always change through secondary succession and that patterns of change can be distinctly site-dependent. Disturbance response patterns in the herbaceous layer of these aspen forests are also site-dependent.     

晋西北丘陵风沙区柠条林下草本植物群落组成和种群生态位变化特征

晋西北丘陵风沙区生态环境脆弱,是我国风沙活动危害最为严重的地区之一。为探究晋西北丘陵风沙区人工柠条林林下草本植物群落组成和种群生态位变化规律,以不同种植年限(撂荒地CK、6、12、18、40、50 a)人工柠条林林下天然草本植物群落作为研究对象,对其群落组成、重要值及种群生态位变化特征进行分析。研究结果表明：(1)不同年限柠条林下天然草本植物组成共记录到22科41属52种,其中,种植前期(0—6 a)林下草本植物以一年生草本植物为优势类群,伴有少数多年生草本植物(3种);种植中期(12—18 a)林下草本植物主要由多年生草本植物(12种)为主,伴生有一年生草本植物(6种)及天然灌木(2种)组成;在种植后期(40—50 a),林下草本植物依然以多年生草本植物(12种)为主。(2)在不同年限柠条林下,草本植物生态位宽度和生态位重叠发生明显变化,种植前期(0—12 a)的优势类群为一年生先锋物种(米蒿和野燕麦),其生态位宽度最宽(9.46、9.34),且与其他物种的生态位重叠程度最大(0.3、0.29);而种植中后期(18—50 a)优势类群变为多年生草本植物,优势种披碱草与其他物种的生态位...     

Lost in the weeds: Ligustrum sinense reduces native plant growth and survival

Plant invasions are often implicated in declines of native plant species. However, common experimental designs have received criticism questioning the assumption that invasive plants are the primary cause for ecosystem deterioration. We used a combination of field observations and a transplant experiment to investigate the influence of an exotic invasive shrub, Ligustrum sinense (Chinese Privet) on native plant species in Piedmont floodplain forests of South Carolina, USA. We conducted vegetation surveys of 12 floodplain forests documenting abundance and cover of all herbaceous and woody plant species. Additionally, we established an experimental garden to compare survival and growth of L. sinense and four common native species transplanted into a mature L. sinense stand and an adjacent uninvaded area over two growing seasons. The vegetation survey demonstrated a strong negative relationship between L. sinense presence and herbaceous vegetation. As L. sinense cover increased, herbaceous cover and height, plant abundance, and native species richness decreased. In our transplant experiment we found drastic effects of L. sinense on native plant seedling survival and growth. Survival for all native species was lower under the L. sinense canopy and native seedling growth was substantially reduced. Results from both the vegetation survey and transplant experiment show that invasion of L. sinense suppresses herbaceous understory and prevents regeneration of native species by reducing seedling survival and growth. With an approach that combines multiple field sites and local site-specific investigations our research provides strong evidence that L. sinense is an agent of change in floodplain forests.     

Bird species richness and abundance in riparian vegetation invaded by exotic Reynoutria spp.

Impacts of invasive alien plant species are threatening biodiversity worldwide and thus it is important to assess their effects on particular groups of organisms. However, such impacts were studied mostly in case of plant or invertebrate communities and our understanding the response of vertebrate species to plant invasions remains incomplete. To improve our knowledge in this respect, we studied bird communities in riparian vegetation along the rivers with different levels of Reynoutria spp. invasion in the Czech Republic. These findings will be interesting for basic ecology enhancing our knowledge of consequences of plant invasions, as well as for conservation practice. We surveyed understory bird species in 26 vegetation blocks along parts of three rivers running from the Beskydy Mountains in spring 2011. We used principal component analysis to assess vegetation structure of particular blocks and the first axis ordinated the blocks according to the degree of invasion by Reynoutria spp. Using generalized linear mixed-effects models we found that counts of Motacilla cinerea, Cinclus cinclus and Sylvia borin, as well as the total bird species richness, significantly decreased with increasing degree of Reynoutria spp. invasion, while Acrocephalus palustris showed the opposite pattern. These results suggest that Reynoutria spp. impacts negatively on the species strictly bond with river banks and habitats specialists, whereas habitat generalist species like Sylvia atricapilla were not affected. Preference of Acrocephalus palustris for Reynoutria spp. corroborates affinity of this species to large invasive herbs observed also in other studies. Our study showed that Reynoutria spp. invasion can reduce species richness of understory birds in riparian communities. Although the distribution of this plant species is still quite limited in central Europe, our results suggest that its more widespread occurrence could potentially threat some river bank bird species. Therefore, we urge for development of management actions that will act counter the Reynoutria spp. invasion.     

Functional and performance comparisons of invasive Hieracium lepidulum and co‐occurring species in New Zealand

Abstract One of the key environmental factors affecting plant species abundance, including that of invasive exotics, is nutrient resource availability. Plant functional response to nutrient availability, and what this tells us about plant interactions with associated species, may therefore give us clues about underlying processes related to plant abundance and invasion. Patterns of abundance of Hieracium lepidulum, a European herbaceous invader of subalpine New Zealand, appear to be related to soil fertility/nutrient availability, however, abundance may be influenced by other factors including disturbance. In this study we compare H. lepidulum and field co‐occurring species for growth performance across artificial nutrient concentration gradients, for relative competitiveness and for response to disturbance, to construct a functional profile of the species. Hieracium lepidulum was found to be significantly different in its functional response to nutrient concentration gradients. Hieracium lepidulum had high relative growth rate, high yield and root plasticity in response to nutrient concentration dilution, relatively low absolute yield, low competitive yield and a positive response to clipping disturbance relative to other species. Based on overall functional response to nutrient concentration gradients, compared with other species found at the same field sites, we hypothesize that H. lepidulum invasion is not related to competitive domination. Relatively low tolerance of nutrient dilution leads us to predict that H. lepidulum is likely to be restricted from invading low fertility sites, including sites within alpine vegetation or where intact high biomass plant communities are found. Positive response to clipping disturbance and relatively high nutrient requirement, despite poor competitive performance, leads us to predict that H. lepidulum may respond to selective grazing disturbance of associated vegetation. These results are discussed in relation to published observations of H. lepidulum in New Zealand and possible tests for the hypotheses raised here.     

Climate change and plant invasions: restoration opportunities ahead?

Rather than simply enhancing invasion risk, climate change may also reduce invasive plant competitiveness if conditions become climatically unsuitable. Using bioclimatic envelope modeling, we show that climate change could result in both range expansion and contraction for five widespread and dominant invasive plants in the western United States. Yellow starthistle ( Centaurea solstitialis ) and tamarisk ( Tamarix spp.) are likely to expand with climate change. Cheatgrass ( Bromus tectorum ) and spotted knapweed ( Centaurea biebersteinii ) are likely to shift in range, leading to both expansion and contraction. Leafy spurge ( Euphorbia esula ) is likely to contract. The retreat of once-intractable invasive species could create restoration opportunities across millions of hectares. Identifying and establishing native or novel species in places where invasive species contract will pose a considerable challenge for ecologists and land managers. This challenge must be addressed before other undesirable species invade and eliminate restoration opportunities.     

连栽杉木林林下植被生物量动态格局

用空间一致时间连续的定位研究方法,在湖南会同杉木林生态系统国家野外科学观测研究站试验基地的第2集水区,对连栽杉木林林下植被生物量进行了12 a的监测,研究了林下植被种类的变化、生物量动态特征、生物量的组成与分布变化格局。结果表明:连栽杉木林在14a生长发育过程中,林下植物种类呈现波动性的减少趋势,其中木本植物物种数下降率为40.0%,草本植物物种数下降率为47.1%。林下植被生物量由杉木林3年生29.48 t/hm²下降至14年生的2.53 t/hm²,其中木本植物生物量由7.07 t/hm²,下降至1.25 t/hm²,下降了82.3%;草本植物由22.41 t/hm²,下降至1.28 t/hm²,下降了94.3%。在此期间,木本与草本植物生物量的高低均出现波动现象。3年生杉木林下木本植物以乔木树种生物量6068.97 kg/hm²最高,占总生物量85.88%,藤本植物生物量736.97 kg/hm²为次,占10.44%,灌木植物生物量259.87 kg/hm²最低,仅占3.68%。14年生杉木林下木本植物以灌木植物生物量881.87 kg/hm²为首,占总生物量70.73%,藤本植物生物量247.07 kg/hm²为次,占19.82%,乔木树种生物量117.87 kg/hm²最少,只占9.45%。3年生杉木林下草本植物以蕨类植物生物量8391.44 kg/hm²最高,占总生物量的37.44%,过路黄生物量36.77 kg/hm²最低,仅占0.16%。杉木14年生时,以芒生物量573.00 kg/hm²最大,占总生物量44.78%,金毛耳草生物量2.93 kg/hm²最小,仅占0.23%。研究结果,可为研究杉木林养分循环、碳平衡、维护和提高林地地力及可持续经营管理提供科学依据。     

Planning Riparian Restoration in the Context of Tamarix Control in Western North America

Throughout the world, the condition of many riparian ecosystems has declined due to numerous factors, including encroachment of non‐native species. In the western United States, millions of dollars are spent annually to control invasions of Tamarix spp., introduced small trees or shrubs from Eurasia that have colonized bottomland ecosystems along many rivers. Resource managers seek to control Tamarix in attempts to meet various objectives, such as increasing water yield and improving wildlife habitat. Often, riparian restoration is an implicit goal, but there has been little emphasis on a process or principles to effectively plan restoration activities, and many Tamarix removal projects are unsuccessful at restoring native vegetation. We propose and summarize the key steps in a planning process aimed at developing effective restoration projects in Tamarix‐dominated areas. We discuss in greater detail the biotic and abiotic factors central to the evaluation of potential restoration sites and summarize information about plant communities likely to replace Tamarix under various conditions. Although many projects begin with implementation, which includes the actual removal of Tamarix, we stress the importance of pre‐project planning that includes: (1) clearly identifying project goals; (2) developing realistic project objectives based on a detailed evaluation of site conditions; (3) prioritizing and selecting Tamarix control sites with the best chance of ecological recovery; and (4) developing a detailed tactical plan before Tamarix is removed. After removal, monitoring and maintenance as part of an adaptive management approach are crucial for evaluating project success and determining the most effective methods for restoring these challenging sites.     

Ungulates increase forest plant species richness to the benefit of non‐forest specialists

Large wild ungulates are a major biotic factor shaping plant communities. They influence species abundance and occurrence directly by herbivory and plant dispersal, or indirectly by modifying plant‐plant interactions and through soil disturbance. In forest ecosystems, researchers’ attention has been mainly focused on deer overabundance. Far less is known about the effects on understory plant dynamics and diversity of wild ungulates where their abundance is maintained at lower levels to mitigate impacts on tree regeneration. We used vegetation data collected over 10 years on 82 pairs of exclosure (excluding ungulates) and control plots located in a nation‐wide forest monitoring network (Renecofor). We report the effects of ungulate exclusion on (i) plant species richness and ecological characteristics, (ii) and cover percentage of herbaceous and shrub layers. We also analyzed the response of these variables along gradients of ungulate abundance, based on hunting statistics, for wild boar (Sus scrofa), red deer (Cervus elaphus) and roe deer (Capreolus capreolus). Outside the exclosures, forest ungulates maintained higher species richness in the herbaceous layer (+15%), while the shrub layer was 17% less rich, and the plant communities became more light‐demanding. Inside the exclosures, shrub cover increased, often to the benefit of bramble (Rubus fruticosus agg.). Ungulates tend to favour ruderal, hemerobic, epizoochorous and non‐forest species. Among plots, the magnitude of vegetation changes was proportional to deer abundance. We conclude that ungulates, through the control of the shrub layer, indirectly increase herbaceous plant species richness by increasing light reaching the ground. However, this increase is detrimental to the peculiarity of forest plant communities and contributes to a landscape‐level biotic homogenization. Even at population density levels considered to be harmless for overall plant species richness, ungulates remain a conservation issue for plant community composition.     

Seasonal responses of avian communities to invasive bush honeysuckles (<Emphasis Type="Italic">Lonicera</Emphasis> spp.)

Invasive bush honeysuckles, Lonicera spp., are widely viewed as undesirable; however, the effects of Lonicera spp. on native fauna are largely unknown. We investigated how breeding and overwintering bird communities respond to the presence of Lonicera spp. by comparing communities in forested areas with Lonicera spp. to those with a native shrub understory. The dense understory created by Lonicera spp. was associated with a change in the breeding bird community. We found large increases in the densities of understory bird species (e.g. northern cardinals) and decreases in select canopy species (e.g. eastern wood-pewees) in Lonicera spp. sites. In winter, we observed greater densities of frugivorous birds (e.g. American robins) likely due to the fruits that remain on Lonicera spp.; however, there was no difference in the community composition between sites with and without Lonicera spp. Given the widespread distribution of Lonicera spp., this invasive species may facilitate the population increase and range expansion of selected bird species. Many bird species appear to utilize Lonicera spp. for nesting and foraging; therefore, its removal should be accompanied by restoring native shrubs that provide needed resources.     

Does understory vegetation reflect the history of fluvial disturbance in a riparian forest?

We have examined the relationship between the history of fluvial disturbance and understory vegetation in a riparian forest. The study site was divided into three sites, by use of aerial photographs and topographical maps, with different histories of fluvial disturbance: (1) Fagus-type on land that has not been flooded for the last 39 years, at least; (2) Populus-type on land that has not been flooded since debris flow occurred 34 years ago; and (3) Salix-type on land that has been flooded periodically from an abandoned channel since debris flow occurred 34 years ago. Species richness in the Salix-type was significantly higher than in the other types. Detrended correspondence analysis revealed obvious floristic differences among the three canopy types. Canonical correspondence analysis showed that herbaceous species were mainly found on lower plots with high moss cover, implying that moss layers may capture seeds transported by the stream. Tall herbs occurred in less shaded plots and on higher plots, suggesting that their rapid growth prevented the occurrence of other species. Fagus-type was dominated by species with ingested fruits which depended on animals for their dispersal. Populus and Salix-types were dominated by species with wind dispersal or no dispersal mechanism, which depended on physical phenomena for dispersal. Attributes of current understory vegetation were connected with historical events, suggesting that riparian vegetation reflects the history of fluvial disturbance.     

Are boreal ecosystems susceptible to alien plant invasion? Evidence from protected areas

Although biological invasion by alien species is a major contributor to loss of indigenous biological diversity, few studies have examined the susceptibility of the boreal biome to invasion. Based on studies of other ecosystems, we hypothesized that alien plants will be restricted to disturbed areas near human activity and will not be found in natural areas of boreal ecosystems in Gros Morne National Park (Canada), a protected area experiencing a wide range of disturbance regimes. The distribution of alien plants in the region was evaluated using surveys, and study sites were established in naturally and anthropogenically disturbed habitats that had been invaded. Within study sites, randomization tests evaluated the importance of disturbance to alien plant invasion by examining changes in environmental conditions and species abundance within various disturbance regimes, while the importance of site characteristics limiting the distribution of alien plants were examined using Canonical Correspondence Analysis. Consistent with studies in a variety of biomes, areas of high disturbance and human activity had the greatest abundance of resources and the highest percentage of alien species. However, contrary to our hypothesis, natural areas of boreal ecosystems were found susceptible to alien plant invasion. Vegetation types vulnerable to invasion include forests, riparian areas, fens, and alpine meadows. Natural disturbance occurring in these vegetation types caused increases in bare ground and/or light availability facilitating alien plant invasion. Although high soil pH was associated with alien plants in these areas, disturbance was not found to cause changes in soil pH, suggesting susceptibility to invasion is pre-determined by bedrock geology or other factors influencing soil pH. Moose (Alces alces), a non-native herbivore, acts as the primary conduit for alien plant invasion in GMNP by dispersing propagules and creating or prolonging disturbance by trampling and browsing vegetation. The recurrent nature of disturbance within the boreal biome and its interaction with site conditions and herbivores enables alien plants to persist away from areas of high human activity. Managers of natural lands should monitor such interactions to decrease the invasion potential of alien plants.     

Plant succession in areas of scorched and blown-down forest after the 1980 eruption of Mount St. Helens,Washington

Abstract. Patterns of plant succession were studied in areas of scorched and blown-down forest resulting from the 1980 eruption of Mount St. Helens, Washington. Changes in species abundance were observed for 7 years in permanent sample plots representing four post-disturbance habitats, or site types. Total plant cover and species richness increased with time on all site types. In blown-down forests supporting snowpack at the time of eruption, understory recovery was dominated by the vegetative regeneration of species persisting through disturbance. In forests without snowpacks, plant survival was poorer. Increases in cover and diversity were dominated first by introduced grasses, then by colonizing forbs characteristic of early successional sites. Epilo-bium angustifolium and Anaphalis margaritacea showed widespread recruitment and clonal expansion throughout the devastated area. As a result, species composition on previously forested sites converged toward that on formerly clearcut sites, where early serai forbs resprouted vigorously from beneath the tephra. Total plant cover and species diversity were poorly correlated with post-disturbance habitat and general site characteristics (e.g. distance from the crater, elevation, slope, and aspect). However, distributions of several life-forms (e.g. low sub-shrubs and tall shrubs) were strongly correlated with depth of burial by tephra and with cover of tree rootwads. Thus, early community recovery may reflect microsite variation or chance survival and recruitment rather than broad-scale gradients in environment or disturbance. Recovery of pre-disturbance composition and structure will undoubtedly be much slower than after other types of catastrophic disturbance. The rate and direction of community recovery will largely depend on the degree to which original understory species survived the eruption.     

Rapid Increases in Forest Understory Diversity and Productivity following a Mountain Pine Beetle (Dendroctonus ponderosae) Outbreak in Pine Forests

The current unprecedented outbreak of mountain pine beetle (Dendroctonus ponderosae) in lodgepole pine (Pinus contorta) forests of western Canada has resulted in a landscape consisting of a mosaic of forest stands at different stages of mortality. Within forest stands, understory communities are the reservoir of the majority of plant species diversity and influence the composition of future forests in response to disturbance. Although changes to stand composition following beetle outbreaks are well documented, information on immediate responses of forest understory plant communities is limited. The objective of this study was to examine the effects of D. ponderosae-induced tree mortality on initial changes in diversity and productivity of understory plant communities. We established a total of 110 1-m² plots across eleven mature lodgepole pine forests to measure changes in understory diversity and productivity as a function of tree mortality and below ground resource availability across multiple years. Overall, understory community diversity and productivity increased across the gradient of increased tree mortality. Richness of herbaceous perennials increased with tree mortality as well as soil moisture and nutrient levels. In contrast, the diversity of woody perennials did not change across the gradient of tree mortality. Understory vegetation, namely herbaceous perennials, showed an immediate response to improved growing conditions caused by increases in tree mortality. How this increased pulse in understory richness and productivity affects future forest trajectories in a novel system is unknown.     

Native perennial grasses show evolutionary response to Bromus tectorum (cheatgrass) invasion

Invasive species can change selective pressures on native plants by altering biotic and abiotic conditions in invaded habitats. Although invasions can lead to native species extirpation, they may also induce rapid evolutionary changes in remnant native plants. We investigated whether adult plants of five native perennial grasses exhibited trait shifts consistent with evolution in response to invasion by the introduced annual grass Bromus tectorum L. (cheatgrass), and asked how much variation there was among species and populations in the ability to grow successfully with the invader. Three hundred and twenty adult plants were collected from invaded and uninvaded communities from four locations near Reno, Nevada, USA. Each plant was divided in two and transplanted into the greenhouse. One clone was grown with B. tectorum while the other was grown alone, and we measured tolerance (ability to maintain size) and the ability to reduce size of B. tectorum for each plant. Plants from invaded populations consistently had earlier phenology than those from uninvaded populations, and in two out of four sites, invaded populations were more tolerant of B. tectorum competition than uninvaded populations. Poa secunda and one population of E. multisetus had the strongest suppressive effect on B. tectorum, and these two species were the only ones that flowered in competition with B. tectorum. Our study indicates that response to B. tectorum is a function of both location and species identity, with some, but not all, populations of native grasses showing trait shifts consistent with evolution in response to B. tectorum invasion within the Great Basin.     

Invading Monotypic Stands of Phalaris arundinacea: A Test of Fire, Herbicide, and Woody and Herbaceous Native Plant Groups

Phalaris arundinacea L. is an aggressive species that can dominate wetlands by producing monotypic stands that suppress native vegetation. In this study invasion windows were created for native species in monotypic stands of P. arundinacea with either fire or herbicide. Three native species groups, herbaceous plants, herbaceous seeds, and woody shrubs, were planted into plots burned or treated with herbicide in the early spring. Fire did not create an effective invasion window for native species; there was no difference in P. arundinacea root and shoot biomass or cover between burned and control plots (p≥ 0.998). Herbicide treatment created an invasion window for native species by reducing P. arundinacea root and shoot biomass for two growing seasons, but that invasion window was fast closing by the end of the second growing season because P. arundinacea shoot biomass had nearly reached the shoot biomass levels in the control plots (p= 0.053). Transplant mortality, frost, and animal herbivory prevented the herbaceous species and woody seedlings from becoming fully established in the plots treated with herbicide during the first year of the experiment. Transplanted monocots had a greater survival than dicots. By the second growing season the herbaceous group had the greatest mean areal cover (5%), compared to the woody seedlings (3%) and seed group (0%). Long‐term monitoring of the plots will determine whether the herbaceous transplants will compete effectively with P. arundinacea and whether the woody species will survive, shade the P. arundinacea, and accelerate forest succession.