Alter Plant Community Composition Research Articles

Intra‐annual rainfall regime shifts competitive interactions between coastal sage scrub and invasive grasses

Changes in rainfall distribution, generally predicted by many climate models, can affect resource dynamics and ecosystem function. While little studied, intra-annual rainfall distribution may have particularly strong effects on competitive interactions. Here, we test whether increased rainfall event size and decreased frequency within a growing season can influence competitive dynamics related to the invasion of exotic annual grasses in California coastal sage scrub (CSS). We hypothesized that larger rainfall events and decreased frequency will increase the competitive ability of native CSS species: a deeper root system will permit greater water use during dry periods between pulses and enhance their resource depletion effect on more shallow-rooted grasses. We planted grass and CSS seedlings in an additive competition design under three rainfall treatments: frequent small events, infrequent large events, and infrequent small events. The first two treatments had the same total rainfall but different frequency, while the second and third treatments had the same frequency but different total rainfall. Rainfall treatment altered the competitive interactions between CSS and grasses. In the first year, the competitive effect of annual grasses on shrub seedlings was strongest under the frequent small rainfall regime where they reduced deep soil moisture and light. In year two, the established shrubs began to exert strong competitive effects on grasses, and these effects were strongest under the infrequent small rainfall regime (low total rain) where they reduced shallow soil moisture and decreased grass stomatal conductance. Results suggest that reductions in both rainfall frequency and total rainfall may be important to competitive interactions, and can alter plant community composition and invasion when species have different rooting depths and different responses to soil moisture.

Effects of Browsing by Captive Elk (Cervus canadensis) on a Midwestern Woody Plant Community

Abstract Elk (Cervus canadensis) can significantly alter plant community composition and reduce plant biodiversity, mass, seedling numbers and sapling growth. Yet, few studies have examined the interactions of reintroduced elk with woody plant communities in eastern North America. To test the hypothesis that elk herbivory would reduce woody plant diversity and recruitment and also change woody plant community composition when compared to deer, we evaluated herbivory effects of a captive elk herd and a free-roaming white-tailed deer (Odocoileus virginianus) population in Land Between the Lakes National Recreation Area in western Kentucky. Elk and deer herbivory did not differ in their effects upon overall woody plant diversity. However, elk browsing significantly altered woody plant community composition (F1,15 = 2.27, P = 0.005), reduced stem heights of Quercus and Cornus genera (Ps < 0.036) and reduced frequencies of Quercus, Nyssa, and Sassafras (Ps < 0.045) when compared to deer. Our results sugges...

Altered Plant-community Composition and Edaphic Features Associated with Plowing in Southern Wisconsin Fens

The potential for land-use legacies to alter current wetland macrophyte communities and edaphic conditions are underexplored. Here we present results of a study describing associations among these factors in southern Wisconsin fens. Plant-community composition, hydrology, and nutrient availability data were recorded from 1) fens that had been plowed, 2) fens that have never been plowed, and 3) fens with plowed and never-plowed areas. Plowed fens had: 1) lower total species richness, 2) higher invasive species richness, 2) lower native graminoid richness, 3) lower shrub richness, 4) lower root-zone volumetric water content, and 5) higher available N and P then never-plowed fens. Most of these same relationships held when comparing plowed and never-plowed areas of the same fen. Non-metric Multidimensional Scaling revealed that the most important community gradient separates graminoid- and fen-specialist-rich communities from those dominated by invasive species. This gradient also separated plowed from never plowed plots. Accordingly, we believe that a history of plowing has fundamentally altered biotic and, potentially, abiotic conditions, with real consequences for wetland management and restoration.

Seasonal Soil Moisture Patterns in Contrasting Habitats in the Willamette Valley, Oregon

Changing seasonal soil moisture regimes caused by global warming may alter plant community composition in sensitive or endangered habitats such as wetlands and oak savannas. To evaluate such changes, an understanding of typical seasonal soil moisture regimes is necessary. The primary objective of this study was to document seasonal soil moisture patterns in herbaceous plant communities across a range of soils typical of Willamette Valley wet prairie, floodplain, and oak savanna habitats. Volumetric soil moisture data were collected periodically from January 2010 through December 2011 at study sites using time domain reflectometry and converted to plant-available soil moisture using moisture release data for each of the soils. In addition, plant communities, soil texture, and soil chemical attributes were evaluated at all sites. Both 2010 and 2011 experienced unusually high spring rainfall, and soils did not begin dry down until about the third week in June. The length of the dry period was similar in both years, but in 2011 it was shifted from mid-July through early September to early August through early October. Well-drained floodplain soils consistently had the lowest soil moisture contents in both years. Wet prairie sites began to dry down shortly after oak savanna sites, and their rates and severity of dry down were similar. These results suggest that the composition of herbaceous plant communities in wet prairie habitats may be influenced more by winter and spring inundation than by summer drought and that the timing of rainfall events during the dry period could influence plant community composition.

Decoupled long‐term effects of nutrient enrichment on aboveground and belowground properties in subalpine tundra

Fertilizer experiments aid understanding of the role of nutrient limitation in tundra ecosystems, and of the effects that global change may exert on them through enhancing nutrient availability. However, little is known about whether fertilizer additions influence tundra ecosystem properties directly, or indirectly through altering plant community composition. To separate these direct and indirect effects, we used an ongoing fertilization experiment in subalpine tundra in northern Sweden initiated in 1989. Here, the slow‐growing dwarf shrub Empetrum hermaphroditum (hereafter Empetrum) dominates, but fertilization causes its replacement by the fast‐growing grass Deschampsia flexuosa (hereafter Deschampsia). Therefore, in 1999, we set up three subplots in every plot of the fertilizer experiment subjected to different treatments, i.e., regular removal of Empetrum or of Deschampsia, or no removal. An interactive effect between fertilizer and removal treatments means that fertilization indirectly influences ecological properties through its effects on the dominant species whereas no interactive effect means that fertilization effects are more likely direct. We measured plant community properties four times between 1999 and 2010, and belowground properties in 2010. Fertilization exerted large effects on nearly all variables aboveground (i.e., vascular plant community properties, bryophyte and lichen cover) and belowground (i.e., measures of nutrient availability, mass of humus and major elements per area, microbial community properties). Meanwhile, loss of either Deschampsia or Empetrum reduced total vascular plant cover, with greatest effects in those fertilization treatments where they otherwise had the most mass. Deschampsia removal affected several other plant community properties, but mostly independently of fertilizer treatment; Empetrum removal had little effect on any other plant community variable. Belowground, both removal treatments reduced humus mass per area but had no effect on any other variable. Our finding of few interactive effects of fertilizer and removal treatments aboveground and none belowground means that fertilizer effects on most community and ecosystem properties operate independently of even large shifts in the plant community. These results also point to a decoupling between aboveground and belowground communities in tundra, and therefore offer insights about the mechanistic basis through which global change impacts on soil nutrient availability may transform tundra ecosystems.

Complex facilitation and competition in a temperate grassland: loss of plant diversity and elevated CO2 have divergent and opposite effects on oak establishment

Encroachment of woody vegetation into grasslands is a widespread phenomenon that alters plant community composition and ecosystem function. Woody encroachment is often the result of fire suppression, but it may also be related to changes in resource availability associated with global environmental change. We tested the relative strength of three important global change factors (CO(2) enrichment, nitrogen deposition, and loss of herbaceous plant diversity) on the first 3 years of bur oak (Quercus macrocarpa) seedling performance in a field experiment in central Minnesota, USA. We found that loss of plant diversity decreased initial oak survival but increased overall oak growth. Conversely, elevated CO(2) increased initial oak seedling survival and reduced overall growth, especially at low levels of diversity. Nitrogen deposition surprisingly had no net effect on survival or growth. The magnitude of these effects indicates that long-term woody encroachment trends may be most strongly associated with those few individuals that survive, but grow much larger in lower diversity patches. Further, while the CO(2) results and the species richness results appear to describe opposing trends, this is due only to the fact that the natural drivers are moving in opposite directions (decreasing species richness and increasing CO(2)). Interestingly, the mechanisms that underlie both patterns are very similar, increased CO(2) and increased species richness both increase herbaceous biomass which (1) increases belowground competition for resources and (2) increases facilitation of early plant survival under a more diverse plant canopy; in other words, both competition and facilitation help determine community composition in these grasslands.

Peatland Microbial Communities and Decomposition Processes in the James Bay Lowlands, Canada

Northern peatlands are a large repository of atmospheric carbon due to an imbalance between primary production by plants and microbial decomposition. The James Bay Lowlands (JBL) of northern Ontario are a large peatland-complex but remain relatively unstudied. Climate change models predict the region will experience warmer and drier conditions, potentially altering plant community composition, and shifting the region from a long-term carbon sink to a source. We collected a peat core from two geographically separated (ca. 200 km) ombrotrophic peatlands (Victor and Kinoje Bogs) and one minerotrophic peatland (Victor Fen) located near Victor Bog within the JBL. We characterized (i) archaeal, bacterial, and fungal community structure with terminal restriction fragment length polymorphism of ribosomal DNA, (ii) estimated microbial activity using community level physiological profiling and extracellular enzymes activities, and (iii) the aeration and temperature dependence of carbon mineralization at three depths (0–10, 50–60, and 100–110 cm) from each site. Similar dominant microbial taxa were observed at all three peatlands despite differences in nutrient content and substrate quality. In contrast, we observed differences in basal respiration, enzyme activity, and the magnitude of substrate utilization, which were all generally higher at Victor Fen and similar between the two bogs. However, there was no preferential mineralization of carbon substrates between the bogs and fens. Microbial community composition did not correlate with measures of microbial activity but pH was a strong predictor of activity across all sites and depths. Increased peat temperature and aeration stimulated CO2 production but this did not correlate with a change in enzyme activities. Potential microbial activity in the JBL appears to be influenced by the quality of the peat substrate and the presence of microbial inhibitors, which suggests the existing peat substrate will have a large influence on future JBL carbon dynamics.

Absence of snow cover reduces understory plant cover and alters plant community composition in boreal forests

Snow regimes affect biogeochemistry of boreal ecosystems and are altered by climate change. The effects on plant communities, however, are largely unexplored despite their influence on relevant processes. Here, the impact of snow cover on understory community composition and below-ground production in a boreal Picea abies forest was investigated using a long-term (8-year) snow cover manipulation experiment consisting of the treatments: snow removal, increased insulation (styrofoam pellets), and control. The snow removal treatment caused longer (118 vs. 57 days) and deeper soil frost (mean minimum temperature -5.5 vs. -2.2°C) at 10 cm soil depth in comparison to control. Understory species composition was strongly altered by the snow cover manipulations; vegetation cover declined by more than 50% in the snow removal treatment. In particular, the dominant dwarf shrub Vaccinium myrtillus (-82%) and the most abundant mosses Pleurozium schreberi (-74%) and Dicranum scoparium (-60%) declined strongly. The C:N ratio in V. myrtillus leaves and plant available N in the soil indicated no altered nitrogen nutrition. Fine-root biomass in summer, however, was negatively affected by the reduced snow cover (-50%). Observed effects are attributed to direct frost damage of roots and/ or shoots. Besides the obvious relevance of winter processes on plant ecology and distribution, we propose that shifts in the vegetation caused by frost damage may be an important driver of the reported alterations in biogeochemistry in response to altered snow cover. Understory plant performance clearly needs to be considered in the biogeochemistry of boreal systems in the face of climate change.

Soil disturbance alters plant community composition and decreases mycorrhizal carbon allocation in a sandy grassland

We have studied how disturbance by ploughing and rotavation affects the carbon (C) flow to arbuscular mycorrhizal (AM) fungi in a dry, semi-natural grassland. AM fungal biomass was estimated using the indicator neutral lipid fatty acid (NLFA) 16:1ω5, and saprotrophic fungal biomass using NLFA 18:2ω6,9. We labeled vegetation plots with (13)CO(2) and studied the C flow to the signature fatty acids as well as uptake and allocation in plants. We found that AM fungal biomass in roots and soil decreased with disturbance, while saprotrophic fungal biomass in soil was not influenced by disturbance. Rotavation decreased the (13)C enrichment in NLFA 16:1ω5 in soil, but (13)C enrichment in the AM fungal indicator NLFA 16:1ω5 in roots or soil was not influenced by any other disturbance. In roots, (13)C enrichment was consistently higher in NLFA 16:1ω5 than in crude root material. Grasses (mainly Festuca brevipila) decreased as a result of disturbance, while non-mycorrhizal annual forbs increased. This decreases the potential for mycorrhizal C sequestration and may have been the main reason for the reduced mycorrhizal C allocation found in disturbed plots. Disturbance decreased the soil ammonium content but did not change the pH, nitrate or phosphate availability. The overall effect of disturbance on C allocation was that more of the C in AM fungal mycelium was directed to the external phase. Furthermore, the functional identity of the plants seemed to play a minor role in the C cycle as no differences were seen between different groups, although annuals contained less AM fungi than the other groups.

Plant community responses to nitrogen addition and increased precipitation: the importance of water availability and species traits

AbstractGlobal nitrogen (N) enrichment and changing precipitation regimes are likely to alter plant community structure and composition, with consequent influences on biodiversity and ecosystem functioning. Responses of plant community structure and composition to N addition and increased precipitation were examined in a temperate steppe in northern China. Increased precipitation and N addition stimulated and suppressed community species richness, respectively, across 6 years (2005–2010) of the manipulative experiment. N addition and increased precipitation significantly altered plant community structure and composition at functional groups levels. The significant relationship between species richness and soil moisture (SM) suggests that plant community structure is mediated by water under changing environmental conditions. In addition, plant height played an important role in affecting the responses of plant communities to N addition, and the effects of increased precipitation on plant community were dependent on species rooting depth. Our results highlight the importance and complexity of both abiotic (SM) and biotic factors (species traits) in structuring plant community under changing environmental scenarios. These findings indicate that knowledge of species traits can contribute to mechanistic understanding and projection of vegetation dynamics in response to future environmental change.

Rehabilitating Downy Brome (Bromus tectorum)–Invaded Shrublands Using Imazapic and Seeding with Native Shrubs

AbstractRehabilitation of downy brome–infested shrublands is challenging once this invasive grass dominates native communities. The effectiveness of imazapic herbicide in reducing downy brome cover has been variable, and there is uncertainty about the impacts of imazapic on native species. We used a before-after-control-impact (BACI) field experiment and greenhouse studies to (1) determine if imazapic herbicide applied at 132 g ai ha−1 (8 oz/ac−1) and seeding with two native shrub species (Wyoming big sagebrush [Artemisia tridentata] and Mexican cliffrose [Purshia mexicana]) reduced downy brome cover and promoted shrub establishment, (2) assess potential effects of imazapic on nontarget plant species and plant community composition, and (3) determine if imazapic affected downy brome or seeded shrub species when applied at different developmental stages. Seeding shrubs, alone, or in combination with imazapic application, did not significantly increase shrub density, possibly because of droughty conditions. In the field, imazapic reduced downy brome cover by 20% and nontarget forb cover by 25% and altered plant community composition the first year after treatment. Imazapic was lethal to downy brome at all growth stages in the greenhouse and reduced shrub germination by 50 to 80%, but older shrub seedlings were more tolerant of the herbicide. We conclude that a one-time application of imazapic combined with seeding shrubs was only slightly effective in rehabilitating areas with high downy brome and thatch cover and resulted in short-term impacts to nontarget species. These results highlight the need to treat downy brome infestations before they become too large. Also, removing thatch prior to treating with imazapic, although likely lethal to the native shrubs we studied, could increase the effectiveness of imazapic.

Nitrogen partitioning between microbes and plants in the shortgrass steppe

Nitrogen (N) and water additions in the shortgrass steppe change the dominance of plant functional types (PFT) that are characterized by different photosynthetic pathways and phenologies. We aimed to examine monthly patterns of plant N and microbial N storage during the growing season, and to assess whether N fertilization last applied 30 years ago alters the timing and magnitude of N storage. We measured plant biomass and N, and microbial biomass N monthly during the growing season. We found differences in temporal patterns of plant and microbial N storage in the control plots, with microbial storage higher than plant storage in July, and the opposite trend in September. Unlike the control plots, the plots fertilized 30 years ago exhibited overlapping peaks of N storage in plants and microbes in August. Seasonal trends indicated that rainfall was an important control over plant and microbial activity at the beginning of the growing season, and that temperature limited these activities at the end of the growing season. PFT affected the amount of microbial N, which was in general higher under C3 grasses than other PFTs, independent of fertilization. Historical resource additions increased plant biomass and N, but had little effect on microbial N. These results highlight the complexity of the microbial response. Changes in climate that influence precipitation timing will affect the temporal pattern for microbial biomass N, while management practices resulting in altered plant community composition will influence the magnitude of microbial biomass N.

Linking field based studies with greenhouse experiments: the impact of Centaurea stoebe (=C. maculosa) in British Columbia grasslands

The grassland community of Lac du Bois Provincial Park in the interior of British Columbia has become increasingly invaded by Centaurea stoebe (=Centaurea maculosa; spotted knapweed). Allelopathy, through the production of the phytotoxin (±)-catechin by C. stoebe roots is believed to be partly responsible for knapweed’s invasive success. We used field sampling and greenhouse experiments to examine two questions: (1) Is increasing abundance of C. stoebe negatively associated with the abundance of specific native grassland species? (2) Do species that exhibit a negative correlation with C. stoebe abundance in the field demonstrate increased levels of susceptibility to application of (±)-catechin during germination? Thirty-eight plots were sampled in the grassland, encompassing areas of low—high knapweed abundance. Seeds from eight native species, exhibiting positive, neutral or negative correlation with knapweed abundance, were treated with three concentrations (0, 0.5, 2.0 mg/mL) of (±)-catechin. Root growth and percent germination were measured over a 6-week period. The results indicate that C. stoebe abundance is negatively correlated with native plant species abundance and may alter plant community composition. Moreover root radical growth was significantly negatively affected by treatment with (±)-catechin in all four native plant species that exhibited a negative correlation with knapweed abundance in the field. Past studies have failed to conclusively link greenhouse results with plant community patterns. Here, we provide a correlative link between plant community composition and tolerance to a phytotoxin.

Impact of invertebrate herbivory in grasslands depends on plant species diversity

Invertebrate herbivores are ubiquitous in most terrestrial ecosystems, and theory predicts that their impact on plant community biomass should depend on diversity and productivity of the associated plant communities. To elucidate general patterns in the relationship between invertebrate herbivory, plant diversity, and productivity, we carried out a long-term herbivore exclusion experiment at multiple grassland sites in a mountainous landscape of central Germany. Over a period of five years, we used above- and belowground insecticides as well as a molluscicide to manipulate invertebrate herbivory at 14 grassland sites, covering a wide range of plant species diversity (13-38 species/m2) and aboveground plant productivity (272-1125 g x m(-2) x yr(-1)), where plant species richness and productivity of the sites were not significantly correlated. Herbivore exclusion had significant effects on the plant communities: it decreased plant species richness and evenness, and it altered plant community composition. In particular, exclusion of belowground herbivores promoted grasses at the expense of herbs. In contrast to our expectation, herbivore effects on plant community biomass were not influenced by productivity. However, effect size of invertebrate herbivores was negatively correlated with plant diversity of the grasslands: the effect of herbivory on biomass tended to be negative at sites of high diversity and positive at sites of low diversity. In general, the effects of aboveground herbivores were relatively small as compared to belowground herbivores, which were important drivers of plant community composition. Our study is the first to show that variation in the effects of invertebrate herbivory on plant communities across a landscape is significantly influenced by plant species richness.

Soil ecosystem functioning under climate change: plant species and community effects

Feedbacks of terrestrial ecosystems to atmospheric and climate change depend on soil ecosystem dynamics. Soil ecosystems can directly and indirectly respond to climate change. For example, warming directly alters microbial communities by increasing their activity. Climate change may also alter plant community composition, thus indirectly altering the soil communities that depend on their inputs. To better understand how climate change may directly and indirectly alter soil ecosystem functioning, we investigated old-field plant community and soil ecosystem responses to single and combined effects of elevated [CO2], warming, and precipitation in Tennessee (USA). Specifically, we collected soils at the plot level (plant community soils) and beneath dominant plant species (plant-specific soils). We used microbial enzyme activities and soil nematodes as indicators for soil ecosystem functioning. Our study resulted in two main findings: (1) Overall, while there were some interactions, water, relative to increases in [CO2] and warming, had the largest impact on plant community composition, soil enzyme activity, and soil nematodes. Multiple climate-change factors can interact to shape ecosystems, but in our study, those interactions were largely driven by changes in water. (2) Indirect effects of climate change, via changes in plant communities, had a significant impact on soil ecosystem functioning, and this impact was not obvious when looking at plant community soils. Climate-change effects on enzyme activities and soil nematode abundance and community structure strongly differed between plant community soils and plant-specific soils, but also within plant-specific soils. These results indicate that accurate assessments of climate-change impacts on soil ecosystem functioning require incorporating the concurrent changes in plant function and plant community composition. Climate-change-induced shifts in plant community composition will likely modify or counteract the direct impact of atmospheric and climate change on soil ecosystem functioning, and hence, these indirect effects should be taken into account when predicting the manner in which global change will alter ecosystem functioning.

Managing plant symbiosis: fungal endophyte genotype alters plant community composition

1. Understanding the extent to which plant genotype influences community composition has important conservation and management value. Here, we add a new layer to current research by testing whether genotypes of plant symbionts influence plant community composition. 2. Tall fescue grass Lolium arundinaceum is planted worldwide for forage, turf, and soil stabilization, has invaded native communities, and commonly hosts the foliar endophytic fungus, Neotyphodium coenophialum. We quantified vegetation development over a 6-year period in replicated field plots of the two most commonly introduced genotypes of the endophyte (KY-31, AR-542) in two tall fescue cultivars (Georgia-5, Jesup). The KY-31 endophyte produces ergot alkaloids toxic to mammalian herbivores, while AR-542 lacks ergots. We expected that the KY-31 endophyte would promote tall fescue dominance by altering mammalian herbivory, that endophyte-free plots would support the highest plant diversity, and that AR-542 endophyte would show intermediate effects. 3. Plots with the KY-31 endophyte supported ∼10% fewer plant species than endophyte-free or AR-542 plots. KY-31 also reduced graminoids and forbs more strongly than AR-542, with the greatest response in Georgia-5. Endophyte genotype did not significantly alter the prevalence of tall fescue; however, absence of the endophyte reduced the biomass of Georgia-5. 4. Both plant and endophyte genetic identity influenced the persistence of endophyte symbiosis. In Georgia-5, the frequency of AR-542 declined more through time than did KY-31; these effects were reversed in Jesup. 5. Endophyte presence reduced herbivory by mammals (voles), but endophyte genotype had no effect, suggesting that voles were not driving endophyte genotype-mediated changes in plant composition, and that other compounds besides ergot alkaloids influenced vole feeding. 6. Synthesis and applications. When endophyte-symbiotic plants are desired, the AR-542 endophyte in the Georgia-5 background would achieve higher native plant diversity and reduce tall fescue invasion of nearby areas. Conversely, Jesup is recommended for increased tall fescue persistence, as both biomass and reproduction exceeded Georgia-5. In summary, our results demonstrate that, like plant genotype, symbiont genotype can also alter vegetation dynamics and plant composition and should be considered when managing for conservation, restoration, forage, turf, re-vegetation or soil stabilization.

Precipitation magnitude and timing differentially affect species richness and plant density in the sotol grassland of the Chihuahuan Desert

Arid and semi-arid environments are dynamic ecosystems with highly variable precipitation, resulting in diverse plant communities. Changes in the timing and magnitude of precipitation due to global climate change may further alter plant community composition in desert regions. In this study, we assessed changes in species richness and plant density at the community, functional group, and species level in response to variation in the magnitude of natural seasonal precipitation and 25% increases in seasonal precipitation [e.g., supplemental watering in summer, winter, or summer and winter (SW)] over a 5-year period in a sotol grassland in the Chihuahuan Desert. Community species richness was higher with increasing winter precipitation while community plant density increased with greater amounts of winter and summer precipitation, suggesting winter precipitation was important for species recruitment and summer precipitation promoted growth of existing species. Herb and grass density increased with increasing winter and summer precipitation, but only grass density showed a significant response to supplemental watering treatments (SW treatment plots had higher grass density). Shrubs and succulents did not exhibit changes in richness or density in response to natural or supplemental precipitation. In this 5-year study, changes in community species richness and density were driven by responses of herb and grass species that favored more frequent small precipitation events, shorter inter-pulse duration, and higher soil moisture. However, due to the long life spans of the shrub and succulent species within this community, 5 years may be insufficient to accurately evaluate their response to variable timing and magnitude of precipitation in this mid-elevation grassland.

Exotic Ecosystem Engineers Change the Emergence of Plants from the Seed Bank of a Deciduous Forest

The anthropogenic spread of exotic ecosystem engineers profoundly impacts native ecosystems. Exotic earthworms were shown to alter plant community composition of the understory of deciduous forests previously devoid of earthworms. We investigated the effect of two exotic earthworm species (Lumbricus terrestris L. and Octolasion tyrtaeum Savigny) belonging to different ecological groups (anecic and endogeic) on the emergence of plants from the seed bank of a northern North American deciduous forest using the seedling emergence method. We hypothesized that (1) exotic earthworms change the seedling emergence from the plant seed bank, (2) L. terrestris increases the emergence of plant seedlings of the deeper soil layer but decreases that of the upper soil layer due to plant seed burial, and (3) O. tyrtaeum decreases plant seedling emergence due the damage of plant seeds. Indeed, exotic earthworms altered the emergence of plant seedlings from the seed bank and the functional composition of the established plant seedlings. Surprisingly, although L. terrestris only marginally affected seedling emergence, O. tyrtaeum changed the emergence of native plant species from the seed bank considerably. In particular, the number of emerging grass and herb seedlings were increased in the presence of O. tyrtaeum in both soil layers. Moreover, the impacts of earthworms depended on the identity of plant functional groups; herb species benefited, whereas legumes suffered from the presence of exotic earthworms. The results highlight the strong effect of invasive belowground ecosystem engineers on aboveground ecosystem characteristics and suggest fundamental changes of ecosystems by human-spread earthworm species.

Introduced plague lessens the effects of an herbivorous rodent on grassland vegetation

Summary 1. Plague, introduced from Eurasia around 1900, today affects many animal species in the western part of North America, including black‐tailed prairie dogs Cynomys ludovicianus, widespread colonial herbivores of the Central Plains. Most studies of introduced diseases describe how they affect susceptible individuals or populations. We examined the indirect community and ecosystem‐level effects of plague to address the hypothesis that frequent plague epizootics have a mediating effect on the important role of prairie dogs in shaping grassland plant communities. 2. We used 25 years of spatially explicit monitoring data on the Pawnee National Grassland (Colorado, USA) to estimate frequency of plague epizootics in prairie dog colonies. We also examined plant community characteristics of young (3–8 years), old (∼20 years) and plague‐extirpated (7–12 years when extirpated) colonies intermittently to determine how colony age and status (active or inactive) influence the effects of prairie dogs on plant communities. 3. Approximately 98% of colonies experienced a plague epizootic within 15 years of continuous activity, nearly half remained inactive for at least 5 years following an epizootic, and less than half attained their pre‐plague area within 10 years of an epizootic. 4. Prairie dogs lowered plant canopy height, reduced plant biomass and altered plant community composition. These effects were most pronounced for older colonies. Plague‐extirpated colonies were not significantly different from their associated off‐colony sites for most variables measured. 5. Synthesis and applications. Our work shows that introduced diseases can indirectly affect community‐ and ecosystem‐level processes through their modification of host species dynamics. Frequent epizootics that lessen the effects of prairie dogs on vegetation probably affect colony‐associated species and grassland food webs. Over 100 animal species are associated with prairie dog colonies because prairie dogs are an important food source and their colonies provide habitat. Prairie dogs are considered competitors with livestock for forage and are exterminated on private lands at considerable expense to ranchers. Extermination may be unwarranted and cost‐ineffective where plague exists because of the high frequency of plague epizootics.

Relative impacts of disturbance and temperature: persistent changes in microenvironment and vegetation in retrogressive thaw slumps

AbstractIn the Low Arctic, a warming climate is increasing rates of permafrost degradation and altering vegetation. Disturbance associated with warming permafrost can change microclimate and expose areas of ion‐rich mineral substrate for colonization by plants. Consequently, the response of vegetation to warming air temperatures may differ significantly from disturbed to undisturbed tundra. Across a latitudinal air temperature gradient, we tested the hypothesis that the microenvironment in thaw slumps would be warmer and more nutrient rich than undisturbed tundra, resulting in altered plant community composition and increased green alder (Alnus viridis subsp. fruticosa) growth and reproduction. Our results show increased nutrient availability, soil pH, snow pack, ground temperatures, and active layer thickness in disturbed terrain and suggest that these variables are important drivers of plant community structure. We also found increased productivity, catkin production, and seed viability of green alder at disturbed sites. Altered community composition and enhancement of alder growth and reproduction show that disturbances exert a strong influence on deciduous shrubs that make slumps potential seed sources for undisturbed tundra. Overall, these results indicate that accelerated disturbance regimes have the potential to magnify the effects of warming temperature on vegetation. Consequently, understanding the relative effects of temperature and disturbance on Arctic plant communities is critical to predicting feedbacks between northern ecosystems and global climate change.