Alter Plant Community Structure Research Articles

Effects of Grazing and Leaf Spot Disease on the Structure and Diversity of Phyllosphere Microbiome Communities in Leymus chinensis.

Leymus chinensis is a high-quality forage with wide distribution. Disease is an important factor affecting the yield and quality of L. chinensis. To investigate the effect of grazing on the phyllosphere microbiome community and leaf spot disease in L. chinensis, high-throughput sequencing technology was used to study the differences in the composition and structure of the phyllosphere fungal and bacterial communities of healthy and diseased leaves under different grazing intensities. The results showed that grazing significantly reduced leaf spot disease incidence and severity. There were significant differences in the phyllosphere microbiome composition between healthy and diseased leaves, and interestingly, diseased leaves showed more complex microbial activity. Grazing altered the relative abundance of micro-organisms and affected microbial dispersal and colonization either directly through behavior or indirectly by altering plant community structure. In this study, we found that the phyllosphere microbiome responded strongly to pathogen infection, and that plants recruited beneficial microbes to protect themselves after disease development. Grazing could regulate microbial community composition and structure, either directly or indirectly, and plays a crucial role in maintaining the health of L. chinensis.

Warming stabilizes alpine ecosystem facing extreme rainfall events by changing plant species composition

Abstract Climate warming and extreme climatic events are threatening ecosystem processes and functions. However, it remains unclear how climate warming changes ecosystem stability in facing extreme rainfall events. Here, we investigated the temporal stability of above‐ground net primary productivity (ANPP) in an alpine meadow via a 5‐year warming experiment, during which two flooding events occurred, in an alpine meadow. We first found that warming significantly increased the temporal stability of ANPP in facing flooding events by increasing resistance to and decreasing recovery from flooding events. Second, we found that warming shifted the plant community structure by increasing the dominance of grasses and reducing species richness and asynchrony. Last, we detected the higher temporal stability of ANPP under warming, which was mainly ascribed to the warming‐induced increase in dominant species stability (Deschampsia caespitosa). Synthesis. These findings indicate that climate warming may mitigate the shock of flooding events on the temporal stability of community productivity via altering plant community structure in alpine grasslands. Our study highlights that to buffer ecosystems from climatic extremes, we should focus on promoting the maintenance or selection of dominant species rather than only focusing on increasing species richness.

Extreme drought alters plant community structure by changing dominant species growth in desert grassland

AbstractQuestionsInterannual precipitation changes, both in amount and frequency, are expected to increase due to climate change, and these changes could dramatically affect the structure and function of ecosystems. However, how plant life‐forms and dominant species regulate the plant community structure after precipitation changes remains unclear.LocationGaolan Experiment Station for Ecology and Agriculture Research of the Northwest Eco‐Environment and Resources Research Institute of the Chinese Academy of Sciences, located in the desert grassland of the western Loess Plateau, China.MethodsTo investigate the effect of precipitation changes on the growth of communities, life‐forms, and species, we used rain shelters and irrigation to create a variety of precipitation gradients (i.e., −40%, −20%, 0% [= CK, i.e., control treatment], +20%, +40% of ambient precipitation) in desert grasslands from 2014 to 2015.ResultsThe plant community density, coverage, and height were significantly decreased in the −40% treatment compared to CK. In contrast, species richness, Shannon–Wiener index, and Pielous' evenness did not respond significantly to the precipitation treatment. In addition, at the level of life‐forms, the density of perennial herbaceous plants was significantly increased by the +40% treatment in 2015, while the richness and density of annual herb were significantly inhibited by extreme drought. The effect on the shrub growth index of increasing and decreasing precipitation compared to the CK was insignificant. Furthermore, at the level of species, the growth of dominant species was obviously suppressed by the −40% and +40% treatment in 2015. However, the positive effect of the +40% treatment on the growth of rare species reached significant levels in 2015. Also, the change in precipitation did not have a significant effect on the growth of companion species. Dominant species primarily drove the response of community‐level plant growth to precipitation changes over the three‐year study period.ConclusionThe response of plant community composition to changes in precipitation in desert grassland is largely regulated by changes in the composition of the dominant species. This finding could also provide a better prediction of the effects of extreme drought on desert grassland plant communities in the scenarios of future global climate change.

Abundance and distribution of arbuscular mycorrhizal fungi associated with oil-yielding plants.

Due to their role in nutrient transmission, arbuscular mycorrhizal fungi (AMF) are widespread plant root symbionts. They may improve plant production by altering plant community structure and function. Therefore, a study was conducted in the state of Haryana to analyze the distribution pattern, diversity, and association of different AMF species with oil-yielding plants. The results of the study revealed the percentage of root colonization, sporulation, and diversity of fungal species associated with the selected 30 oil-yielding plants. The percentage root colonization ranged from 0% to 100%, the highest in Helianthus annuus (100.00 ± 0.00) and Zea mays (100.00 ± 0.00) and the least in Citrus aurantium (11.87 ± 1.43). At the same time, there was no root colonization in the Brassicaceae family. The number of AMF spores present in 50 g of soil samples varied from 17.41 ± 5.28 to 497.2 ± 8.38, with maximum spore population in Glycine max (497.2 ± 8.38) and minimum in Brassica napus (17.41 ± 5.28). Besides, the presence of several species of different genera of AMF was reported in all the studied oil-yielding plants, that is, 60 AMF belonging to six genera viz. Acaulospora, Entrophospora, Glomus, Gigaspora, Sclerocystis, and Scutellospora were observed. Overall, this study will promote AMF usage in oil-yielding plants.

Plant community re-organization and increased productivity due to multi-year nutrient enrichment of a coastal grassland.

Nutrient enrichment alters plant community structure and function at a global scale. Coastal plant systems are expected to experience increased rates of nitrogen and phosphorus deposition by 2100, caused mostly by anthropogenic activity. Despite high density of studies investigating connections between plant community structure and ecosystem function in response to nutrient addition, inconsistencies in system response based on the ecosystem in question calls for more detailed analyses of nutrient impacts on community organization and resulting productivity response. Here, we focus on nutrient addition impacts on community structure and organization as well as productivity of different lifeforms in a coastal grassland. We established long-term nutrient enrichment plots in 2015 consisting of control (C), nitrogen (N), phosphorus (P), and nitrogen + phosphorus (NP) treatments. In 2017 we collected graminoid and forb productivity, root productivity, and community composition for each plot. We found no N x P interaction, but N enrichment was a significant main effect on productivity, highlighting N limitation in coastal systems. Importantly, nutrient enrichment treatments did not alter root productivity. However, all treatments caused significant differences in community composition. Using rank abundance curves, we determined that community composition differences were driven by increased dominance of nitrophilous graminoids, re-organization of subordinate species, and species absences in N and NP plots. Results of this study highlight how coastal grassland communities are impacted by nutrient enrichment. We show that community re-organization, increased dominance, and absence of critical species are all important mechanisms that reflect community-level impacts of nutrient enrichment in our coastal grassland site.

Experimental warming reduces ecosystem resistance and resilience to severe flooding in a wetland.

Climate warming and extreme hydrological events are threatening the sustainability of wetlands across the globe. However, whether climate warming will amplify or diminish the impact of extreme flooding on wetland ecosystems is unknown. Here, we show that climate warming significantly reduced wetland resistance and resilience to a severe flooding event via a 6-year warming experiment. We first found that warming rapidly altered plant community structure by increasing the dominance of low-canopy species. Then, we showed that warming reduced the resistance and resilience of vegetation productivity to a 72-cm flooding event. Last, we detected slower postflooding carbon processes, such as gross ecosystem productivity, soil respiration, and soil methane emission, under the warming treatment. Our results demonstrate how severe flooding can destabilize wetland vegetation structure and ecosystem function under climate warming. These findings indicate an enhanced footprint of extreme hydrological events in wetland ecosystems in a warmer climate.

Grazing regime alters plant community structure via patch‐scale diversity in semiarid grasslands

AbstractSelective grazing of livestock creates lightly and heavily grazed vegetation patches, which together contribute to the whole community in grazed grasslands. The intermediate disturbance hypothesis (IDH) predicts moderate grazing intensity can increase species diversity. However, grazing patchiness complicates predicted responses to grazing intensity from ecological theory and may influence how various management regimes affect biodiversity at the whole community scale. We examined effects of management regime and grazing intensity on plant species diversity, community composition, aboveground net primary production (ANPP), and soil compaction. Two management regimes (season‐long continuous grazing and grazing and mowing alternated annually) and seven levels of grazing intensity were applied over the past 10 yr. We assessed how α diversity within patches and β diversity across patches contributed to the diversity of the whole plot and how these relationships responded to both grazing intensity and management regime. We further divided β diversity into nestedness and replacement components across lightly and heavily grazed patches within plots. The mixed grazing–mowing regime differed from the continuous grazing regime in that the former had a higher number of palatable species, higher species evenness, and higher Shannon‐Wiener diversity, in both lightly and heavily grazed patches and the whole plots, and especially at moderate and high grazing intensities. The continuous grazing regime and the mixed grazing–mowing regime did not differ in total β diversity. However, the nestedness component of total β diversity was dominant in the continuous grazing regime. In contrast, species richness and ANPP did not differ significantly between the two management regimes, though soil hardness in heavily grazed patches was significantly higher under the continuous grazing regime than the mixed grazing–mowing regime. Loss of rare species under both management regimes, even at low to moderate grazing intensities, suggests that selective grazing and patch formation may not conform with the IDH. Our study indicates that the mixed grazing–mowing regime is more sustainable for long‐term grassland management than the continuous grazing regime by controlling the creation of heavily grazed patches. These findings, integrating α‐β‐γ diversity and patch‐scale approaches, provide a more thorough evaluation than the intermediate disturbance hypothesis of grazing management in terms of sustainability and biodiversity conservation in semiarid regions.

Mining and quarrying activities tend to favor stress-tolerant plants

Anthropogenic activities such as mining and agricultural practices may alter plant community structure and functioning. Grime’s Competitors–Stress–Ruderal (CSR) strategy can be used to detect plant community transformations caused by environmental changes. We examined the effects of mining and quarrying activities on CSR strategies, species diversity, functional composition (community-weighted mean – CWM), and functional diversity (Rao’s and Mason’s indexes) of plant species from nearby an active Cu-Pb-Zn mine site and an agricultural site away from the mine.We determined the ecological strategies of 34 species using 13 functional traits. We found that ruderal strategy (e.g., high R-scores) was dominant in the agricultural site, whereas stress-tolerant species (e.g., high S-scores) were abundant in the mine site according to “StrateFy” that is a globally calibrated CSR analysis tool. Community-weighted mean (CWM) trait values were greater in the agricultural site where has relatively higher soil fertility than in the stressful mine site (e.g., low pH and heavy metal stress). Although species diversity was higher in the mine site, functional diversity in the agricultural site was slightly higher than that in the mine site as indicated by greater Rao’s and Mason’s index values.Our findings suggest that the effect of anthropogenic activities might be detected using CSR strategies both at species and community levels. Mining activities may favor stress-tolerant species, whereas agricultural practices (e.g., fertilization and mowing disturbance) may favor ruderal species in accordance with the CSR theory predictions. “StrateFy” CSR analysis tool can help us to quickly track shifts in ecological strategies of plants in response to changes in the environment and land-use. Therefore, CSR strategies can be used as bioindicators of anthropogenic activities. Moreover, under increasing abiotic stress, functional diversity of plant communities may decrease which can have crucial effects on community-level responses and eventually ecosystem functioning and services.

Foundation Species Loss Affects Leaf Breakdown and Aquatic Invertebrate Resource Use in Black Ash Wetlands

The invasion by emerald ash borer (Agrilus planipennis) of extensive black ash wetlands in the upper Great Lakes region of North America is expected to alter plant community structure and composition and, therefore, abiotic factors like temperature and hydrology. We conducted two experiments to examine how changes in leaf litter could alter ecosystem function via 1) changes in litter breakdown and 2) aquatic invertebrate feeding. For the first experiment, we placed litter bags containing black ash (Fraxinus nigra), swamp white oak (Quercus bicolor), and lake sedge (Carex lacustris), in either control or clear-cut black ash plots. We found that black ash litter broke down 2-3 times faster than other species and broke down faster in control plots than in clear-cuts. There was no effect of clear cutting on swamp white oak or lake sedge breakdown rates. For the second experiment, we fed shredding caddisfly larvae (Limnephilus indivisus) one of six species: black ash, swamp white oak, lake sedge, balsam poplar (Populus balsamifera), American elm (Ulmus americana), or speckled alder (Alnus incana) for 14 days. Caddisflies had the highest survival and greater resource use when given black ash or speckled alder, which are abundant in black ash wetlands. These results suggest that loss of black ash will alter ecosystem processes via changes in the physical environment, changes in leaf litter properties, and changes in shredder processing rates of leaf litter.

Microtopography-induced ecohydrological effects alter plant community structure

Microtopography affects radiation, reflection, and hydrological processes, thereby forming different microhabitats. This study aimed to determine how these processes influence the plot-scale ecosystem (~0.1–1000 m2). Soil was sampled at 0–10 cm and 10–25 cm on depressions, gentle and steep slopes, and gullies on an artificial hillslope. Moisture, temperature, ammonium, nitrate, and plant-available phosphorus (P) were measured. Plant species distributions were evaluated using quadrats and plant nitrogen (N) and P were quantified. Resource richness, evenness, and diversity were compared for all microtopographies. Seasonal dynamics of moisture, nitrate, and available phosphorus indicate that leaching processes shape resource heterogeneity at the plot-scale. Soil nutrient levels widely differed among microtopographies during the non-growing season, but these differences decreased during the growing season. Elevated soil nitrate (but not available P) increased plant nitrate and N:P ratios on steep slopes. Higher soil available P on steep slopes was associated with an increase in the relative abundance of legumes, and subsequently stimulated the accumulation of soil N. The hydrological pathway contributed towards shaping plant community structure by regulating soil resource distribution across microtopographies. In conclusion, this study demonstrated that slight differences in microtopography (e.g., gentle versus steep slopes) alter the soil environment and plant community; therefore, sampling and treatments should be implemented under similar microtopographic conditions to avoid heterogeneity, even at such small scales.

Community composition, structure and productivity in response to nitrogen and phosphorus additions in a temperate meadow

Global nitrogen (N) enrichment likely alters plant community composition and increases productivity, consequently affecting ecosystem stability. Meanwhile, the effects of N addition on plant community composition and productivity are often influenced by phosphorus (P) nutrition, as the effects of N and P addition and interactions between N and P on plant community structure and productivity are still not well understood. An in situ experiment with N and P addition was conducted in a temperate meadow in northeastern China from 2013 to 2016. The responses of plant community composition, structure, functional group cover, richness and productivity to N and P additions were examined. N addition significantly reduced species richness and diversity but increased aboveground net primary productivity (ANPP) during the four-study-year period. P addition exerted no significant impact on species richness, diversity or ANPP but reduced cover of grasses and increased legume cover. Under N plus P addition, P addition alleviated the negative effects of N addition on community structure by increasing species richness and covers of legume and forbs. N and P additions significantly altered plant community structure and productivity in the functional groups. N addition significantly increased the cover of gramineous and reduced the cover of legume, P addition significantly increased legume cover. Our observations revealed that soil nutrient availability regulates plant community structure and ANPP in response to nutrient enrichment caused by anthropogenic activities in the temperate meadow. Our results highlight that the negative influence of N deposition on plant community composition might be alleviated by P input in the future.

Fire history influences large-herbivore behavior at circadian, seasonal, and successional scales.

Recurrent environmental changes often prompt animals to alter their behavior leading to predictable patterns across a range of temporal scales. The nested nature of circadian and seasonal behavior complicates tests for effects of rarer disturbance events like fire. Fire can dramatically alter plant community structure, with important knock-on effects at higher trophic levels, but the strength and timing of fire's effects on herbivores remain unclear. We combined prescribed fire treatments with fine-scale location data to quantify herbivore responses to fire across three temporal scales. Between 2001 and 2003, 26 stands of fir (Abies spp.) and Douglas-fir (Pseudotsuga menziesii) were thinned and burned; 27 similar stands were left untreated as experimental controls. Analyzing female elk (Cervus canadensis) locations across 21yr (1996-2016), we found crepuscular, seasonal, and successional shifts in behavioral responses to fire. Elk displayed "commuting" behavior, avoiding burns during the day, but selecting them at night. Elk selection for burns was strongest in early summer and the relative probability of elk using burns peaked quickly (5yr post burn) before gradually returning to pre-treatment levels (15yr post burn). Our results demonstrate that fire history has complex, persistent effects on herbivore behavior, and suggest that herbivores benefit from heterogeneous landscapes containing a range of successional stages.

Fire rather than nitrogen addition affects understory plant communities in the short term in a coniferous-broadleaf mixed forest.

Increasing fire risk and atmospheric nitrogen (N) deposition have the potential to alter plant community structure and composition, with consequent impacts on biodiversity and ecosystem functioning. This study was conducted to examine short‐term responses of understory plant community to burning and N addition in a coniferous‐broadleaved mixed forest of the subtropical‐temperate transition zone in Central China. The experiment used a pair‐nested design, with four treatments (control, burning, N addition, and burning plus N addition) and five replicates. Species richness, cover, and density of woody and herbaceous plants were monitored for 3 years after a low‐severity fire in the spring of 2014. Burning, but not N addition, significantly stimulated the cover (+15.2%, absolute change) and density (+62.8%) of woody species as well as herb richness (+1.2 species/m2, absolute change), cover (+25.5%, absolute change), and density (+602.4%) across the seven sampling dates from June 2014 to October 2016. Light availability, soil temperature, and prefire community composition could be primarily responsible for the understory community recovery after the low‐severity fire. The observations suggest that light availability and soil temperature are more important than nutrients in structuring understory plant community in the mixed forest of the subtropical‐temperate transition zone in Central China. Legacy woody and herb species dominated the understory vegetation over the 3 years after fire, indicating strong resistance and resilience of forest understory plant community and biodiversity to abrupt environmental perturbation.

Predators in the plant-soil feedback loop: aboveground plant-associated predators may alter the outcome of plant-soil interactions.

Plant-soil feedback (PSF) can structure plant communities, promoting coexistence (negative PSF) or monodominance (positive PSF). At higher trophic levels, predators can alter plant community structure by re-allocating resources within habitats. When predator and plant species are spatially associated, predators may alter the outcome of PSF. Here, I explore the influence of plant-associated predators on PSF using a generalised cellular automaton model that tracks nutrients, plants, herbivores and predators. I explore key contingencies in plant-predator associations such as whether predators associate with live vs. senesced vegetation. Results indicate that plant-associated predators shift PSF to favour the host plant when predators colonise live vegetation, but the outcome of PSF will depend upon plant dispersal distance when predators colonise dead vegetation. I apply the model to two spider-associated invasive plants, finding that spider predators should shift PSF dynamics in a way that inhibits invasion by one forest invader, but exacerbates invasion by another.

Logging debris and herbicide treatments improve growing conditions for planted Douglas-fir on a droughty forest site invaded by Scotch broom

Abstract Logging debris has the potential to benefit forest regeneration by increasing resource availability, modifying microclimate, and altering plant community structure. To understand potential mechanisms driving these benefits, we initiated research at a forested site on the Olympic Peninsula, WA that contained the invasive, nonnative competitor, Scotch broom (Cytisus scoparius). Immediately after harvesting the stand of mature coast Douglas-fir (Pseudotsuga menziesii var. menziesii) in late 2011, two levels of logging debris retention were created on replicated plots: 18.9 and 9.0 Mg ha−1, with debris depths averaging 32 and 17 cm, respectively. Within each plot, three herbicide treatments (aminopyralid (A), triclopyr ester (T), and A + T) and a non-sprayed control were applied to split plots in August 2012. Douglas-fir seedlings were planted in early 2013, and microclimate and seedling performance were monitored through 2016. During the growing seasons of 2012–2014, soil water content was greater and soil temperature was lower under heavy debris than under light debris. Survival of planted Douglas-fir seedlings declined an average of 45 and 11 percentage points after intense summer droughts in 2015 and 2016, respectively, but it averaged 7–10 percentage points greater in heavy debris than in light debris during this period. Douglas-fir stem diameter growth was consistently greater in heavy debris than in light debris, with the exception of treatment A + T where diameter did not differ between debris treatments. A reciprocal regression model (R2 = 0.55) predicted that total stem volume of Douglas-fir increased from 19 to 84 dm3 ha−1 as Scotch broom cover decreased from 20% to 0% as a result of the logging debris and herbicide treatments. There were limited treatment effects on mineral soil chemical and physical properties, but forest floor mass and nutrient content were increased in the heavy debris treatment. Five years after forest harvesting (2016), logging debris mass in heavy debris differed little from that in light debris at study initiation, indicating a substantial reduction in fuels and the potential for severe wildfire. Results suggest that, on gravelly soils and possibly other droughty forest ecosystems in the Pacific Northwest, heavy debris will benefit planted Douglas-fir by improving growing conditions and by limiting abundance of nonnative competitors, such as Scotch broom.

Effects of grazing on spatiotemporal variations in community structure and ecosystem function on the grasslands of Inner Mongolia, China

Grasslands worldwide are suffering from overgrazing, which greatly alters plant community structure and ecosystem functioning. However, the general effects of grazing on community structure and ecosystem function at spatial and temporal scales has rarely been examined synchronously in the same grassland. Here, during 2011–2013, we investigated community structure (cover, height, and species richness) and aboveground biomass (AGB) using 250 paired field sites (grazed vs. fenced) across three vegetation types (meadow, typical, and desert steppes) on the Inner Mongolian Plateau. Grazing, vegetation type, and year all had significant effects on cover, height, species richness, and AGB, although the primary factor influencing variations in these variables was vegetation type. Spatially, grazing significantly reduced the measured variables in meadow and typical steppes, whereas no changes were observed in desert steppe. Temporally, both linear and quadratic relationships were detected between growing season precipitation and cover, height, richness, or AGB, although specific relationships varied among observation years and grazing treatments. In each vegetation type, the observed community properties were significantly correlated with each other, and the shape of the relationship was unaffected by grazing treatment. These findings indicate that vegetation type is the most important factor to be considered in grazing management for this semi-arid grassland.

Browsing by an invasive herbivore promotes development of plant and soil communities during primary succession

Summary Invasive herbivores can strongly affect ecosystems by reducing or removing native plant species, and early in primary successions they could have enduring consequences for plant community assembly and ecosystem functioning, although this has seldom been explored. Invasive brushtail possums (Trichosurus vulpecula) browse from ground levels to forest canopies in New Zealand, including on plant species common early in primary succession. We used exclosures (fenced on the top as well as the sides) and corresponding control plots to study effects of brushtail possums on primary successions on bare landslide surfaces in the South Island of New Zealand. We then conducted repeated measurements of the plant community over 11 years and at the end also quantified soil abiotic and biotic properties. Above‐ground, plant biomass increased from close to zero to > 1000 g dry mass m−2 after 11 years in both exclosures and control plots, with significantly more biomass of the N2‐fixing tree Carmichaelia odorata in the controls. Brushtail possums reduced the biomass in the control plots of small‐stature grasses and sedges in the first 3 years. Plant species richness was greater in control than exclosure plots throughout the experiment. Below‐ground, soil carbon and nitrogen concentrations and the biomass of all major soil microbial groups were significantly greater in the control than exclosure plots, although soil nematode abundances were not. Below‐ground effects could be attributed to the greater biomass of Carmichaelia in control plots. Synthesis. Our study shows that an invasive herbivore exerted strong effects on the rate and trajectory of primary succession by altering plant community assembly. Herbivore exclusion resulted in reduced biomass of a key N2‐fixing tree that drives forest development, with important below‐ground consequences. These results support a view that restoration of ecosystems needs to consider the consequences of altering plant community structure before removing invasive herbivores.

Natural and experimental tests of trophic cascades: gray wolves and white-tailed deer in a Great Lakes forest.

Herbivores can be major drivers of environmental change, altering plant community structure and changing biodiversity through the amount and species of plants consumed. If natural predators can reduce herbivore numbers and/or alter herbivore foraging behavior, then predators may reduce herbivory on sensitive plants, and a trophic cascade will emerge. We have investigated whether gray wolves (Canis lupus) generate such trophic cascades by reducing white-tailed deer (Odocoileus virginianus) herbivory on saplings and rare forbs in a northern mesic forest (Land O' Lakes, WI). Our investigation used an experimental system of deer exclosures in areas of high and low wolf use that allowed us to examine the role that wolf predation may play in reducing deer herbivory through direct reduction in deer numbers or indirectly through changing deer behavior. We found that in areas of high wolf use, deer were 62 % less dense, visit duration was reduced by 82 %, and percentage of time spent foraging was reduced by 43 %; in addition, the proportion of saplings browsed was nearly sevenfold less. Average maple (Acer spp.) sapling height and forb species richness increased 137 and 117 % in areas of high versus low wolf use, respectively. The results of the exclosure experiments revealed that the negative impacts of deer on sapling growth and forb species richness became negligible in high wolf use areas. We conclude that wolves are likely generating trophic cascades which benefit maples and rare forbs through trait-mediated effects on deer herbivory, not through direct predation kills.

Plant community response to nitrogen and phosphorus enrichment varies across an alpine tundra moisture gradient

Background: The extent to which nutrient availability influences plant community composition and dynamics has been a focus of ecological enquiry for decades.Aims: Results from a long-term nitrogen (N) and phosphorus (P) addition experiment in alpine tundra were used to evaluate the importance of the two nutrients in structuring plant communities in three communities that differed in their snow cover amounts and duration and soil moisture characteristics.Methods: A factorial N and P experiment was established in three meadows differing in initial vegetation composition and soil moisture. Plant and soil characteristics were measured after 20 years, and the dissimilarity among meadows and treatments were measured using permutational analysis of variance.Results: Plant species richness declined uniformly across the three meadow types and in response to N and N + P additions, while both evenness and the Shannon diversity index finding indicated that nutrient additions had the highest impact on moister habitats. Overall, N impacts overshadowed changes attributed to P additions, and the N and N + P plots in wet meadow sites were the least diverse and scored the lowest dissimilarity averages among treatments. Dissimilarity estimates indicated that the control and P plots in the dry meadow community were more distinct in composition than all other plots, and especially those in the moist or wet meadows. Above-ground biomass of grasses and sedges (graminoids) increased with N additions while forbs appeared to show responses dictated in part by the graminoid responses. The most abundant grass species of moist and wet meadow, Deschampsia cespitosa, dominated N and N + P plots of the wet sites, but did not show a N response in moist areas in spite of its general abundance in moist meadow. Competition from other plant species in the moist areas likely diminished the D. cespitosa response and contributed to the resilience of the community to nutrient enrichment.Conclusions: Initial community composition, as influenced by the specific moisture regime, appears to control the extent to which changes in nutrient resources can alter plant community structure. Long-term fertilization tends to support most but not all findings obtained from shorter-termed efforts, and wet meadows exhibit the largest changes in plant species numbers and composition when chronically enriched with N.

The consequences of multiple resource shifts on the productivity and composition of alpine tundra communities: inferences from a long-term snow and nutrient manipulation experiment

Background: Gradients in the amounts and duration of snowpack and resulting soil moisture gradients have been associated with different plant communities across alpine landscapes.Aims: The extent to which snow additions could alter plant community structure, both alone and in combination with nitrogen (N) and phosphorus (P) additions, provided an empirical assessment of the strength of these variables on structuring the plant communities of the alpine tundra at Niwot Ridge, Colorado Front Range.Methods: A long-term snow fence was used to study vegetation changes in responses to snowpack, both alone and in conjunction with nutrient amendments, in plots established in dry and moist meadow communities in the alpine belt. Species richness, diversity, evenness and dissimilarity were evaluated after 20 years of treatments.Results: Snow additions, alone, reduced species richness and altered species composition in dry meadow plots, but not in moist meadow; more plant species were found in the snow-impacted areas than in nearby controls. Changes in plant community structure to N and N + P additions were influenced by snow additions. Above-ground plant productivity in plots not naturally affected by snow accumulation was not increased, and the positive responses of plant species to nutrient additions were reduced by snow addition. Plant species showed individualistic responses to changes in snow and nutrients, and indirect evidence suggested that competitive interactions mediated responses. A Permanova analysis demonstrated that community dissimilarity was affected by snow, N, and P additions, but with these responses differing by community type for snow and N. Snow influenced community patterns generated by N, and finally, the communities impacted by N + P were significantly different than those affected by the individual nutrients.Conclusions: These results show that changes in snow cover over a 20-year interval produce measureable changes in community composition that concurrently influence and are influenced by soil nutrient availability. Dry meadow communities exhibit more sensitivity to increases in snow cover whereas moist meadow communities appear more sensitive to N enrichment. This study shows that the dynamics of multiple limiting resources influence both the productivity and composition of alpine plant communities, with, species, life form, and functional traits mediating these responses.