Alter Plant Community Composition Research Articles

Nutrient addition shifts plant community composition towards earlier flowering species in some prairie ecoregions in the U.S. Central Plains.

The distribution of flowering across the growing season is governed by each species’ evolutionary history and climatic variability. However, global change factors, such as eutrophication and invasion, can alter plant community composition and thus change the distribution of flowering across the growing season. We examined three ecoregions (tall-, mixed, and short-grass prairie) across the U.S. Central Plains to determine how nutrient (nitrogen (N), phosphorus, and potassium (+micronutrient)) addition alters the temporal patterns of plant flowering traits. We calculated total community flowering potential (FP) by distributing peak-season plant cover values across the growing season, allocating each species’ cover to only those months in which it typically flowers. We also generated separate FP profiles for exotic and native species and functional group. We compared the ability of the added nutrients to shift the distribution of these FP profiles (total and sub-groups) across the growing season. In all ecoregions, N increased the relative cover of both exotic species and C3 graminoids that flower in May through August. The cover of C4 graminoids decreased with added N, but the response varied by ecoregion and month. However, these functional changes only aggregated to shift the entire community’s FP profile in the tall-grass prairie, where the relative cover of plants expected to flower in May and June increased and those that flower in September and October decreased with added N. The relatively low native cover in May and June may leave this ecoregion vulnerable to disturbance-induced invasion by exotic species that occupy this temporal niche. There was no change in the FP profile of the mixed and short-grass prairies with N addition as increased abundance of exotic species and C3 graminoids replaced other species that flower at the same time. In these communities a disturbance other than nutrient addition may be required to disrupt phenological patterns.

Small mammal activity alters plant community composition and microbial activity in an old‐field ecosystem

AbstractHerbivores modify their environment by consuming plant biomass and redistributing materials across the landscape. While small mammalian herbivores, such as rodents, are typically inconspicuous, their impacts on plant community structure and chemistry can be large. We used a small mammal exclosure experiment to explore whether rodents in a southeastern old field directly altered the aboveground plant species composition and chemistry, and indirectly altered the belowground soil community composition and activity. In general, when rodents were excluded, C3 graminoids increased in cover and biomass, contributing toward a shift in plant species composition relative to plots where rodents were present. The plant community chemistry also shifted; plant fiber concentration and carbon : nitrogen were higher, whereas plant nitrogen concentration was lower in exclosure plots relative to access plots. While microbial community enzyme activity increased when rodents were excluded, no significant changes in the fungal : bacterial or potential nitrogen mineralization occurred between treatments. Our results show that rodents can rapidly influence aboveground plant community composition and chemistry, but their influence on belowground processes may require plant inputs to the soil to accumulate over longer periods of time.

Management of Grazing Intensity in the Semi‐Arid Rangelands of Southern Australia: Effects on Soil and Biodiversity

AbstractOvergrazing contributes to rangeland degradation altering plant community composition, erosion and biodiversity. Little unanimity in the literature exists on the effects of livestock grazing on soil carbon and biodiversity, in part, due to uncontrolled grazing pressure from native and feral animals. Paired paddock contrasts at three, long‐term (>8 years) study locations in the southern Australian rangelands were used to examine the effects of managing grazing intensity through the use of exclusion fencing and rotational grazing on soil organic carbon (SOC), soil nitrogen (TN), ground cover and biodiversity (flora and invertebrates). Grazing management had no effect on SOC or TN on grey soils (Vertisols), but for red soils (Lixisols), significantly higher levels of SOC were found for both the 0 to 5 and 5 to 10‐cm soil depths (0·3% and 0·27% respectively) and associated with increased TN. We found strong and consistent relationships among SOC and higher perennial (p < 0·001), higher litter (p < 0·05) cover and close proximity to trees (p < 0·05). Managing grazing intensity resulted in significantly higher perennial ground cover (p < 0·001) on Vertisols (8·9 to 11%) and Lixisols (12·5 to 15%) and higher plant diversity (both native and exotic) but negatively impacted invertebrate diversity, indicating trade‐offs between production and resources. We provide evidence that the effects of grazing management on SOC are mediated by ground cover and increased organic matter supply and/or reduced soil carbon redistribution (erosion), which indicates that the management of grazing intensity may provide a tool to avoid soil carbon loss in rangelands. Copyright © 2016 John Wiley & Sons, Ltd.

Imposing antecedent global change conditions rapidly alters plant community composition in a mixed-grass prairie.

Global change drivers are altering climatic and edaphic conditions of ecosystems across the globe, and we expect novel plant communities to become more common as a result. In the Colorado Front Range, compositional changes have occurred in the mixed-grass prairie plant community in conjunction with shifts in winter precipitation and atmospheric nitrogen (N) deposition. To test whether these environmental changes have been responsible for the observed plant community change, we conducted an in situ manipulative experiment in a mixed-grass meadow near Boulder, CO. We simulated historical conditions by reducing N availability (+500g C m(-2) year(-1)) and winter precipitation (with rainout shelters) for 2years (2013-2014) and compared vegetation response to these treatments with that of ambient conditions. The site experienced an extreme precipitation event in autumn 2013 which allowed comparison of an exceptionally wet year with an average year. We measured pre-treatment species composition in 2012, and treatment responses in the spring and summer of 2013 and 2014. As predicted, simulating historical low N-winter dry conditions resulted in a plant community dominated by historically abundant species. Cool-season introduced species were significantly reduced in low N-winter dry plots, particularly the annual plants Bromus tectorum and Alyssum parviflorum. These same species responded strongly to the extreme precipitation event with large increases, while native grasses and forbs showed little change in productivity or composition under varying climatic or edaphic conditions. This work provides clear evidence linking on-going global change drivers to altered plant community composition in an otherwise relatively undisturbed grassland ecosystem.

Response of AM fungi spore population to elevated temperature and nitrogen addition and their influence on the plant community composition and productivity.

To examine the influence of elevated temperature and nitrogen (N) addition on species composition and development of arbuscular mycorrhizal fungi (AMF) and the effect of AMF on plant community structure and aboveground productivity, we conducted a 5-year field experiment in a temperate meadow in northeast China and a subsequent greenhouse experiment. In the field experiment, N addition reduced spore population diversity and richness of AMF and suppressed the spore density and the hyphal length density (HLD). Elevated temperature decreased spore density and diameter and increased the HLD, but did not affect AMF spore population composition. In the greenhouse experiment, AMF altered plant community composition and increased total aboveground biomass in both elevated temperature and N addition treatments; additionally, AMF also increased the relative abundance and aboveground biomass of the grasses Leymus chinensis (Poaceae) and Setaria viridis (Gramineae) and significantly reduced the relative abundance and aboveground biomass of the Suaeda corniculata (Chenopodiaceae). Although elevated temperature and N addition can affect species composition or suppress the development of AMF, AMF are likely to play a vital role in increasing plant diversity and productivity. Notably, AMF might reduce the threat of climate change induced degradation of temperate meadow ecosystems.

Contingency in Defoliation and Moisture Effects on Northern Mixedgrass Prairie Composition and Diversity

Little is known about how defoliation intensity and frequency alter plant community composition and diversity in northern Great Plains mixedgrass communities. We evaluated defoliation effects in combination with watering on vascular plant composition and diversity in two contrasting ecological sites, a drier upland and more mesic lowland, in the Dry Mixedgrass natural subregion of Alberta, Canada. Treatments were applied for three growing seasons (2010 through 2012, inclusive) and included defoliation regimes of high intensity at high frequency, high intensity at low frequency, low intensity at high frequency, and defoliation deferred until the end of the growing season. Moisture regimes were ambient and elevated. Defoliation rather than moisture was the primary determinant of plant composition after 3 yr, particularly in the lowland site. Watering effects on composition were more apparent in the drier upland. All growing season defoliation regimes markedly altered composition relative to the deferred control, with relatively minor differences in composition among growing season defoliation treatments, particularly in the mesic lowland site. We conclude that growing season defoliation alters mixedgrass composition by reducing canopy dominant grasses (Pascopyrum smithii and Hesperostipa comata) and releasing shorter-statured grasses and forbs, which can either increase or decrease diversity depending on site (edaphic) conditions and the relative dominance of midgrasses and shortgrasses (Koeleria macrantha and Bouteloua gracilis). Finally, increased moisture did not ameliorate defoliation effects during the growing season, suggesting compositional responses were predictable and independent of growing season defoliation regime.

Nitrogen (N) and phosphorus (P) resorption of two dominant alpine perennial grass species in response to contrasting N and P availability

Nutrient additions may affect plant nutrient resorption and hence alter plant community composition and nutrient cycling in ecosystems. However, there is still much debate about how changes in nutrient availability affects nutrient resorption. Field experiments were conducted in 2013 on the Kunlun Mountain alpine grassland in northwestern China to investigate the variation between two dominant grasses Stipa capillata and Seriphidium rhodanthum in term of their nutrient resorption response to contrasting N and P addition levels. Results showed increasing N addition reduced N resorption efficiency and proficiency, and the N:P resorption ratio for both grass species, but increased P resorption efficiency and proficiency for only S. rhodanthum. Phosphorus addition increased P and N resorption efficiency of S. rhodanthum. In contrast, P addition decreased P resorption efficiency but had no impact on N resorption efficiency of S. capillata. Our results suggest that nutrient resorption responses to nutrient availability differed between perennial grass species. For both species, P additions reduced P resorption proficiency but increased N resorption proficiency. Thus, nutrient resorption is sensitive to soil nutrient availability and is an important strategy for nutrient conservation strategy for alpine grasses.

Rainfall variability and nitrogen addition synergistically reduce plant diversity in a restored tallgrass prairie

Summary Climate change is expected to bring fewer, larger rainfall events and prolonged droughts (i.e. increased rainfall variability). Concurrently, the burning of fossil fuels and reliance on nitrogen (N) fertilizers are expected to continue to increase N availability in many ecosystems. These changes in water and N availability have the potential to alter plant community composition and structure. We manipulated rainfall variability and N inputs in a restored tallgrass prairie over the course of two growing seasons. Greater rainfall variability led to wetter soils throughout the majority of both growing seasons and provided punctuated relief from a severe drought that occurred during the first three months of the experiment. Both rainfall variability‐ and fertilization‐induced increases in resource availability favoured fast‐growing, deeply rooted C3 forbs, particularly the dominant Solidago canadensis, at the expense of species adapted to low resource conditions, particularly C4 grasses and N‐fixing forbs. This change in community composition decreased plant community diversity and evenness in plots receiving both supplemental N and more variable rainfall. Synthesis and applications. These results suggest that future increases in rainfall variability and nitrogen (N) deposition could synergistically alter the structure of prairie restorations and jeopardize restoration targets related to increasing floral diversity. Mitigating N availability in restoration sites may help to maintain prairie diversity as rainfall patterns become more variable.

Plant community shifts caused by feral swine rooting devalue Florida rangeland

Invasive species threaten agriculture by changing agroecosystem structure and function, reducing habitat value, decreasing biodiversity and ecosystem services and increasing management costs. Grazing lands in south central Florida are a mosaic of sown pastures, native grasslands, wetlands and woodlands that provide a variety of ecosystem services. Disturbance of these pastures and native grasslands by invasive feral swine (Sus scrofa) can have negative consequences for both economic productivity and biodiversity. In this study, we show that the effect of rooting on plant diversity depends on ecosystem type and initial levels of plant species diversity. For example, in native grassland pasture, rooting was initially associated with declines in plant species richness, while in sown pastures, rooting was associated with more sustained increases in plant species richness. In both sown pastures and native grasslands, swine rooting altered plant community composition reducing agricultural productivity. Forage grasses were primarily associated with unrooted areas, whereas low quality forage species or nuisance species were found in rooted areas. We provide an example of monetary losses that cattle ranches can incur when feral swine are abundant on the landscape and control is minimal. Ranch managers and government agencies are encouraged to consider implementing more stringent feral swine management programs to minimize negative effects of feral swine rooting on ecological and economic value of grazing lands.

Climate change alters plant biogeography in Mediterranean prairies along the West Coast, USA.

Projected changes in climate are expected to have widespread effects on plant community composition and diversity in coming decades. However, multisite, multifactor climate manipulation studies that have examined whether observed responses are regionally consistent and whether multiple climate perturbations are interdependent are rare. Using such an experiment, we quantified how warming and increased precipitation intensity affect the relative dominance of plant functional groups and diversity across a broad climate gradient of Mediterranean prairies. We implemented a fully factorial climate manipulation of warming (+2.5-3.0 °C) and increased wet-season precipitation (+20%) at three sites across a 520-km latitudinal gradient in the Pacific Northwest, USA. After seeding with a nearly identical mix of native species at all sites, we measured plant community composition (i.e., cover, richness, and diversity), temperature, and soil moisture for 3 years. Warming and the resultant drying of soils altered plant community composition, decreased native diversity, and increased total cover, with warmed northern communities becoming more similar to communities further south. In particular, after two full years of warming, annual cover increased and forb cover decreased at all sites mirroring the natural biogeographic pattern. This suggests that the extant climate gradient of increasing heat and drought severity is responsible for a large part of the observed biogeographic pattern of increasing annual invasion in US West Coast prairies as one moves further south. Additional precipitation during the rainy season did little to relieve drought stress and had minimal effects on plant community composition. Our results suggest that the projected increase in drought severity (i.e., hotter, drier summers) in Pacific Northwest prairies may lead to increased invasion by annuals and a loss of forbs, similar to what has been observed in central and southern California, resulting in novel species assemblages and shifts in functional composition, which in turn may alter ecosystem functions.

Elk, sagebrush, and saprotrophs: indirect top-down control on microbial community composition and function.

Saprotrophic microbial communities in soil are primarily structured by the availability of growth-limiting resources (i.e., plant detritus), a bottom-up ecological force. However, foraging by native ungulates can alter plant community composition and the nature of detritus entering soil, plausibly exerting an indirect, top-down ecological force that shapes both the composition and function of soil microbial communities. To test this idea, we used physiological assays and molecular approaches to quantify microbial community composition and function inside and outside of replicate, long-term (60-80 yr) winter-foraging exclosures in sagebrush steppe of Wyoming, USA. Winter foraging exclusion substantially increased shrub biomass (2146 g/m2 vs. 87 g/m2), which, in turn, increased the abundance of bacterial and fungal genes with lignocellulolytic function; microbial respiration (+50%) and net N mineralization (+70%) also were greater in the absence of winter foraging. Our results reveal that winter foraging by native, migratory ungulates in sagebrush steppe exerts an indirect, top-down ecological force that shapes the composition and function of soil microbial communities. Because approximately 25% of the Earth's land surface is influenced by grazing animals, this indirect top-down ecological force could function to broadly shape the community membership and physiological capacity of saprotrophic microbial communities in shrub steppe.

Non-Consumptive Interactions between Grasshoppers (Orthoptera: Acrididae) and Wolf Spiders (Lycosidae) Produce Trophic Cascades in an Old-Field Ecosystem

Abstract Top-down control by spider predators on grasshopper herbivores can produce trophic cascades, which may enhance plant biomass and alter plant community composition. These trophic level effects may be the result of either predator reduction in prey numbers (i.e., consumptive effects) or decreased prey foraging time in response to predator presence (i.e., non-consumptive effects). However, predator-prey interactions can be context dependent and do not always affect the plant trophic level. We conducted a field and laboratory experiment in a Northern Wisconsin (USA) old field ecosystem to uncover whether consumptive or non-consumptive effects of spider predation on grasshopper herbivores result in a trophic cascade, and if so to determine the underlying mechanisms that drive these trophic cascades. In a field experiment, four treatments examined the effects of multiple trophic-level interactions on plant biomass: 1) control treatment of vegetation only, 2) a two trophic-level interaction (grasshopper...

Reproductive allocation in plants as affected by elevated carbon dioxide and other environmental changes: a synthesis using meta-analysis and graphical vector analysis.

Reproduction is an important life history trait that strongly affects dynamics of plant populations. Although it has been well documented that elevated carbon dioxide (CO2) in the atmosphere greatly enhances biomass production in plants, the overall effect of elevated CO2 on reproductive allocation (RA), i.e., the proportion of biomass allocated to reproductive structures, is little understood. We combined meta-analysis with graphical vector analysis to examine the overall effect of elevated CO2 on RA and how other environmental factors, such as low nutrients, drought and elevated atmospheric ozone (O3), interacted with elevated CO2 in affecting RA in herbaceous plants. Averaged across all species of different functional groups and environmental conditions, elevated CO2 had little effect on RA (-0.9%). RA in plants of different reproductive strategies and functional groups, however, differed in response to elevated CO2. For example, RA in iteroparous wild species decreased by 8%, while RA in iteroparous crops increased significantly (+14%) at elevated CO2. RA was unaffected by CO2 in plants grown with no stress or in low-nutrient soils. RA decreased at elevated CO2 and elevated O3, but increased in response to elevated CO2 in drought-stressed plants, suggesting that elevated CO2 could ameliorate the adverse effect of drought on crop production to some extent. Our results demonstrate that elevated CO2 and other global environmental changes have the potential to greatly alter plant community composition through differential effects on RA of different plant species and thus affect the dynamics of natural and agricultural ecosystems in the future.

Characterizing a contentious management tool: the effects of a grass-specific herbicide on the silvery blue butterfly

Selective herbicides are a conservation tool employed to reduce invasive vegetation and improve habitat for native plants and animals. However, herbicides may negatively affect non-target organisms such as butterflies through direct chemical exposure or by altering plant community composition and structure. We evaluate the effect of the grass-specific herbicide fluazifop-p-butyl on behavior and demographic responses of the silvery blue butterfly (Glaucopsyche lygdamus) in the field and also quantify effects on reproductive behavior in the greenhouse. We find that in the first few months after an early spring application, herbicide decreases vertical grass structure but does not have a positive or negative net effect on adult behavior, egg deposition, larval density, pupal weight, or ant-tending association for the silvery blue. Our greenhouse oviposition choice trials corroborate field findings and indicate that females do not show preference for unsprayed host plants. Selective herbicides create a vegetative structure preferred by butterflies and do not negatively affect the silvery blue when applied in the early spring. Appropriate timing of herbicide application is likely the key to avoiding adverse effects on vulnerable butterfly life stages. Depending on the longevity of the vegetative reduction, strategic herbicide application may be useful for restoring prairie communities in concert with other restoration tools; however, further testing on additional butterfly species is an imperative precursor to large-scale spraying.

Mutualistic rhizobia reduce plant diversity and alter community composition.

Mutualistic interactions can be just as important to community dynamics as antagonistic species interactions like competition and predation. Because of their large effects on both abiotic and biotic environmental variables, resource mutualisms, in particular, have the potential to influence plant communities. Moreover, the effects of resource mutualists such as nitrogen-fixing rhizobia on diversity and community composition may be more pronounced in nutrient-limited environments. I experimentally manipulated the presence of rhizobia across a nitrogen gradient in early assembling mesocosm communities with identical starting species composition to test how the classic mutualism between nitrogen-fixing rhizobia and their legume host influence diversity and community composition. After harvest, I assessed changes in α-diversity, community composition, β-diversity, and ecosystem properties such as inorganic nitrogen availability and productivity as a result of rhizobia and nitrogen availability. The presence of rhizobia decreased plant community diversity, increased community convergence (reduced β-diversity), altered plant community composition, and increased total community productivity. These community-level effects resulted from rhizobia increasing the competitive dominance of their legume host Chamaecrista fasciculata. Moreover, different non-leguminous species responded both negatively and positively to the presence of rhizobia, indicating that rhizobia are driving both inhibitory and potentially facilitative effects in communities. These findings expand our understanding of plant communities by incorporating the effects of positive symbiotic interactions on plant diversity and composition. In particular, rhizobia that specialize on dominant plants may serve as keystone mutualists in terrestrial plant communities, reducing diversity by more than 40%.

Changes in plant community composition, not diversity, during a decade of nitrogen and phosphorus additions drive above‐ground productivity in a tallgrass prairie

SummaryNutrient additions typically increase terrestrial ecosystem productivity, reduce plant diversity and alter plant community composition; however, the effects ofPadditions and interactions betweenNandPare understudied.We added bothN(10 g m−2) and three levels ofP(2.5, 5 and 10 g m−2) to a native, ungrazed tallgrass prairie burned biennially in northeasternKansas,USA, to determine the independent and interactive effects ofNandPon plant community composition and above‐ground net primary productivity (ANPP).After a decade of nutrient additions, we found few effects ofPalone on plant community composition,Nalone had stronger effects, andNandPadditions combined resulted in much larger effects than either alone. The changes in the plant community were driven by decreased abundance ofC4grasses, perhaps in response to altered interactions with mycorrhizal fungi, concurrent with increased abundance of non‐N‐fixing perennial and annual forbs. Surprisingly, this large shift in plant community composition had little effect on plant community richness, evenness and diversity.The shift in plant composition withNandPcombined had large but variable effects onANPPover time. Initially,NandNandPcombined increased above‐ground productivity ofC4grasses, but after 4 years, productivity returned to ambient levels as grasses declined in abundance and the community shifted to dominance by non‐N‐fixing and annual forbs. Once these forbs increased in abundance and became dominant,ANPPwas more variable, with pulses in forb production only in years when the site was burned.Synthesis. We found that a decade ofNandPadditions interacted to drive changes in plant community composition, which had large effects on ecosystem productivity but minimal effects on plant community diversity. The large shift in species composition increased variability inANPPover time as a consequence of the effects of burning. Thus, increased inputs ofNandPto terrestrial ecosystems have the potential to alter stability of ecosystem function over time, particularly within the context of natural disturbance regimes.

Limits to understory plant restoration following fuel-reduction treatments in a piñon-juniper woodland.

National fuel-reduction programs aim to reduce the risk of wildland fires to human communities and to restore forest and rangeland ecosystems to resemble their historical structure, function, and diversity. There are a number of factors, such as seed bank dynamics, post-treatment climate, and herbivory, which determine whether this latter goal may be achieved. Here, we examine the short-term (2 years) vegetation response to fuel-reduction treatments (mechanical mastication, broadcast burn, and pile burn) and seeding of native grasses on understory vegetation in an upland piñon-juniper woodland in southeast Utah. We also examine how wildlife herbivory affects the success of fuel-reduction treatments. Herbaceous cover increased in response to fuel-reduction treatments in all seeded treatments, with the broadcast burn and mastication having greater increases (234 and 160 %, respectively) in herbaceous cover than the pile burn (32 %). In the absence of seeding, herbaceous cover only increased in the broadcast burn (32 %). Notably, fuel-reduction treatments, but not seeding, strongly affected herbaceous plant composition. All fuel-reduction treatments increased the relative density of invasive species, especially in the broadcast burn, which shifted the plant community composition from one dominated by perennial graminoids to one dominated by annual forbs. Herbivory by wildlife reduced understory plant cover by over 40 % and altered plant community composition. If the primary management goal is to enhance understory cover while promoting native species abundance, our study suggests that mastication may be the most effective treatment strategy in these upland piñon-juniper woodlands. Seed applications and wildlife exclosures further enhanced herbaceous cover following fuel-reduction treatments.

The role of invasive pine on changes of plant composition and functional traits in a coastal dune ecosystem

ABSTRACT In this article we investigate how invasion by Pinus taeda L. alters plant community composition and functional traits in sand dunes. In invaded psammophilous vegetation in southern Brazil, we randomly selected 15 P . taeda individuals and measured their age based on the number of growth rings. For each tree, we sampled 1 m 2 quadrats beneath and 2 m away of its crown. In each quadrat, we assessed non-graminoid plant composition and performed a multivariate analysis of variance. Data on species’ functional traits were scaled up to community level, and their variations were compared to tree age by linear regression. Species’ composition and functional traits under P . taeda crown were different from control plots. Tree age affected the presence of thorns, potential height, and functional diversity. P. taeda individuals likely change the micro-habitat beneath their crowns, thereby benefiting species with some traits and increasing functional diversity of plants that are not characteristic of open dune communities. © 2014 Associacao Brasileira de Ciencia Ecologica e Conservacao. Published by Elsevier Editora Ltda.

Evergreen shrubs dominate responses to experimental summer warming and fertilization in Canadian mesic low arctic tundra

Summary Climate change in arctic tundra is projected to increase soil fertility, which may alter plant community composition and ecosystem processes by shifting niche space to favour particular species' life‐history strategies. The rate and magnitude of change in soil fertility may be critical to determining plant community responses, and so effects of slow increases in nutrient availability due to climate warming may differ substantially from those of chronic high‐level fertilizer additions. We investigated above‐ and below‐ground plant biomass responses to experimental summer warming and above‐ground responses to nutrient additions (low‐level N and factorial N and P) in a mesic birch hummock tundra community in the central Canadian Low Arctic after eight years of experimental treatment. Plant community biomass responses to experimental warming were fundamentally different from those of high‐level N and/or P additions, mainly due to opposing effects on the evergreen shrubs. Evergreen shrub above‐ground biomass increased 66% with greenhouse warming, but decreased on average 70% with high‐level N and/or P additions, driven by the strong responses of Rhododendron subarcticum. Because of this evergreen response, greenhouse‐warming increased total above‐ground biomass by 32% and total below‐ground biomass by 70%, but did not significantly change the total above‐ground/below‐ground biomass ratio. However, warming increased the shoot/root ratio of Betula glandulosa threefold. Increased soil fertility created interactions between N and P availability, whereby increased P availability led to a substantial increase in inorganic N availability. Meanwhile, the growth of several species that span a range of different functional groups was stimulated by the separate N and P additions. These factorial fertilization results highlight the importance of understanding climate warming impacts on availability of both of these nutrients in order to predict plant community responses. Synthesis. Our results strongly suggest that the trajectory of mesic tundra vegetation change with warming depends critically on the rate of increase in soil fertility. The relatively large greenhouse‐induced biomass increase in evergreen compared to deciduous shrubs suggests that carbon balance and albedo feedbacks to warming will be restricted in mesic tundra ecosystems, at least in their early responses to climate change.

Impact of Phytophthora-dieback on birds in Banksia woodlands in south west Western Australia

Invasive plant pathogens have impacted forest and woodland systems globally and can negatively impact biodiversity. The soil-borne plant pathogen Phytophthora cinnamomi is listed as one of the world’s worst invasive species and alters plant community composition and habitat structure. Few studies have examined how these Phytophthora-induced habitat changes affect faunal communities. We examined bird communities in Banksia woodland with, and without, Phytophthora dieback in a biodiversity hotspot, southwestern Australia. Seven sites along dieback fronts, with paired 1-ha plots in diseased and healthy vegetation, were surveyed monthly for birds over seven months. Vegetation assessments showed that diseased sites had reduced plant species richness, litter, shrub, tree and canopy cover, high bare ground and significantly lower flowering scores, than healthy sites. Bird community composition differed significantly between diseased and healthy sites, although total bird abundance, total species richness and foraging guilds, did not. Average species richness of birds per survey and the abundance of brown honeyeaters, western spinebills and silvereyes was lower in diseased than healthy sites. The tawny-crowned honeyeater had higher abundances in diseased sites. Similarity matrices of habitat structure, flowering scores and bird assemblages were congruent, indicating that habitat structural differences were influencing bird community composition. Our results suggest that this pathogen is potentially a serious threat to avian biodiversity and especially for nectarivores, and populations in fragmented landscapes. Since elimination of the pathogen is not currently possible, management should focus on methods of preventing its spread until techniques to eliminate the pathogen are developed.