The shift in plant species composition in a subarctic mountain birch forest floor due to climate change would modify the biogenic volatile organic compound emission profile

Background and aims

Mountain birch forests dominate in the Subarctic but little is known of their non-methane biogenic volatile organic compound (BVOC) emissions. The dwarf shrubs Empetrum hermaphroditum, Vaccinium myrtillus and Vaccinium uliginosum co-dominate in the forest floors of these forests. The abundance of these three dwarf shrubs relative to each other could be affected by climate warming expected to increase nutrient availability by accelerating litter decomposition and nutrient mineralization. We 1) compared the BVOC emission profiles of vegetation covers dominated by E. hermaphroditum and V. myrtillus plus V. uliginosum in a subarctic mountain birch forest floor, 2) distinguished the BVOCs emitted from plants and soil and 3) measured how the BVOC emissions from the different vegetation covers differed under darkness.

Methods

BVOCs were sampled during two growing seasons using a conventional ecosystem chamber-based method, collected on adsorbent and analyzed with gas chromatography–mass spectrometry.

Results

High abundance of E. hermaphroditum increased the sesquiterpene emissions. Soil released fewer different BVOCs than controls (i.e. natural vegetation) but the total emission rates were similar. Darkness did not affect the emissions. Carbon emitted as BVOCs was less than 0.2% of the CO₂ exchange.

Conclusions

Our results suggest that sesquiterpene emissions from subarctic mountain birch forest floors would be reduced following an increased abundance of V. myrtillus and V. uliginosum with climate change because these species respond rapidly to increased nutrient availability.     

The impact of temperature regimes on development,dormancy breaking and germination of dwarf shrub seeds from arctic,alpine and boreal sites

It has been suggested that the infrequent sexual reproduction of arctic dwarf shrubs might be related to the harsh environmental conditions in which they live. If this is the case, then increases in temperature resulting from global climate change might drastically affect regeneration of arctic species. We examined whether recruitment of Empetrum nigrum ssp. hermaphroditum and Vaccinium uliginosum (hereafter E. nigrum and V. uliginosum) was affected by temperature during three reproductive stages: seed development, dormancy breakage and germination. Seeds were collected from an arctic, an alpine (only E. nigrum) and a boreal site with different climates; stored at different winter temperatures and incubated for germination at different temperatures. Seeds of V. uliginosum developed in the boreal region had a higher percentage germination than did seeds developed in the Arctic. In contrast, seeds of E. nigrum from the arctic site had a higher or similar percentage germination than did seeds from the alpine and boreal sites. Increased winter temperatures had no significant effect on resulting germination percentage of E. nigrum. However, V. uliginosum seeds from the arctic site suffered increased fungal attack (and thus decreased germination) when they were stratified under high winter temperatures. Seeds of both species increased germination with increased incubation temperatures. Our results suggest that both species would increase their germination in response to warmer summers. Longer summers might also favour the slow-germinating E. nigrum. However, increased winter temperatures might increase mortality due to fungal attack in V. uliginosum ecotypes that are not adapted to mild winters.     

Niche differentiation with respect to light utilization among coexisting dwarf shrubs in a subarctic woodland

Summary Canopy structure, shoot design, and photosynthetic light recruitment were used to compare four coexisting dwarf shrub species with respect to light utilization. All four species showed different shoot designs which probably result in different light interception properties. Leaves of Vaccinium uliginosum showed the highest levels of photosynthetic light saturation but in situ the shoots of this species reached their maximum photosynthetic rate at the lowest photon flux densities. No consistent differences with respect to photosynthetic light responses were found between deciduous and evergreen species. At sites dominated by one of the deciduous species (Vaccinium uliginosum or V. myrtillus), the two evergreen species studied (V. vitis-idaea and Empetrum hermaphroditum) occurred in the understory, i.e., with their leaf distribution slightly below that of the deciduous species. Sites dominated by one of the evergreen species showed less vertical differentiation in leaf distribution between species.     

The functional quality of decomposing litter outputs from an Arctic plant community is affected by long-term exposure to enhanced UV-B

Plant exposure to enhanced UV-B radiation typically induces changes in leaf secondary metabolite profiles which will be inherited in litter, affecting litter breakdown and the carbon (C) dynamics of sensitive plant communities. A key enzyme in the decomposition process is phenol oxidase which is influenced by litter quality and, hence, a decomposition bioindicator. Here we investigated dwarf shrub litter decomposition following experimental community exposure to enhanced UV-B over two decades in the Swedish sub-Arctic. We examined the hypothesis that foliar UV-B exposure would alter litter quality to elevate phenol oxidase activity. This was tested in the field by measuring phenol oxidase activity in freshly collected mixed-community litter from under our experimental vegetation. A laboratory mesocosm was next used in a decomposition assay to investigate individual species responses over eight weeks, with an emphasis on the quality of leachate outputs from decomposing litter (from Empetrum hermaphroditum, Vaccinium vitis-idaea, Vaccinium uliginosum). In the assay bi-weekly collections of leachate were analysed for phenol oxidase activity, together with total phenolics and dissolved organic C (DOC). At the end of the assay litter mass loss and respired C were also determined. The initial assessment on field mixed-community litter found no enhanced UV-B treatment (henceforth: ‘UV-B treatment’) effect on phenol oxidase activity. However, in the controlled laboratory mesocosm assay, significant species-specific effects of the UV-B treatment were evident, with increased phenol oxidase activity in V. vitis-idaea leachate (P < 0.001) and a significant reduction (P = 0.05) in respired C. Leachate DOC release from the UV-B treatment was greater in both Vaccinium species and the effect on V. uliginosum was significant (P < 0.05). The UV-B treatment had no effect on the total phenolic concentration of litter or leachates for any species, but there were significant differences in leachate total phenolics, both over time and between species. Also the initial phenolic concentration in leachates from the decomposing litter of E. hermaphroditum was greater than both Vaccinum species. Results suggest a species specific role for UV-B in influencing enzyme function and decomposition, dependent on individual traits. This has implications for decomposition dynamics in this system and more widely. Our study highlights the value of using a laboratory assay to gain a mechanistic understanding the species level impacts of a global change factor (UV-B) on decomposition, which are otherwise obscured by community-level responses and difficult to determine under field conditions.     

The impacts of UV-B radiation on the regeneration of a sub-arctic heath community

The effects of ultraviolet-B radiation on regeneration after disturbance of a natural sub-arctic heathland have been investigated. Areas of pristine dwarf shrub heath were denuded of all above ground biomass in 1992 and exposed to enhanced UV-B (simulating a 15% depletion of the ozone layer). The resulting regenerated stem and leaf growth parameters were measured after four years on three dwarf shrubs, Vaccinium myrtillus, V. uliginosum and V. vitis-idaea and the grass Calamagrostis lapponica; leaves of the three dwarf shrubs were also analysed for UV-absorbing compounds and carbohydrates. Regeneration irrespective of treatment was slow, with Empetrum hermaphroditum failing to regenerate at all. Vaccinium myrtillus showed the most rapid regeneration attaining much of its original biomass in four years. There was a significant interaction between UV-B and year of regeneration in V. myrtillus; annual stem length increment showed an initial stimulation of 75% under enhanced UV-B in the first year of regeneration while a reduction of 16% was observed in the fourth year. Both V. uliginosum and V. vitis-idaea showed a reduction in annual stem length increment as regeneration progressed with a greater than 50% reduction in stem increment in the fourth year of regeneration compared to the first. Vaccinium uliginosum also showed an initial reduction in stem length increment of 40% under enhanced UV-B. None of the species were affected by enhanced UV-B in terms of total regenerated stem and leaf biomass or UV-absorbing compounds in regenerated leaf tissue. Total leaf carbohydrate and the ethanol/water soluble fraction in V. uliginosum were significantly increased by 29% and 31% respectively under enhanced UV-B. This suggests either a stimulation of photosynthesis or a reduction in sink size for photo-assimilates. Results are discussed in the context of the extremely slow regeneration of sub-arctic heath communities and the implications of contrasting UV-B effects on the regenerative ability of different species.     

Tight Coupling Between Shoot Level Foliar N and P,Leaf Area,and Shoot Growth in Arctic Dwarf Shrubs Under Simulated Climate Change

Nutrient availability limits productivity of arctic ecosystems, and this constraint means that the amount of nitrogen (N) in plant canopies is an exceptionally strong predictor of vegetation productivity. However, climate change is predicted to increase nutrient availability leading to increases in carbon sequestration and shifts in community structure to more productive species. Despite tight coupling of productivity with canopy nutrients at the vegetation scale, it remains unknown how species/shoot level foliar nutrients couple to growth, or how climate change may influence foliar nutrients–productivity relationships to drive changes in ecosystem carbon gain and community structure. We investigated the influence of climate change on arctic plant growth relationships to shoot level foliar N and phosphorus (P) in three dominant subarctic dwarf shrubs using an 18-year warming and nutrient addition experiment. We found a tight coupling between total leaf N and P per shoot, leaf area and shoot extension. Furthermore, a steeper shoot length-leaf N relationship in deciduous species (Vaccinium myrtillus and Vaccinium uliginosum) under warming manipulations suggests a greater capacity for nitrogen to stimulate growth under warmer conditions in these species. This mechanism may help drive the considerable increases in deciduous shrub cover observed already in some arctic regions. Overall, our work provides the first evidence at the shoot level of tight coupling between foliar N and P, leaf area and growth i.e. consistent across species, and provides mechanistic insight into how interspecific differences in alleviation of nutrient limitation will alter community structure and primary productivity in a warmer Arctic.     

An alpine treeline in a carbon dioxide-rich world: synthesis of a nine-year free-air carbon dioxide enrichment study

We evaluated the impacts of elevated CO₂ in a treeline ecosystem in the Swiss Alps in a 9-year free-air CO₂ enrichment (FACE) study. We present new data and synthesize plant and soil results from the entire experimental period. Light-saturated photosynthesis (A _max) of ca. 35-year-old Larix decidua and Pinus uncinata was stimulated by elevated CO₂ throughout the experiment. Slight down-regulation of photosynthesis in Pinus was consistent with starch accumulation in needle tissue. Above-ground growth responses differed between tree species, with a 33 % mean annual stimulation in Larix but no response in Pinus. Species-specific CO₂ responses also occurred for abundant dwarf shrub species in the understorey, where Vaccinium myrtillus showed a sustained shoot growth enhancement (+11 %) that was not apparent for Vaccinium gaultherioides or Empetrum hermaphroditum. Below ground, CO₂ enrichment did not stimulate fine root or mycorrhizal mycelium growth, but increased CO₂ effluxes from the soil (+24 %) indicated that enhanced C assimilation was partially offset by greater respiratory losses. The dissolved organic C (DOC) concentration in soil solutions was consistently higher under elevated CO₂ (+14 %), suggesting accelerated soil organic matter turnover. CO₂ enrichment hardly affected the C–N balance in plants and soil, with unaltered soil total or mineral N concentrations and little impact on plant leaf N concentration or the stable N isotope ratio. Sustained differences in plant species growth responses suggest future shifts in species composition with atmospheric change. Consistently increased C fixation, soil respiration and DOC production over 9 years of CO₂ enrichment provide clear evidence for accelerated C cycling with no apparent consequences on the N cycle in this treeline ecosystem.     

Long-term recruitment dynamics of arctic dwarf shrub communities in coastal east Greenland

Warming-induced biological and ecological responses have been reported from high-northern latitude sites, where changes in dwarf shrub communities translate into complex vegetation-climate feedbacks. Most of the available Arctic tree-ring evidence is, however, restricted to a limited number of species and locations. A combination of wood anatomical and ‘dendro’-ecological techniques provides insights into past growth rates, recruitment dynamics and even community assemblages of Arctic vegetation. Here, we use thin sectioning and ring counting of 1432 dwarf shrub samples from eight species and two tundra regions in coastal east Greenland to assess community recruitment history and its relation to climate. Site and species-specific annual stem increments, as well as estimated plant ages, range from 0.013-0.720 mm and from 4 to 204 years, respectively. The mean ring width is 0.086 mm, with a mean age of 50 years. Decadal-scale recruitment dynamics of the studied vegetation cover respond to Greenlandic summer temperature variations back to the late 19th century (r = 0.7; 1881–2000).     

Microbe-plant competition, allelopathy and arctic plants

Michelsen et al. (1995) present results of an experiment in which aqueous leaf extracts of three arctic woody plant species were found to inhibit growth and nutrient acquisition of three graminoid species, and suggested that microbial nutrient immobilisation, rather than allelopathy, was responsible for the observed trends. In doing this they also question previous work proposing that the Arctic dwarf shrub Empetrum hermaphroditum is allelopathic. We suggest that their conclusions are not unequivocally supported by their data. Firstly we indicate that the approaches used for estimating microbial nutrient immobilisation are questionable. Secondly we indicate that most of the trends that they discussed are based on data in which the treatments and controls are not significantly different for the majority of cases. Finally we respond specifically to their criticisms of previous work on E. hermaphroditum. While the question of how arctic plants interact is an interesting one, we conclude that this question cannot be answered by their data. Received: 20 November 1995 /Accepted: 10 July 1996     

Growth and reproduction of the alpine grasshopper <Emphasis Type="Italic">Miramella alpina</Emphasis> feeding on CO<Subscript>2</Subscript>-enriched dwarf shrubs at treeline

The consequences for plant-insect interactions of atmospheric changes in alpine ecosystems are not well understood. Here, we tested the effects of elevated CO₂ on leaf quality in two dwarf shrub species (Vaccinium myrtillus and V. uliginosum) and the response of the alpine grasshopper (Miramella alpina) feeding on these plants in a field experiment at the alpine treeline (2,180 m a.s.l.) in Davos, Switzerland. Relative growth rates (RGR) of M. alpina nymphs were lower when they were feeding on V. myrtillus compared to V. uliginosum, and were affected by elevated CO₂ depending on plant species and nymph developmental stage. Changes in RGR correlated with CO₂-induced changes in leaf water, nitrogen, and starch concentrations. Elevated CO₂ resulted in reduced female adult weight irrespective of plant species, and prolonged development time on V. uliginosum only, but there were no significant differences in nymphal mortality. Newly molted adults of M. alpina produced lighter eggs and less secretion (serving as egg protection) under elevated CO₂. When grasshoppers had a choice among four different plant species grown either under ambient or elevated CO₂, V. myrtillus and V. uliginosum consumption increased under elevated CO₂ in females while it decreased in males compared to ambient CO₂-grown leaves. Our findings suggest that rising atmospheric CO₂ distinctly affects leaf chemistry in two important dwarf shrub species at the alpine treeline, leading to changes in feeding behavior, growth, and reproduction of the most important insect herbivore in this system. Changes in plant-grasshopper interactions might have significant long-term impacts on herbivore pressure, community dynamics and ecosystem stability in the alpine treeline ecotone.     

Demographic variation of dwarf birch (<Emphasis Type="Italic">Betula nana</Emphasis>) in communities dominated by <Emphasis Type="Italic">Ledum palustre</Emphasis> and <Emphasis Type="Italic">Vaccinium uliginosum</Emphasis>

The structure and demographic processes were compared in shrub communities to test the effects of vegetation succession on population growth, fecundity and abundance of the dwarf birch (Betula nana L.), which is a rare and endangered plant species in Poland and a glacial relict in Central Europe. The effects of Ledum palustre L. and Vaccinium uliginosum L. were studied in the Linje nature reserve in Chełmińskie Lake District (northern Poland), in three permanent plots on a peat bog. Vegetative growth and reproduction of B. nana were lower in plant communities dominated by L. palustre and V. uliginosum, than in a reference site. Fecundity was also lower, despite the fact that the percentage share of potentially fertile age groups was similar in all study sites. Mortality of ramets was independent of vegetation, both for juvenile and mature stages. The results confirm that B. nana is intolerant of shade, and it is more abundant in vegetation without competitors. Light limitation can lead to its decline, primarily by a decrease in vegetative growth. Sexual reproduction may be negatively affected by shade, but it plays only small role in population growth. Butterfly larvae can destroy inflorescences, and thus contribute to low effectiveness of sexual reproduction. Increasing density of shrubs and trees in peat bogs can reduce the abundance of dwarf birch, and can lead to the extinction of its local populations.     

Impacts of multiple extreme winter warming events on sub‐Arctic heathland: phenology,reproduction, growth,and CO2 flux responses

Extreme weather events can have strong negative impacts on species survival and community structure when surpassing lethal thresholds. Extreme, short‐lived, winter warming events in the Arctic rapidly melt snow and expose ecosystems to unseasonably warm air (for instance, 2–10 °C for 2–14 days) but upon return to normal winter climate exposes the ecosystem to much colder temperatures due to the loss of insulating snow. Single events have been shown to reduce plant reproduction and increase shoot mortality, but impacts of multiple events are little understood as are the broader impacts on community structure, growth, carbon balance, and nutrient cycling. To address these issues, we simulated week‐long extreme winter warming events – using infrared heating lamps and soil warming cables – for 3 consecutive years in a sub‐Arctic heathland dominated by the dwarf shrubs Empetrum hermaphroditum, Vaccinium vitis‐idaea (both evergreen) and Vaccinium myrtillus (deciduous). During the growing seasons after the second and third winter event, spring bud burst was delayed by up to a week for E. hermaphroditum and V. myrtillus, and berry production reduced by 11–75% and 52–95% for E. hermaphroditum and V. myrtillus, respectively. Greater shoot mortality occurred in E. hermaphroditum (up to 52%), V. vitis‐idaea (51%), and V. myrtillus (80%). Root growth was reduced by more than 25% but soil nutrient availability remained unaffected. Gross primary productivity was reduced by more than 50% in the summer following the third simulation. Overall, the extent of damage was considerable, and critically plant responses were opposite in direction to the increased growth seen in long‐term summer warming simulations and the ‘greening’ seen for some arctic regions. Given the Arctic is warming more in winter than summer, and extreme events are predicted to become more frequent, this generates large uncertainty in our current understanding of arctic ecosystem responses to climate change.     

Impacts of extreme winter warming events on plant physiology in a sub‐Arctic heath community

Insulation provided by snow cover and tolerance of freezing by physiological acclimation allows Arctic plants to survive cold winter temperatures. However, both the protection mechanisms may be lost with winter climate change, especially during extreme winter warming events where loss of snow cover from snow melt results in exposure of plants to warm temperatures and then returning extreme cold in the absence of insulating snow. These events cause considerable damage to Arctic plants, but physiological responses behind such damage remain unknown. Here, we report simulations of extreme winter warming events using infrared heating lamps and soil warming cables in a sub‐Arctic heathland. During these events, we measured maximum quantum yield of photosystem II (PSII), photosynthesis, respiration, bud swelling and associated bud carbohydrate changes and lipid peroxidation to identify physiological responses during and after the winter warming events in three dwarf shrub species: Empetrum hermaphroditum, Vaccinium vitis‐idaea and Vaccinium myrtillus. Winter warming increased maximum quantum yield of PSII, and photosynthesis was initiated for E. hermaphroditum and V. vitis‐idaea. Bud swelling, bud carbohydrate decreases and lipid peroxidation were largest for E. hermaphroditum, whereas V. myrtillus and V. vitis‐idaea showed no or less strong responses. Increased physiological activity and bud swelling suggest that sub‐Arctic plants can initiate spring‐like development in response to a short winter warming event. Lipid peroxidation suggests that plants experience increased winter stress. The observed differences between species in physiological responses are broadly consistent with interspecific differences in damage seen in previous studies, with E. hermaphroditum and V. myrtillus tending to be most sensitive. This suggests that initiation of spring‐like development may be a major driver in the damage caused by winter warming events that are predicted to become more frequent in some regions of the Arctic and that may ultimately drive plant community shifts.     

Separation of allelopathy and resource competition by the boreal dwarf shrub Empetrum hermaphroditum Hagerup

An experimental technique was used to separate and evaluate the magnitude of allelopathic interference relative to resource competition by the boreal dwarf shrub Empetrum hermaphroditum Hagerup (Empetraceae). To test for resource competition and allelopathy, respectively, Scots pine (Pinus sylvestris L.) seedlings were grown in both the greenhouse and in the field over a 3 year period, in four different treatments within E. hermaphroditum vegetation: (1) PVC tubes were used to reduce effects of interspecific below-ground competition; (2) activated carbon was spread on the soil to adsorb toxins leached from E. hermaphroditum litter and green leaves, thus reducing effects of allelopathic interference; (3) E. hermaphroditum vegetation was left untreated to evaluate inhibiting effects when both allelopathy and resource competition were present; (4) PVC tubes, placed in E. hermaphroditum vegetation spread with activated carbon were used to determine growth of seedlings when both allelopathy and resource competition were reduced. Scots pine seedlings grown in untreated vegetation (with both root competition and allelopathy present) had the lowest shoot length and dry weight; seedlings with both allelopathy and root competition reduced (activated carbon in tube) were the largest. Reducing either root competition alone (tube treatment) or allelopathy alone (carbon treatment) produced seedlings of intermediate size, but reduced competition had a greater effect than reduced allelopathy (although, in the greenhouse, significantly so only for root biomass). In the greenhouse experiment, biomass production of seedlings grown free of both interactions (carbon in tube) was greater than the simple sum of the growth response to the individual interactions (tube treatment and carbon treatment, respectively). Larger shoot:root ratios were also found when pine seedlings were grown without tubes (i.e. when resource competition was occurring). In the field, the removal of allelopathy (carbon treatments) increased shoot:root ratio when compared to the removal of resource competition. The study showed that two different interference mechanisms of E. hermaphroditum can be separated and quantified, and that below-ground competition and allelopathy by E. hermaphroditum are both important factors retarding growth of Scots pine.     

Ambient UV-B radiation reduces PSII performance and net photosynthesis in high Arctic Salix arctica

Ambient ultraviolet-B (UV-B) radiation potentially impacts the photosynthetic performance of high Arctic plants. We conducted an UV-B exclusion experiment in a dwarf shrub heath in NE Greenland (74°N), with open control, filter control, UV-B filtering and UV-AB filtering, all in combination with leaf angle control. Two sites with natural leaf positions had ground angles of 0° (‘level site’) and 45° (‘sloping site’), while at a third site the leaves were fixed in an angle of 45° to homogenize the irradiance dose (‘fixed leaf angle site’). The photosynthetic performance of the leaves was characterized by simultaneous gas exchange and chlorophyll fluorescence measurements and the PSII performance through the growing season was investigated with fluorescence measurements. Leaf harvest towards the end of the growing season was done to determine the specific leaf area and the content of carbon, nitrogen and UV-B absorbing compounds. Compared to a 60% reduced UV-B irradiance, the ambient solar UV-B reduced net photosynthesis in Salix arctica leaves fixed in the 45° position which exposed leaves to maximum natural irradiance. Also a reduced Calvin Cycle capacity was found, i.e. the maximum rate of electron transport (J_max) and the maximum carboxylation rate of Rubisco (V_cmax), and the PSII performance showed a decreased quantum yield and increased energy dissipation. A parallel response pattern and reduced PSII performance at all three sites indicate that these responses take place in all leaves across position in the vegetation. These findings add to the evidence that the ambient solar UV-B currently is a significant stress factor for plants in high Arctic Greenland.     

Variability of polar scurvygrass Cochlearia groenlandica individual traits along a seabird influenced gradient across Spitsbergen tundra

In the resource-limited Arctic environment, vegetation developing near seabird colonies is exceptionally luxuriant. Nevertheless, there are very few detailed quantitative studies of any specific plant species responses to ornithogenic manuring. Therefore, we studied variability of polar scurvygrass Cochlearia groe nlandica individual biomass and leaf width along a seabird influenced gradient determining environmental conditions for vegetation in south-west Spitsbergen. We found seabird colony effect being a paramount factor responsible for augmented growth of C. groenlandica. The species predominated close to the colony and reached the highest mean values of individual biomass (1.4 g) and leaf width (26.6 mm) 10 m below the colony. Its abundance and size declined towards the coast. Both C. groenlandica individual traits significantly decreased with distance from the colony, soil water and organic matter content and increased with guano deposition, soil δ ¹⁵N, conductivity, acidity and nitrate, phosphate and potassium ion content. Our study supports the hypothesis that seabirds have fundamental importance for vegetation growth in poor Arctic environment. Highly plastic species such as C. groenlandica may be a useful instrument in detecting habitat condition changes, for instance resulting from climate change.     

Growth performance of two deciduous Vaccinium species in relation to nutrient status in a subalpine heath

Net primary production (NPP) of two Vaccinium species (V. myrtillus and V. uliginosum) was determined in three subalpine heath communities on the Northern Apennines (N. Italy). The main objective of the study was to test the hypothesis that two species sharing the same plant functional type (deciduous dwarf shrub) have similar growth performances along environmental gradients. The second objective was to assess whether, and to what extent, NPP of the two species was associated with functional and morphological traits, which can affect plant growth in relation to nutrient status. Total community NPP in the three communities was closely related to soil nutrient availability. NPP of V. uliginosum did not vary among communities, while that of V. myrtillus peaked in the most fertile habitat. The N:P ratio in the whole plant as well as in the leaves of the two shrubs exactly mirrored the among-community pattern in soil phosphate concentration. In particular, the foliar N:P ratio in both V. uliginosum and V. myrtillus was >16 in the poorer sites, which indicates P limitation. I concluded that the growth response of the two shrubs in relation to soil nutrient availability is individualistic. Growth of V. myrtillus is P-limited while that of V. uliginosum is not.     

A native C<Subscript>3</Subscript> grass alters fuels and fire spread in montane grassland of South Africa

Although most fire research in plant ecology focuses on vegetation responses to burning, shifts in plant community composition wrought by climate change can change wildland fuelbeds and affect fire behaviour such that the nature of fire in these systems is altered. Changes that introduce substantially different fuel types can alter the spatial extent of fire, with potential impacts on community succession and biodiversity. Montane grasslands of sub-Saharan Africa are threatened by climate change because species distributions can shift with climatically determined ranges. We studied the impact of patches of the temperate C₃ grass Festuca costata in C₄-dominated grassland at the transition between their subalpine ranges in South Africa’s Drakensberg. We used empirical data on fuel moisture and fuel load across F. costata-dominated patches in a C₄-dominated matrix in fire spread models to predict the effect of larger, higher-moisture F. costata patches on the spatial extent of fire. Results indicate F. costata reduces fire spread and burn probability in F. costata patches, and the effect increases as live fuel moisture increases and patches get larger. However, as a native species, F. costata does not appear to have the extreme, fire-suppressing effect of non-native C₃ grasses in other C₄ grasslands. Instead, F. costata patches likely increase variability in the spatial extent of fire in this C₄-dominated grassland, which likely translates to spatial variability on vegetation succession.     

Evidence of enhanced freezing damage in treeline plants during six years of CO2 enrichment and soil warming

Climate change and elevated atmospheric CO₂ levels could increase the vulnerability of plants to freezing. We analyzed tissue damage resulting from naturally occurring freezing events in plants from a long–term in situ CO₂ enrichment (+ 200 ppm, 2001–2009) and soil warming (+ 4°C since 2007) experiment at treeline in the Swiss Alps (Stillberg, Davos). Summer freezing events caused damage in several abundant subalpine and alpine plant species in four out of six years between 2005 and 2010. Most freezing damage occurred when temperatures dropped below –1.5°C two to three weeks after snow melt. The tree Larix decidua and the dwarf shrubs Vaccinium myrtillus and Empetrum hermaphroditum showed more freezing damage under experimentally elevated CO₂ and/or temperatures than under control conditions. Soil warming induced a 50% die‐back of E. hermaphroditum during a single freezing event due to melting of the protective snow cover. Although we could not identify a clear mechanism, we relate greater freezing susceptibility to a combination of advanced plant phenology in spring and changes in plant physiology. The climate record since 1975 at the treeline site indicated a summer warming by 0.58°C/decade and a 3.5 days/decade earlier snow melt, but no significant decrease in freezing events during the vegetation period. Therefore, in a warmer climate with higher CO₂ levels but constant likelihood of extreme weather events, subalpine and alpine plants may be more susceptible to freezing events, which may partially offset expected enhanced growth with global change. Hence, freezing damage should be considered when predicting changes in growth of alpine plants or changes in community composition under future atmospheric and climate conditions.