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Global change drivers can interact in synergistic ways, yet the interactive effect of global change drivers, such as climatic warming and species invasions, on plant pollination are poorly represented in experimental studies. We paired manipulative experiments to probe two mechanistic pathways through which plant invasion and warming may alter phenology and reproduction of native plant species. In the first, we tested how experimental warming (+1.7°C) modulated flowering phenology and how this affected flowering overlap between a native plant (Dracophyllum subulatum) and an invasive plant (Calluna vulgaris L.). In the second experiment, we explored how variation in the ratio of native to invasive flowers, and the overall quantity of resources in a floral patch, affected the reproduction of the native species. We hypothesized that the flowering overlap of native and invasive plants would be altered by warming, given that invading plants typically exhibit greater phenological plasticity than native plants. Further, we hypothesized that pollination of native plant flowers would decrease in floral patches dominated by invasive plant flowers, but that this effect would depend on total floral density in the patch. As predicted, the invasive plant had a stronger phenological response to experimental warming than the native plant, resulting in increased flowering overlap between the native the invasive plants. There was a four-fold increase in the number of native flowers co-flowering with high densities of invasive flowers suggesting native plant competition for pollinators with invasive plants under a warmed climate. In the second experiment, we found depressed seed masses of the native species in high density floral patches that were dominated by invasive flowers relative to high density floral patches dominated by native flowers. At low floral densities, seed mass of native plants was unaffected by invasion. Together, these results demonstrate that by increasing their phenological overlap, warming may enhance the magnitude of existing competition for pollination exerted by an invasive plant on a native plant, particularly in plant patches with high floral density. Our results illustrate a novel pathway through which global change drivers can operate synergistically to alter an important ecosystem service: pollination., (© 2019 by the Ecological Society of America.)

Extreme climatic events are among the drivers of recent declines in plant biomass and productivity observed across Arctic ecosystems, known as "Arctic browning." These events can cause landscape-scale vegetation damage and so are likely to have major impacts on ecosystem CO 2 balance. However, there is little understanding of the impacts on CO 2 fluxes, especially across the growing season. Furthermore, while widespread shoot mortality is commonly observed with browning events, recent observations show that shoot stress responses are also common, and manifest as high levels of persistent anthocyanin pigmentation. Whether or how this response impacts ecosystem CO 2 fluxes is not known. To address these research needs, a growing season assessment of browning impacts following frost drought and extreme winter warming (both extreme climatic events) on the key ecosystem CO 2 fluxes Net Ecosystem Exchange (NEE), Gross Primary Productivity (GPP), ecosystem respiration (R eco ) and soil respiration (R soil ) was carried out in widespread sub-Arctic dwarf shrub heathland, incorporating both mortality and stress responses. Browning (mortality and stress responses combined) caused considerable site-level reductions in GPP and NEE (of up to 44%), with greatest impacts occurring at early and late season. Furthermore, impacts on CO 2 fluxes associated with stress often equalled or exceeded those resulting from vegetation mortality. This demonstrates that extreme events can have major impacts on ecosystem CO 2 balance, considerably reducing the carbon sink capacity of the ecosystem, even where vegetation is not killed. Structural Equation Modelling and additional measurements, including decomposition rates and leaf respiration, provided further insight into mechanisms underlying impacts of mortality and stress on CO 2 fluxes. The scale of reductions in ecosystem CO 2 uptake highlights the need for a process-based understanding of Arctic browning in order to predict how vegetation and CO 2 balance will respond to continuing climate change., (© 2018 The Authors. Global Change Biology Published by John Wiley & Sons Ltd.)

Nitrogen (N) dynamic is one of the main controlling factors of responses to climate change in N‐limited terrestrial ecosystems, which rely on nutrient recycling and retention. In this study we investigate the N partitioning in ecosystem compartments of a grassland heath, and the impact of multiple climate change factors on long‐term N retention after 15N pulse labelling. The impacts of elevated carbon dioxide (eCO2), warming and drought and the treatments in combination on ecosystem N retention were investigated in a field scale manipulation experiment. A 6‐year time‐course was assessed by pulse‐labelling with the stable N isotope 15N and by sampling after 1 day, 1 year and 6 years. After 6 years we observed that the total ecosystem retained 42% of the amended 15N across treatments (recovery of the amended 15N in the pool). The fate of the applied 15N was mainly stabilization in soil, with 36% 15N recovery in soil, while the plant compartment and microbial biomass each retained only 1%–2% of the added 15N. This suggests a moderate retention of N, for all treatments, as compared to similar long‐term studies of forest ecosystems. A decreased ammonium and vegetation N pool combined with higher 15N retention in the soil under eCO2 treatments suggests that eCO2 promoted processes that immobilize N in soil, while warming counteracted this when combined with eCO2. Drought treatments contrastingly increased the vegetation N pool. We conclude that as the organic soil layer has the main capacity for N storage in a temperate heathland‐grassland, it is important for buffering nutrient availability and maintaining a resilient ecosystem. However, the full treatment combination of drought, warming and eCO2 did not differ in 15N recovery from the control, suggesting unchanged long‐term consequences of climate change on retention of pulse added N in this ecosystem. [ABSTRACT FROM AUTHOR]

Winter temperatures are projected to increase in Central Europe. Subsequently, snow cover will decrease, leading to increased soil temperature variability, with potentially different consequences for soil frost depending on e.g. altitude. Here, we experimentally evaluated the effects of increased winter soil temperature variability on the root associated mycobiome of two plant species (Calluna vulgaris and Holcus lanatus) at two sites in Germany; a colder and wetter upland site with high snow accumulation and a warmer and drier lowland site, with low snow accumulation. Mesocosm monocultures were set-up in spring 2010 at both sites (with soil and plants originating from the lowland site). In the following winter, an experimental warming pulse treatment was initiated by overhead infrared heaters and warming wires at the soil surface for half of the mesocosms at both sites. At the lowland site, the warming treatment resulted in a reduced number of days with soil frost as well as increased the average daily temperature amplitude. Contrary, the treatment caused no changes in these parameters at the upland site, which was in general a much more frost affected site. Soil and plant roots were sampled before and after the following growing season (spring and autumn 2011). High-throughput sequencing was used for profiling of the root-associated fungal (ITS marker) community (mycobiome). Site was found to have a profound effect on the composition of the mycobiome, which at the upland site was dominated by fast growing saprotrophs (Mortierellomycota), and at the lowland site by plant species-specific symbionts (e.g. Rhizoscyphus ericae and Microdochium bolleyi for C. vulgaris and H. lanatus respectively). The transplantation to the colder upland site and the temperature treatment at the warmer lowland site had comparable consequences for the mycobiome, implying that winter climate change resulting in higher temperature variability has large consequences for mycobiome structures regardless of absolute temperature of a given site. [ABSTRACT FROM AUTHOR]

Background and Aims Plant secondary metabolites play critical roles in plant stress tolerance and adaptation, and are known to be influenced by the environment and climate changes, yet the impacts and interactions of multiple climate change components are poorly understood, particularly under natural conditions. Methods Accumulation of phenolics and emissions of volatile organic compounds (VOCs) were assessed on heather, Calluna vulgaris , an abundant evergreen dwarf shrub in European heathlands, after 6 years of exposure to elevated CO2, summer drought and nighttime warming. Key Results Drought alone had the strongest effects on phenolic concentrations and compositions, with moderate effects of elevated CO2 and temperature. Elevated CO2 exerted the greatest impact on VOC emissions, mainly by increasing monoterpene emissions. The response magnitudes varied among plant tissue types and chemical constituents, and across time. With respect to interactive effects of the studied climate change components, the interaction between drought and elevated CO2 was most apparent. Drought mainly reduced phenolic accumulation and VOC emissions, while elevated CO2 mitigated such effects. Conclusions In natural ecosystems, co-occurring climate factors can exert complex impacts on plant secondary metabolite profiles, which may in turn alter ecosystem processes. [ABSTRACT FROM AUTHOR]

Extreme climatic events are among the drivers of recent declines in plant biomass and productivity observed across Arctic ecosystems, known as "Arctic browning." These events can cause landscape‐scale vegetation damage and so are likely to have major impacts on ecosystem CO2 balance. However, there is little understanding of the impacts on CO2 fluxes, especially across the growing season. Furthermore, while widespread shoot mortality is commonly observed with browning events, recent observations show that shoot stress responses are also common, and manifest as high levels of persistent anthocyanin pigmentation. Whether or how this response impacts ecosystem CO2 fluxes is not known. To address these research needs, a growing season assessment of browning impacts following frost drought and extreme winter warming (both extreme climatic events) on the key ecosystem CO2 fluxes Net Ecosystem Exchange (NEE), Gross Primary Productivity (GPP), ecosystem respiration (Reco) and soil respiration (Rsoil) was carried out in widespread sub‐Arctic dwarf shrub heathland, incorporating both mortality and stress responses. Browning (mortality and stress responses combined) caused considerable site‐level reductions in GPP and NEE (of up to 44%), with greatest impacts occurring at early and late season. Furthermore, impacts on CO2 fluxes associated with stress often equalled or exceeded those resulting from vegetation mortality. This demonstrates that extreme events can have major impacts on ecosystem CO2 balance, considerably reducing the carbon sink capacity of the ecosystem, even where vegetation is not killed. Structural Equation Modelling and additional measurements, including decomposition rates and leaf respiration, provided further insight into mechanisms underlying impacts of mortality and stress on CO2 fluxes. The scale of reductions in ecosystem CO2 uptake highlights the need for a process‐based understanding of Arctic browning in order to predict how vegetation and CO2 balance will respond to continuing climate change. Extreme climatic events are among the drivers of vegetation damage and decline observed across Arctic ecosystems in recent years; termed "Arctic browning." Although these events can cause landscape‐scale vegetation damage, their impacts on ecosystem CO2 balance are little understood. Here, it is demonstrated that these events can have major impacts on CO2 balance, considerably reducing the carbon sink capacity of the ecosystem across the growing season. These impacts can be similar when associated with shoot mortality, or with shoot stress responses. [ABSTRACT FROM AUTHOR]

Understanding the response of soil respiration to climate variability is critical to formulate realistic predictions of future carbon (C) fluxes under different climate change scenarios. There is growing evidence that the influence of long-term climate variability in C fluxes from terrestrial ecosystems is modulated by adjustments in the aboveground-belowground links. Here, we studied the inter-annual variability in soil respiration from a wet shrubland going through successional change in North Wales (UK) during 13 years. We hypothesised that the decline in plant productivity observed over a decade would result in a decrease in the apparent sensitivity of soil respiration to soil temperature, and that rainfall variability would explain a significant fraction of the inter-annual variability in plant productivity, and consequently, in soil respiration, due to excess-water constraining nutrient availability for plants. As hypothesised, there were parallel decreases between plant productivity and annual and summer CO emissions over the 13-year period. Soil temperatures did not follow a similar trend, which resulted in a decline in the apparent sensitivity of soil respiration to soil temperature (apparent Q values decreased from 9.4 to 2.8). Contrary to our second hypothesis, summer maximum air temperature rather than rainfall was the climate variable with the greatest influence on aboveground biomass and annual cumulative respiration. Since summer air temperature and rainfall were positively associated, the greatest annual respiration values were recorded during years of high rainfall. The results suggest that adjustments in plant productivity might have a critical role in determining the long-term-sensitivity of soil respiration to changing climate conditions. [ABSTRACT FROM AUTHOR]

Background and Aims: As different plant species support different soil communities, assessment of climate change impacts on soil communities must consider how these impacts depend on the vegetation. We investigate soil community responses to global change under different co-occurring plant species. Methods: In a heathland FACE-experiment, we modelled projected global changes by increasing atmospheric CO concentration (to 510 ppm) and soil temperature by 1 °C at 2 cm depth (with night-time reflectance curtains) and reducing summer precipitation for 2-5 weeks annually (with rain-out curtains). We assessed nematode community trophic composition and bacterial and fungal abundance in soil under the dominant plant species Calluna vulgaris and Deschampsia flexuosa in the spring, 10 months after the last experimental drought period. Results: Fungal dominance increased relative to bacterial under elevated CO. Similarly, elevated CO increased abundance of fungivorous nematodes relative to bacterivorous nematodes. Decreased precipitation did not affect the abundances of microorganisms or nematodes, and there were only few effects of warming. Fungivorous nematodes dominated under C. vulgaris, whereas bacterivorous nematodes were relatively more abundant under D. flexuosa. Abundances of plant feeding and omnivorous/predatory nematodes were higher under D. flexuosa than under C. vulgaris. Conclusions: This supports the hypothesis that more carbon will flow through fungal than bacterial channels when below-ground allocation of photosynthetically-derived C increases without a corresponding soil nitrogen increase. Further, as different plant species are associated with distinct belowground decomposer communities, vegetation shifts induced by future CO and climate regimes will also shift belowground communities with consequences for decomposition dynamics. [ABSTRACT FROM AUTHOR]

Evaluating the response of soil organic matter decomposition to warming and changes in rainfall is critical to assess the likelihood of proposed positive feedbacks from the terrestrial to the atmospheric system. The response of soil respiration and extracellular activities (EEAs) to long-term warming and recurrent summer drought was studied in a wet shrubland ecosystem in Wales (UK), after 13 years of climate change simulation in a whole-ecosystem experiment. Over a year soil respiration, temperature and moisture was monitored in the field. During the summer season, coinciding with maximum soil respiration rates, soil inorganic N and P, microbial biomass and the extracellular activities (EEAs) of a selection of enzymes involved in C, N and P cycling were analysed. Based on previous field measurements of C and N mineralization, we expected a stronger response of C-cycling EEAs, in comparison to N-cycling EEAs, to drought and warming, and a greater sensitivity of C-cycling EEAs to drought than to warming. Drought had a clear impact on soil respiration during the summer season. However, the availability of inorganic N or P was not significantly affected by the treatments. Microbial biomass and C:N ratio also remained unchanged. In contrast to one of our hypothesis, C-cycling EEAs measured under non-optimal conditions that simulated soil environment in the field (pH of 4.1 and with a temperature incubation of 10 °C) showed no significant differences due to long-term warming and recurring drought treatments. Possibly, this assay approach may have obscured treatment effects on the soil enzyme pool. Our results highlight the need for developing methods for the in-situ analysis of EEAs to determine rates of reactions. [ABSTRACT FROM AUTHOR]

The dominant plant species of European heathlands Calluna vulgaris is considered vulnerable to drought and enhanced nitrogen (N) loads. However, impacts may vary across the distribution range of Calluna heathlands. We tested the hypothesis that Calluna of southern and eastern marginal populations (MP) are more resistant to drought events than plants of central populations (CP), and that this is mainly due to trait differences such as biomass allocation patterns. Furthermore, we hypothesised that N fertilisation can offset differences in drought susceptibility between CP and MP. We conducted a full-factorial 2-year greenhouse experiment with Calluna plants of CP and MP and quantified growth responses in terms of biomass production, allocation and tissue δC signatures. Biomass production, shoot-root ratios and tissue δC values of 1-year-old plants were higher for CP than for MP, indicating a higher drought susceptibility of CP. These trait differences were not observed for 2-year-old plants. N fertilisation increased shoot-root ratios of 1- and 2-year-old plants and across populations due to a stimulation of the aboveground biomass allocation. As a consequence, population-related differences in drought susceptibility were offset for N-fertilised plants. We concluded that Calluna plants originating from different populations developed adaptive traits to local climates, which determined their drought sensitivity. However, the higher drought resistance of MP can be attenuated by an N-induced increase in shoot-root ratios. This suggests that analyses on plant growth responses to global change should include multi-factor approaches with a focus on different populations throughout a species' distribution range. [ABSTRACT FROM AUTHOR]

Climate change and atmospheric deposition of nitrogen (N) affect the biodiversity patterns and functions of ecosystems world-wide. While many single-factor studies have quantified ecosystem responses to single global change drivers, less is known about the interaction effects of these drivers on ecosystem functions., Here, we present the results of a three-year field and a two-year glasshouse experiment, in which we assessed responses of Calluna vulgaris heathlands to the single and combined effects of drought events (D) and N fertilization (D: 25% precipitation reduction in the field experiment and 20-50% soil water content reduction in the glasshouse experiment; N fertilization: 35 kg N ha−1 year−1)., We examined the effects of D and N treatments on growth responses of the dominant dwarf shrub Calluna vulgaris (in terms of biomass production and allocation, tissue δ13C signatures and C:N ratios) in relation to two plant life-history stages and different 'ecotypes' (sub-Atlantic vs. subcontinental heathlands)., Plant responses varied strongly with life-history stage, and the interaction of N and D showed lower effects than would be expected based on additive responses to single factors. While D treatments had no effects on Calluna in the building phase (ca. ten-year-old plants), seedlings (particularly one-year-old plants) were highly susceptible to drought. Differences in response patterns were attributable to the high shoot-root ratios typical of young Calluna plants. These ratios decreased with progressing life history as a result of increasing below-ground biomass investments. Below-ground biomass production and shoot-root ratios differed between plants from the different heathland sites., Tissue δ13C signatures decreased and C:N ratios increased with plant age as a result of decreasing evaporative demands (per unit root biomass). N fertilization increases the shoot-root ratios and thereby the drought susceptibility of Calluna plants., Synthesis. Our findings suggest that plant responses to global change are difficult to anticipate by means of single-factor studies or by focusing on a single life-history stage. This highlights the need for global change research to include multiple factors and life-history stages when assessing an ecosystem's susceptibility to shifts in environmental conditions. [ABSTRACT FROM AUTHOR]

Climate change and nitrogen deposition affect biodiversity and ecosystem functioning, but interactive effects of these global change drivers are poorly understood. We analysed single and interactive effects of nitrogen (N) fertilisation and drought on the growth performance ofCalluna vulgaris. We measured biomass production and allocation, tissue nutrient (N, phosphorus (P) and carbon (C)) concentrations, N allocation patterns (using15N tracer) and plant's water status (using δ 13C signatures) as response variables in a 2-year greenhouse experiment. N fertilisation increased biomass production and biomass shoot:root ratios.15N allocation patterns indicated an increasing aboveground N allocation following N fertilisation. Tissue δ 13C signatures were higher in N-fertilised plants. Plant responses to drought were weak. We found strong antagonistic interaction effects of N fertilisation and drought for biomass production. δ 13C values peaked when N-fertilised plants were subjected to drought, indicating that N fertilisation increased the evaporative demands ofCallunaplants, likely due to increased biomass shoot:root ratios, which in turn resulted in higher drought susceptibility. As an important consequence, even slight drought events may weaken the competitiveness ofCallunawhen interacting with enhanced airborne N loads. Single-factor studies, thus, need to be complemented by multi-factor analyses to assess conceivable impacts of co-occurring global change drivers. [ABSTRACT FROM AUTHOR]

The article discusses a study on the sustained impact of drought on organo-mineral soils from wet shrublands in North Wales. Topics include the influence of stimulation of soil respiration by drought and climate warming on soil structure, the impact of bryophytes on soil moisture loss and moderating soil carbon dioxide levels, and the impact that changes in sub-dominant vegetation and soil physical properties have on determining climate change impacts on on soil C dynamics.

Background and aims: Accurate predictions of nutrient acquisition by plant roots and mycorrhizas are critical in modelling plant responses to climate change. Methods: We conducted a field experiment with the aim to investigate root nutrient uptake in a future climate and studied root production by ingrowth cores, mycorrhizal colonization, and fine root N and P uptake by root assay of Deschampsia flexuosa and Calluna vulgaris. Results: Net root growth increased under elevated CO, warming and drought, with additive effects among the factors. Arbuscular mycorrhizal colonization increased in response to elevated CO, while ericoid mycorrhizal colonization was unchanged. The uptake of N and P was not increased proportionally with root growth after 5 years of treatment. Conclusions: While aboveground biomass was unchanged, the root growth was increased under elevated CO. The results suggest that plant production may be limited by N (but not P) when exposed to elevated CO. The species-specific response to the treatments suggests different sensitivity to global change factors, which could result in changed plant competitive interactions and belowground nutrient pool sizes in response to future climate change. [ABSTRACT FROM AUTHOR]

Background and aims: Shrublands are ecosystems vulnerable to climate changes, with key functions such as carbon storage likely to be affected. In dwarf shrublands dominated by Calluna vulgaris, the aboveground carbon allocation is associated with community age and phase of development. As the Calluna community grows older, a shift to net biomass loss occurs and it was hypothesized this would result in carbon stock increases within the soil. Methods: The interaction of community age with ecosystem carbon stocks was investigated through a chronosequence study on three Calluna communities, aged 11, 18 and 27 years. Results: Aboveground Calluna carbon stock increased significantly from the 11 year community (0.73 kg C m) to the 18 year community (1.11 kg C m) but did not significantly change from 18 to 27 years (1.0 kg C m), indicating a net carbon gain that corresponded with the growth phase of the Calluna plants. Moss was also found to be a relatively large contributor to aboveground carbon stock (e.g. 30 % in the Young community). Moss has often been excluded in aboveground assessments on Calluna heathlands which may have led to previous stock underestimation. Belowground carbon stocks to 25 cm were six to nine times greater than in the aboveground pools. For example in the Young community, 8 % of the carbon stock was located aboveground, 35 % in the organic layer and 55 % in the mineral soil. Conclusions: Increased heathland age resulted in increased aboveground carbon stock until peak production was reached at approximately 18 years of age. However, the proportionally large belowground carbon stock eclipsed any aboveground effect when total carbon stocks were considered. The investigation emphasized both the importance of including the mineral soil in sampling programs and of consider all major species, such as bryophytes, and vegetation age in carbon stock assessments. [ABSTRACT FROM AUTHOR]

Increases in the emissions and associated atmospheric deposition of nitrogen (N) have the potential to cause significant changes to the structure and function of N-limited ecosystems. Here, we present the results of a long-term (13 year) experiment assessing the impacts of N addition (30 kg ha−1 yr−1) on a UK lowland heathland under a wide range of environmental conditions, including the occurrence of prolonged natural drought episodes and a severe summer fire. Our findings indicate that elevated N deposition results in large, persistent effects on Calluna growth, phenology and chemistry, severe suppression of understorey lichen flora and changes in soil biogeochemistry. Growing season rainfall was found to be a strong driver of inter-annual variation in Calluna growth and, although interactions between N and rainfall for shoot growth were not significant until the later phase of the experiment, N addition exacerbated the extent of drought injury to Calluna shoots following naturally occurring droughts in 2003 and 2009. Following a severe wildfire at the experimental site in 2006, heathland regeneration dynamics were significantly affected by N, with a greater abundance of pioneering moss species and suppression of the lichen flora in plots receiving N additions. Significant interactions between climate and N were also apparent post fire, with the characteristic stimulation in Calluna growth in +N plots suppressed during dry years. Carbon (C) and N budgets demonstrate large increases in both above- and below-ground stocks of these elements in N-treated plots prior to the fire, despite higher levels of soil microbial activity and organic matter turnover. Although much of the organic material was removed during the fire, pre-existing treatment differences were still evident following the burn. Post fire accumulation of below-ground C and N stocks was increased rapidly in N-treated plots, highlighting the role of N deposition in ecosystem C sequestration. [ABSTRACT FROM AUTHOR]

Climate change scenarios predict simultaneously increase in temperature, altered precipitation patterns and elevated atmospheric CO concentration, which will affect key ecosystem processes and plant growth and species interactions. In a large-scale experiment, we investigated the effects of in situ exposure to elevated atmospheric CO concentration, increased temperature and prolonged drought periods on the plant biomass in a dry heathland (Brandbjerg, Denmark). Results after 3 years showed that drought reduced the growth of the two dominant species Deschampsia flexuosa and Calluna vulgaris. However, both species recovered quickly after rewetting and the drought had no significant effect on annual aboveground biomass production. We did not observe any effects of increased temperature. Elevated CO stimulated the biomass production for D. flexuosa in one out of three years but did not influence the standing biomass for either D. flexuosa or the ecosystem as more litter was produced. Treatment combinations showed little interactions on the measured parameters and in particular elevated CO did not counterbalance the drought effect on plant growth, as we had anticipated. The plant community did not show any significant responses to the imposed climate changes and we conclude that the two heathland species, on a short time scale, will be relatively resistant to the changes in climatic conditions. [ABSTRACT FROM AUTHOR]

Abstract: Increased temperature, atmospheric CO2 and change in precipitation patterns affect plant physiological and ecosystem processes. In combination, the interactions between these effects result in complex responses that challenge our current understanding. In a multi-factorial field experiment with elevated CO2 (CO2, FACE), nighttime warming (T) and periodic drought (D), we investigated photosynthetic capacity and PSII performance in the evergreen dwarf shrub Calluna vulgaris and the grass Deschampsia flexuosa in a temperate heath ecosystem. Photosynthetic capacity was evaluated using A/C i curves, leaf nitrogen content and chlorophyll-a fluorescence OJIP induction curves. The PSII performance was evaluated via the total performance index PItotal, which integrates the function of antenna, reaction centers, electron transport and end-acceptor reduction according to the OJIP-test. The PSII performance was negatively influenced by high air temperature, low soil water content and high irradiance dose. The experimental treatments of elevated CO2 and prolonged drought generally down-regulated J max, V cmax and PItotal. Recovery from these depressions was found in the evergreen shrub after rewetting, while post-rewetting up-regulation of these parameters was observed in the grass. Warming effects acted indirectly to improve early season J max, V cmax and PItotal. The responses in the multi-factorial experimental manipulations demonstrated complex interactive effects of T×CO2, D×CO2 and T×D×CO2 on photosynthetic capacity and PSII performance. The impact on the O–J, J–I and I–P phases which determine the response of PItotal are discussed. The single factor effects on PSII performance and their interactions could be explained by parallel adjustments of V cmax, J max and leaf nitrogen in combination. Despite the highly variable natural environment, the OJIP-test was very robust in detecting the impacts of T, D, CO2 and their interactions. This study demonstrates that future climate will affect fundamental plant physiological processes in a way that is not predictable from single factor treatments. The interaction effects that were observed depended upon both the growth strategy of the species considered, and their ability to adjust during drought and rewetting periods. [Copyright &y& Elsevier]

Temperate terrestrial ecosystems are currently exposed to increased atmospheric CO2 and progressive climatic changes with increased temperature and periodical drought. We here present results from a field experiment, where the effects of these three main climate change related factors are investigated solely and in all combinations at a temperate heathland. Significant responses were found in the top soils below the two dominant species ( Calluna vulgaris and Deschampsia flexuosa). During winter incubation, microbial immobilization of N and ammonification rate decreased in response to warming in Deschampsia soil, and microbial immobilization of N and P decreased in warmed Calluna soil. Warming tended to increase microbial N and P in Calluna but not in Deschampsia soil in fall, and more microbial C was accumulated under drought in Calluna soil. The effects of warming were often counteracted or erased when combined with CO2 and drought. Below Deschampsia, the net nitrification rate decreased in response to drought and, while phosphorus availability and microbial P immobilization decreased, but nitrification increased in response to elevated CO2. Furthermore, leaf litter decomposition of both species decreased in response to drought. These complex changes in availability and release of nutrients from soil organic matter turnover and mineralization in response to elevated CO2 and climate change may influence the future plant carbon sequestration and species composition at temperate heathlands. [ABSTRACT FROM AUTHOR]

Abstract: Ring diameters and ring number counts were recorded from Calluna stem cross-sections harvested during an N manipulation experiment. The application of dendrochronological methods permitted accurate ageing of shrubs growing in a heathland of known age. Linear regression analyses demonstrated a highly significant association between increasing levels of N input and stem diameter in both watered and droughted plots (droughted in 1997 only). N inputs were identified as the most significant environmental stressor, appearing to speed ageing vis the Calluna growth cycle. Drought sensitivity was noted in the high N treatment plots (120kgNha−1 yr−1) with decreased stem diameters. This may have implications in the response of Calluna growth under current climate change scenarios. [Copyright &y& Elsevier]

Plant ecophysiological changes in response to climatic change may be different in northern and southern European countries because different abiotic factors constrain plant physiological activity. We studied the effects of experimental warming and drought on the photosynthetic performance of two ericaceous shrubs (Erica multifloraandCalluna vulgaris) along a European gradient of temperature and precipitation (UK, Denmark, The Netherlands, and Spain). At each site, a passive warming treatment was applied during the night throughout the whole year, whereas the drought treatment excluded rain events over 6-10 weeks during the growing season. We measured leaf gas exchange, chlorophyllafluorescence, and leaf carbon isotope ratio (d13C) during the growing seasons of 1999 and 2000. Leaf net photosynthetic rates clearly followed a gradient from northern to southern countries in agreement with the geographical gradient in water availability. Accordingly, there was a strong correlation between net photosynthetic rates and the accumulated rainfall over the growing season. Droughted plants showed lower leaf gas exchange rates than control plants in the four sites. Interestingly, although leaf photosynthetic rates decreased along the precipitation gradient and in response to drought treatment, droughted plants were able to maintain higher leaf photosynthetic rates than control plants in relation to the accumulated rainfall over the months previous to the measurements. Droughted plants also showed higher values of potential photochemical efficiency (Fv/Fm) in relation to controls, mainly at midday. The warming treatment did not affect significantly any of the studied instantaneous ecophysiological variables. [ABSTRACT FROM AUTHOR]

Climate change may affect ecosystem functioning through increased temperatures or changes in precipitation patterns. Temperature and water availability are important drivers for ecosystem processes such as photosynthesis, carbon translocation, and organic matter decomposition. These climate changes may affect the supply of carbon and energy to the soil microbial population and subsequently alter decomposition and mineralization, important ecosystem processes in carbon and nutrient cycling. In this study, carried out within the cross-European research project CLIMOOR, the effect of climate change, resulting from imposed manipulations, on carbon dynamics in shrubland ecosystems was examined. We performed a14C-labeling experiment to probe changes in net carbon uptake and allocation to the roots and soil compartments as affected by a higher temperature during the year and a drought period in the growing season. Differences in climate, soil, and plant characteristics resulted in a gradient in the severity of the drought effects on net carbon uptake by plants with the impact being most severe in Spain, followed by Denmark, with the UK showing few negative effects at significance levels ofp= 0.10. Drought clearly reduced carbon flow from the roots to the soil compartments. The fraction of the14C fixed by the plants and allocated into the soluble carbon fraction in the soil and to soil microbial biomass in Denmark and the UK decreased by more than 60%. The effects of warming were not significant, but, as with the drought treatment, a negative effect on carbon allocation to soil microbial biomass was found. The changes in carbon allocation to soil microbial biomass at the northern sites in this study indicate that soil microbial biomass is a sensitive, early indicator of drought- or temperature-initiated changes in these shrubland ecosystems. The reduced supply of substrate to the soil and the response of the soil microbial biomass may help to... [ABSTRACT FROM AUTHOR]

We used a novel, nonintrusive experimental system to examine plant responses to warming and drought across a climatic and geographical latitudinal gradient of shrubland ecosystems in four sites from northern to southern Europe (UK, Denmark, The Netherlands, and Spain). In the first two years of experimentation reported here, we measured plant cover and biomass by the pinpoint method, plant14C uptake, stem and shoot growth, flowering, leaf chemical concentration, litterfall, and herbivory damage in the dominant plant species of each site. The two years of approximately 1°C experimental warming induced a 15% increase in total aboveground plant biomass growth in the UK site. Both direct and indirect effects of warming, such as longer growth season and increased nutrient availability, are likely to be particularly important in this and the other northern sites which tend to be temperature-limited. In the water-stressed southern site, there was no increase in total aboveground plant biomass growth as expected since warming increases water loss, and temperatures in those ecosystems are already close to the optimum for photosynthesis. The southern site presented instead the most negative response to the drought treatment consisting of a soil moisture reduction at the peak of the growing season ranging from 33% in the Spanish site to 82% in The Netherlands site. In the Spanish site there was a 14% decrease in total aboveground plant biomass growth relative to control. Flowering was decreased by drought (up to 24% in the UK and 40% in Spain). Warming and drought decreased litterfall in The Netherlands site (33% and 37%, respectively) but did not affect it in the Spanish site. The tissue P concentrations generally decreased and the N/P ratio increased with warming and drought except in the UK site, indicating a progressive importance of P limitation as a consequence of warming and drought. The magnitude of the response to warming and drought was thus very... [ABSTRACT FROM AUTHOR]

Summary: Fffects of climatic warming on plants are likely to be different for different species, with consequences for interactions between species, changes in community structure and interactions between different communities. This study reports the effects of increased temperature and N addition on heather, an extensive and important species in the uplands of NW Europe, and bracken, a species of global importance, often regarded as a weed. Air temperature was increased by up to 0.83 °C using open-topped polythene tents, and additional N was provided as NH4NO3 at a rate of 50 kg N ha−1 yr−1, at a field site in Upper Teesdale in northern England, both in pure canopies, and at the boundary between the species. Biomass of deciduous bracken fronds, with its relatively fast within-year response potential, was significantly increased by higher growing temperatures, while biomass of the perennial rhizomes, which represent growth over a number of years, was not altered after three years. Additional N had little effect, possibly due to the high efficiency with which bracken recycles nutrients. Overall, bracken is likely to benefit from the currently rising temperatures in Britain. Direct effects of the temperature and N treatments on heather were small: heather in the pure canopy responded only marginally to the treatments. However, it showed considerable reductions in vigour in the boundary plots. The results highlight the importance of the biological environment in perturbation experiments. We conclude that heather will probably be further displaced by bracken in a warmer climate. In the third year of the study albedo was much higher over bracken than over heather dominated areas. This means that if there are shifts in the boundary between them due to climate change, there is a potential for the modification of feedbacks to climate change, but the magnitude depends on a number of competing processes. Die Effekte der Klimaerwärmung auf Pflanzen sind wahrscheinlich unterschiedlich bei verschiedenen Pflanzen, mit Konsequenzen für die Interaktionen zwischen den Arten, für die Veränderungen in der Gemeinschaftsstruktur und für die Interaktionen zwischen verschiedenen Gemeinschaften. Diese Studie berichtet über die Effekte zunehmender Temperatur und N-Düngung auf Heide, eine ausgedehnte und wichtige Art der Hochländer Nordwest-Europas, sowie auf Adlerfarn, eine Art mit globaler Bedeutung, die oft als Unkraut betrachtet wird. In Upper Teesdale im nördlichen England wurde in einem Feldversuch sowohl in geschlossenen Beständen als auch an der Grenze zwischen den Arten die Lufttemperatur durch den Gebrauch von oben offenen Polyäthylen-Zelten um bis zu 0,83 °C erhöht und zusätzlicher Stickstoff in Form von NH4NO 3 mit einer Rate von 50 kg N ha−1 yr−1 zur Verfügung gestellt. Die Biomasse des Laubanteils der Adlerfarnwedel, mit ihrem relativ schnellen Reaktionspotential innerhalb des Jahres, war durch die höheren Wachstumstemperaturen signifikant erhöht, während die Biomasse der perennierenden Rhizome, die das Wachstum über eine Anzahl von Jahren präsentieren, nach drei Jahren nicht verändert war. Zusätzlicher N hatte nur einen geringen Effekt, möglicherweise aufgrund der hohen Effektivität mit der Adlerfarn Nährstoffe zurückgewinnt. Insgesamt wird Adlerfarn wahrscheinlich von den gegenwärtig steigenden Temperaturen in Großbritannien begünstigt. Die direkten Effekte der Temperatur und der N-Behandlungen auf die Heide waren klein: Heide in reinen Beständen reagierte nur geringfügig auf die Behandlungen. Sie zeigte jedoch eine beachtliche Reduzierung der Vitalität in den Grenzflächen. Die Ergebnisse heben die Wichtigkeit der biologischen Umgebung bei Störungsexperimenten hervor. Wir schließen daraus, dass in einem warmen Klima die Heide wahrscheinlich zunehmend durch Adlerfarn ersetzt wird. In den von Adlerfarn dominierten Arealen war im dritten Jahr der Studie die Einstrahlung viel größer als in den von Heide dominierten Arealen. Wenn es aufgrund der Klimaveränderung Verschiebungen der Grenzen zwischen beiden Arten gibt, bedeutet dies, dass ein Potential für Modifikationen in der Reaktion auf Klimaveränderungen vorhanden ist. Die Größenordnung hängt jedoch von einer Anzahl konkurrierender Prozesse ab. [Copyright &y& Elsevier]

Three Calluna vulgaris (L.) Hull heathlands (1100, 1500 and 1780 m altitude) are studied in order to establish and estimate parameters which will be integrated in a discrete event simulation of heathland functioning in the case of a 2 °C climate warming. The sites, situated in the Chaîne des Puys and the Massif du Sancy (France), present similar conditions for Calluna growth: bedrock (trachyt), exposure and slope, so that they can be compared on a climatic basis. Main parameters sampled are: age distributions, life expectancies, layering probabilities, annual growths, biomasses (standing crop), C/N rates and potential respiratory of soils. In addition, experiments were carried out on germination, hypocotyle elongation and growth of mycorrhizal fungi in order to estimate the inhibitory-to-growth action against the substitution process. On one stand (exhibiting gaps) the patch structure of Calluna plants was recorded and mapped. The rising of biomass expected is calculated according to the Aerts's model, as a function of nitrogen availability which would increase under a scenario of climate warming. From a similar point of view, layering probabilities, life expectancies and inhibition of seedlings survival modelling – under a linear model assumption – are proposed. [ABSTRACT FROM AUTHOR]

Abstract Heather plants (Calluna vulgaris (L.) Hull) were grown for two years (April 1994–October 1996) under ambient or enhanced ultraviolet-B radiation (UVB: 280–315 nm), provided as a modulated treatment simulating a 15% ozone depletion (seasonally adjusted). Populations of the psyllid (Strophingia ericae (Curtis)) were measured before treatment and at yearly intervals thereafter. Before treatment there was no significant difference in the psyllid populations between treatments, or between the experimental and source populations. Enhanced UVB progressively produced a reduction in S. ericae populations compared with controls over 27 months. Analyses of C, N, total water soluble phenolics, total free amino acids and measurements of leaf angles and distances between leaflets demonstrated no effects of UVB treatment. However, concentrations of the amino acid isoleucine were lower (28%) in C. vulgaris exposed to the enhanced UVB treatment. Over the duration of the experiment the psyllid population structure at Lancaster changed from that typical of the upland site of origin (two-year cycle with overlapping cohorts) to a one-year life cycle typical of lowland sites, but this was independent of UVB treatments. Reduced isoleucine might explain the negative effects of elevated UVB on psyllid population numbers, but the precise effects of UVB on host chemistry and morphology are unknown. The problem of interpreting herbivore responses to enhanced UVB treatments in the field is discussed. [ABSTRACT FROM AUTHOR]