Niche construction mediates climate effects on recovery of tundra heathlands after extreme event.

Climate change is expected to increase the frequency and intensity of extreme events in northern ecosystems. The outcome of these events across the landscape, might be mediated by species effects, such as niche construction, with likely consequences on vegetation resilience. To test this hypothesis, we simulated an extreme event by removing aboveground vegetation in tundra heathlands dominated by the allelopathic dwarf shrub Empetrum nigrum, a strong niche constructor. We tested the hypothesis under different climate regimes along a 200-km long gradient from oceanic to continental climate in Northern Norway. We studied the vegetation recovery process over ten years along the climatic gradient. The recovery of E. nigrum and subordinate species was low and flattened out after five years at all locations along the climatic gradient, causing low vegetation cover at the end of the study in extreme event plots. Natural seed recruitment was low at all sites, however, the addition of seeds from faster growing species did not promote vegetation recovery. A soil bioassay from 8 years after the vegetation was removed, suggested the allelopathic effect of E. nigrum was still present in the soil environment. Our results provide evidence of how a common niche constructor species can dramatically affect ecosystem recovery along a climatic gradient after extreme events in habitats where it is dominant. By its extremely slow regrowth and it preventing establishment of faster growing species, this study increases our knowledge on the possible outcomes when extreme events harm niche constructors in the tundra.

In vitro axenic germination and cultivation of mixotrophic Pyroloideae (Ericaceae) and their post-germination ontogenetic development.

BACKGROUND AND AIMS: Pyroloids, forest sub-shrubs of the Ericaceae family, are an important model for their mixotrophic nutrition, which mixes carbon from photosynthesis and from their mycorrhizal fungi. They have medical uses but are difficult to cultivate ex situ; in particular, their dust seeds contain undifferentiated, few-celled embryos, whose germination is normally fully supported by fungal partners. Their germination and early ontogenesis thus remain elusive. METHODS: An optimized in vitro cultivation system of five representatives from the subfamily Pyroloideae was developed to study the strength of seed dormancy and the effect of different media and conditions (including light, gibberellins and soluble saccharides) on germination. The obtained plants were analysed for morphological, anatomical and histochemical development. KEY RESULTS: Thanks to this novel cultivation method, which breaks dormancy and achieved up to 100 % germination, leafy shoots were obtained in vitro for representatives of all pyroloid genera (Moneses, Orthilia, Pyrola and Chimaphila). In all cases, the first post-germination stage is an undifferentiated structure, from which a root meristem later emerges, well before formation of an adventive shoot. CONCLUSIONS: This cultivation method can be used for further research or for ex situ conservation of pyroloid species. After strong seed dormancy is broken, the tiny globular embryo of pyroloids germinates into an intermediary zone, which is functionally convergent with the protocorm of other plants with dust seeds such as orchids. Like the orchid protocorm, this intermediary zone produces a single meristem: however, unlike orchids, which produce a shoot meristem, pyroloids first generate a root meristem.

Sphagnum Species Modulate their Phenolic Profiles and Mycorrhizal Colonization of Surrounding Andromeda polifolia along Peatland Microhabitats.

Sphagnum mosses mediate long-term carbon accumulation in peatlands. Given their functional role as keystone species, it is important to consider their responses to ecological gradients and environmental changes through the production of phenolics. We compared the extent to which Sphagnum phenolic production was dependent on species, microhabitats and season, and how surrounding dwarf shrubs responded to Sphagnum phenolics. We evaluated the phenolic profiles of aqueous extracts of Sphagnum fallax and Sphagnum magellanicum over a 6-month period in two microhabitats (wet lawns versus dry hummocks) in a French peatland. Phenolic profiles of water-soluble extracts were measured by UHPLC-QTOF-MS. Andromeda polifolia mycorrhizal colonization was quantified by assessing the intensity of global root cortex colonization. Phenolic profiles of both Sphagnum mosses were species-, season- and microhabitat- dependant. Sphagnum-derived acids were the phenolics mostly recovered; relative quantities were 2.5-fold higher in S. fallax than in S. magellanicum. Microtopography and vascular plant cover strongly influenced phenolic profiles, especially for minor metabolites present in low abundance. Higher mycorrhizal colonization of A. polifolia was found in lawns as compared to hummocks. Mycorrhizal abundance, in contrast to environmental parameters, was correlated with production of minor phenolics in S. fallax. Our results highlight the close interaction between mycorrhizae such as those colonizing A. polifolia and the release of Sphagnum phenolic metabolites and suggest that Sphagnum-derived acids and minor phenolics play different roles in this interaction. This work provides new insight into the ecological role of Sphagnum phenolics by proposing a strong association with mycorrhizal colonization of shrubs.

The spread of Kalmia angustifolia on black spruce forest cutovers contributes to the spatial heterogeneity of soil resources.

Kalmia angustifolia is a boreal ericaceous shrub that can rapidly spread on black spruce forest cutovers in eastern Canada, where CPRS (i.e. Cutting with Protection of Regeneration and Soils") is practiced. The proliferation of Kalmia often coincides with a reduction in the growth rate of regenerating black spruce seedlings. We report on a study where we compared the local effects of Kalmia and black spruce seedling patches (i.e. two types of "Vegetation") on chemical and biochemical soil properties in CPRS cutovers within mesic spruce-moss and xeric spruce-lichen ecosystems, as well as in four mature spruce-moss forests (i.e. three "Site Types"). Results from 13C-CPMAS-NMR revealed lower O-alkyl C (i.e. carbohydrates), higher aromatic C (i.e. lignin and other phenolics) and higher carbonyl-C (i.e. amide-C and carboxyl groups) in spruce-moss than in spruce-lichen forest floors (F-horizon). In spite of these distinctions, we observed only a small number of Site Type x Vegetation interactions controlling soil properties. Vegetation had a significant effect on ten forest floor properties. Most notably, Kalmia patches had higher concentrations of condensed tannins and lower mineral N cycling. On the other hand, Site Type had a relatively greater effect on the deeper podzolic-B horizons, where mineral N and microbial activity were higher in mature spruce-moss forests than in the cutovers. Green and senescent Kalmia leaves collected at these sites had higher N, tannin and phenolic concentrations than green and senescent spruce needles. A 25 month litter bag study found lower decomposition of Kalmia leaf litter in spruce patches on spruce-lichen cutovers compared to spruce patches on spruce-moss cutovers, or to Kalmia patches on spruce-lichen cutovers. Given that black spruce seedlings obtain most of their nutrients from the forest floor, our results suggest that CPRS may have long-term negative effects on black spruce forest productivity if the spread of Kalmia is left unchecked.

Xylogenesis reveals the genesis and ecological signal of IADFs in Pinus pinea L. and Arbutus unedo L.

Background and Aims: Mediterranean trees have patterns of cambial activity with one or more pauses per year, leading to intra-annual density fluctuations (IADFs) in tree rings. We analysed xylogenesis (January 2015-January 2016) in Pinus pinea L. and Arbutus unedo L., co-occurring at a site on Mt. Vesuvius (southern Italy), to identify the cambial productivity and timing of IADF formation. Methods: Dendrochronological methods and quantitative wood anatomy were applied and enabled IADF identification and classification. Key Results: We showed that cambium in P. pinea was productive throughout the calendar year. From January to March 2015, post-cambial (enlarging) earlywood-like tracheids were observed, which were similar to transition tracheids. The beginning of the tree ring was therefore not marked by a sharp boundary between latewood of the previous year and the new xylem produced. True earlywood tracheids were formed in April. L-IADFs were formed in autumn, with earlywood-like cells in latewood. In A. unedo, a double pause in cell production was observed, in summer and winter, leading to L-IADFs in autumn as well. Moreover, the formation of more than one IADF was observed in A. unedo. Conclusions: Despite having completely different wood formation models and different life strategies, the production of earlywood, latewood and IADF cells was strongly controlled by climatic factors in the two species. Such cambial production patterns need to be taken into account in dendroecological studies to interpret climatic signals in wood from Mediterranean trees.

Phyto-toponyms of Arbutus unedo L. and their distribution in Sardinia (Italy).

The study shows the results of an inventory of place names connected to Arbutus unedo L., a Mediterranean species, widespread throughout Sardinia. The main aim was to compare the past distribution of place names, referring to the strawberry tree, to the current distribution of the species on the island. In addition, we investigated the meaning and the diversity of these local place names in the various communities. The result was a collection of 432 phyto-toponyms. 248 of them were used for an analysis of their distribution in the habitats, indicated on the Map of the Nature System in Sardinia, defined on the basis of the current vegetation typology. The persistence of the species in the various habitats was either confirmed or negated with in site investigations and interviews. 47.5% of municipalities have place names related to the strawberry tree. Of the 248 phyto-toponyms, 127 fall in the habitats where the species currently persists proving a correspondence between their regional distribution and the current distribution of the species. The remaining 121 phyto-toponyms fall in habitats where the strawberry tree is currently absent. Most of them are found in man-made habitats where man has transformed the forest cover which previously included the strawberry tree. This study also contributes to promoting and conserving the linguistic heritage of local communities.

High Arctic summer warming tracked by increased Cassiope tetragona growth in the world's northernmost polar desert.

Rapid climate warming has resulted in shrub expansion, mainly of erect deciduous shrubs in the Low Arctic, but the more extreme, sparsely vegetated, cold and dry High Arctic is generally considered to remain resistant to such shrub expansion in the next decades. Dwarf shrub dendrochronology may reveal climatological causes of past changes in growth, but is hindered at many High Arctic sites by short and fragmented instrumental climate records. Moreover, only few High Arctic shrub chronologies cover the recent decade of substantial warming. This study investigated the climatic causes of growth variability of the evergreen dwarf shrub Cassiope tetragona between 1927 and 2012 in the northernmost polar desert at 83°N in North Greenland. We analysed climate-growth relationships over the period with available instrumental data (1950-2012) between a 102-year-long C. tetragona shoot length chronology and instrumental climate records from the three nearest meteorological stations, gridded climate data, and North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) indices. July extreme maximum temperatures (JulTemx ), as measured at Alert, Canada, June NAO, and previous October AO, together explained 41% of the observed variance in annual C. tetragona growth and likely represent in situ summer temperatures. JulTemx explained 27% and was reconstructed back to 1927. The reconstruction showed relatively high growing season temperatures in the early to mid-twentieth century, as well as warming in recent decades. The rapid growth increase in C. tetragona shrubs in response to recent High Arctic summer warming shows that recent and future warming might promote an expansion of this evergreen dwarf shrub, mainly through densification of existing shrub patches, at High Arctic sites with sufficient winter snow cover and ample water supply during summer from melting snow and ice as well as thawing permafrost, contrasting earlier notions of limited shrub growth sensitivity to summer warming in the High Arctic.

EFFECTS OF COLD STRATIFICATION AND GA3 ON GERMINATION OF ARBUTUS UNEDO SEEDS OF THREE PROVENANCES.

BACKGROUND: Arbutus unedo is a valuable Mediterranean shrub as an ornamental plant as well as fruit tree. Fresh fruits of A. unedo are a good source of antioxidants, of vitamins C, E and carotenoids and also are characterized by the high content of mineral elements. MATERIALS AND METHODS: The effects of gibberellic acid (GA3) and cold stratification (CS) on seed germination performance were investigated in A. unedo seeds collected from three provenances in the Northern part of Greece. Seeds of each provenance were soaked in solutions of GA3 (500, 1000 or 2000 ppm) for 24 h and subsequently were subjected to CS at 3 - 5°C for 0, 1, 2, and 3 months. RESULTS: Non-stratified seeds of the three A. unedo provenances which were not treated with GA3 solutions exhibited very low germination. However, seed germination was significantly improved after a one-month period of CS. Similarly, the non-stratified seeds of all three provenances became non-dormant after the treatment with 2000 ppm GA3 and they germinated at high percentages. However, in untreated seeds with GA3, after a one-month CS period the seeds of the Pieria provenance exhibited higher germination percentage than that of Rodopi provenance seeds. Furthermore, in non-stratified seeds, the Pieria provenance seeds treated with GA3 germinated at higher percentages and more rapidly than those of the other two provenances. CONCLUSION: The results indicated that untreated seeds exhibited very low germination at 20/25°C. However, in all three provenances seed germinability was significantly improved by a one-month period of CS or treatment of seeds with 2000 ppm GA3. Furthermore, there was a considerable variability among seed provenances in response to the treatments which were applied.

ermination and Clonal Propagation of the Endemic Shrub Corema album, a ulnerable Species with Conservation Needs and Commercial Interest.

In this study, we aimed to explore regeneration possibilities of Corema album (L.) D. Don by determining germination mechanisms and testing vegetative propagation methods. We analyzed seed viability under natural conditions, carried out germination treatments and a greenhouse experiment to study clonal propagation. We confirmed that C. album seeds present physiological dormancy, broken by ingestion by natural dispersers (rabbits and foxes), and that seed viability under natural conditions is lost after one year. In vitro germination was better achieved with a 200 ppm gibberellic acid treatment. Clonal propagation proved to be a successful technique for the production of C. album. Treating cuttings with IBA 0.2, w/v, at 20% resulted in the highest rooting percentage, while planting rooted cuttings in a substrate of perlite with vermiculite 1:1 was essential for plant survival. Our results show that both germination pretreatments and cutting propagation are powerful tools for the production of this valuable species. Both methods could be incorporated for population regeneration in natural habitats, and for the potential establishment of the species as a new crop for consumption and pharmacological purposes.

Biogenic volatile organic compound emissions along a high arctic soil moisture gradient.

Emissions of biogenic volatile organic compounds (BVOCs) from terrestrial ecosystems are important for the atmospheric chemistry and the formation of secondary organic aerosols, and may therefore influence the climate. Global warming is predicted to change patterns in precipitation and plant species compositions, especially in arctic regions where the temperature increase will be most pronounced. These changes are potentially highly important for the BVOC emissions but studies investigating the effects are lacking. The aim of this study was to investigate the quality and quantity of BVOC emissions from a high arctic soil moisture gradient extending from dry tundra to a wet fen. Ecosystem BVOC emissions were sampled five times in the July-August period using a push-pull enclosure technique, and BVOCs trapped in absorbent cartridges were analyzed using gas chromatography-mass spectrometry. Plant species compositions were estimated using the point intercept method. In order to take into account important underlying ecosystem processes, gross ecosystem production, ecosystem respiration and net ecosystem production were measured in connection with chamber-based BVOC measurements. Highest emissions of BVOCs were found from vegetation communities dominated by Salix arctica and Cassiope tetragona, which had emission profiles dominated by isoprene and monoterpenes, respectively. These results show that emissions of BVOCs are highly dependent on the plant cover supported by the varying soil moisture, suggesting that high arctic BVOC emissions may affect the climate differently if soil water content and plant cover change.

Factors Affecting Diet Variation in the Pyrenean Rock Ptarmigan (Lagopus muta pyrenaica): Conservation Implications.

The Pyrenean rock ptarmigan (Lagopus muta pyrenaica) lives at one of the southernmost limits of the ptarmigan range. Their small population sizes and the impacts of global changes are limiting factors in the conservation of this threatened subspecies. An effective conservation policy requires precise basic knowledge of a species' food and habitat requirements, information that is practically non-existent for this Pyrenean population. Here, we describe the diet of a ptarmigan population in the Eastern Pyrenees, the environmental factors influencing its variability and the relationship between diet floristic composition and quality. Diet composition was determined by microhistological analysis of faeces and diet quality was estimated from free-urate faecal N content. Our results show that grouse diet is based mainly on arctic-alpine shrubs of the Ericaceae family, as well as dwarf willows (Salix spp.) and Dryas octopetala. The most frequently consumed plant species was Rhododendron ferrugineum, but its abundance in the diet was negatively related to the diet nitrogen content. Conversely, the abundance of Salix spp., grass leaves and arthropods increased the nitrogen content of the diet. Seasonality associated with snow-melting contributed the most to variability in the Pyrenean ptarmigan diet, differentiating winter from spring/summer diets. The latter was characterised by a high consumption of dwarf willows, flowers, arthropods and tender forb leaves. Geographic area and sex-age class influenced diet variability to a lesser extent. Current temperature increases in the Pyrenees due to global warming may reduce the persistence and surface area of snow-packs where preferred plants for rock ptarmigan usually grow, thus reducing food availability. The high consumption of Rh. ferrugineum characterised the diet of the Pyrenean population. Given the toxicity of this plant for most herbivores, its potential negative effect on Pyrenean ptarmigan populations should be evaluated.

Stability and metabolism of Arbutus unedo bioactive compounds (phenolics and antioxidants) under in vitro digestion and colonic fermentation.

The natural antioxidants of Arbutus unedo highlight the importance of this fruit as natural source of bioactive compounds. In the present study, to evaluate the stability of phenolic compounds, ascorbic acid and fat-soluble antioxidants (α-tocopherol, ß-carotene and lutein), in vitro gastrointestinal digestion was applied to A. unedo fruit. After that, the non-absorbable fraction was anaerobically incubated with human faeces and the metabolic pathway for gallotannins, ellagitannins, flavan-3-ols and anthocyanins from A. unedo fruit was proposed. The results showed that the presence of pectin from the fruit hampered the solubilization of the phenolic compounds (with exception of gallic and ellagic acids) and fat-soluble vitamins during gastric digestion. Degradation of pectin-gel during the duodenal digestion favored the release of the phenolic compounds and fat-soluble antioxidants to the media. The catabolic activity of human microbiota led to the generation of a wide range of simple phenols, such as p-hydroxybenzoic acid and catechol, derived from the catabolism of gallotannins, ellagitannins, flavan-3-ols and anthocyanins.

Effects of methanol extracts from roots, leaves, and fruits of the Lebanese strawberry tree (Arbutus andrachne) on cardiac function together with their antioxidant activity.

CONTEXT: Several plant-derived natural products have been used in clinical phase for applications in neurological, cardiovascular, and inflammatory diseases. Arbutus andrachne L. (Ericacea) is an evergreen shrub native to the Mediterranean region. Traditionally, the fruits and leaves of Arbutus tree are well known and used as antiseptics, diuretics, blood tonic, and laxatives. OBJECTIVE: Data regarding the biological effects of compounds derived from the Lebanese Arbutus andrachne are not available. In the present work, we studied the antioxidant activity of methanol extracts of leaves, fruits, and roots of the plant against electrolysis; together with their effects on the cardiodynamics of isolated perfused rabbit hearts. MATERIALS AND METHODS: In vitro electrolysis of the different root, leaves, and fruits methanol extracts was evaluated by the amount of free radicals that has been reduced by increasing the concentration of root extracts ranging from 0.5 to 2 mg after 1, 2, 3, and 4 min. Left ventricular pressure (LVP), heart rate (HR), and coronary flow (CR) were investigated in isolated rabbit heart after administration of 0.5, 1, 2, and 2 mg of each methanol extracts plotted against time (0, 0.5, 1.5, 5, and 10 min), according to the Langendorff method. Lipid peroxidation study was performed by the colorimetric method on myocard tissue after incubation with 500 µl of the different methanol extracts. The amount of MDA was determined at 500 nm absorbance after 5 min incubation. RESULTS: Among the different methanol extracts, the roots showed the highest in vitro antioxidant activity, particularly observed at concentration of 2 mg which completely inhibits free radical generation after 4 min. LVP decreases by 32% at the dose of 2 mg of root extracts after 5 min. No significant effect was observed by the three tested extracts on the heart rate. The three methanol extracts did not show any significant effect on the coronary flow. Moreover, the roots show an increase in the coronary flow at a concentration of 1 and 2 mg/ml during 1 min. Electrolysis on heart tissue treated with the roots extracts shows a decrease in the MDA level from 70.51 ± 6.71 to 48.58 ± 4.15 nmole/g of tissue. DISCUSSION AND CONCLUSION: Methanol extracts of the roots possess antihypertensive effect that may result from its ability to decrease the LVP together with its protective role by inhibiting free radical generation and significantly decreasing the MDA level of heart tissue.

Interactive effects of UV radiation and reduced precipitation on the seasonal leaf phenolic content/composition and the antioxidant activity of naturally growing Arbutus unedo plants.

The effects of UV radiation and rainfall reduction on the seasonal leaf phenolic content/composition and antioxidant activity of the Mediterranean shrub Arbutus unedo were studied. Naturally growing plants of A. unedo were submitted to 97% UV-B reduction (UVA), 95% UV-A+UV-B reduction (UV0) or near-ambient UV levels (UVBA) under two precipitation regimes (natural rainfall or 10-30% rainfall reduction). Total phenol, flavonol and flavanol contents, levels of eight phenols and antioxidant activity [DPPH(●) radical scavenging and Cu (II) reducing capacity] were measured in sun-exposed leaves at the end of four consecutive seasons. Results showed a significant seasonal variation in the leaf content of phenols of A. unedo, with the lowest values found in spring and the highest in autumn and/or winter. Leaf ontogenetic development and/or a possible effect of low temperatures in autumn/winter may account for such findings. Regardless of the watering regime and the sampling date, plant exposure to UV-B radiation decreased the total flavanol content of leaves, while it increased the leaf content in quercitrin (the most abundant quercetin derivative identified). By contrast, UV-A radiation increased the leaf content of theogallin, a gallic acid derivative. Other phenolic compounds (two quercetin derivatives, one of them being avicularin, and one kaempferol derivative, juglanin), as well as the antioxidant activity of the leaves, showed different responses to UV radiation depending on the precipitation regime. Surprisingly, reduced rainfall significantly decreased the total amount of quantified quercetin derivatives as well as the DPPH scavenging activity in A. unedo leaves. To conclude, present findings indicate that leaves of A. unedo can be a good source of antioxidants throughout the year, but especially in autumn and winter.

Low among-provenance differences in structural and functional plasticity in response to nutrients in saplings of the circum-Mediterranean tree Arbutus unedo L.

The Mediterranean region is an area of special interest for conservation where the incidence of multiple drivers of global change is expected to increase. One of the factors predicted to change is soil-nutrient availability, an essential factor for plant growth. Thus, study of the effects of variation in this parameter is especially relevant in species with a circum-Mediterranean distribution, such as Arbutus unedo L., in which the different provenances grow in different habitats, which must differ in nutritional conditions. We aimed to determine the effect of provenance on plasticity, to establish whether structural and morphological traits differ in the level of plasticity and to assess how nutrients affect the photosynthetic light response. In a common garden experiment, we studied seven provenances from the circum-Mediterranean range of A. unedo and established two nutrient treatments (low and high nutrient availability). We measured physiological and structural traits in 1-year-old sapling and determined a phenotypic plasticity index (PPI) to quantify the level of plasticity, whereas the radiation effects were tested by construction and analysis of light response curves. Interestingly, provenance did not explain a significant amount of variance, but the plasticity was four times higher for the structural traits than for the physiological traits. Therefore, the plasticity to nutrient availability will not favour or prevent the expansion or contraction of the range of any of these provenances of A. unedo. Furthermore, the structural plasticity demonstrated the ability of the strawberry tree to optimize resource allocation, whereas the physiology remained stable, thus avoiding extra expenditure. The study findings also suggest that increased availability of nutrients would improve the performance of the species during the Mediterranean summer, characterized by high irradiance. These abilities will be key to the survival of saplings of the species under the future scenario of changes in nutrient availability.

Phenological mismatch with abiotic conditions implications for flowering in Arctic plants.

Although many studies have examined the phenological mismatches between interacting organisms, few have addressed the potential for mismatches between phenology and seasonal weather conditions. In the Arctic, rapid phenological changes in many taxa are occurring in association with earlier snowmelt. The timing of snowmelt is jointly affected by the size of the late winter snowpack and the temperature during the spring thaw. Increased winter snowpack results in delayed snowmelt, whereas higher air temperatures and faster snowmelt advance the timing of snowmelt. Where interannual variation in snowpack is substantial, changes in the timing of snowmelt can be largely uncoupled from changes in air temperature. Using detailed, long-term data on the flowering phenology of four arctic plant species from Zackenberg, Greenland, we investigate whether there is a phenological component to the temperature conditions experienced prior to and during flowering. In particular, we assess the role of timing of flowering in determining pre-flowering exposure to freezing temperatures and to the temperatures-experienced prior to flowering. We then examine the implications of flowering phenology for flower abundance. Earlier snowmelt resulted in greater exposure to freezing conditions, suggesting an increased potential for a mismatch between the timing of flowering and seasonal weather conditions and an increased potential for negative consequences, such as freezing 'damage. We also found a parabolic relationship between the timing of flowering and the temperature experienced during flowering after taking interannual temperature effects into account. If timing of flowering advances to a cooler period of the growing season, this may moderate the effects of a general warming trend across years. Flower abundance was quadratically associated with the timing of flowering, such that both early and late flowering led to lower flower abundance than did intermediate flowering. Our results indicate that shifting the timing of flowering affects the temperature experienced during flower development and flowering beyond that imposed by interannual variations in climate. We also found that phenological timing may affect flower abundance, and hence, fitness. These findings suggest that plant population responses to future climate change will be shaped not only by extrinsic climate forcing, but also by species' phenological responses.

Contrasting impacts of continuous moderate drought and episodic severe droughts on the aboveground-biomass increment and litterfall of three coexisting Mediterranean woody species.

Climate change is predicted to increase the aridity in the Mediterranean Basin and severely affect forest productivity and composition. The responses of forests to different timescales of drought, however, are still poorly understood because extreme and persistent moderate droughts can produce nonlinear responses in plants. We conducted a rainfall-manipulation experiment in a Mediterranean forest dominated by Quercus ilex, Phillyrea latifolia, and Arbutus unedo in the Prades Mountains in southern Catalonia from 1999 to 2014. The experimental drought significantly decreased forest aboveground-biomass increment (ABI), tended to increase the litterfall, and decreased aboveground net primary production throughout the 15 years of the study. The responses to the experimental drought were highly species-specific. A. unedo suffered a significant reduction in ABI, Q. ilex experienced a decrease during the early experiment (1999-2003) and in the extreme droughts of 2005-2006 and 2011-2012, and P. latifolia was unaffected by the treatment. The drought treatment significantly increased branch litterfall, especially in the extremely dry year of 2011, and also increased overall leaf litterfall. The drought treatment reduced the fruit production of Q. ilex, which affected seedling recruitment. The ABIs of all species were highly correlated with SPEI in early spring, whereas the branch litterfalls were better correlated with summer SPEIs and the leaf and fruit litterfalls were better correlated with autumn SPEIs. These species-specific responses indicated that the dominant species (Q. ilex) could be partially replaced by the drought-resistant species (P. latifolia). However, the results of this long-term study also suggest that the effect of drought treatment has been dampened over time, probably due to a combination of demographic compensation, morphological and physiological acclimation, and epigenetic changes. However, the structure of community (e.g., species composition, dominance, and stand density) may be reordered when a certain drought threshold is reached.

Contrasting growth responses of dominant peatland plants to warming and vegetation composition.

There is growing recognition that changes in vegetation composition can strongly influence peatland carbon cycling, with potential feedbacks to future climate. Nevertheless, despite accelerated climate and vegetation change in this ecosystem, the growth responses of peatland plant species to combined warming and vegetation change are unknown. Here, we used a field warming and vegetation removal experiment to test the hypothesis that dominant species from the three plant functional types present (dwarf-shrubs: Calluna vulgaris; graminoids: Eriophorum vaginatum; bryophytes: Sphagnum capillifolium) contrast in their growth responses to warming and the presence or absence of other plant functional types. Warming was accomplished using open top chambers, which raised air temperature by approximately 0.35 °C, and we measured air and soil microclimate as potential mechanisms through which both experimental factors could influence growth. We found that only Calluna growth increased with experimental warming (by 20%), whereas the presence of dwarf-shrubs and bryophytes increased growth of Sphagnum (46%) and Eriophorum (20%), respectively. Sphagnum growth was also negatively related to soil temperature, which was lower when dwarf-shrubs were present. Dwarf-shrubs may therefore promote Sphagnum growth by cooling the peat surface. Conversely, the effect of bryophyte presence on Eriophorum growth was not related to any change in microclimate, suggesting other factors play a role. In conclusion, our findings reveal contrasting abiotic and biotic controls over dominant peatland plant growth, suggesting that community composition and carbon cycling could be modified by simultaneous climate and vegetation change.

Partial mycoheterotrophy in Pyroleae: nitrogen and carbon stable isotope signatures during development from seedling to adult.

Mycoheterotrophic plants (MHP) are divided into non-photosynthesizing full MHP and green-leaved partial or initial MHP. We investigated (13)C and (15)N isotope enrichment in five putatively partial MHP species in the tribe Pyroleae (Ericaceae): Chimaphila umbellata, Moneses uniflora, Orthilia secunda, Pyrola chlorantha and Pyrola minor, sampled from forest sites on Öland, Sweden. For M. uniflora and P. chlorantha, we investigated isotope signatures of subterranean seedlings (which are mycoheterotrophic), to examine how the use of seedlings instead of full MHP species (Hypopitys monotropa) as reference species affects the assessment of partial mycoheterotrophy. Our main findings were as follows: (1) All investigated Pyroleae species were enriched in (15)N compared to autotrophic reference plants. (2) significant fungal-derived C among the Pyroleae species was found for O. secunda and P. chlorantha. For the remaining species of C. umbellata, M. uniflora and P. minor, isotope signatures suggested adult autotrophy. (3) C and N gains, calculated using seedlings as a full MHP reference, yielded qualitatively similar results as when using H. monotropa as a reference. However, the estimated differences in C and N gains became larger when using seedlings as an MHP reference. (4) A previously unknown interspecific variation in isotope signature occurs during early ontogeny, from seed production to developing seedlings. Our findings suggest that there is a variation among Pyroleae species concerning partial mycoheterotrophy in adults. Adult autotrophy may be most common in Pyroleae species, and these species may not be as dependent on fungal-derived nutrients as some green orchids.

Rapid carbon turnover beneath shrub and tree vegetation is associated with low soil carbon stocks at a subarctic treeline.

Climate warming at high northern latitudes has caused substantial increases in plant productivity of tundra vegetation and an expansion of the range of deciduous shrub species. However significant the increase in carbon (C) contained within above-ground shrub biomass, it is modest in comparison with the amount of C stored in the soil in tundra ecosystems. Here, we use a 'space-for-time' approach to test the hypothesis that a shift from lower-productivity tundra heath to higher-productivity deciduous shrub vegetation in the sub-Arctic may lead to a loss of soil C that out-weighs the increase in above-ground shrub biomass. We further hypothesize that a shift from ericoid to ectomycorrhizal systems coincident with this vegetation change provides a mechanism for the loss of soil C. We sampled soil C stocks, soil surface CO2 flux rates and fungal growth rates along replicated natural transitions from birch forest (Betula pubescens), through deciduous shrub tundra (Betula nana) to tundra heaths (Empetrum nigrum) near Abisko, Swedish Lapland. We demonstrate that organic horizon soil organic C (SOCorg ) is significantly lower at shrub (2.98 ± 0.48 kg m(-2) ) and forest (2.04 ± 0.25 kg m(-2) ) plots than at heath plots (7.03 ± 0.79 kg m(-2) ). Shrub vegetation had the highest respiration rates, suggesting that despite higher rates of C assimilation, C turnover was also very high and less C is sequestered in the ecosystem. Growth rates of fungal hyphae increased across the transition from heath to shrub, suggesting that the action of ectomycorrhizal symbionts in the scavenging of organically bound nutrients is an important pathway by which soil C is made available to microbial degradation. The expansion of deciduous shrubs onto potentially vulnerable arctic soils with large stores of C could therefore represent a significant positive feedback to the climate system.