59 research outputs found

    Relative contribution of groundwater to plant transpiration estimated with stable isotopes

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    Water stored underground in the saturated and subsurface zones below the soil are important sources of water for plants in water-limited ecosystems. The presence of deep-rooted plants worldwide, however, suggests that the use of groundwater is not restricted to arid and seasonally dry ecosystems. We compiled the available data (71 species) on the relative contribution of groundwater to plant water estimated using stable isotopes and mixing models, which provided information about relative groundwater use, and analyzed their variation across different climates, seasons, plant types, edaphic conditions, and landscape positions. Plant use of groundwater was more likely at sites with a pronounced dry season, and represented on average 49 per cent of transpired water in dry seasons and 28 per cent in wet seasons. The relative contribution of groundwater to plant-water uptake was higher on rocky substrates (saprolite, fractured bedrock), which had reduced groundwater uptake when this source was deep belowground. In addition, we found that the connectivity between groundwater pools and plant water may be quantitatively larger and more widespread than reported by recent global estimations based on isotopic averaged values. Earth System Models should account for the feedbacks between transpiration and groundwater recharge

    Sequence of plant responses to droughts of different timescales : lessons from holm oak (Quercus ilex) forests

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    imbalance-P paper conctact with Adrià Barbeta: [email protected] functional traits of plants in regions of the world with a Mediterranean climate have been shaped to tolerate periods of water deficit. These species are adapted to summer droughts but may not be able to cope with future increases in drought intensity, duration, and/or frequency. Here, we review the mechanisms and traits of drought resistance and recovery of the well-studied holm oak (Quercus ilex), which we propose as a model species for Mediterranean-type ecosystems. Our aim was to understand the differences and links between the responses of Q. ilex to summer droughts, extreme droughts, and long-term drought experiments. A main goal was to provide an integral picture of drought responses across organisational and temporal scales for identifying the most relevant processes that are likely to contribute to determining the future of Mediterranean vegetation. Evidence from long-term drought experiments showed that acclimation processes from the molecular (e.g. epigenetic changes) to the ecosystem level (e.g. reductions in stand density) mitigate the effects of drought. Changes in leaf morphology and hydraulics, leaf-to-shoot allometry, and root functioning are among the key mechanisms for overcoming increasing drought. The duration of drought determines its severity in terms of canopy loss and stem mortality. Although Q. ilex can vigorously resprout after such episodes, its resilience may be subsequently reduced. In the future, higher frequency of return of extreme droughts will challenge thus the capacity of these forests to recover. The insights provided by this review of the complex interplay of processes that determine the response of trees to droughts of different duration, intensity, and frequency will also help us to understand the likely responses of other resprouting angiosperms in seasonally dry ecosystems that share similar functional traits with Q. ilex

    Stem mortality and forest Dieback in a 20-years experimental Drought in a Mediterranean holm oak forest

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    Climatic warming is predicted to increase the intensity and duration of extreme weather such as droughts and heat waves. Climate change could therefore increase stem mortality and forest dieback in many ecosystems around the world, especially in semi-arid forests. We investigated the influence of climatic conditions and a throughfall displacement experimental treatment (15% decrease in the amount of soil moisture) on the intensity of stem mortality in a Mediterranean forest. We also investigated the use of remotely sensed data from MODIS as a tool to estimate the intensity of stem mortality and a possible strategy of forest management to mitigate this forest dieback induced by climate change. Stem mortality was higher when mean annual temperature was higher and rainfall was lower, especially during spring and summer. Mortality was higher and more affected by the drought treatment in the holm oak, Quercus ilex, than in co-occurring species of tall shrubs better adapted to drier conditions. Two spectral MODIS indices (NDVI and EVI) were good predictors of stem mortality, but others (GPP, PsnNet, and NPP specifically calculated to predict forest health and development) were not clearly correlated with stem mortality. Selective stem thinning strongly reduced stem mortality (especially in Q. ilex) by buffering the effects of climate change on forest structure. Finally, the possible future substitution of the current dominant species of this forest, Q. ilex, by species of tall shrubs better adapted to drought is also discusse

    Efectes de l'estassada en un alzinar amb baixa disponibilitat d'aigua

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    El document és una contribució a les "V Jornades sobre el Bosc de Poblet i les Muntanyes de Prades" que van tenir lloc el 16 i 17 de novembre de 2018 al Museu de la Vida Rural (l'Espluga de Francolí, Tarragona)". Podeu consultar-ne les actes, publicades sota una llicència CC BY-NC-ND, a http://parcsnaturals.gencat.cat/web/.content/home/poblet/coneix-nos/centre_de_documentacio/fons_documental/5a_jornades/V-Jornades_Poblet_Prades.pdfEn un alzinar natural s'ha realitzat una estassada selectiva per a estudiar el seu ús com a eina per a reduir la mortalitat d'arbres, l'augment de la productivitat forestal, i invertir la regressió recent que està patint l'espècie dominant (l'alzina, Quercus ilex) degut al canvi climàtic. Durant els 4 anys estudiats, l'estassada ha comportat un augment de la productivitat i una disminució de la mortalitat de troncs en aquest alzinar, contribuint al manteniment del funcionament de l'ecosistema sota les condicions climàtiques canviants. Q. ilex ha estat l'única espècie que ha experimentat un major creixement dels troncs, independentment del seu tamany, i una disminució de la mortalitat degut a l'estassada selectiva. En canvi, el creixement i la mortalitat de troncs no ha variat significativament en les altres espècies arbustives, més resistents a la sequera que Q. ilex. Els nostres resultats posen de manifest la utilitat de l'estassada selectiva com a eina per a augmentar la productivitat forestal i contribuir a la conservació d'aquest bosc mediterrani. Altres beneficis de l'aclarida selectiva, com la disminució del risc d'incendi forestal i l'increment de la disponibilitat d'aigua per a la població, també han estat discutits en aquest trebal

    Effects of thinning in a water-limited holm oak forest

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    A natural holm oak forest was selectively thinned to test thinning as a tool to reduce tree mortality, increase productivity, and reverse the recent regression of the dominant species (Quercus ilex) induced by climate change. Thinning increased aboveground productivity and reduced stem mortality in this Mediterranean forest during four years just after thinning, contributing to the maintenance of forest functioning under changing climatic conditions. Q. ilex was the only species positively affected by the thinning: stem growth increased for all stem sizes, and mortality was significantly lower in thinned plots. On the contrary, mortality rates of Phillyrea latifolia and Arbutus unedo were not significantly lower. Stem growth increased for P. latifolia only in the smallest stem-size class. Our results highlight the suitability of selective thinning for improving the forest productivity and ensuring the conservation of Mediterranean coppices. Other benefits of selective thinning, such as a decrease in the risk of fire dispersion and an increase in the water supply for human populations, are also discussed

    Long-term experimental drought combined with natural extremes accelerate vegetation shift in a Mediterranean holm oak forest

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    Increasing drought combined with natural extremes are expected to accelerate forest die-off and shifts in vegetation in the Mediterranean Basin. However, fewer studies have explored these climate-driven changes in forest ecosystems. A long-term (17-year) experimental drought (−30% precipitation) was established in a Mediterranean holm oak forest with high (H) and low (L) canopies to determine the changes in stem mortality, recruitment and composition shifts. Experimental drought increased annual stem mortality rate at the community level for both H- and L-canopies. Natural drought amplified the effects of experimental drought on stem mortality at the community level and of Q. ilex for H- and L-canopies. The timescales of natural drought, however, varied substantially with canopy types and species, with shorter timescales in L- than H-canopy and for Q. ilex than P. latifolia. Furthermore, experimental drought combined with natural extremes amplified the increases in stem mortality and decreases in growth for L-canopy. Contrasting responses between Q. ilex and P. latifolia for the relative in abundance and growth were observed in L-canopy and drought treatment reinforced the vegetation shift favoring P. latifolia. These findings suggest continuous drought regimes accelerated a vegetation shift, implying potential consequences for the functions and services for water-limited forest ecosystems

    Balance between carbon gain and loss under long-term drought: impacts on foliar respiration and photosynthesis in Quercus ilex L

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    Terrestrial carbon exchange is a key process of the global carbon cycle consisting of a delicate balance between photosynthetic carbon uptake and respiratory release. We have, however, a limited understanding how long-term decreases in precipitation induced by climate change affect the boundaries and mechanisms of photosynthesis and respiration. We examined the seasonality of photosynthetic and respiratory traits and evaluated the adaptive mechanism of the foliar carbon balance of Quercus ilex L. experiencing a long-term rainfall-exclusion experiment. Day respiration (R d) but not night respiration (R n) was generally higher in the drought treatment leading to an increased R d/R n ratio. The limitation of mesophyll conductance (g m) on photosynthesis was generally stronger than stomatal limitation (g s) in the drought treatment, reflected in a lower g m/g s ratio. The peak photosynthetic activity in the drought treatment occurred in an atypical favourable summer in parallel with lower R d/R n and higher g m/g s ratios. The plant carbon balance was thus strongly improved through: (i) higher photosynthetic rates induced by g m; and (ii) decreased carbon losses mediated by R d. Interestingly, photosynthetic potentials (V c,max, J max, and TPU) were not affected by the drought treatment, suggesting a dampening effect on the biochemical level in the long term. In summary, the trees experiencing a 14-year-long drought treatment adapted through higher plasticity in photosynthetic and respiratory traits, so that eventually the atypical favourable growth period was exploited more efficiently

    Mortalitat de troncs i decaïment forestal a l'alzinar mediterrani

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    El bosc mediterrani està sotmès a unes condicions climàtiques cada cop més càlides i amb una disponibilitat d'aigua més escassa. L'alzina pot ser substituïda per altres espècies arbustives més resistents a la sequera. Tanmateix, una aclarida selectiva de troncs realitzada a l'alzinar de Prades ens mostra com aquesta gestió permet reduir la mortalitat d'arbres i preservar l'actual estructura de l'alzinar i els seus serveis ecosistèmics. La Unitat d'Ecologia Global del CREAF s'ha centrat en dos tractaments experimentals de reducció d'aigua i disminució d'arbres per estudiar-ne les repercussions i per gestionar pròxims escenaris ecosistèmics.El bosque mediterráneo está sometido a unas condiciones climáticas cada vez más cálidas y con una disponibilidad de agua más escasa. La encina puede ser substituida por otras especies arbustivas más resistentes a la sequía. No obstante, un aclareo selectivo de troncos realizado en el encinar de Prades nos muestra como esta gestión permite reducir la mortalidad de árboles y preservar la actual estructura del encinar y sus servicios ecosistémicos. La Unidad de Ecología Global del CREAF se ha centrado en dos tratamientos experimentales de reducción de agua y disminución de árboles para analizar las repercusiones y para gestionar próximos escenarios ecosistémicos.Mediterranean forest is progressively submitted to drier climatic conditions accompanied with a decrease in water availability. Holm oak could be substituted by other shrub species more resistant to drought. However, a selective stem thinning conducted in Prades holm oak forest shows as a management to decrease tree mortality and for the preservation of holm oak structure and their ecosystem services. The Global Ecology Unit of CREAF has focused on two experimental treatments for tree and water reduction to see their impact and to manage upcoming ecosystem scenarios

    Contrasting trait syndromes in angiosperms and conifers are associated with different responses of tree growth to temperature on a large scale

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    Recent large-scale studies of tree growth in the Iberian Peninsula reported contrasting positive and negative effects of temperature in Mediterranean angiosperms and conifers. Here we review the different hypotheses that may explain these trends and propose that the observed contrasting responses of tree growth to temperature in this region could be associated with a continuum of trait differences between angiosperms and conifers. Angiosperm and conifer trees differ in the effects of phenology in their productivity, in their growth allometry, and in their sensitivity to competition. Moreover, angiosperms and conifers significantly differ in hydraulic safety margins, sensitivity of stomatal conductance to vapor-pressure deficit (VPD), xylem recovery capacity or the rate of carbon transfer. These differences could be explained by key features of the xylem such as non-structural carbohydrate content (NSC), wood parenchymal fraction or wood capacitance. We suggest that the reviewed trait differences define two contrasting ecophysiological strategies that may determine qualitatively different growth responses to increased temperature and drought. Improved reciprocal common garden experiments along altitudinal or latitudinal gradients would be key to quantify the relative importance of the different hypotheses reviewed. Finally, we show that warming impacts in this area occur in an ecological context characterized by the advance of forest succession and increased dominance of angiosperm trees over extensive areas. In this context, we examined the empirical relationships between the responses of tree growth to temperature and hydraulic safety margins in angiosperm and coniferous trees. Our findings suggest a future scenario in Mediterranean forests characterized by contrasting demographic responses in conifer and angiosperm trees to both temperature and forest succession, with increased dominance of angiosperm trees, and particularly negative impacts in pines

    Satellite data as indicators of tree biomass growth and forest dieback in a Mediterranean holm oak forest

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    Context : In the framework of climate change, decreased tree growth and enhanced mortality induced by hot and dry conditions are increasing in many forests around the world, and particularly in Mediterranean forests.- Aims : Our aim was to estimate tree growth and mortality in a Mediterranean holm oak forest, using remote sensing data from MODIS. - Methods: We monitored annual increases of aboveground biomass by measuring tree basal area, and we determined tree mortality by counting dead stems. We analyzed the relationships between forest growth and mortality with mean annual values of some MODIS products and meteorological data. - Results: Mortality and increases of aboveground biomass correlated well with precipitation, September standardized precipitation/evapotranspiration indices (SPEI), and some MODIS products such as NDVI and enhanced vegetation index EVI. Other MODIS products such as gross primary production (GPP) and net photosynthesis, however, showed no clear relationship with tree mortality or measured increases of biomass. - Conclusion: The MODIS products as proxies of ecosystemic productivity (gross primary productivity, net photosynthesis) were weakly correlated with biomass increase, and did not reflect the mortality following the drought of autumn 2011. Nevertheless, NDVI and EVI were efficient indicators of forest productivity and diebac
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