Species-specific drought effects on fl ower and fruit production in a Mediterranean holm oak forest

7 páginas, 3 figuras, 1 tablaA holm oak forest was exposed to an experimental drought (reduction of 15 per cent soil moisture as predicted for this area for the next decades by General Circulation Models and ecophysiological models) during 7 years to elucidate the reproductive responses of the dominant species Quercus ilex L., Arbutus unedo L. and Phillyrea latifolia L. Soil moisture was partially reduced by plastic strips intercepting rainfall and by ditch exclusion of water runoff. During the period studied, meteorological conditions and soil moisture were continuously monitored, together with fl ower and fruit production in the three dominant species. In Q. ilex and A. unedo, fl ower and specially fruit production were strongly correlated with annual rainfall, but not in P. latifolia. The experimental drought reduced fl ower and fruit production in Q. ilex by 30 per cent and 45 per cent, respectively. Reductions in fl ower and fruit production were not signifi cant in A. unedo and were not observed in P. latifolia. A decrease in production of reproductive structures and the different response of the species studied to a decrease in water availability could induce important changes in the competitive ability of the different species and in the long term in the community species composition and future distribution of these Mediterranean species.This research was fi nancially supported by CGL 2004-01402/BOS and GCL 2006-04025/BOS grants from the Spanish Government, a Fundación Banco Bilbao Vizcaya 2004 grant, a Catalan Government SGR2005-00312 grant and the European project Assesing Large scale Risks for biodiversity with tested Methods (contract 506675, European Union sixth framework programme).Peer reviewe

Tree growth, mortality, and above-ground biomass accumulation in a holm oak forest under a five-year experimental field drought

9 páginas. 7 figurasA holm oak forest was exposed to an experimental drought during 5 years to elucidate the growth responses of the dominant species Quercus ilex, Arbutus unedo and Phillyrea latifolia. Soil water availability was partially reduced, about 15% as predicted for this area for the next decades by GCM and ecophysiological models, by plastic strips intercepting rainfall and by ditch exclusion of water runoff. The stem diameter increment was highly correlated with annual rainfall in all species, and drought treatment strongly reduced the diameter increment of Q. ilex (41%) and specially of A. unedo (63%), the species showing higher growth rates. Stem mortality rates were highly correlated with previous stem density, but drought treatment increased mortality rates in all species. Q. ilex showed the highest mortality rates (9% and 18% in control and drought plots, respectively), and P. latifolia experienced the lowest mortality rates (1% and 3% in control and drought plots, respectively). Drought strongly reduced the increment of live aboveground biomass during these 5 years (83%). A. unedo and Q. ilex experienced a high reduction in biomass increment by drought, whereas P. latifolia biomass increment was insensitive to drought. The different sensitivity to drought of the dominant species of the holm oak forest may be very important determining their future development and distribution in a drier environment as expected in Mediterranean areas for the next decades. These drier conditions could thus have strong effects on structure (species composition) and functioning (carbon uptake and biomass accumulation) of these Mediterranean forests.This research was financially supported by MEC projects CLI97-0344, REN2003-04871, and CGL 2004-01402/BOS from the Spanish Government, and by the European project ALARM (Contract 506675, EU sixth framework programme).Peer reviewe

Leaf mass per area ratio in Quercus ilex leaves under a wide range of climatic conditions. The importance of low temperatures

6 páginas, 4 figuras, 1 tabla.The Digital Climatic Atlas and the Ecological and the Forestry Inventory of Catalonia (NE Spain) were analysed to study the climate effect on leaf mass per area (LMA) and leaf area index (LAI) on Quercus ilex L., one of the most widely spread tree species in the Mediterranean region. 195 sites in this region of 31,895 km2 were considered. The relationship between climatic variables (total annual rainfall, mean annual temperature, mean minimum winter temperatures, and mean annual solar radiation) and LMA and LAI were analysed by simple and multiple regressions. LMA was higher in the drier sites and specially in the colder sites. There was also a significant correlation between solar radiation and LMA. On the contrary, LAI values, which were negatively correlated with LMA values, were lower in drier and colder sites, and were not significantly affected by solar radiation. The results highlight that high LMA values do not seem to be a specific protection to dry conditions but to a wide range of environmental stress factors, including low temperatures.This research was financially supported by MEC projects REN2003- 04871, and CGL2004-01402/BOS from the Spanish Government, by the European project ALARM (Contract 506675, EU sixth framework programme) and by a Fundacio´n BBVA 2004 and a SGR2005-00312 AGAUR-Generalitat de Catalunya grants.Peer reviewe

Seasonal ultrasonic acoustic emissions of Quercus ilex L. trees in a Mediterranean forest

4 páginas, 2 figuras.Ultrasonic acoustic emissions were measured in Quercus ilex trees of a Mediterranean forest in Catalonia (NE Spain) each season from summer of 2004 to autumn of 2005. Acoustic emissions were maximum during hot and dry summer periods. Acoustic emissions started below 17% soil moisture, 0.85 RWC, and 2.5 MPa leaf water potential. They were negatively correlated with soil moisture and leaf water potential. The relationship between acoustic emissions and leaf water potential was the strongest, indicating that xylem tension is the most important factor inducing both cavitation (acoustic emissions) and a decrease in leaf water potential. Future increase of xylem cavitation derived from climate change may result in growth and survival limitations for this species in the drier southern limits of its current distribution.This research was financially supported by MEC REN2003-04871 and CGL 2004-01402/BOS grants from the Spanish Government, a Fundacio´n BBVA 2004 grant, a Catalan Government SGR2005-00312 grant, and the European project ALARM (Contract 506675, EU sixth framework programme).Peer reviewe

Changes in leaf d 13 C and d 15 N for three Mediterranean tree species in relation to soil water availability

8 páginas, 5 figuras.A rain exclusion experiment simulating drought conditions expected in Mediterranean areas for the following decades (15% decrease in soil moisture) was conducted in a Mediterranean holm oak forest to study the response of leaf d 13 C, d 15 N, and N concentrations to the predicted climatic changes for the coming decades. Plant material was sampled in 2000, 2003, 2004, and 2005 in eight plots: four of them were control plots and the other four plots received the rain exclusion treatment. Although there was a negative relationship between d 13 C and soil moisture, for each species and year, the rain exclusion treatment did not have any significant effect on d 13 C, and therefore on the intrinsic water use efficiency (iWUE) of the three dominant species: Phillyrea latifolia, Arbutus unedo, and Quercus ilex. On the other hand, rain exclusion clearly increased the d 15 N values in the three species studied, probably indicating higher N losses at the soil level leading to a 15 N enrichment of the available N. It suggested that rain exclusion exerted a greater effect on the nitrogen biogeochemical cycle than on the carbon assimilation process. d 15 N values were inversely correlated with summer soil moisture in Q. ilex and A. unedo, but no relationship was observed in P. latifolia. This latter species showed the lowest iWUE values, but it was the only species with no decrease in annual basal increment in response to the rain exclusion treatment, and it also had the highest resistance to the hot and dry conditions projected for the Mediterranean basin in the coming decades. The different strategies to resist rain exclusion conditions of these species could induce changes in their competitive ability and future distribution. The losses of N from the ecosystem may further limit plant growth and ecosystem functioning.This research was financially supported by MEC CGL 2004-01402/BOS, and CGL 2006-04025/BOS grants from the Spanish Government, a Catalan Government SGR2005-00312 grant, and the European projects ALARM (Contract GOTCECT-506675) and NITROEUROPE (Contract GOCE 017841).Peer reviewe

Drought’s impact on Ca, Fe, Mg, Mo and S concentration and accumulation patterns in the plants and soil of a Mediterranean evergreen Quercus ilex forest.

21 páginas, 5 tablas, 10 figuras.We conducted a 6-year field manipulation drought experiment in an evergreen Quercus ilex forest where we simulated the drought predicted by GCM and ecophysiological models for the coming decades (an average of 15% soil moisture reduction). We thereby tested the hypothesis that enhanced drought will change Ca, Fe, Mg, Mo and S availability, concentrations and accumulation patterns in Mediterranean ecosystems. The strongest effects of drought occurred in the soil. Drought increased the total soil concentrations of S, the soil extract concentrations of Fe, Mg and S, the Mg saturation in the soil exchangeable complex and tended to increase the percentage base saturation of the soil exchangeable complex. These increased soil concentrations were related to a decrease of plant uptake capacity and not to an increase of soil enzyme activity, which in fact decreased under drier conditions. Drought increased leaf Mg concentrations in the three dominant species although only significantly in Quercus ilex and Arbutus unedo (20 and 14%, respectively). In contrast, drought tended to decrease Ca in Phillyrea latifolia (18%) and Ca and Fe concentrations in the wood of all three species. Drought increased Ca and Fe concentrations in the roots of Quercus ilex (26 and 127%). There was a slight general trend to decrease total biomass accumulation of nutrients that depend on water flux such as Mg, Fe and S. This effect was related to a decrease of soil moisture that reduced soil flow, and to a decrease in photosynthetic capacity, sap flow, transpiration and growth, and therefore plant uptake capacity under drought observed in Quercus ilex and Arbutus unedo. On the contrary, drought increased Mo accumulation in aboveground biomass in Phillyrea latifolia and reduced Mo accumulation in Arbutus unedo by reducing growth and wood Mo concentrations (51%). Phillyrea latifolia showed a great capacity to adapt to drier conditions, with no decrease in growth, an increase of Mo uptake capacity and a decrease in leaf Ca concentration, which was related to a decrease in transpiration under drought. The results indicate asymmetrical changes in species capacity to accumulate these elements, which are likely to produce changes in inter-specific competitive relations among dominant plant species and in their nutritional quality as food sources. The results also indicate that drought tended to decrease nutrient content in aboveground biomass, mainly through the decrease in growth and transpiration of the most sensitive species and caused an increase in the availability of these nutrients in soil. Thus, drought decreased the ecosystem’s capacity to retain Mg, Fe and S, facilitating their loss in torrential rainfalls.This research was supported by Spanish Government CGL2004-01402/BOS and CGL2006-04025/BOS grants, the EC Integrated FP6 ALARM (GOCE-CT-2003- 506675), FP6 NEU NITROEUROPE (GOCE017841) Projects, and a Fundación BBVA 2004 and a Catalan government SGR2005-00312 grant.Peer reviewe

Density and length of stomatal amd epidermal cells in “living fossil” trees grown under elevated CO2 and a polar light regime.

5 páginas, 2 figuras.During the Cretaceous and early Tertiary, when the climate was warm and the atmospheric CO2 concentration ([CO2]) was at least double that of the present-day, polar forests populated high latitude landmasses. We investigated the density and length of stomata and other epidermal cells of two deciduous and three evergreen "living fossil" tree species representative of these ancient forests. These tree species were grown in a simulated Cretaceous high latitude environment at either ambient (400 ppmv) or elevated (800 ppmv) [CO2] during four years. After 4 years growing at elevated [CO2], the leaf stomatal density and index (percentage of leaf epidermal cells that are stomata) of these plants were similar to those of their counterparts growing at ambient [CO2]. While the CO2 enrichment only modified the stomatal pore length in two of the five studied species, it increased significantly the overall length of the epidermal cells of all the species, reducing their density. These results revealed that leaf epidermal cells of these "living fossil" species were more sensitive than stomata to an experimental doubling of atmospheric CO2 concentration.and David J. Beerling for allowing us to participate in the experiment. This study was supported by a European Union Marie Curie postdoctoral fellowship to L.L. (contract N EVK2eCTe2002e50022), by the Spanish Government CGL2006-4025/BOS, CGL2007-64583/BOS and Consolider-Ingenio MONTES CSD2008-00040 projects, the Catalan government SGR2009-458 project, and the Spanish National Research Council CSIC-PIF08-006-3 project.Peer reviewe

Increased water-use efficiency during the 20th century did not translate into enhanced tree growth.

12 páginas, 3 figuras, 1 tabla.The goals of this study are: (1) to determine whether increasing atmospheric CO2 concentrations and changing climate increased intrinsic water use efficiency (iWUE, as detected by changes in D 13 C) over the last four decades; and if it did increase iWUE, whether it led to increased tree growth (as measured by tree-ring growth); (2) to assess whether CO2 responses are biome dependent due to different environmental conditions, including availability of nutrients and water; and (3) to discuss how the findings of this study can better inform assumptions of CO2 fertilization and climate change effects in biospheric and climate models.This research was supported by Spanish Government CGL2006- 04025/BOS and Consolider-Ingenio Montes CSD2008-00040 grants, and a Catalan Government SGR2009-458 grant. The work is a contribution to the research agenda of the Global Carbon Project of the Earth System Science Partnership. J.G.C. acknowledges the support of the Australian Climate Change Science Program.Peer reviewe

Experimental drought reduced acid and alkaline phosphatase activity and increased organic extractable P in soil in a Quercus ilex Mediterranean forest.

12 páginas, 6 figuras, 3 tablas.A six-year (1999–2005) experiment of drought manipulation was conducted in aQuercusilexMediterraneanforest (Southern Catalonia) to simulate predicted climatic conditions projected for the decades to come. The aim was to investigate the direct and indirect effects of drought conditions on acid and alkalinephosphataseactivity in soil and on P concentrations in soil, leaves and litter throughout the year. Soilacidphosphataseactivity was higher than soilalkalinephosphataseactivity. Droughtreducedacidphosphataseactivity in soil in all seasons, including summer and winter, the seasons with less biological activity due to water and cold stress. Reductions of soil water content between 13 and 29% reducedsoilacidphosphataseactivity between 22 and 27% depending on the season. Droughtreducedalkalinephosphataseactivity (by 28%) only in winter. Soilacid and alkalinephosphataseactivities were positively correlated with soil water content in all seasons. In contrast short-term available-P which increased under drought in several seasons was weakly correlated with soilphosphataseactivities. As a result, immediately/short-term available-P concentration ratios decreased in all the seasons (between 10 and 71%). Droughtincreased foliar P concentration and reduced the C/P concentration ratio in litter fall of the dominant tree Q. ilex. Drought also decreased the ratio between organic C and short-term available-P in soil. The results show that soilphosphataseactivity is more directly dependent on changes in water availability than on changes in its substrate, short-term available-P. These effects of drought have several implications: the accumulation in the soil of labile P not directly available to plants, the increase in potential P losses from leaching and erosion during the torrential rainfalls typical of the Mediterranean climate, and changes in plant, litter and soil C:P stoichiometry that may lead to changes in soil trophic chains.This research was supported by the Spanish Government CGL2004-01402/BOS and CGL2006-04025/BOS grants, the EC Integrated project FP6 ALARM (GOCE-CT-2003-506675), a Fundación BBVA 2004 and a Catalan government SGR2005-00312 grant.Peer reviewe

Dynamics of non-structural carbohydrates in three Mediterranean woody species following long-term experimental drought

Stored non-structural carbohydrates (NSC) have been proposed as a key determinant of drought resistance in plants. However, the evidence for this role is controversial, as it comes mostly from observational, short-term studies. Here, we take advantage of a long-term experimental throughfall reduction to elucidate the response of NSC to increased drought 14 years after the beginning of the treatment in three Mediterranean resprouter trees (Quercus ilex L., Arbutus unedo L. and Phillyrea latifolia L.). In addition, we selected 20 Q. ilex individuals outside the experimental plots to directly assess the relationship between defoliation and NSC at the individual level. We measured the seasonal course of NSC concentrations in leaves, branches and lignotuber in late winter, late spring, summer, and autumn 2012. Total concentrations of NSC were highest in the lignotuber for all species. In the long-term drought experiment we found significant depletion in concentrations of total NSC in treatment plots only in the lignotuber of A. unedo. At the same time, A. unedo was the only species showing a significant reduction in BAI under the drought treatment during the 14 years of the experiment. By contrast, Q. ilex just reduced stem growth only during the first 4 years of treatment and P. latifolia remained unaffected over the whole study period. However, we found a clear association between the concentrations of NSC and defoliation in Q. ilex individuals sampled outside the experimental plots, with lower total concentrations of NSC and lower proportion of starch in defoliated individuals. Taken together, our results suggest that stabilizing processes, probably at the stand level, may have been operating in the long-term to mitigate any impact of drought on NSC levels, and highlight the necessity to incorporate long-term experimental studies of plant responses to drought.This study was supported by the Spanish government through grants CGL2010-16373 and CGL2010-17172, Consolider Ingenio MONTES (CSD2008-00040), and by the Catalan government through grant SGR2009-458.Peer reviewedPeer Reviewe