Water resource partitioning,stem xylem hydraulic properties,and plant water use strategies in a seasonally dry riparian tropical rainforest

This study investigated seasonal variation in the origin of water used by plants in a riparian tropical rainforest community and explored linkages between plant water source, plant xylem hydraulic conductivity and response to the onset of dry conditions. The study focused on five co-dominant canopy species, comprising three tree species (Doryphora aromatica, Argyrodendron trifoliolatum, Castanospora alphandii) and two climbing palms (Calamus australis and Calamus caryotoides). Stable isotope ratios of oxygen in water (¹⁸O) from soil, groundwater, stream water and plant xylem measured in the wet season and the subsequent dry season revealed water resource partitioning between species in the dry season. Measurement of stem-area-specific hydraulic conductivity (K_S) in the wet season and subsequent dry season showed a significant dry-season loss of K_S in three of the five species (Castanospora alphandii, Calamus australis and C. caryotoides) and a decrease in mean K_S for all species. This loss of hydraulic conductivity was positively correlated with the difference between wet-season and dry-season midday leaf water potentials and with leaf carbon isotope discrimination, indicating that plants that were less susceptible to loss of conductivity had greater control over transpiration rate and were more water-use efficient.     

Carbon isotope discrimination,water-use efficiency,growth, and mortality in a natural shrub population

In order to scale up from the ecophysiological characters of individual plants to population-level questions, we need to determine if character patterns in natural populations are stable through time, and if the characters are related to growth and survival. We investigated these questions in a 3-year study for one character, integrated water-use efficiency (WUE) as estimated by carbon isotope discrimination () in a population of the Great Basin shrub, Chrysothamnus nauseosus. WUE was a conservative character for a given plant within and across seasons, and a previously documented difference between two size classes (represented by juveniles and adults) was maintained; smaller juveniles had a lower WUE than larger adults. The lower WUE of juveniles was often accompanied by higher rates of photosynthesis and stomatal conductance as compared to adults even though juveniles generally had more negative xylem pressure potentials. Although many discussions of the role of WUE in natural populations have been based on the expectation that higher WUE (lower ) is generally associated with less growth, we found no such relation-ship for juvenile plants in this population (i.e was not positively correlated with height increase). In addition, juvenile plant mortality was not correlated with . Although there were stable patterns of WUE for plants in this population, the positive correlation between WUE and size, and the lack of a negative correlation between WUE and height growth, make it unlikely that the WUE of an individual plant will be related in a simple manner to its growth and survival in the population.     

Leaf productivity along a precipitation gradient in lowland Panama: patterns from leaf to ecosystem

Moisture availability has the potential to affect tropical forest productivity at scales ranging from leaf to ecosystem. We compared data for leaf photosynthetic, chemical and structural traits of canopy trees, litterfall production and seasonal availability of soil water at four sites across a precipitation gradient (1,800–3,500 mm year^–1) in lowland Panamanian forest to determine how productivity at leaf and ecosystem scales may be related. We found stronger seasonality in soil water potential at drier sites. Values were close to zero at all sites during the wet season and varied between a minimum of –2.5 MPa and –0.3 MPa at the driest and wettest sites, respectively, during the dry season. Leaf photosynthesis and nitrogen concentration decreased with increasing precipitation, whereas leaf thickness increased with increasing precipitation. Leaf toughness and fiber/N ratios increased with increasing precipitation indicating reduced nutritional content and palatability with precipitation. Seasonality of litter production and quality decreased with increasing precipitation, but the amount of litterfall produced was not substantially different among sites. It appears that in Neotropical forest, moisture availability is associated with leaf photosynthetic and defensive traits that influence litterfall timing and quality. Therefore, variation in leaf physiological traits has the potential to influence decomposition and nutrient cycling through effects on litter quality.     

Ecological and ecophysiological patterns in a pre-altiplano shrubland of the Andean Cordillera in northern Chile

We report on community structural, ecophysiological, phenological, and morphological measurements made on woody plant species in the high elevation pre-altiplano zone on the western slopes of the Andean Cordillera of northern Chile. Notwithstanding extreme conditions of low rainfall, high atmospheric vapour demand and diurnal temperature fluctuation, a diversity of habitats (associated with drainage and slope aspect), appreciable local plant species richness (28 woody perennial plant species in a small area), and an array of adaptive morphological ecophysiological and phenological traits are present among woody species in these shrublands. Family diversity was low with four families accounted for 82% of the species. A range of gas exchange and watering use efficiency strategies was present ranging from highly efficient CAM species with a carbon isotope discrimination (Δ) of 3.7–7.5‰ through C₃ species with varying stomatal and gas exchange characteristics with a Δ of 14.4 to 19.8‰. Drought-deciduous small-leaved amphistomatic species from arid slopes generally had high stomatal conductance and high carbon assimilation rates during the rainy season. These drought deciduous species were largely leafless and, with one exception had low water potentials, during the dry season. Wash and less xeric site species commonly had broader evergreen to semi-evergreen leaves, higher dry season water potentials, and relatively consistent and moderate rates of gas exchange throughout the year. For all species, intrinsic water use efficiency (as estimated from the inverse of c _i :c _a ratio) correlated positively with mean leaf width (broader leaves had a lower higher intrinsic WUE) and dry season water potential. The charismatic high altitude tree, Polylepis rugulosa (Roaceae), had a population structure that suggests highly episodic establishment of seedlings, likely in sequences of wet years. Little of the area of these significant shrublands is currently protected. It would be desirable to add areas of pre-altiplano shrublands to adjacent national parks to ensure the persistence of these important communities. This revised version was published online in June 2006 with corrections to the Cover Date.     

Seasonal and inter-annual variations of gas exchange in thirteen woody species along a climatic gradient in the Mediterranean island of Mallorca

We studied the influence of summer drought and winter temperatures on seasonal and spatial variations of light-saturated net photosynthesis and stomatal conductance in Mediterranean woody species. We measured variations in leaf gas exchange over 3 years in 13 Mediterranean trees and shrubs, located at four different sites along a climate gradient of temperature and precipitation in the island of Mallorca (West Mediterranean basin). Net photosynthesis and stomatal conductance were at a maximum during spring, autumn or winter and at a minimum during summer in most sites, species and years. Nevertheless, important spatial, temporal and species-specific variations were observed. Summer drought limitation to gas exchange was greatest in the dry part of the transect, where many species showed their maximum gas exchange rate during winter. In contrast, winter temperatures limited gas exchange of many species at the wet and cool end of the transect, while summer depression of gas exchange was shorter and less pronounced. These results suggest that the effect on carbon fixation and productivity by the predicted future increase of aridity in the Mediterranean basin will depend on whether gas exchange is mostly limited by summer drought or by low winter temperatures.     

Leaf carbon isotope discrimination and vegetative responses of Dryas octopetala to temperature and water manipulations in a High Arctic polar semi-desert,Svalbard

Integrative ecophysiological and vegetative responses of Dryas octopetala were measured in response to field perturbations of temperature, precipitation and their interactions in a polar semi-desert in Svalbard, Norway (79°N, 12°E). Leaf carbon isotope discrimination (), total leaf nitrogen concentration and leaf development were determined for photosynthetic leaves collected during the last week of August 1991, after one season of manipulations. Individual leaf weight and the total mass of leaf tissue were significantly lower when water was added, irrespective of temperature regime. Leaf carbon isotope discrimination and estimated long-term c _i/c _avalues (the ratio of CO₂ concentration in leaf intercellular spaces to that in the atmosphere) were significantly higher under all three field manipulation treatments, and was significantly reduced when Dryas was grown under drought conditions in a related greenhouse study. Nitrogen concentrations of plants from the field experiment were significantly lower under warmed conditions regardless of water regime. Our results indicate that changes in environmental conditions in high arctic settings will result in alterations of Dryas leaf gas exchange, as expressed by increases in carbon isotope discrimination, which may be accompanied by shifts in leaf nitrogen content and leaf biomass.     

Physiological and structural changes in the chloroplast of the green alga Micrasterias denticulata induced by UV-B simulation

Exposure of postmitotic growing and non-growing cells of the unicellular green alga Micrasterias denticulata to different UV-B cut-off wavelengths together with simulated sunlight in a sun simulator has revealed a marked resistence of the algae against strong irradiation. While down to a cut-off wavelength of 284 nm irradiated during the most sensitive stage of cell development chloroplast ultrastructure remains unaffected, severe changes in arrangement and structure of stroma and grana thylakoids occur only at the lowest cut-off wavelengths of 280 and 275 nm. The structural alterations end up in a more or less complete desintegration of grana and stroma thylakoids with the remaining membraneous structures appearing in negative staining thus indicating drastic changes in membrane composition. Photosynthetic activity determined by chlorophyll fluorescence (ratio of variable to maximal fluorescence) and oxygen evolution responded more sensitively to UV-B irradiation. With decreasing UV cut-off wavelengths and prolonged incubation a decrease of photochemistry of PS II occured reaching its lowest values after 60 min at 275 and 280 nm. Oxygen production was even maintained under strong UV irradiation with a cut-off wavelenght of 275 nm up to 15 min. With prolonged UV-B treatment any activity was lost. HPLC separations of pigments exhibited the appearance of break-down products (mainly derivatives of chl b and chl a) with decreasing cut-off wavelength and increasing exposure time. The xanthophyll cycle pigments seemed to be unaffected at least for an irradiation period of 60 to 90 min at low UV cut-offs. Possible mechanisms of UV stress avoidance or protection are discussed with regard to the varying altitudes of the natural habitats of the algae.     

Seasonal changes in needle water content and needle ABA concentration of Japanese red pine, Pinus densiflora, in declining forests on Mt. Gokurakuji, Hiroshima prefecture, Japan

To evaluate the effects of air pollution on the decline of Pinus densiflora forests, various research has been conducted around Mt. Gokurakuji (34° 23N, 132° 19E, 693 m a.s.l.) north of the Seto Inland Sea, west Japan. To investigate the mechanisms responsible for decreases in photosynthesis (Pn) and stomatal conductance (gl), ¹³C of needles and seasonal changes in the water content (WC) and abscisic acid concentration (ABA) of needles were measured in various stands. The ¹³C values were less negative in declining stands and younger needles. ABA and WC were not correlated with each other. WC decreased consistently with needle age while the ABA showed a minimum in August and a smaller content in older needles. Monthly precipitation and the daily maximum vapor pressure were not correlated with ABA and WC. In declining stands, WC and ABA tended to be higher and lower, respectively, than in nondeclining stands. These results suggest that the trees in declining stands received less water stress than those in nondeclining stands and the differences in gl and ¹³C are not caused by the difference in water stress. The possibilities of the effects of air pollution and the infection of pine-wood nematode on the physiological decline on the pine needles are discussed.     

Physiological and morphological variation in Metrosideros polymorpha, a dominant Hawaiian tree species, along an altitudinal gradient: the role of phenotypic plasticity

Metrosideros polymorpha, a dominant tree species in Hawaiian ecosystems, occupies a wide range of habitats. Complementary field and common-garden studies of M. polymorpha populations were conducted across an altitudinal gradient at two different substrate ages to ascertain if the large phenotypic variation of this species is determined by genetic differences or by phenotypic modifications resulting from environmental conditions. Several characteristics, including ecophysiological behavior and anatomical features, were largely induced by the environment. However, other characteristics, particularly leaf morphology, appeared to be mainly determined by genetic background. Common garden plants exhibited higher average rates of net assimilation (5.8 μmol CO₂ m⁻² s⁻¹) and higher average stomatal conductance (0.18 mol H₂O m⁻² s⁻¹) than their field counterparts (3.0 μmol CO₂ m⁻² s⁻¹, and 0.13 mol H₂O m⁻² s⁻¹ respectively). Foliar δ¹³C of most common-garden plants was similar among sites of origin with an average value of −26.9‰. In contrast, mean values of foliar δ¹³C in field plants increased substantially from −29.5‰ at low elevation to −24.8‰ at high elevation. Leaf mass per unit area increased significantly as a function of elevation in both field and common garden plants; however, the range of values was much narrower in common garden plants (211–308 g m⁻² for common garden versus 107–407 g m⁻² for field plants). Nitrogen content measured on a leaf area basis in common garden plants ranged from 1.4 g m⁻² to 2.4 g m⁻² and from 0.8 g m⁻² to 2.5 g m⁻² in field plants. Photosynthetic nitrogen use efficiency (PNUE) decreased 50% with increasing elevation in field plants and only 20% in plants from young substrates in the common garden. This was a result of higher rates of net CO₂ assimilation in the common garden plants. Leaf tissue and cell layer thickness, and degree of leaf pubescence increased significantly with elevation in field plants, whereas in common garden plants, variation with elevation of origin was much narrower, or was entirely absent. Morphological characteristics such as leaf size, petiole length, and internode length decreased with increasing elevation in the field and were retained when grown in the common garden, suggesting a potential genetic basis for these traits. The combination of environmentally induced variability in physiological and anatomical characteristics and genetically determined variation in morphological traits allows Hawaiian M. polymorpha to attain and dominate an extremely wide ecological distribution not observed in other tree species. Received: 12 March 1997 / Accepted: 27 August 1997     

Changes in drought response strategies with ontogeny in Quercus rubra: implications for scaling from seedlings to mature trees

We investigated scaling of physiological parameters between age classes of Quercus rubra by combining in situ field measurements with an experimental approach. In the in situ field study, we investigated changes in drought response with age in seedlings, juveniles, and mature trees of Q. rubra. Throughout the particularly dry summer of 1995 and the unusually wet summer of 1996 in New England, we measured water potential of leaves (Ψ_Leaf) and gas exchange of plants at three sites at the Harvard Forest in Petersham, Massachusetts. In order to determine what fraction of the measured differences in gas exchange between seedlings and mature trees was due to environment versus ontogeny, an experiment was conducted in which seedlings were grown under light and soil moisture regimes simulating the environment of mature trees. The photosynthetic capacity of mature trees was three-fold greater than that of seedlings during the wet year, and six-fold greater during the drought year. The seedling experiment demonstrated that the difference in photosynthetic capacity between seedlings and mature trees is comprised equally of an environmental component (50%) and an ontogenetic component (50%) in the absence of water limitation. Photosynthesis was depressed more severely in seedlings than in mature trees in the drought year relative to the wet year, while juveniles showed an intermediate response. Throughout the drought, the predawn leaf water potential (Ψ_PD) of seedlings became increasingly negative (–0.4 to –1.6 MPa), while that of mature trees became only slightly more negative (–0.2 to –0.5 MPa). Again, juveniles showed an intermediate response (–0.25 to –0.8 MPa). During the wet summer of 1996, however, there was no difference in Ψ_PD between seedlings, juveniles and mature trees. During the dry summer of 1995, seedlings were more responsive to a major rain event than mature trees in terms of Ψ_Leaf , suggesting that the two age classes depend on different water sources. In all age classes, instantaneous measurements of intrinsic water use efficiency (WUE_i), defined as C assimilation rate divided by stomatal conductance, increased as the drought progressed, and all age classes had higher WUE_i during the drought year than in the wet year. Mature trees, however, showed a greater ability to increase their WUE_i in response to drought. Integrated measurements of WUE from C isotope discrimination (Δ) of leaves indicated higher WUE in mature trees than juveniles and seedlings. Differences between years, however, could not be distinguished, probably due to the strong bias in C isotope fractionation at the time of leaf production, which occurred prior to the onset of drought conditions in 1995. From this study, we arrive at two main conclusions: Received: 14 July 1999 / Accepted: 10 January 2000     

The effect of water and nitrogen amendments on photosynthesis,leaf demography,and resource-use efficiency in Larrea tridentata,a desert evergreen shrub

Summary In the Chihuahuan Desert of southern New Mexico, both water and nitrogen limit the primary productivity of Larrea tridentata, a xerophytic evergreen shrub. Net photosynthesis was positively correlated to leaf N, but only in plants that received supplemental water. Nutrient-use efficiency, defined as photosynthetic carbon gain per unit N invested in leaf tissue, declined with increasing leaf N. However, water-use efficiency, defined as the ratio of photosynthesis to transpiration, increased with increasing leaf N, and thus these two measures of resource-use efficiency were inversely correlated. Resorption efficiency was not significantly altered over the nutrient gradient, nor was it affected by irrigation treatments. Leaf longevity decreased significantly with fertilization although the absolute magnitude of this decrease was fairly small, in part due to a large background of insect-induced mortality. Age-specific gas exchange measurements support the hypothesis that leaf aging represents a redistribution of resources, rather than actual deterioration or declining resource-use efficiency.     

Physiological determinants of growth rate in response to phosphorus supply in wild and cultivated Hordeum species

Summary Under favorable nutrition, accessions of the weedy barleygrass (Hordeum leporinum and H. glaucum) had a higher relative growth rate (RGR) than did accessions of cultivated barley (H. vulgare) or its wild progenitor (H. spontaneum). RGR was not positively correlated with the presumed level of soil fertility at the collection site of an accession either within or among species. RGR was reduced more strongly by low-P supply in the progenitor than in the crop or weed, indicating that selection of cultivars to grow in fertile soils had not reduced their potential to grow effectively under low-P conditions. Seed and embryo masses were more important than RGR in determining plant size. Relative differences among assessions in plant size declined with time, because (1) accessions with small seeds had a higher RGR, and (2) RGR of large-seeded accessions declined with time. Absolute growth rate correlated positively with leaf area and negatively with photosynthetic rate per unit leaf area. Under favorable nutrition, maximum photosynthetic rate correlated negatively with leaf length and therefore was higher in the weeds than in the crop or progenitor accessions. P absorption potential did not differ consistently among species but generally increased in response to P stress. Cultivars produced a few tall tillers, whereas weeds and progenitors produced many small tillers. The cultivar had a larger proportion of reproductive tillers, allocated a larger proportion of biomass to grain, and produced larger grains than did the weedy accession. By contrast, the weed began maturing seeds sooner, produced more reproductive tillers, and produced more grains per car and per plant than did the cultivar. The study suggests two major conclusions: (1) A low RGR is not an adaptation to low P supply in annual Hordeum species. (2) Seed size is the major determinant of early plant size between accessions in these Hordeum species under favorable nutrition. However, large seed size indirectly results in a low RGR because of the inverse relationship between plant size and RGR and results in a low photosynthetic rate because of the inverse relationship between leaf size and photosynthesis.     

Carbon isotope composition in relation to leaf gas exchange and environmental conditions in Hawaiian Metrosideros polymorpha populations

Summary Carbon isotope composition, photosynthetic gas exchange, and nitrogen content were measured in leaves of three varieties of Metrosideros polymorpha growing in sites presenting a variety of precipitation, temperature and edaphic regimes. The eight populations studied could be divided into two groups on the basis of their mean foliar ¹³C values, one group consisting of three populations with mean ¹³C values ca.-26 and another group with ¹³C values ca.-28. Less negative ¹³C values appeared to be associated with reduced physiological availability of soil moisture resulting from hypoxic conditions at a poorly drained high elevation bog site and from low precipitation at a welldrained, low elevation leeward site. Gas exchange measurements indicated that foliar ¹³C and intrinsic wateruse efficiency were positively correlated. Maximum photosynthetic rates were nearly constant while maximum stomatal conductance varied substantially in individuals with foliar ¹³C ranging from-29 to-24. In contrast with the patterns of ¹³C observed, leaf nitrogen content appeared to be genetically determined and independent of site characteristics. Photosynthetic nitrogenuse efficiency was nearly constant over the range of ¹³C observed, suggesting that a compromise between intrinsic water- and N-use efficiency did not occur. In one population variations in foliar ¹³C and gas exchange with leaf cohort age, caused the ratio of intercellular to atmospheric partial pressure of CO₂ predicted from gas exchange and that calculated from ¹³C to be in close agreement only in the two youngest cohorts of fully expanded leaves. The results indicated that with suitable precautions concerning measurement protocol, foliar ¹³C and gas exchange measurements were reliable indicators of potential resource use efficiency by M. polymorpha along environmental gradients.     

Effects of grazing and topography on long-term vegetation changes in a Mediterranean ecosystem in Israel

The dynamics of Mediterranean vegetation over 28 years was studied in the Northern Galilee Mountains, Israel, in order to identify and quantify the major factors affecting it at the landscape scale. Image analysis of historical and current aerial photographs was used to produce high resolution digital vegetation maps (pixel size = 30 cm) for an area of 4 km² in the Galilee Mountains, northern Israel. GIS tools were used to produce corresponding maps of grazing regime, topographic indices and other relevant environmental factors. The effects of those factors were quantified using a multiple regression analyses. Major changes in the vegetation occurred during the period studied (1964–1992); tree cover increased from 2% in 1964 to 41% in 1992, while herbaceous vegetation cover decreased from 56% in 1964 to 24% in 1992. Grazing, topography and initial vegetation cover were found to significantly affect present vegetation patterns. Both cattle grazing and goat grazing reduced the rate of increase in tree cover, yet even intensive grazing did not halt the process. Grazing affected also the woody-herbaceous vegetation dynamics, reducing the expansion of woody vegetation. Slope, aspect, and the interaction term between these two factors, significantly affected vegetation pattern. Altogether, 56% and 72% of the variability in herbaceous and tree cover, respectively, was explained by the regression models. This study indicates that spatially explicit Mediterranean vegetation dynamics can be predicted with fair accuracy using few biologically important environmental variables.     

Water use efficiency and carbon isotope composition of plants in a cold desert environment

Summary The effects of the availabilities of water and nitrogen on water use efficiency (WUE) of plants were investigated in a sagebrush steppe. The four species studied wereArtemisia tridentata (shrub),Ceratoides lanata (suffrutescent shrub),Elymus lanceolatus (rhizomatous grass), andElymus elymoides (tussock grass). Water and nitrogen levels were manipulated in a two-by-two factorial design resulting in four treatments: control (no additions), added water, added nitrogen, and added water and nitrogen. One instantaneous and two long-term indicators of WUE were used to testa priori predictions of the ranking of WUE among treatments. The short-term indicator was the instantaneous ratio of assimilation to transpiration (A/E). The long-term measures were 1) the slope of the relationship between conductance to water vapor and maximum assimilation and 2) the carbon isotope composition (¹³C) of plant material. Additional water decreased WUE, whereas additional nitrogen increased WUE. For both A/E and ¹³C, the mean for added nitrogen alone was significantly greater than the mean for added water alone, and means for the control and added water and nitrogen fell in between. This ranking of WUE supported the hypothesis that both water and nitrogen limit plant gas exchange in this semiarid environment. The short- and long-term indicators were in agreement, providing evidence in support of theoretical models concerning the water cost of carbon assimilation.     

Seasonal changes in net photosynthesis rates and photosynthetic capacity in leaves of Cistus salvifolius,a European mediterranean semi-deciduous shrub

Summary During five different periods between Nov. 1982 and Aug. 1983, the diurnal patterns exhibited in photosynthetic CO₂ uptake and stomatal conductance were observed under natural conditions on twigs of Cistus salvifolius, a Mediterranean semi-deciduous shrub which retains a significant proportion of its leaves through the summer drought. During the same periods, net photosynthesis at saturating CO₂ partial pressure was measured on the same twigs as a function of irradiance at different temperatures. From these data, photosynthetic capacity, defined here as the CO₂- and light-saturated net photosynthesis rate, was obtained as a function of leaf temperature. C. salvifolius is a winter growing species, shoot growth being initiated in Nov. and continuing through May. Photosynthetic capacity was quite high in Nov., March and June, exceeding 40 mol m^-2 s^-1 at optimum temperature. In Dec., photosynthetic capacity was somewhat reduced, perhaps due to low night-time temperatures (<5°C) during the measurement period. In Aug., capacity in oversummering shoots at optimum temperature fell to less than 8 mol m^-2 s^-1, due to water trees and perhaps leaf aging. Seasonal changes in maximal photosynthetic rates under ambient conditions were similar, and like those found in co-occurring evergreen sclerophylls. Like the evergreens, Cistus demonstrated considerable stomatal control of transpirational water loss, particularly in oversummering leaves. During each measurement period except Aug. when capacity was quite low, the maximum rates of net photosynthesis measured under ambient conditions were less than half the measured photosynthetic capacities at comparable temperatures, suggesting an apparent excess nitrogen investment in the photosynthetic apparatus.     

Competition and drought limit the response of water-use efficiency to rising atmospheric carbon dioxide in the Mediterranean fir <Emphasis Type="Italic">Abies pinsapo</Emphasis>

The gas-exchange and radial growth responses of conifer forests to climatic warming and increasing atmospheric CO₂ have been widely studied. However, the modulating effects of variables related to stand structure (e.g., tree-to-tree competition) on those responses are poorly explored. The basal-area increment (BAI) and C isotope discrimination (C stable isotope ratio; δ¹³C) in the Mediterranean fir Abies pinsapo were investigated to elucidate the influences of stand competition, atmospheric CO₂ concentrations and climate on intrinsic water-use efficiency (WUEi). We assessed the variation in δ¹³C of tree-rings from dominant or co-dominant trees subjected to different degrees of competition. A high- (H) and a low-elevation (L) population with contrasting climatic constraints were studied in southern Spain. Both populations showed an increase in long-term WUEi. However, this increase occurred more slowly at the L site, where a decline of BAI was also observed. Local warming and severe droughts have occurred in the study area over the past 30 years, which have reduced water availability more at lower elevations. Contrastingly, trees from the H site were able to maintain high BAI values at a lower cost in terms of water consumption. In each population, trees subjected to a higher degree of competition by neighboring trees showed lower BAI and WUEi than those subjected to less competition, although the slopes of the temporal trends in WUEi were independent of the competitive micro-environment experienced by the trees. The results are consistent with an increasing drought-induced limitation of BAI and a decreasing rate of WUEi improvement in low-elevation A. pinsapo forests. This relict species might not be able to mitigate the negative effects of a decrease in water availability through a reduction in stomatal conductance, thus leading to a growth decline in the more xeric sites. An intense and poorly asymmetric competitive environment at the stand level may also act as an important constraint on the adaptive capacity of these drought-sensitive forests to climatic warming. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Grape harvest and yield responses to inter-annual changes in temperature and precipitation in an area of north-east Spain with a Mediterranean climate

This study presents an analysis of temperature and precipitation trends and their impact on grape harvests in the Penedès region (NE Spain). It includes analyses of maximum, minimum and mean daily temperatures (for both the growing and ripening seasons) and daily rainfall (for the hydrological year, the growing season and each phenological stage) for three observatories in the immediate area. We analysed a series of factors: beginning and end harvest dates; the day on which a given potential alcoholic degree was reached; and yield for several varieties of grape grown in the area in relation to climatic variables. Maximum temperatures increased at all the observatories, with greater values being recorded in recent years (1996–2009) than in previous decades (1960s–2000s): we observed increases in average growing season temperatures of 0.11°C per year for the period 1996–2009 vs 0.04°C per year for the period 1960–2009 at Vilafranca del Penedès. These temperature changes were due mainly to increases in maximum temperatures and an increase in the incidence of extreme heat (number of days with T > 30°C). Crop evapotranspiration also increased significantly during the same period. The Winkler index also increased, so the study area would correspond to region IV according to that climatic classification. There were no significant trends in annual rainfall but rainfall recorded between bloom and veraison decreased significantly at the three observatories, with the greatest decrease corresponding to the period 1996–2009. The dates on which harvests started and ended showed a continuous advance (of between −0.7 and −1.1 days per year, depending on the variety), which was significantly correlated with the average mean and maximum daily growing season temperatures (up to −7.68 days for 1°C increase). Winegrape yield was influenced by the estimated water deficit (crop evapotranspiration minus precipitation) in the bloom-veraison period; this value increased due to a reduction in precipitation and an increase in evapotranspiration. Yield may have been reduced by up to 30 kg/ha for each millimetre increase in the estimated water deficit. Under these conditions, new strategies need to be followed in this area in order to maintain grape quality and yield.