Seasonal Differences in Photochemical Efficiency and Chlorophyll and Carotenoid Contents in Six Mediterranean Shrub Species under Field Conditions

The effects of summer and winter stress on the chlorophyll and carotenoid contents and photosystem 2 efficiency were examined in six Mediterranean scrub species. These six species belong to two different plant functional types: drought semi-deciduous (Halimium halimifolium L., Rosmarinus officinalis L., Erica scoparia L.) and evergreen sclerophylls (Juniperus phoenicea L., Pistacia lentiscus L., Myrtus communis L.). Two sites with different water availability were chosen. In the xerophytic site, despite they belong to two different functional types, R. officinalis and J. phoenicea showed a similar response. These were the most affected species in summer. H. halimifolium showed optimal values of F_v/F_m and non-significant seasonal changes in xanthophyll content. In the mesic site, E. scoparia and M. communis were apparently the most affected species by winter climatic conditions. P. lentiscus presented a pattern similar to H. halimifolium, except for elevated F₀ values. In all the studied species, lutein plus zeaxanthin content was negatively correlated with F_v/F_m in summer and with leaf water potential, thus indicating that the thermal dissipation of energy was a general pattern for all species. Under stress, plant response is more species-specific than dependent on its functional type.     

Different vulnerabilities of Quercus ilex L. to freeze- and summer drought-induced xylem embolism: an ecological interpretation

Quercus ilex L. growing in the southern Mediterranean Basin region is exposed to xylem embolism induced by both winter freezing and summer drought. The distribution of the species in Sicily could be explained in terms of the different vulnerability to embolism of its xylem conduits. Naturally occurring climatic conditions were simulated by: (1) maintaining plants for 3h at ambient temperatures of 0, -1.5, -2.5, -5.0 and -11°C; and (2) allowing plants to dry out to ratios of their minimum diurnal leaf water potentials (Ψ₁) to that at the turgor loss point (Ψ_tlp) of 0.6, 0.9, 1.05, 1.20 and 1.33. The loss of hydraulic conductivity of one-year-old twigs reached 40% at -1.5°C and at Ψ₁/Ψ_tlP= 1.05. Recovery from these strains was almost complete 24 h after the release of thermal stress or after one irrigation, respectively. More severe stresses reduced recovery consistently. The percentages of xylem conduits embolized following application of the two stresses, were positively related to xylem conduit diameter. The capability of the xylem conduits to recover from stress was positively related to the conduit diameter in plants subjected to summer drought, but not in the plants subjected to winter freezing stress. The ecological significance of the different vulnerabilities to embolism of xylem conduits under naturally occurring climatic conditions is discussed.     

Drought increases freezing tolerance of both leaves and xylem of Larrea tridentata

Drought and freezing are both known to limit desert plant distributions, but the interaction of these stressors is poorly understood. Drought may increase freezing tolerance in leaves while decreasing it in the xylem, potentially creating a mismatch between water supply and demand. To test this hypothesis, we subjected Larrea tridentata juveniles grown in a greenhouse under well‐watered or drought conditions to minimum temperatures ranging from ?8 to ?24 °C. We measured survival, leaf retention, gas exchange, cell death, freezing point depression and leaf‐specific xylem hydraulic conductance (k_l). Drought‐exposed plants exhibited smaller decreases in gas exchange after exposure to ?8 °C compared to well‐watered plants. Drought also conferred a significant positive effect on leaf, xylem and whole‐plant function following exposure to ?15 °C; drought‐exposed plants exhibited less cell death, greater leaf retention, higher k_l and higher rates of gas exchange than well‐watered plants. Both drought‐exposed and well‐watered plants experienced 100% mortality following exposure to ?24 °C. By documenting the combined effects of drought and freezing stress, our data provide insight into the mechanisms determining plant survival and performance following freezing and the potential for shifts in L. tridentata abundance and range in the face of changing temperature and precipitation regimes.     

Additive effects of a potentially invasive grass and water stress on the performance of seedlings of gypsum specialists

Question: What is the combined effect of two drivers of local biodiversity changes (presence of a potentially invasive species and seasonal drought) on the performance of seedlings of plants from gypsum habitats under experimental conditions? Location: A controlled microcosm reconstruction of natural assemblages of gypsum plant communities from central Spain. Methods: We evaluated the effects of a potentially invasive grass (Lolium rigidum) and water stress on the survival, height growth and biomass of five woody species (Colutea hispanica, Gypsophila struthium, Thymus lacaitae, Lepidium subulatum and Helianthemum squamatum) from semi‐arid gypsum ecosystems. Seedlings of the five species were grown with or without the potential invader and under three watering regimes: early stress — simulating an advanced summer, late stress — simulating the characteristic timing of current summer drought and well‐watered. Results: Seedling survival and performance were negatively affected by the presence of the potential invader. Early stress had larger impacts on the gypsum species than late stress. No interactions were found between factors for any of the study variables, and responses to both factors were found to be species‐specific. Conclusions: The lack of interactions between factors indicates that the presence of the potentially invasive grass and water stress had additive effects in our study system. The negative impact of early water stress draws attention to the possible consequences of the advances of summer drought predicted for Mediterranean ecosystems. Finally, the differential responses found for the study species suggest that plant communities will not respond as a unit to global change, leading to significant changes in species composition and dominance.     

Seasonal physiological plasticity and recovery capacity after summer stress in Mediterranean scrub communities

Under natural conditions the overlapping of multiple stressors may initiate coordinated ecophysiological responses in Mediterranean species. Seasonal plasticity may enable plants to better cope with adverse environmental conditions and/or resource variability. In this article, we study the seasonal responses of 12 woody species in two sites of differing water availability, in a Mediterranean-type climate. Plants were measured for water potential, photochemical efficiency, photosynthetic pigments and leaf proline content throughout the year. The results revealed that species presented different ecophysiological strategies, even when sharing the same area. In the xerophytic site, some species suffered severe water stress (−12 MPa and F _v/F _m lower than 0.3), while others exhibited optimal values of F _v/F _m and only moderate water stress. All the plants recovered after the first autumn rains. In the hygrophytic site, some sclerophyll species did not exhibit signs of water stress, but did suffer photoinhibition in clear winter days. A plasticity index was calculated to provide an integrated value of species plasticity. In summer, plasticity was higher in the xerophytic site, while in winter it was higher in the hygrophytic site. Ordination analysis of the physiological traits supports the traditional gradient of Mediterranean strategies from drought semideciduous to evergreen sclerophyll species, although spiny legume species formed an independent functional group. The functional responses of species clearly differ among plant communities according to the prevailing site stressors, but no unique pattern emerges. Species combine traits in broader strategies according to previous evolutionary story exhibiting a certain amount of trade among traits, each contributing to alleviate a part of the plant stress.     

Seasonal changes in the freezing behavior of xylem ray parenchyma cells in four boreal hardwood species

The freezing behavior of xylem ray parenchyma cells in several boreal hardwood species, namely, Betula platyphylla, Populus canadensis, P. sieboldii, and Salix sachalinensis, was examined by differential thermal analysis (DTA), cryo-scanning electron microscopy (Cryo-SEM), and freeze-fracture replica electron microscopy. Although DTA profiles of samples harvested in summer and in winter suggested that the xylem ray parenchyma cells in all four species responded to freezing stress by extracellular freezing, Cryo-SEM showed clearly that the xylem ray parenchyma cells in all these species responded to freezing stress by shallow supercooling in summer and by extracellular freezing in winter. It is suggested that DTA failed to reveal the true freezing behavior of xylem ray parenchyma cells because of an overlap of temperature ranges between the high-temperature exotherm and the low-temperature exotherm and/or because of the limited extent of the LTE. The seasonal changes in freezing behavior of xylem ray parenchyma cells in all these boreal species, which are results of seasonal cold acclimation, support the hypothesis that a gradual shift of freezing behavior in xylem ray parenchyma cells from shallow supercooling in hardwood species that grow in tropical zones to extracellular freezing in hardwood species that grow in cold areas might be a result of the evolutionary adaptation of hardwood species to cold climates. Copyright 1999 Academic Press.     

Seasonal changes in photosynthesis, antioxidant systems and ELIP expression in a thermonastic and non-thermonastic Rhododendron species: A comparison of photoprotective strategies in overwintering plants

Leaves of overwintering evergreen rhododendrons are typically exposed to freezing temperatures and high light during winters which can potentially result in photon flux exceeding that required for photochemistry. This excess energy, if not dissipated as heat or fluorescence, may cause photooxidative damage to PSII. The goal of this study is to compare the photoprotection strategies during seasonal cold acclimation (CA) in two Rhododendron species (R. catawbiense Michx. and R. ponticum L.) that are divergent in their leaf freezing tolerance and thermonastic behaviour (temperature-induced leaf movement). R. catawbiense exhibits thermonasty while R. ponticum does not. Differences in leaf freezing tolerance (LT₅₀), photosynthesis, photoinhibition, early light-induced proteins (ELIPs) gene expression, and accumulation of antioxidant metabolites and enzymes during seasonal CA were investigated. During seasonal CA, maximum photosynthetic rate (P_max) and maximum quantum efficiency of PSII (F_v/F_m) were significantly down-regulated. Compared with R. catawbiense, R. ponticum showed less photoinhibition and higher overall accumulation (in magnitude) of antioxidant systems while R. catawbiense exhibited more efficient up-regulation of ELIP expression and antioxidant system (i.e., greater efficiency of increasing this pool in winter months relative to the summer levels). The two species respond differently to winter conditions and have evolved strategies to avoid, reduce and/or tolerate photooxidative stress in winter. These include down-regulation of photosynthesis and up-regulation of ELIPs and antioxidant systems, together with specialized leaf anatomy and thermonasty behaviour.     

Seasonal Patterns of Photosystem 2 Photochemical Efficiency in Evergreen Sclerophylls and Drought Semi-Deciduous Shrubs under Mediterranean Field Conditions

Photochemical efficiency of photosystem 2 (PS2), assessed from in situ chlorophyll (Chl) fluorescence measurements, was seasonally monitored in five evergreen sclerophyll and five malacophyllous drought semi-deciduous species, co-occurring in the same Mediterranean field site. In evergreen sclerophylls, a considerable drop in the variable (F_v) to maximum (F_m) Chl fluorescence ratio coincided with the lowest winter temperatures, indicating low PS2 efficiency during this period. Summer drought caused a comparatively slight decrease in F_v/F_m and only in three of the five evergreen sclerophyll species tested. In drought semi-deciduous shrubs, the winter drop in F_v/F_m was much less conspicuous. During the summer, and in spite of the severe and prolonged desiccation of their malacophyllous leaves, F_v/F_m was maintained high and only in one species the PS2 efficiency was transiently suppressed, when the leaf relative water content became lower than 30 %. Thus evergreen sclerophylls are more prone to photoinhibition by low winter temperatures, while the sensitivity of drought semi-deciduals depends on the extent and duration of summer drought. This revised version was published online in September 2006 with corrections to the Cover Date.     

Comparative ecophysiological responses to drought of two shrub and four tree species from karst habitats of southwestern China

Drought stress is one of the most important factors in limiting the survival and growth of plants in the harsh karst habitats of southwestern China, especially at the seedling establishment stage. The ecophysiological response to drought stress of native plants with different growth forms is useful for re-vegetation programs. Two shrub and four tree species were studied, including Pyracantha fortuneana (evergreen shrub), Rosa cymosa (deciduous shrub), Cinnamomum bodinieri (evergreen tree), and other three deciduous trees, Broussonetia papyrifera, Platycarya longipes, and Pteroceltis tatarinowii. The seedlings were randomly assigned to four drought treatments, i.e., well-watered, mild drought stress, moderate drought stress, and severe drought stress. Leaf water relations, gas exchange, chlorophyll fluorescence, and growth of the seedlings were investigated. Under severe drought stress, the two shrubs with low leaf area ratio (LAR) maintained higher water status, higher photosynthetic capacity, and larger percent biomass increase than the most of the trees. The two shrubs also had lower specific leaf area, greater intrinsic water use efficiency, and thermal dissipation than the trees. This suggested that the two shrubs had high tolerance to severe drought and were suitable for re-vegetation in harsh habitats. The evergreen C. bodinieri exhibited higher leaf mass ratio (LMR) and LAR than the deciduous species under mild and moderate stress. However, the low maximum quantum efficiency of PSII photochemistry (F _v/F _m) and net assimilation rate, and the sharp decreases of water potential, LMR, LAR, and biomass under severe stress indicated C. bodinieri’s weak tolerance to severe drought. In response to drought stress, the three deciduous trees revealed sharp reductions of biomass due to the large drought-induced decreases of gas exchange, LAR, and LMR. Under drought conditions, the deciduous trees minimized water loss by stomatal closure and by reducing transpiration leaf area and light harvesting through shedding leaves. This suggested that the three deciduous trees were more sensitive to water availability than the shrubs and used avoidance strategies against drought stress. However, the better growth performance of the deciduous trees than that of the shrubs under favorable conditions suggested that deciduous trees could be suitable for habitats with mild and temporary drought stress.     

Ecophysiological studies of Sonoran Desert plants

Summary The gas exchange and water relations of two Sonoran Desert plants were measured throughout a 12 month period. Seasonal photosynthesis patterns of both species followed the seasonal variation in soil and plant water potential. Acacia greggii, a winter deciduous shrub, appears to be fall active since the day-long mean photosynthesis rate was maximum, i.e., 16 mg CO₂ dm^-2 h^-1, at this time. Cercidium microphyllum, a drought deciduous and chlorophyllous-stemmed tree, also appears to be mainly fall active. For this species the day-long mean photosynthesis rate was not in excess of 14 mg CO₂ dm^-2 h^-1. Both species initiate leaf production in the spring and neither experiences severe plant water stress. Seasonal minima of dawn plant water potential were-44 and-32 bars for the shrub and tree species, respectively. The two species differ slightly in their tolerance of heat and water stress, since foliated plants of Acacia greggii maintain summer gross photosynthesis.All of the aboveground organs for plants of C. microphyllum are capable of exogenous ¹⁴CO₂ assimilation. This species appears to be unique in the magnitude of the photosynthetic production contributed by stems. Seasonal production by stems, leaves and flowers/fruits averaged 72, 24 and 4% of the total carbon gain per tree, respectively. Aboveground gross primary production was over 4.5-fold greater than aboveground net primary production. This difference between these two production estimates is likely due to the very small foliar biomass maintained throughout the year and the energy expenditure required to maintain the metabolically active cells of the chlorophyllous stems.     

Osmotic and elastic adjustments in cold desert shrubs differing in rooting depth: coping with drought and subzero temperatures

Physiological adjustments to enhance tolerance or avoidance of summer drought and winter freezing were studied in shallow- to deep-rooted Patagonian cold desert shrubs. We measured leaf water potential (Ψ_L), osmotic potential, tissue elasticity, stem hydraulic characteristics, and stomatal conductance (g _S) across species throughout the year, and assessed tissue damage by subzero temperatures during winter. Species behavior was highly dependent on rooting depth. Substantial osmotic adjustment (up to 1.2?MPa) was observed in deep-rooted species exhibiting relatively small seasonal variations in Ψ_L and with access to a more stable water source, but having a large difference between predawn and midday Ψ_L. On the other hand, shallow-rooted species exposed to large seasonal changes in Ψ_L showed limited osmotic adjustment and incomplete stomatal closure, resulting in turgor loss during periods of drought. The bulk leaf tissue elastic modulus (ε) was lower in species with relatively shallow roots. Daily variation in g _S was larger in shallow-rooted species (more than 50?% of its maximum) and was negatively associated with the difference between Ψ_L at the turgor loss point and minimum Ψ_L (safety margin for turgor maintenance). All species increased ε by about 10?MPa during winter. Species with rigid tissue walls exhibited low leaf tissue damage at ?20?°C. Our results suggest that osmotic adjustment was the main water relationship adaptation to cope with drought during summer and spring, particularly in deep-rooted plants, and that adjustments in cell wall rigidity during the winter helped to enhance freezing tolerance.     

Seasonal variability of forest sensitivity to heat and drought stresses: A synthesis based on carbon fluxes from North American forest ecosystems

Climate extremes such as heat waves and droughts are projected to occur more frequently with increasing temperature and an intensified hydrological cycle. It is important to understand and quantify how forest carbon fluxes respond to heat and drought stress. In this study, we developed a series of daily indices of sensitivity to heat and drought stress as indicated by air temperature (T_a) and evaporative fraction (EF). Using normalized daily carbon fluxes from the FLUXNET Network for 34 forest sites in North America, the seasonal pattern of sensitivities of net ecosystem productivity (NEP), gross ecosystem productivity (GEP) and ecosystem respiration (RE) in response to T_a and EF anomalies were compared for different forest types. The results showed that warm temperatures in spring had a positive effect on NEP in conifer forests but a negative impact in deciduous forests. GEP in conifer forests increased with higher temperature anomalies in spring but decreased in summer. The drought‐induced decrease in NEP, which mostly occurred in the deciduous forests, was mostly driven by the reduction in GEP. In conifer forests, drought had a similar dampening effect on both GEP and RE, therefore leading to a neutral NEP response. The NEP sensitivity to T_a anomalies increased with increasing mean annual temperature. Drier sites were less sensitive to drought stress in summer. Natural forests with older stand age tended to be more resilient to the climate stresses compared to managed younger forests. The results of the Classification and Regression Tree analysis showed that seasons and ecosystem productivity were the most powerful variables in explaining the variation of forest sensitivity to heat and drought stress. Our results implied that the magnitude and direction of carbon flux changes in response to climate extremes are highly dependent on the seasonal dynamics of forests and the timing of the climate extremes.     

The physiological implications of primary xylem organization in two ferns

Xylem structure and function are well described in woody plants, but the implications of xylem organization in less‐derived plants such as ferns are poorly understood. Here, two ferns with contrasting phenology and xylem organization were selected to investigate how xylem dysfunction affects hydraulic conductivity and stomatal conductance (g_s). The drought‐deciduous pioneer species, Pteridium aquilinum, exhibits fronds composed of 25 to 37 highly integrated vascular bundles with many connections, high g_s and moderate cavitation resistance (P₅₀ = ?2.23 MPa). By contrast, the evergreen Woodwardia fimbriata exhibits sectored fronds with 3 to 5 vascular bundles and infrequent connections, low g_s and high resistance to cavitation (P₅₀ = ?5.21 MPa). Xylem‐specific conductivity was significantly higher in P. aqulinium in part due to its wide, efficient conduits that supply its rapidly transpiring pinnae. These trade‐offs imply that the contrasting xylem organization of these ferns mirrors their divergent life history strategies. Greater hydraulic connectivity and g_s promote rapid seasonal growth, but come with the risk of increased vulnerability to cavitation in P. aquilinum, while the conservative xylem organization of W. fimbriata leads to slower growth but greater drought tolerance and frond longevity.     

Effects of long-term experimental night-time warming and drought on photosynthesis, <Emphasis Type="Italic">F</Emphasis>v/<Emphasis Type="Italic">F</Emphasis>m and stomatal conductance in the dominant species of a Mediterranean shrubland

We conducted a night-time warming and drought field experiment for 7 years (1999–2005) in a Mediterranean shrubland. We focused on the two dominant shrub species, Erica multiflora L. and Globularia alypum L. and the tree Pinus halepensis L. and the final years to study the effects of the experimental night-time warming and drought on Fv/Fm, photosynthesis, and stomatal conductance. Warming treatment increased mean air temperature and mean soil temperature through the years by an average of 0.7 and 0.9°C respectively, and drought treatment reduced soil moisture through the years by an average of 19%. Warming tended to increase photosynthetic rates in E. multiflora, G. alypum and P. halepensis mostly in the cold seasons, when plants were more limited by temperature, as shown by the lowest values of Fv/Fm being detected in winter in the three studied species. A negative effect of warming was only detected for E. multiflora in summer 2003. Drought treatment generated different responses of net photosynthetic rates depending on the species, season and year. Stomatal conductance showed the same pattern as photosynthesis for the three studied species, displaying seasonal and inter-annual variability, although with an overall negative effect of drought for P. halepensis. Photosynthetic rates decreased significantly in the dry winter 2005 and spring 2005 in comparison to the same seasons of 2003 and 2004. There were positive correlations between the photosynthetic rates in different seasons for E. multiflora, G. alypum and P. halepensis and the soil moisture of the week prior to measurements. The great variation in the photosynthetic rates was thus explained in a significant part by soil moisture levels. The lowest Fv/Fm values usually corresponded with lowest stomatal conductances suggesting that drought stress could be associated to stress by low temperatures in winter.     

Seasonal dynamics in the stable carbon isotope composition δ¹³C from non-leafy branch, trunk and coarse root CO₂ efflux of adult deciduous (Fagus sylvatica) and evergreen (Picea abies) trees

Respiration is a substantial driver of carbon (C) flux in forest ecosystems and stable C isotopes provide an excellent tool for its investigation. We studied seasonal dynamics in δ¹³C of CO₂ efflux (δ¹³C_E) from non‐leafy branches, upper and lower trunks and coarse roots of adult trees, comparing deciduous Fagus sylvatica (European beech) with evergreen Picea abies (Norway spruce). In both species, we observed strong and similar seasonal dynamics in the δ¹³C_E of above‐ground plant components, whereas δ¹³C_E of coarse roots was rather stable. During summer, δ¹³C_E of trunks was about ?28.2‰ (Beech) and ?26.8‰ (Spruce). During winter dormancy, δ¹³C_E increased by 5.6–9.1‰. The observed dynamics are likely related to a switch from growth to starch accumulation during fall and remobilization of starch, low TCA cycle activity and accumulation of malate by PEPc during winter. The seasonal δ¹³C_E pattern of branches of Beech and upper trunks of Spruce was less variable, probably because these organs were additionally supplied by winter photosynthesis. In view of our results and pervious studies, we conclude that the pronounced increases in δ¹³C_E of trunks during the winter results from interrupted access to recent photosynthates.     

Seasonal contrasting changes of foliar concentrations of terpenes and other volatile organic compound in four dominant species of a Mediterranean shrubland submitted to a field experimental drought and warming

To test the effect of forecasted drought and warming conditions for the next decades by GCM and ecophysiological models on foliar concentrations of volatile organic compounds (VOCs) and especially of volatile terpenes, we studied four typical Mediterranean woody plants (Pinus halepensis L., Pistacia lentiscus L., Rosmarinus officinalis L. and Globularia alypum L.) under a field experimental drought and warming generated using automatically sliding curtains. Terpenes were detected in the four studied species (R. officinalis L., P halepensis L., Pistacia lentiscus L. and G. alypum L.). In general, maximum concentrations of terpenes were found in the coldest periods and minimum concentrations in the summer. Their concentrations ranged between 0.003 mg g^?1 DM (eugenol) in G. alypum under drought conditions and 37 mg g^?1 DM in R. officinalis under control conditions. Main volatile terpenes found in all studied species except in G. alypum were α‐pinene, camphene, β‐pinene, β‐phellandrene and caryophyllene. In general, VOC leaf concentrations increased when soil moisture increased and decreased when air temperature increased. However, contrasting not consistent responses to the drought and warming treatments were found among species, seasons and years. For example, in P. halepensis, the concentrations decreased in response to drought in winter and instead increased in summer. Contrarily, drought decreased concentrations in summer and increased them in winter in Pistacia lentiscus. In any case, the data on seasonal VOC concentration in Mediterranean woody species provided here will add new knowledge of seasonal variation in essential oil contents of these species. These data might help in the study of flammability of Mediterranean ecosystems and in improving prediction algorithms, inventories and modelling of monoterpene emissions in response to climate change, which mostly do not consider the changes in concentration under drought stress. However, the lack of general and consistent response patterns to increasing drought and warming among species, seasons and years found here makes this task difficult.     

A global analysis of plant recovery performance from water stress

Plant post‐drought recovery performance is essential to predict shifts in ecosystem dynamics and production during frequent climate change‐driven drought events. Yet, it is not clear how post‐drought recovery is related to evolutionary and geographic variations in plants. In this study, we generated a global data set of post‐drought recovery performance in 140 plant species from published studies. We quantified the plant post‐drought recovery performance by calculating a recovery index for multiple plant physiological and hydraulic parameters, including leaf water potential, net photosynthetic rate, leaf hydraulic conductance and shoot biomass. The magnitude of recovery among four plant functional types (deciduous angiosperms, evergreen angiosperms, gymnosperms, and crops), two plant growth forms (shrubs and trees), two water management strategies (isohydric and anisohydric), four xylem porosity types (diffuse, ring, semi‐ring and tracheid), and four major biomes (dry sclerophyll forest, boreal forest, temperate forest and tropical/subtropical forest) were compared. We found the inability to completely recover immediately after severe water stress is ubiquitous across all plant functional types and growth forms, while the rate and magnitude of post‐drought recovery varied greatly across different plant taxonomic categories and geographic ranges. In general, plant hydraulic architecture, leaf anatomy and physiology affect plants’ propensity towards recovery, and reflect evolutionary consequences of plant adaptation to their habitat. Due to the essential role of plant functional traits in regulating carbon storage in each biome, a better understanding plant post‐drought recovery performance could improve our predictions on ecosystem productivity in a rapidly changing climate.     

Different responses to drought and freeze stress of Quercus ilex L. growing along a latitudinal gradient

The vulnerability to drought and freeze stress was measured in young plants of Quercus ilex L. growing in the field in two natural sites within the Italian distribution area of this species, i.e. Sicily (Southern Mediterranean Basin) and Venezia Giulia (Northeastern Italy), respectively. In particular, the resistance strategies adopted by Q. ilex to withstand the two stresses were estimated in terms of seasonal and/or diurnal changes in leaf conductance to water vapour (g_L), water potential (_L) and relative water content (RWC) as well as of xylem embolism in the stem and root hydraulic conductance (K_RL). Sicilian (SI) plants showed to reduce water loss by stomatal closure (g_L decreased) in summer, thus maintaining average RWCs at 88–90%. Moreover, SI plants showed considerable resistance to xylem cavitation in the stem (the loss of hydraulic conductance, PLC, was less than 12% throughout the year) and to maintain the hydraulic conductance of their roots (K_RL), constantly high even in summer. Plants growing in Venezia Giulia (VG plants), on the contrary, underwent leaf dehydration in the winter due to freeze stress so that RWC measured in April was still 78% on a diurnal basis. This was apparently due to consistent xylem embolism in the stem. In fact, PLC was as high as 40% between November and March. Only in the summer was PLC similar to that recorded in SI plants. Moreover, K_RL of VG plants decreased in November from about 1.5 to 0.8×10^–4 kg s^–1 m^–2 MPa^–1, i.e. about 50%, and in February K_RL dropped further to 0.4×10^–4 kg s^–1 m^–2 MPa^–1. On the basis of the above, we conclude that: (a) Q. ilex was more sensitive to freeze than to drought stress so that freeze stress can be considered as a factor limiting the distribution area of this species; (b) drought and freeze stress were faced by Q. ilex adopting two different resistance strategies, i.e. drought avoidance based on water saving in Sicily and freeze tolerance in Venezia Giulia.     

Climatic controls on the carbon and water balances of a boreal aspen forest, 1994–2003

The carbon and water budgets of boreal and temperate broadleaf forests are sensitive to interannual climatic variability and are likely to respond to climate change. This study analyses 9 years of eddy‐covariance data from the Boreal Ecosystem Research and Monitoring Sites (BERMS) Southern Old Aspen site in central Saskatchewan, Canada and characterizes the primary climatic controls on evapotranspiration, net ecosystem production (F_NEP), gross ecosystem photosynthesis (P) and ecosystem respiration (R). The study period was dominated by two climatic extremes: extreme warm and cool springs, which produced marked contrasts in the canopy duration, and a severe, 3‐year drought. Annual F_NEP varied among years from 55 to 367 g C m⁻² (mean 172, SD 94). Interannual variability in F_NEP was controlled primarily by factors that affected the R/P ratio, which varied between 0.74 and 0.96 (mean 0.87, SD 0.06). Canopy duration enhanced P and F_NEP with no apparent effect on R. The fraction of annual photosynthetically active radiation (PAR) that was absorbed by the canopy foliage varied from 38% in late leaf‐emergence years to 51% in early leaf‐emergence years. Photosynthetic light‐use efficiency (mean 0.0275, SD 0.026 mol C mol⁻¹ photons) was relatively constant during nondrought years but declined with drought intensity to a minimum of 0.0228 mol C mol⁻¹ photons during the most severe drought year. The impact of drought on F_NEP varied with drought intensity. Years of mild‐to‐moderate drought suppressed R while having little effect on P, so that F_NEP was enhanced. Years of severe drought suppressed both R and P, causing either little change or a subtle reduction in F_NEP. The analysis produced new insights into the dominance of canopy duration as the most important biophysical control on F_NEP. The results suggested a simple conceptual model for annual F_NEP in boreal deciduous forests. When water is not limiting, annual P is controlled by canopy duration via its influence on absorbed PAR at constant light‐use efficiency. Water stress suppresses P, by reducing light‐use efficiency, and R, by limiting growth and/or suppressing microbial respiration. The high photosynthetic light‐use efficiency showed this site to be a highly productive boreal deciduous forest, with properties similar to many temperate deciduous forests.