Variability in growth, carbon isotope composition, leaf gas exchange and hydraulic traits in the eastern Mediterranean cedars Cedrus libani and C. brevifolia.

Four Turkish provenances and five Lebanese provenances of Cedrus libani A. Rich. and one Cypriot provenance of C. brevifolia Henry were compared during the third year of growth in a controlled-climate greenhouse after exposure to a well-watered or moderate-drought treatment. Effects of treatment on CO(2) assimilation (A), stomatal conductance (g(s)), (13)C isotope composition (delta(13)C), growth and biomass were assessed. Hydraulic conductivity and shoot vulnerability to cavitation were measured in well-watered plants only. The Lebanese provenances of C. libani had the highest growth rates, but were the most sensitive to drought. The Turkish provenances of C. libani showed moderate growth rates and moderate drought sensitivity. Cedrus brevifolia had the lowest growth rate and was least sensitive to drought. For each provenance, mean biomass values were positively correlated with delta(13)C and intrinsic water-use efficiency (A/g(s)), and negatively correlated with g(s). Drought reduced growth and favored carbon storage in roots, increasing the ratio of root biomass to aboveground biomass. The drought treatment increased delta(13)C and A/g(s). Specific hydraulic conductivity (K(s)) was similar for the provenance groups, whereas leaf-specific conductivity (K(l)) was lower in the Lebanese provenances than in the other provenances. Within each provenance group, provenances with the highest K(l) were most susceptible to xylem cavitation, but were also the most productive. Growth and drought adaptation were linked with precipitation in each provenance's native range.

Interactive effects of elevated CO2 concentration and nitrogen supply on partitioning of newly fixed 13C and 15N between shoot and roots of pedunculate oak seedlings (Quercus robur).

Pedunculate oak (Quercus robur L.) seedlings were grown for 3 or 4 months (second- and third-flush stages) in greenhouses at two atmospheric CO2 concentrations ([CO2]) (350 or 700 micromol mol(-1)) and two nitrogen fertilization regimes (6.1 or 0.61 mmol N l(-1) nutrient solution). Combined effects of [CO2] and nitrogen fertilization on partitioning of newly acquired carbon (C) and nitrogen (N) were assessed by dual 13C and 15N short-term labeling of seedlings at the second- or third-flush stage of development. In the low-N treatment, root growth, but not shoot growth, was stimulated by elevated [CO2], with the result that shoot/root biomass ratio declined. At the second-flush stage, overall seedling biomass growth was increased (13%) by elevated [CO2] regardless of N fertilization. At the third-flush stage, elevated [CO2] increased growth sharply (139%) in the high-N but not the low-N treatment. Root/shoot biomass ratios were threefold higher in the low-N treatment relative to the high-N treatment. At the second-flush stage, leaf area was 45-51% greater in the high-N treatment than in the low-N treatment. At the-third flush stage, there was a positive interaction between the effects of N fertilization and [CO2] on leaf area, which was 93% greater in the high-N/elevated [CO2] treatment than in the low-N/ambient [CO2] treatment. Specific leaf area was reduced (17-25%) by elevated [CO2], whereas C and N concentrations of seedlings increased significantly in response to either elevated [CO2] or high-N fertilization. At the third-flush stage, acquisition of C and N per unit dry mass of leaf and fine root was 51 and 77% greater, respectively, in the elevated [CO2]/high-N fertilization treatment than in the ambient [CO2]/low-N fertilization treatment. However, there was dilution of leaf N in response to elevated [CO2]. Partitioning of newly acquired C and N between shoot and roots was altered by N fertilization but not [CO2]. More newly acquired C and N were partitioned to roots in the low-N treatment than in the high-N treatment.

Competition for water between walnut seedlings (Juglans regia) and rye grass (Lolium perenne) assessed by carbon isotope discrimination and delta18O enrichment.

Container-grown walnut seedlings (Juglans regia L.) were subjected to competition with rye grass (Lolium perenne L.) and to a 2-week soil drying cycle. One and 2 weeks after the beginning of the drought treatment, H2 18O (delta approximately equals +100%) was added to the bottom layer of soil in the plant containers to create a vertical H2 18O gradient. Rye grass competition reduced aboveground and belowground biomass of the walnut seedlings by 60%, whereas drought had no effect. The presence of rye grass reduced the dry weight of walnut roots in the upper soil layer and caused a 50% reduction in lateral root length. Rye grass competition combined with the drought treatment reduced walnut leaf CO2 assimilation rate (A) and leaf conductance (gw) by 20 and 39%, respectively. Transpiration rates in rye grass, both at the leaf level and at the plant or tiller level, were higher than in walnut seedlings. Leaf intrinsic water-use efficiency (A/gw) of walnut seedlings increased in response to drought and no differences were observed between the single-species and mixed-species treatments, as confirmed by leaf carbon isotope discrimination measurements. Measurement of delta18O in soil and in plant xylem sap indicated that the presence of rye grass did not affect the vertical profile of soil water uptake by walnut seedlings. Walnut seedlings and rye grass withdrew water from the top and middle soil layers in well-watered conditions, whereas during the drought treatment, walnut seedlings obtained water from all soil layers, but rye grass took up water from the bottom soil layer only.

Interspecific variability of δ13C among trees in rainforests of French Guiana: functional groups and canopy integration.

The interspecific variability of sunlit leaf carbon isotope composition (δ13C), an indicator of leaf intrinsic water-use efficiency (WUE, CO2 assimilation rate/leaf conductance for water vapour), was investigated in canopy trees of three lowland rainforest stands in French Guiana, differing in floristic composition and in soil drainage characteristics, but subjected to similar climatic conditions. We sampled leaves with a rifle from 406 trees in total, representing 102 species. Eighteen species were common to the three stands. Mean species δ13C varied over a 6.0‰ range within each stand, corresponding to WUE varying over about a threefold range. Species occurring in at least two stands displayed remarkably stable δ13C values, suggesting a close genetic control of species δ13C. Marked differences in species δ13C values were found with respect to: (1) the leaf phenology pattern (average δ13C=-29.7‰ and -31.0‰ in deciduous-leaved and evergreen-leaved species, respectively), and (2) different types of shade tolerance defined by features reflecting the plasticity of growth dynamics with respect to contrasting light conditions. Heliophilic species exhibited more negative δ13C values (average δ13C=-30.5‰) (i.e. lower WUE) than hemitolerant species (-29.3‰). However, tolerant species (-31.4‰) displayed even more negative δ13C values than heliophilic ones. We could not provide a straightforward ecophysiological interpretation of this result. The negative relationship found between species δ13C and midday leaf water potential (Ψwm) suggests that low δ13C is associated with high whole tree leaf specific hydraulic conductance. Canopy carbon isotope discrimination (Δ A ) calculated from the basal area-weighed integral of the species δ13C values was similar in the three stands (average Δ A =23.1‰), despite differences in stand species composition and soil drainage type, reflecting the similar proportions of the three different shade-tolerance types among stands.

Functional diversity in an Amazonian rainforest of French Guyana: a dual isotope approach (δ15N and δ13C).

Functional aspects of biodiversity were investigated in a lowland tropical rainforest in French Guyana (5°2'N, annual precipitation 2200 mm). We assessed leaf δ15N as a presumptive indicator of symbiotic N2 fixation, and leaf and wood cellulose δ13C as an indicator of leaf intrinsic water-use efficiency (CO2 assimilation rate/leaf conductance for water vapour) in dominant trees of 21 species selected for their representativeness in the forest cover, their ecological strategy (pioneers or late successional stage species, shade tolerance) or their potential ability for N2 fixation. Similar measurements were made in trees of native species growing in a nearby plantation after severe perturbation (clear cutting, mechanical soil disturbance). Bulk soil δ15N was spatially quite uniform in the forest (range 3-5‰), whereas average leaf δ15N ranged from -0.3‰ to 3.5‰ in the different species. Three species only, Diplotropis purpurea, Recordoxylon speciosum (Fabaceae), and Sclerolobium melinonii (Caesalpiniaceae), had root bacterial nodules, which was also associated with leaf N concentrations higher than 20 mg g-1. Although nodulated trees displayed significantly lower leaf δ15N values than non-nodulated trees, leaf δ15N did not prove a straightforward indicator of symbiotic fixation, since there was a clear overlap of δ15N values for nodulated and non-nodulated species at the lower end of the δ15N range. Perturbation did not markedly affect the difference δ15Nsoil - δ15Nleaf, and thus the isotopic data provide no evidence of an alteration in the different N acquisition patterns. Extremely large interspecific differences in sunlit leaf δ13C were observed in the forest (average values from -31.4 to -26.7‰), corresponding to intrinsic water-use efficiencies (ratio CO2 assimilation rate/leaf conductance for water vapour) varying over a threefold range. Wood cellulose δ13C was positively related to total leaf δ13C, the former values being 2-3‰ higher than the latter ones. Leaf δ13C was not related to leaf δ15N at either intraspecific or interspecific levels. δ13C of sunlit leaves was highest in shade hemitolerant emergent species and was lower in heliophilic, but also in shade-tolerant species. For a given species, leaf δ13C did not differ between the pristine forest and the disturbed plantation conditions. Our results are not in accord with the concept of existence of functional types of species characterized by common suites of traits underlying niche differentiation; rather, they support the hypothesis that each trait leads to a separate grouping of species.

Effects of dessication on post-planting stress in bare-root Corsican pine seedlings.

We examined the post-planting consequences of pre-planting exposure stress on two-year-old, bare-root Corsican pine (Pinus nigra Arnold. ssp. laricio var. Corsicana) seedlings. Seedlings were lifted from a nursery and exposed to ambient conditions for periods of up to 192 h before being planted in minirhizotrons. Exposure decreased seedling water potential, CO(2) assimilation rate, leaf conductance and new root elongation, and increased mortality after planting. During exposure, needle total nonstructural carbohydrates (TNC) concentration (expressed on a dry mass basis) decreased by 0.31 mg g(dm) (-1) h(-1); however, needle and root TNC concentrations remained high (> 100 mg g(dm) (-1)) at planting, even in those treatments leading to severe seedling mortality. More than 90% of the seedlings with predawn water potentials lower than -1.3 MPa at planting did not elongate new roots and did not survive, whereas a similar percentage of seedlings with a predawn water potential above this value at planting elongated new roots and survived, suggesting that this value corresponds to a turgor threshold below which new root formation is inhibited. At planting, embolization of xylem conduits in roots and shoots was low for seedlings in all of the exposure treatments.

Interseasonal comparison of CO2 concentrations, isotopic composition, and carbon dynamics in an Amazonian rainforest (French Guiana).

Canopy CO2 concentrations in a tropical rainforest in French Guiana were measured continuously for 5 days during the 1994 dry season and the 1995 wet season. Carbon dioxide concentrations ([CO2]) throughout the canopy (0.02-38 m) showed a distinct daily pattern, were well-stratified and decreased with increasing height into the canopy. During both seasons, daytime [CO2] in the upper and middle canopy decreased on average 7-10 µmol mol-1 below tropospheric baseline values measured at Barbados. Within the main part of the canopy (≥ 0.7 m), [CO2] did not differ between the wet and dry seasons. In contrast, [CO2] below 0.7 m were generally higher during the dry season, resulting in larger [CO2] gradients. Supporting this observation, soil CO2 efflux was on average higher during the dry season than during the wet season, either due to diffusive limitations and/or to oxygen deficiency of root and microbial respiration. Soil respiration rates decreased by 40% after strong rain events, resulting in a rapid decrease in canopy [CO2] immediately above the forest floor of about 50␣µmol mol-1. Temporal and spatial variations in [CO2]canopy were reflected in changes of δ13Ccanopy and δ18Ocanopy values. Tight relationships were observed between δ13C and δ18O of canopy CO2 during both seasons (r 2 > 0.86). The most depleted δ13Ccanopy and δ18Ocanopy values were measured immediately above the forest floor (δ13C = -16.4‰; δ18O = 39.1‰ SMOW). Gradients in the isotope ratios of CO2 between the top of the canopy and the forest floor ranged between 2.0‰ and 6.3‰ for δ13C, and between 1.0‰ and 3.5‰ for δ18O. The δ13Cleaf and calculated c i/c a of foliage at three different positions were similar for the dry and wet seasons indicating that the canopy maintained a constant ratio of photosynthesis to stomatal conductance. About 20% of the differences in δ13Cleaf within the canopy was accounted for by source air effects, the remaining 80% must be due to changes in c i/c a. Plotting 1/[CO2] vs. the corresponding δ13C ratios resulted in very tight, linear relationships (r 2 = 0.99), with no significant differences between the two seasons, suggesting negligible seasonal variability in turbulent mixing relative to ecosystem gas exchange. The intercepts of these relationships that should be indicative of the δ13C of respired sources were close to the measured δ13C of soil respired CO2 and to the δ13C of litter and soil organic matter. Estimates of carbon isotope discrimination of the entire ecosystem, Δe, were calculated as 20.3‰ during the dry season and as 20.5‰ during the wet season.

Responses of Antioxidative Systems to Drought Stress in Pendunculate Oak and Maritime Pine as Modulated by Elevated CO2.

The aim of the present study was to investigate the effects of an enhanced CO2 concentration alone or in combination with drought stress on antioxidative systems of a deciduous (oak; Quercus robur) and an evergreen (pine; Pinus pinaster) tree species. The seedlings were grown for one season in a greenhouse in tunnels supplied with 350 or 700 [mu]L L-1 CO2. The experiment was repeated in a second year. Antioxidants, protective enzymes, soluble protein, and pigments showed considerable fluctuations in different years. Elevated CO2 caused significant reductions in the activities of superoxide dismutases in both oak and pine. The activities of ascorbate peroxidase and catalase were also reduced in most cases. The activities of dehydroascorbate reductase, monodehydroascorbate radical reductase, glutathione reductase, and guaiacol peroxidase were affected little or not at all by elevated CO2. When the trees were subjected to drought stress by withholding water, the activities of antioxidative enzymes decreased in leaves of pine and oak grown at ambient CO2 and increased in plants grown at elevated CO2 concentrations. The present results suggest that growth in elevated CO2 might reduce oxidative stress to which leaf tissues are normally exposed and enhance metabolic flexibility to encounter increased stress by increases in antioxidative capacity.

Interactive effects of elevated CO(2) and soil drought on growth and transpiration efficiency and its determinants in two European forest tree species.

The responses of growth and transpiration efficiency (W = biomass accumulation/water consumption) to ambient and elevated atmospheric CO(2) concentrations (350 and 700 micro mol mol(-1), respectively) were investigated under optimal nutrient supply in well-watered and in drought conditions in two temperate-forest tree species: Quercus petraea Liebl. and Pinus pinaster Ait. Under well-watered conditions, doubling the CO(2) concentration for one growing season increased biomass growth by 138% in Q. petraea and by 63% in P. pinaster. In contrast, under drought conditions, elevated CO(2) increased biomass growth by only 47% in Q. petraea and had no significant effect on biomass growth in P. pinaster. Transpiration efficiency was higher in Q. petraea than in P. pinaster in all treatments. This difference was linked (i) to lower carbon isotope discrimination (Delta), and thus lower values of the intercellular/ambient CO(2) concentration (c(i)/c(a)) ratio, in Q. petraea, (ii) to lower values of leaf mass ratio (LMR, leaf mass/whole plant mass), which we suggest was positively related to the proportion of daytime carbon fixation lost by respiration (Phi), in Q. petraea, and (iii) to slightly lower C concentrations in Q. petraea than in P. pinaster. The CO(2)-promoted increase in W was higher in Q. petraea (+80%) than in P. pinaster (+50%), and the difference was associated with a more pronounced decrease in Phi in response to elevated CO(2) in Q. petraea than in P. pinaster, which could be linked with the N dilution effect observed in Q. petraea. Because Phi also directly affects growth, the CO(2)-induced enhancement of Phi in Q. petraea is a crucial determinant of the growth stimulation observed in this species. Leaf gas exchange regulation was not the only factor involved in the responses of growth and W to elevated CO(2) and drought, other physiological processes that have crucial roles include carbon and N allocation and respiration.

Pioneer and late stage tropical rainforest tree species (French Guiana) growing under common conditions differ in leaf gas exchange regulation, carbon isotope discrimination and leaf water potential.

Leaf gas exchange rates, predawn Ψwp and daily minimum Ψwm leaf water potentials were measured during a wet-to-dry season transition in pioneer (Jacaranda copaia, Goupia glabra andCarapa guianensis) and late stage rainforest tree species (Dicorynia guianensis andEperua falcata) growing in common conditions in artificial stands in French Guiana. Carbon isotope discrimination (Δ) was assessed by measuring the stable carbon isotope composition of the cellulose fraction of wood cores. The Δ values were 2.7‰ higher in the pioneer species than in the late stage species. The calculated time integratedC i values derived from the Δ values averaged 281 µmol mol-1 in the pioneers and 240 µmol mol-1 in the late stage species. The corresponding time-integrated values of intrinsinc water-use efficiency [ratio CO2 assimilation rate (A)/leaf conductance (g)] ranged from 37 to 47 mmol mol-1 in the pioneers and the values were 64 and 74 mmol mol-1 for the two late stage species. The high Δ values were associated-at least inJ. copaia-with high maximumg values and with high plant intrinsinc specific hydraulic conductance [C≔g/(Ψwm-Ψwp], which could reflect a high competitive ability for water and nutrient uptake in the absence of soil drought in the pioneers. A further clear discriminating trait of the pioneer species was the very sensitive stomatal response to drought in the soil, which might be associated with a high vulnerability to cavitation in these species. From a methodological point of view, the results show the relevance of Δ for distinguishing ecophysiological functional types among rainforest trees.

Planting stress, water status and non-structural carbohydrate concentrations in Corsican pine seedlings.

Two-year-old Corsican pine (Pinus nigra ssp. laricio var. Corsicana) seedlings were either well watered or subjected to a moderate drought for one month before being lifted from the nursery bed on October 9 and transplanted. Well-watered, non-transplanted seedlings served as controls. Needle predawn water potential (Psi(wp)), non-structural carbohydrate concentrations and plant development (survival, bud break, shoot elongation) were assessed before and during the first growing season after transplanting. On April 16, just before bud break, Psi(wp) was lower for the well-watered + transplanted and drought-conditioned + transplanted seedlings (Psi(wp) = -1.45 and -1.83 MPa, respectively) than for the controls (Psi(wp) = -0.56). There was a close relationship between the Psi(wp) measured on April 16 and bud break, shoot elongation and plant survival during the following growing period. Above a Psi(wp) of -1.1 MPa, all plants developed normally. Between -1.1 MPa and -1.6 MPa, bud break, and thus shoot elongation, did not occur in all plants. Between -1.6 MPa and -2.1 MPa, the plants were characterized by the absence of shoot growth, but mortality was zero. Below -2.1 MPa, there was a large increase in plant mortality. On April 16, starch concentrations were markedly lower in the roots of transplanted seedlings than in the controls. There was a positive correlation between Psi(wp) and root starch concentration. The Psi(wp) (-2.3 MPa) at which complete starch depletion was observed in the roots corresponded to the Psi(wp) below which plants did not survive. These results suggest that mechanisms specifically linked to altered water status and metabolic processes associated with altered carbohydrate status are involved in transplanting stress; however, it was not possible to disentangle the two effects. Drought conditioning did not lead to a marked increase in soluble carbohydrate concentrations, as reported for other species, and did not increase plant tolerance to transplanting stress.

Photosynthesis Decrease and Stomatal Control of Gas Exchange in Abies alba Mill. in Response to Vapor Pressure Difference.

The responses of steady state CO(2) assimilation rate (A), transpiration rate (E), and stomatal conductance (g(s)) to changes in leaf-to-air vapor pressure difference (DeltaW) were examined on different dates in shoots from Abies alba trees growing outside. In Ecouves, a provenance representative of wet oceanic conditions in Northern France, both A and g(s) decreased when DeltaW was increased from 4.6 to 14.5 Pa KPa(-1). In Nebias, which represented the dry end of the natural range of A. alba in southern France, A and g(s) decreased only after reaching peak levels at 9.0 and 7.0 Pa KPa(-1), respectively. The representation of the data in assimilation rate (A) versus intercellular CO(2) partial pressure (C(i)) graphs allowed us to determine how stomata and mesophyll photosynthesis interacted when DeltaW was increased. Changes in A were primarily due to alterations in mesophyll photosynthesis. At high DeltaW, and especially in Ecouves when soil water deficit prevailed, A declined, while C(i) remained approximately constant, which may be interpreted as an adjustment of g(s) to changes in mesophyll photosynthesis. Such a stomatal control of gas exchange appeared as an alternative to the classical feedforward interpretation of E versus DeltaW responses with a peak rate of E. The gas exchange response to DeltaW was also characterized by considerable deviations from the optimization theory of IR Cowan and GD Farquhar (1977 Symp Soc Exp Biol 31: 471-505).