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The stable C isotope composition (δ13C) of CO2 respired by trunks was examined in a mature temperate deciduous oak forest ( Quercus petraea). Month-to-month, day-to-day and diurnal, measurements were made to determine the range of variations at different temporal scales. Trunk growth and respiration rates were assessed. Phloem tissue was sampled and was analysed for total organic matter and soluble sugar 13C composition. The CO2 respired by trunk was always enriched in 13C relative to the total organic matter, sometimes by as much as 5‰. The δ13C of respired CO2 exhibited a large seasonal variation (3.3‰), with a relative maximum at the beginning of the growth period. The lowest values occurred in summer when the respiration rates were maximal. After the cessation of radial trunk growth, the respired CO2 δ13C values showed a progressive increase, which was linked to a parallel increase in soluble sugar content in the phloem tissue ( R = 0.95; P < 0.01). At the same time, the respiration rates declined. This limited use of the substrate pool might allow the discrimination during respiration to be more strongly expressed. The late-season increase in CO2 δ13C might also be linked to a shift from recently assimilated C to reserves. At the seasonal scale, CO2 δ13C was negatively correlated with air temperature ( R = −0.80; P < 0.01). The diurnal variation sometimes reached 3‰, but the range and the pattern depended on the period within the growing season. Contrary to expectations, diurnal variations were maximal in winter and spring when the leaves were missing or not totally functional. By contrast to the seasonal scale, these diurnal variations were not related to air temperature or sugar content. Our study shows that seasonal and diurnal variations of respired 13C exhibited a similar large range but were probably explained by different mechanisms. [ABSTRACT FROM AUTHOR]

Leaves from several desert and woodland species, including gymnosperms and angiosperms with both C3 and C4 physiology, were analyzed to detect trends in δ13Cleaf with elevation and slope aspect along two transects in southeastern Utah and south-central New Mexico, USA. The main difference between the two transects is the steeper elevational gradient for mean annual and summer precipitation in the southern transect. For any given species, we found that isotopic differences between individual plants growing at the same site commonly equal differences measured for plants along the entire altitudinal gradient. In C3 plants, δ13Cleaf values become slightly enriched at the lowest elevations, the opposite of trends identified in more humid regions. Apparently, increasing water-use efficiency with drought stress offsets the influence of other biotic and abiotic factors that operate to decrease isotopic discrimination with elevation. For some species shared by the two transects (e.g., Pinus edulis and Cercocarpus montanus), δ13Cleaf values are dramatically depleted at sites that receive more than 550 mm mean annual precipitation, roughly the boundary (pedalfer-pedocal) at which soils commonly fill to field capacity in summer and carbonates are leached. We hypothesize that, in summer-wet areas, this may represent the boundary at which drought stress overtakes other factors in determining the sign of δ13Cleaf with elevation. The opposition of isotopic trends with elevation in arid versus humid regions cautions against standard correction for elevation in comparative studies of δ13Cleaf. [ABSTRACT FROM AUTHOR]

In this study, sun leaf carbon isotope composition (δ13C) of two co-occurring woody Mediterranean species ( Quercus pubescens Willd., a deciduous oak, and Q. ilex L., an evergreen one) was investigated on four sites with different water availability. The total range of δ13C values was 4.4 and 3.1‰ for Q. pubescens and Q. ilex respectively. The intra-site variability was about 3‰. Total mean per species was equal. There were significant differences among sites, but at each site means of δ13C were not significantly different between species. A simple physiological model predicts no difference in intrinsic water-use efficiency (WUEi) between evergreen and deciduous oaks. The relationship between site means of δ13C and water parameters suggests that there is a leaf functional adjustment with respect to available water resource. No correlation was found between δ13C and the contents of any mass-based biochemical constituent. Nevertheless there was a significant correlation between δ13C and leaf mass per area of Q. ilex. For both species, there is also a positive correlation between leaf δ13C and individual crown area, i.e. a structural characteristic at tree level. Causal relations between δ13C and plant-environment interactions are discussed. [ABSTRACT FROM AUTHOR]

We tested whether variation in growth of native koa ( Acacia koa) forest along a rainfall gradient was attributable to differences in leaf area index (LAI) or to differences in physiological performance per unit of leaf area. Koa stands were studied on western Kauai prior to Hurricane Iniki, and ranged from 500 to 1130 m elevation and from 850 to 1800 mm annual precipitation. Koa stands along the gradient had basal area ranging from 8 to 42 m/ha, LAI ranging from 1.4 to 5.4, and wood increment ranging from 0.7 to 7.1 tonnes/ha/year. N, P, and K contents by weight of sun leaves (phyllodes) were negatively correlated with specific leaf mass (SLM, g m) across sites; on a leaf area basis, N increased whereas P and K decreased with SLM. LAI, aboveground woody biomass increment, and production per unit leaf area ( E) increased as phyllode δC became more negative. The δC data suggested that intrinsic water-use efficiency (ratio of assimilation to conductance) increased as water availability decreased. In five of the six sites, phyllode P contents increased as LAI increased, but biomass increment and E were not correlated with phyllode nutrient contents, suggesting that productivity was limited more by water than by nutrient availability. Because vapor pressure deficits increased with decreasing elevation, actual water-use efficiency (ratio of assimilation to transpiration) was lower at drier, low-elevation sites. There was a trade-off between intrinsic water-use efficiency and production per unit of canopy N or P across the gradient. In summary, koa responds to water limitation both by reducing stand LAI and by adjusting gas exchange, which results in increased intrinsic water-use efficiency but decreased E. [ABSTRACT FROM AUTHOR]

The carbon isotope compositions of samples of Kalanchoë species collected at the natural stands in Madagascar were determined. The results suggest that all species of the genus Kalanchoë are capable of crassulacean acid metabolism. The observed δC values cover the whole range from −10 to −30‰. This high diversity of the δC values was found among the species of the genus as well as, in certain cases, within a single species. This suggest that the CAM patterns in Kalanchoë are generally very flexible. The δC values show a clear correlation with the climate of the habitats from where the samples derived. Values indicative of CO fixation taking place exclusively during the night were found in the dry regions of Madagascar, whereas δC values indicative of mixed CO fixation during night and day or of CO fixation entirely during the day are distributed in the humid zones. [ABSTRACT FROM AUTHOR]

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 were Artemisia tridentata (shrub), Ceratoides lanata (suffrutescent shrub), Elymus lanceolatus (rhizomatous grass), and Elymus 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 test a 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 (δC) of plant material. Additional water decreased WUE, whereas additional nitrogen increased WUE. For both A/E and δC, 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. [ABSTRACT FROM AUTHOR]

The carbon isotope compositions of samples of Kalanchoë species collected at the natural stands in Madagascar were determined. The results suggest that all species of the genus Kalanchoë are capable of crassulacean acid metabolism. The observed δ 13 C values cover the whole range from -10 to -30‰. This high diversity of the δ 13 C values was found among the species of the genus as well as, in certain cases, within a single species. This suggest that the CAM patterns in Kalanchoë are generally very flexible. The δ 13 C values show a clear correlation with the climate of the habitats from where the samples derived. Values indicative of CO 2 fixation taking place exclusively during the night were found in the dry regions of Madagascar, whereas δ 13 C values indicative of mixed CO 2 fixation during night and day or of CO 2 fixation entirely during the day are distributed in the humid zones.