Báo cáo lâm nghiệp: "Gas exchange and water relations of evergreen and deciduous tropical savanna trees"

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Gas exchange and water relations of evergreen and deciduous tropical savanna trees G. Goldstein F. Rada P. Rundel’ A. Azocar 2 12 2 A. Orozco 1 Laboratory of Biomedical and Environmental Sciences, University of California Los Angeles, 900 Veteran Ave., Los Angeles, CA 90024, U.S.A., and 2 Departamento de Biologia, Facultad de Ciencias, Universidad de Los Andes, Merida, Venezuela ther evaluate water use efficiency. Our Introduction main hypotheses are that: 1) the vascular system of the evergreen trees is more effi- Many neotropical savannas with pro- cient than the vascular system of the deci- nounced wet/dry seasonality and well- duous species for water transport; 2) the drained soils are characterized by the structural basis for the high efficiency in presence of both evergreen and decidu- water transport of the evergreen species ous trees. The evergreen species grow as is more related to the cross-sectional area isolated individuals in the oligotrophic soils of the conducting tissue per surface area which predominate, while the deciduous of supplied leaves (Huber values) than to species form small forest ’islands’ located intrinsic properties of the vascular system, on patches of richer soil (Sarmiento, such as large vessels; 3) despite the fact 1984). The trees in these forest islands that the leaves of the evergreen plants are mostly drought deciduous, dropping are more scleromorphic and longer lived, its their leaves at the onset of the dry season. C0 assimilation rates are as high or even 2 In contrast to the more pliant foliage of the higher, than the photosynthetic rates of deciduous species, evergreen trees tend deciduous trees; and 4) water and nitro- to have scleromorphic leaves. An ad- gen use efficie!ncies are similar between ditional structural difference is that ever- the 2 groups of species. Some of these green species have relatively large root hypotheses contradict current notions systems allowing them access to soil concerning leai life span and physiological water throughout the rainless period behavior of the plant species. (Medina, 1982; Sarmiento et al., 1985). The purpose of this study was to investi- gate gas exchange characteristics, water Materials and Methods relations and vascular hydraulic properties of 2 evergreen and 2 drought deciduous Two evergreen and 2 deciduous woody species tree species. In addition, carbon isotope studied in the Venezuelan Ilanos (200 m were ratios of leaf tissue were measured to fur- elev., 9°37’N and 70°12’W). Curatella america-
dif- and the 2 evergreen Byrsonima crassifolia, significantly water potentials were not na species, initiate leaf renewal during the middle ferent between the 2 groups of species of the dry season, when the old leaves start to higher efficiency of water suggesting a senesce. The average leaf life span is approxi- transport in the evergreen species. mately 14 mo. Leaf longevity of the 2 drought deciduous species, Genipa caruto and Cochlo- Physiological estimates of hydraulic spermum vitifolium is shorter. Leaf production properties in terminal stem sections of starts with the onset of the rainy season, and several branches support the hypothesis leaves last for about 8-9 mo, at which time that resistances to water flow in the liquid water is no longer available in the upper part of the soil profile. phase were significantly smaller in the A portable system was used to measure gas evergreen than in deciduous savanna exchange in the field (LCA-2 system). Gas (Table I). Leaf-specific conductivities trees exchange calculations are according to von (LSCs - hydraulic conductivity per leaf Caemmerer and Farquhar (1981). Leaf water surface area supplied) were higher in the potential was measured with a pressure cham- evergreen plants. Terminal branches were ber. Hydraulic properties were estimated using methods outlined in Zimmermann (1978) and used to compare the hydraulic conductivi- Goldstein et al. (1987). Sap flow velocity was ty among species because smaller measured with a heat pulse apparatus. Carbon branches tend to be less efficient in water isotope ratios of leaf tissues are reported in6 transportation and represent, therefore, units relative to PDB standard. the hydraulic constriction or bottleneck for water movement in the plant. Table I summarizes information on the Results and Discussion xylem anatomy, the ratio between the xylem transverse section and the total sur- exhibited Evergreen species generally face area of the supported leaves and the higher rates of water loss than deciduous sap flux density predicted by Poiseuille’s species (Fig. 1The rate of water loss law for ideal capillaries. Regression analy- was determined both by extrapolations of sis for LSC versus all the anatomical and the porometer measurements on an area morphological variables indicates that the basis and by calculation from heat pulse Huber value is the best predictor of LSC measurements. Both estimates of volume- 2 (r 0.87). An important intrinsic charac- = tric water flux tend to agree. Despite dif- teristic of the water flow system, such as ferences in transpiration rates, minimum the mean vessel diameter for example,
not significantly correlated with C.SC. amount of vascular tissue, rather than the was It appears that the increased hydraulic ef- consequence of wider and more efficient ficiency of evergreen tropical savanna conducting vessels. species is a consequence of relatively low Photosynthetic rates and instantaneous total leaf surface area compared to the water use effic:iencies were monitored in
the field during the wet season, when both Mooney, 1986). Large differences in and groups of species support active leaves. specific leaf weight (leaf mass to area Neither the photosynthetic capacity nor ratio) between deciduous and evergreen the water use efficiency of the deciduous plants result in larger differences in photo- species was higher relative to the ever- synthetic rates expressed on a weight green species (Fig. 2); integration of gas basis. Carbon isotope ratios of leaf tissue measured to further evaluate water exchange measurements during the were efficiency. Table II shows that the 0 C 13 course of the day generally suggests that use the photosynthetic rates of the evergreen values of several evergreen and decidu- species tend to be slightly higher (data not ous savanna trees, including the previous 4 species 0 values were in the range of C 13 shown). We have expressed gas ex- change measurements on an area-base - 27 to -31 %, and that there were no because light interception and gas significant differences between deciduous exchange with the atmosphere are area- and Furthermore, trees. evergreen based there were no significant differences in Field phenomena (however, see
instantaneous water use efficiencies period for photosynthesis. In this regard, (WUE) between evergreen and deciduous the evergreen strategy can compensate for higher maintenance costs; however, it trees. The small differences between INUE as estimated by gas exchange and appears that increased life span does not amortize additional maintenance costs if as estimated by the relative amount of photosynthetic rates of evergreen species carbon stable isotopes can be attributed to differences in nighttime respiration rates are low. and differences in timing of leaf construc- tion (dry season for the evergreen trees versus wet season for the deciduous Acknowledgments trees). The 2 evergreen species have more study was supported in part by a CONICIT This efficient systems for water transport than grant no. S1-1588 and by an NSF grant do the 2 deciduous woody species. In (BSR-86-15575). We are grateful to C. Swift for her comments on the manuscript. the high evaporative demand savanna environment, water transport efficiency is advantageous because it permits mainten- of high stomatal conductance ance References without turgor loss, particularly during the dry season when evaporative demand is Field C. & Mooney H.A. (1986) The photosyn- higher. It is possible that relatively high thesis-nitrogen relationship in wild plants. In: conductances may be critical for the main- On the Economy of Plant Form and Function tenance of a favorable carbon and nutrient (Givnish T.J., ed.), Cambridge University Press, Cambridge, pp. :?5-55 balance in the evergreen species. Com- Goldstein G., Flada F. & Catalan A. (1987) pared to deciduous trees, evergreen trees Water transport efficiency in stems of evergreen have a much higher maintenance cost due and deciduous savanna trees. In: Proceedings to the presence of an extensive root sys- of the International Conference on Measure- tem and scleromorphic leaves. An in- ment of Soil and Plant Water Status, Vol. 2. creased leaf life span increases the time Plants. Utah State University, pp. 267-274
BioL Rev. 60, cies in neotropical Physiological ecology of neo- Medina E. (1982) savannas. plants. In: Ecology of Tropical 315-355 tropical savanna Savannas, Ecological Studies 42. (Huntley B.J. Caemmerer S. & Farquhar G.D. (1981) von & Walker B.H., eds.), Springer-Verlag, Berlin, Some relationships between the biochemistry of pp. 308-355 photosynthesis and the gas exchange of Sarmiento G. (1984) In: The Ecology of Neo- leaves. Planta 153, 376-387 tropical Savannas. Harvard University Press, Cambridge, pp. 235 Zimmermann M.H. (1978) Hydraulic architec- ture of some diffuse-porous trees. Can. J. Bot Sarmiento G., Goldstein G. & Meinzer F. (1985) Adaptative strategies of woody spe- 56, 2286-2295