Model results for ae estimates display several important features reflecting global trends in vegetation composition and water status (Fig. 3). Generally, ecosystems in cool, wet environments exhibit the greatest ae values, whereas C4-dominated, tropical grasslands show the lowest ae estimates. However, within the area of C3-dominated vegetation, there is a wide range of ae values, representative of changing water status of the vegetation and plant adaptation to these environments. Dry deserts have characteristically low ae values due to the plants' water conserving strategy. High ratios of assimilation to stomatal conductance result in low C/Ca ratios, and therefore in low foliar carbon discrimination (Farquhar et al, 1989). Ecosystems in Mediterranean-type winterrain climates show a higher ae because most of plant production takes place in the wet winter season (i.e., in the Mediterranean basin). A study of natural vegetation and unirrigated agriculture on Crete showed that Mediterranean plants had unexpectedly depleted foliar 5I3C ratios and therefore high leaf-carbon discriminations (Kaplan, unpublished data). In contrast, dry savannas and woodlands in summer-rain areas have lower ae values in apparent response to the higher evaporative demand (i.e., in Australia). In some temperate deserts that are dominated by C3 grasses and C3 shrubs, the presence of C4 woody shrubs, which are simulated in this model, further depresses the ae signal from that region (i.e., southwestern U.S.A.).
Latitudinal averages of flux-weighted ae estimates display a tri-modal distribution with ae values being greatest in the boreal zones of both hemispheres and the humid tropics (Fig. 4). Due to large longitudinal variations within a latitudinal band of 0.5°, the moist, highly productive tropical rain forests (between 10° N and S) overshadow any signal from C4 equatorial grasslands such as those in East Africa. In the boreal zone (between 55° and 80° N), ae is generally high because of low ratios of assimilation to stomatal conductance (i.e., intrinsic water-use efficiencies) of boreal plants. In addition, waterlogged soils due to permafrost and low evaporative demand in maritime influenced areas (i.e., Chile, Alaska) tend to increase ae. Boreal forests dominated by Larix, Betula, Sorbus and other deciduous species show even higher ae values than evergreen taiga, presumably because of their physiological ability to transpire more water and hence achieve a higher C-JC.d ratio. These model results are corroborated by field studies on leaf 3I3C ratios (Lloyd and Farquhar, 1994, Michelsen etal, 1996).
10 12 14 16 18 20 22 24 flux-weighted %0
FIGURE 3 Modeled ecosystem carbon-isotope discrimination. See Sec. 3 for details on BIOME3.5.
Mean values for modeled Ae estimates are lowest in subtropical C4-dominated grasslands, especially in Africa and Australia. In the temperate prairies of central North America and Eurasia, C4 grasslands are only seasonally dominant and share latitude bands with both forests and deserts. Thus, in these regions Ae values are correspondingly intermediate.
both estimates using free tropospheric air measurements (Trolier et al, 1996; Bakwin et al, 1998) showed lower standard deviations than the estimate based on canopy air measurements from 50 different ecosystem studies (0.4 and 0.8%o vs 2.2%o). Mean SL,CER values from these three independent estimates ranged between — 24.7 and — 25.9%o, with a global mean SL,CER value of — 25.3 ±
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