Ecophysiological Information from Ae

It is well established that leaf carbon-isotope discrimination provides valuable information about the ratio of leaf assimilation to stomatal conductance (intrinsic water-use efficiency; Farquhar et at, 1989). Based on field observations of Ac, Buchmann ct al. (1998) postulated that ecosystem discrimination might increase with decreasing ratio of ecosystem assimilation to canopy conductance (A/Gc, in mol CO, per mol II,0). Using BIOME3.5, we tested this hypothesis for the 22 biomes simulated by the model (Fig. 5).

We found a strong negative relationship of Ac with A/Gc. Within the C, biomes, deserts and tropical grasslands show low Ae values, but high A/Gc ratios. Tropical savannas have a much lower A/Gc ratio, mainly due to the mixture with tropical, drought-deciduous C? trees. Obviously, this new vegetation component increases the water loss from savanna regions compared to pure C , ecosystems. Differences in physiological regulation of the gas exchange between the two photosynthetic pathways is probably the main reason for the drop in A/Gc (Ehleringer ct al., 1997). However, effects due to the mixture with deciduous or semideciduous trees and shrubs might also be responsible for this pattern in savannas (Larcher, 1994). Furthermore, deciduous and semideciduous tropical forests and woodlands show lower A/Gc ratios but similar Ae compared to evergreen tropical forests. Broad leaf and needle leaf evergreen forests in temperate regions also exhibit higher A/Gc ratios than deciduous temperate forests, at the same

BIOME3.5

BIOME3.5

Ecosystem discrimination, Ae [%„]

FIGURE 5 Relationship between At, estimates and modeled ratio of ecosystem assimilation to canopy conductance (A/Gc; using BIOME3.5). 1, tropical evergreen forest; 2, tropical semideciduous forest; 3, tropical deciduous forest/woodland; 4, temperate broadleaf evergreen forest; 5, temperate deciduous forest; 6, temperate conifer forest; 7, warm mixed forest; 8, cool mixed forest; 9, cold mixed forest; 10, evergreen taiga/montane forest; 11, deciduous taiga/montane forest; 12, tropical savanna; 13, temperate sclerophyll woodland; 14, temperate woodland; 15, tropical grassland; 16, temperate grassland; 17, desert: shrubland and steppe; 18, steppe tundra; 19, shrub tundra; 20, dwarf shrub tundra; 21, prostrate shrub tundra; 22, cushion-forb, lichen and moss tundra.

Ecosystem discrimination, Ae [%„]

FIGURE 5 Relationship between At, estimates and modeled ratio of ecosystem assimilation to canopy conductance (A/Gc; using BIOME3.5). 1, tropical evergreen forest; 2, tropical semideciduous forest; 3, tropical deciduous forest/woodland; 4, temperate broadleaf evergreen forest; 5, temperate deciduous forest; 6, temperate conifer forest; 7, warm mixed forest; 8, cool mixed forest; 9, cold mixed forest; 10, evergreen taiga/montane forest; 11, deciduous taiga/montane forest; 12, tropical savanna; 13, temperate sclerophyll woodland; 14, temperate woodland; 15, tropical grassland; 16, temperate grassland; 17, desert: shrubland and steppe; 18, steppe tundra; 19, shrub tundra; 20, dwarf shrub tundra; 21, prostrate shrub tundra; 22, cushion-forb, lichen and moss tundra.

Ae value. Thus, the advantage of the evergreen life form that is well established at the plant and the leaf levels (Aerts, 1995) also shows at the ecosystem level. The model's simulation of leaf area index, which is optimized for A/Gc, is closely correlated to Ae in most cases, indicating the importance of canopy structure and canopy roughness for determining Ae (unpublished mss.).

Highest A/Gc within boreal and arctic biomes was modeled for steppe tundra. Its A/Gc ratio was of a similar magnitude as that of tropical grasslands, but with a higher corresponding Ae estimate. Tundra Ae values of about 12%o indicated that the C, vegetation was probably growing under conditions of low water availability (i.e., in polar deserts). The highest Ae estimate coupled with the lowest A/Gc was found for deciduous taiga forests, clearly indicating the isotopic signature of C, vegetation and high water supply during the growing season. Both boreal ecosystems span almost the entire range of the observed ecosystem discrimination and the ratio of ecosystem assimilation to canopy conductance, representing the two end-members of this negative relationship between Ae and AGC for C3 vegetation.

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