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Fig. 2.13 Ranges of carbonisotope ratios for C3-, C4- and CAM species. Measurements from 285 grasses (Poaceae) with 47 C3 species and 238 C4 species and from 513 CAM plants. (Unpub. data sets of H. Ziegler)

that in C4- and CAM-plants, for each CO2-molecule fixed, a smaller number of H2O-molecules is lost via transpiration as compared to C3-photosynthesis. Thus, in addition to the 13C-content, the 2H- and 18O -content of the plants is also affected. Within the different modes of photosynthesis there are subtleties dependent on the diffusive limitation imposed on evapotranspiration (see above) or in the relative utilization of RuBPC and PEPC in CAM, and generally in the use of water. All this is reflected in isotope composition. Thus, also various ecotypes of plants, like halo-phytes and xerophytes can be differentiated. It must be noted for the latter, however, that such comparisons are only allowed between C3-species because these effects are highly overridden by the carbon isotope effects of CO2 fixation by RuBPC and PEPC, respectively.

Another example from metabolism is represented by N-nutrition of plants. For nitrogen isotope effects the situation often is very complex and straightforward conclusions are difficult (Martinelli et al. 1999; Adams and Grierson 2001). Differences in nitrogen uptake mechanisms and in the pathways of assimilation and recycling of nitrogen in the plants can greatly affect 815N values (Evans 2001). N-salts in the soil tend to enrich the heavier isotope 15N, as compared to atmospheric gaseous N2, and therefore one can recognize symbiotic N2-fixers (e.g. legumes with root nodules) by the lower 15N content. Analyses of the natural abundance of 15N in soils have also served to document forest-to-pasture chronologies and record changes of land-use pattern in the western Amazon Basin in Brazil (Piccolo et al. 1994).

The flow of various isotopes through the biomass of plants also affects the transfer into other compartments of ecosystems. This allows the study of food webs, habitat preferences in wandering animals, and even the analysis of eating and drinking habits in human populations and individuals. For the latter it usually suffices to analyze the organic matter of hairs or finger- and toenails.

Since the analysis of dry matter of the organisms is often sufficient, one can cover large geographic areas with sampling even from remote regions using simple equipment. Even collections in herbaria may be used. In this way, for example one can arrive at conclusions about the large-scale ecological distribution of modes of photosynthesis. C4-grasses dominate in tropical savannas, their relative abundance declines with increasing altitude (Tieszen et al. 1979; Medina 1982; see Sect. 10.1.1.2).

An impressive example, if not for a whole continent, is given for the rather large tropical island of Madagascar of 590,000 km2 (Kluge et al. 1991). Combining the Klimadiagramm method with the stable-isotope technique, the distribution of CAM among species of the genus Kalanchoe has been studied and related to climatic zones and vegetation types of the island. There are 52 species of Kalan-choe in Madagascar, of which all are either obligate or facultative CAM plants. There is high flexibility among the species to obtain a variable amount of carbon by direct CO2 fixation via RuBPC, and this is reflected by increasingly negative 813C values, whereas primary C02-fixation dominated by PEPC leads to less negative 8 13C values. The large scale effects deduced from the analysis present a very close correlation of 8 13 C values in the dry matter of Kalanchoe species and climate and vegetation zones on the island. Less negative (CAM-like) values are dominant in the drier zones with evergreen dry forest, deciduous woodland, savannas and xerophilous thornbush, while more negative (C3-like) values prevail in the wetter zones with evergreen rainforest and montane forest (Fig. 2.14). The example illustrates the close relations between climate, vegetation types and prevalence of the water conserving CAM-mode of photosynthesis in a given genus. It even allows some views into the evolutionary history of the genus Kalanchoe. Comparisons of the phytogeographic distribution of C3- and CAM-species of Kalanchoe in Madagascar with the phylogeny of different subgenera based on morphological and molecular characteristics and the evolution of CAM in the CAM-species show that CAM has only evolved once (monophyletically) from the C3-species of the moister regions and that the more drought resistant CAM-species then have conquered the drier regions of the island (Kluge et al. 1991, 2001; Gehrig et al. 2001).

Large-scale isotope effects, however, may also result from transfer rates. Thus, respiration and photosynthesis of organisms determine the vertical 13CO2-gradient in tropical rainforests (Medina etal. 1986). Richey et al. (1990) have proposed to use 18O-analyses to assess the large-scale consequences of the destruction of tropical

Fig. 2.14A, B Climate zones related to Klimadiagramm distribution (A) and vegetation types (B) of Madagascar. The vegetation map (B) contains points indicating ranges of 813 C values as explained in the inset. Note that the closed symbols marking the more negative 813 C values are concentrated in the wetter regions, and the open symbols marking less negative 813 C values are accumulated in the drier vegetation units. The inset also gives the frequency of 813C values for the samples collected on the island for three combinations of vegetation units as indicated. (After Kluge et al. 1991)

Fig. 2.14A, B Climate zones related to Klimadiagramm distribution (A) and vegetation types (B) of Madagascar. The vegetation map (B) contains points indicating ranges of 813 C values as explained in the inset. Note that the closed symbols marking the more negative 813 C values are concentrated in the wetter regions, and the open symbols marking less negative 813 C values are accumulated in the drier vegetation units. The inset also gives the frequency of 813C values for the samples collected on the island for three combinations of vegetation units as indicated. (After Kluge et al. 1991)

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Renewable Energy 101

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