The Geography of Phytochemical Races

2.6 South America 75
Fig. 2.46 Map of Argentina showing Tagetes minuta sites
comfortably within a collection of papers involving secondary metabolites, it is
suffi ciently interesting that I will break the rules! The work is that of Cabido et al.
(1997) and concerns the distribution of C 3 and C 4 grasses along an altitudinal gradient
in central Argentina. It is well known that C 3 grasses tend to occur in cooler
habitats, whereas C 4 grasses tend to occur in warmer habitats. In the present study,
139 species were scored as a function of relative temperature at eight sites along
an elevation gradient that extended from 350 m to ca. 2100 m in the Córdoba
Mountains (32°60′S, 65°50′W) of north-central Argentina (Fig. 2.46). Fifty-nine
of the species encountered were shown to be C 3 , with the remaining 80 established
as C 4 by the presence of Kranz anatomy. The C 3 species belonged to Aveneae,
Bromeae, Meliceae, Poeae, Stipeae, and Triciceae. Grass species exhibiting the
C 4 syndrome belonged to Andropogoneae, Aristideae, Cynodonteae, Eragrostideae,
and Pappophoreae. Members of Paniceae, which consists of both C 3 and C 4
species, were observed at all sites except the highest one (ca. 2100 m). Although
C 3 species were found at the 1400 m, 1600 m, 1800 m, and 1900 m sites, C 4 species
outnumbered them at all sites except at the 1800 m level, where each was
represented by two species. The three lowest sites, at 300 m, 650 m, and 1000 m,
afforded only C 4 species.
The strong correlation between type of photosynthesis and environment is in
accord with other studies of the distribution of C 3 and C 4 species. For example, an
earlier study in a more arid region of Argentina (Cavagnaro, 1988), involving 31
grass genera, showed that the C 4 grasses, representing 19 genera, occur at lower
elevations with the remaining 12 genera restricted to higher sites. The division line
was located roughly between 1100 m and 1600 m. Rundel (1980) observed a similar
sensitivity to the environment, in terms of elevation, in a study of 65 species
of native and introduced grasses in the Hawaiian Islands, where C 3 species predominate
(40 species) above 1400 m, with the C 4 species inhabiting sites at lower
elevations. Other examples of photosynthetic pathway variation as a function of
environment can be found in the work of Chazdon (1978) in Costa Rica, Hattersley
(1983) in Australia, Wentworth (1983) in southeastern Arizona, and Schwartz and
Redman (1988) in the boreal forests of northwestern Canada.