Common generalizations concerning the ecologic significance of C 4 photosynthesis were tested in a study of plant gas exchange, productivity, carbon balance, and water use in monospecific communities of C 3 and C 4 salt desert shrubs. Contrary to expectations, few of the hypotheses concerning the performance of C 4 species were supported. Like the C 3 species, Ceratoides lanata , the C 4 shrub, Atriplex confertifolia , initiated growth and photosynthetic activity in the cool spring months and also exhibited maximum photosynthetic rates at this time of year. To compete successfully with C 3 species, Atriplex may have been forced to evolve the capacity for photosynthesis at low temperatures prevalent during the spring when moisture is most abundant. Maximum photosynthetic rates of Atriplex were lower than those of the C 3 species. This was compensated by a prolonged period of low photosynthetic activity in the dry late summer months while Ceratoides became largely inactive. However, the annual photosynthetic carbon fixation per ground area was about the same in these two communities composed of C 3 and C 4 shrubs. The C 4 species did not exhibit greater leaf diffusion resistance than the C 3 species. The photosynthesis/transpiration ratios of the two species were about the same during the period of maximum photosynthetic rates in the spring. During the warm summer months the C 4 species did have superior photosynthesis/transpiration ratios. Yet, since Ceratoides completed a somewhat greater proportion of its annual carbon fixation earlier in the season, the ratio of annual carbon fixation/transpiratory water loss in the two communities was about the same. Atriplex did incorporate a greater percentage of the annual carbon fixation into biomass production than did Ceratoides . However, this is considered to be a reflection of properties apart from the C 4 photosynthetic pathway. Both species displayed a heavy commitment of carbon to the belowground system, and only about half of the annual moisture resource was utilized in both communities.
Oecologia – Springer Journals
Published: Dec 1, 1977
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