The rates of foliar dark respiration and net photosynthesis in attached leaves of 25 C3, C4, and C3-C4 intermediate dicotyledonous weed species were determined using the infrared gas analyzer. The ratio of dark respiration to photosynthesis per unit leaf area in attached leaves of each species was inversely proportional to leaf age. Highly significant, positive linear correlation was observed between the rates of foliar dark respiration and net photosynthetic CO2 uptake in dicot weeds irrespective of the photosynthetic type. The higher foliar dark respiration rate found in some of the weed species can be attributed in part to the higher carbohydrate levels as generated by a rapid photosynthetic CO2 assimilation. The significance of higher dark respiration rate in relation to carbon and energy economy of weeds is discussed.

Southern African savannas are mixed plant communities where C3 trees co-exist with C4 grasses. Here foliar δ 15N and δ 13C were used as indicators of nitrogen uptake and of water use efficiency to investigate the effect of the rainfall regime on the use of nitrogen and water by herbaceous and woody plants in both dry and wet seasons. Foliar δ 15N increased as aridity rose for both C3 and C4 plants for both seasons, although the magnitude of the increase was different for C3 and C4 plants and for two seasons. Soil δ 15N also significantly increased with aridity. Foliar δ 13C increased with aridity for C3 plants in the wet season but not in the dry season, whereas in C4 plants the relationship was more complex and non-linear. The consistently higher foliar δ 15N for C3 plants suggests that C4 plants may be a superior competitor for nitrogen. The different foliar δ 13C relationships with rainfall may indicate that the C3 plants have an advantage when competing for water resources. The differences in water and nitrogen use likely collectively contribute to the tree–grass coexistence in savannas. Such differences facilitate interpretations of palaeo-vegetation composition variations and help predictions of vegetation composition changes under future climatic scenarios.

13C/12C and 18O/16O ratios of aragonite shells of modern land snails from the southern Great Plains of North America were measured for samples from twelve localities in a narrow east–west corridor that extended from the Flint Hills in North Central Oklahoma to the foothills of the Sangre de Cristo Mountains in Northern New Mexico, USA. Across the study area, shell δ18O values (PDB scale) ranged from −4.1‰ to 1.2‰, while δ13C values ranged from −13.2‰ to 0.0‰. δ18O values of the shell aragonite were predicted with a published, steady state, evaporative flux balance model. The predicted values differed (with one exception) by less than 1‰ from locality averages of measured δ18O values. This similarity suggests that relative humidity at the time of snail activity is an important control on the δ18O values of the aragonite and emphasizes the seasonal nature of the climatic information preserved in the shells. Correlated δ13C values of coexisting Vallonia and Gastrocopta suggest similar feeding habits and imply that these genera can provide information on variations in southern Great Plains plant ecology. Although there is considerable scatter, multispecies, transect average δ13C values of the modern aragonite shells are related to variations in the type of photosynthesis (i.e., C3, C4) in the local plant communities. The results of this study emphasize the desirability of obtaining isotope ratios representing averages of many shells in a locale to reduce possible biases associated with local variations among individuals, species, etc., and thus better represent the “neighborhood” scale temporal and/or spatial environmental variations of interest in studies of modern and ancient systems.