RESUMO
The decline of atmospheric CO2 over the last 65 million years (Ma) resulted in the 'CO2-starvation' of terrestrial ecosystems and led to the widespread distribution of C4 plants, which are less sensitive to CO2 levels than are C3 plants. Global expansion of C4 biomass is recorded in the diets of mammals from Asia, Africa, North America, and South America during the interval from about 8 to 5 Ma. This was accompanied by the most significant Cenozoic faunal turnover on each of these continents, indicating that ecological changes at this time were an important factor in mammalian extinction. Further expansion of tropical C4 biomass in Africa also occurred during the last glacial interval confirming the link between atmospheric CO2 levels and C4 biomass response. Changes in fauna and flora at the end of the Miocene, and between the last glacial and interglacial, have previously been attributed to changes in aridity; however, an alternative explanation for a global expansion of C4 biomass is CO2 starvation of C3 plants when atmospheric CO2 levels dropped below a threshold significant to C3 plants. Aridity may also have been a factor in the expansion of C4 ecosystems but one that was secondary to, and perhaps because of, gradually decreasing CO2 concentrations in the atmosphere. Mammalian evolution in the late Neogene, then, may be related to the CO2 starvation of C3 ecosystems.
Assuntos
Evolução Biológica , Dióxido de Carbono , Ecossistema , Mamíferos , África , Animais , Ásia , Biomassa , Clima , Dieta , América do Norte , América do Sul , TempoRESUMO
Canopy CO2 concentrations in a tropical rainforest in French Guiana were measured continuously for 5 days during the 1994 dry season and the 1995 wet season. Carbon dioxide concentrations ([CO2]) throughout the canopy (0.02-38 m) showed a distinct daily pattern, were well-stratified and decreased with increasing height into the canopy. During both seasons, daytime [CO2] in the upper and middle canopy decreased on average 7-10 µmol mol-1 below tropospheric baseline values measured at Barbados. Within the main part of the canopy (≥ 0.7 m), [CO2] did not differ between the wet and dry seasons. In contrast, [CO2] below 0.7 m were generally higher during the dry season, resulting in larger [CO2] gradients. Supporting this observation, soil CO2 efflux was on average higher during the dry season than during the wet season, either due to diffusive limitations and/or to oxygen deficiency of root and microbial respiration. Soil respiration rates decreased by 40% after strong rain events, resulting in a rapid decrease in canopy [CO2] immediately above the forest floor of about 50⣵mol mol-1. Temporal and spatial variations in [CO2]canopy were reflected in changes of δ13Ccanopy and δ18Ocanopy values. Tight relationships were observed between δ13C and δ18O of canopy CO2 during both seasons (r 2 > 0.86). The most depleted δ13Ccanopy and δ18Ocanopy values were measured immediately above the forest floor (δ13C = -16.4; δ18O = 39.1 SMOW). Gradients in the isotope ratios of CO2 between the top of the canopy and the forest floor ranged between 2.0 and 6.3 for δ13C, and between 1.0 and 3.5 for δ18O. The δ13Cleaf and calculated c i/c a of foliage at three different positions were similar for the dry and wet seasons indicating that the canopy maintained a constant ratio of photosynthesis to stomatal conductance. About 20% of the differences in δ13Cleaf within the canopy was accounted for by source air effects, the remaining 80% must be due to changes in c i/c a. Plotting 1/[CO2] vs. the corresponding δ13C ratios resulted in very tight, linear relationships (r 2 = 0.99), with no significant differences between the two seasons, suggesting negligible seasonal variability in turbulent mixing relative to ecosystem gas exchange. The intercepts of these relationships that should be indicative of the δ13C of respired sources were close to the measured δ13C of soil respired CO2 and to the δ13C of litter and soil organic matter. Estimates of carbon isotope discrimination of the entire ecosystem, Δe, were calculated as 20.3 during the dry season and as 20.5 during the wet season.
RESUMO
In the Atacama Desert of northern Chile, cacti of the genus Copiapoa occur in extensive, relatively dense, monospecific stands. The spatial distribution patterns within several stands of Copiapoa cinerea v. columna-alba were analyzed for evidence of competitive interactions among individuals. There was no indication that competition was affecting stand density. Mean annual precipitation is only 25 mm/yr, but estimates of maximum possible water use within a stand were much lower. It was shown that the time between rainstorms is long compared to the water storage capacity of the plant stems, especially in the case of small individuals. We conclude that population densities in Copiapoa are limited by the difficulty of establishment, not by competition.