Effect of Low Glacial Atmospheric CO ₂ on Tropical African Montane Vegetation

Abstract

Estimates of glacial-interglacial climate change in tropical Africa have varied widely. Results from a process-based vegetation model show how montane vegetation in East Africa shifts with changes in both carbon dioxide concentration and climate. For the last glacial maximum, the change in atmospheric carbon dioxide concentration alone could explain the observed replacement of tropical montane forest by a scrub biome. This result implies that estimates of the last glacial maximum tropical cooling based on tree- line shifts must be revised.

A few prospective ideas on climate reconstruction: from a statistical single proxy approach towards a multi-proxy and dynamical approach

“…A number of physiological and palaeoecological studies (e.g. Jolly and Haxeltine, 1997;Cowling and Sykes, 1999) have proved that plant-climate interactions are sensitive to the atmospheric CO 2 concentration, and we know, from ice cores (EPICA, 2004), that this concentration is presently much higher than ever during the past 740 000 yr. Consequently modern samples collected under high CO 2 concentration are hardly good analogues for low CO 2 periods. Moreover, pollen assemblages are noisy and sometimes biased records of the climate variables, because (1) pollen productivity is not equal to vegetation productivity, (2) pollen assemblages are disturbed by pollen grain transportation, (3) a pollen taxon is not a univocal species, (4) and the species are not affected by a single climatic variables.…”

Amazonian paleoecological histories: one hill, three watersheds

“…Over portions of their range, C3 plants would be expected to be replaced by C4 plants. It is even argued that the replacement of trees with grasses in montane Africa may have owed more to lowered CO2 concentration than temperature depression (Jolly and Haxeltine, 1997).…”

Do Low CO2 Concentrations Affect Pollen-Based Reconstructions of LGM Climates? A Response to “Physiological Significance of Low Atmospheric CO2 for Plant–Climate Interactions” by Cowling and Sykes

“…This effect may have been strong enough to influence the taxonomic composition of plant communities (Idso, 1989), and may in part explain why the LGM forests of eastern North America were more open than at present (Cowling, 1999;Davis, 1989). In tropical Africa, low CO 2 concentrations may explain the observed lowering of montane vegetation zones during the LGM (Jolly and Haxeltine, 1997;Street-Perrott et al, 1997).…”

“…In some parts of the world, such shifts could be misinterpreted to reflect changes in temperature. Using a process-based vegetation model, Jolly and Haxeltine (1997) showed that the effects of CO 2 could explain much of the observed lowering of African treelines at the LGM. However, they noted that if CO 2 is a factor in controlling the position of African treelines at the LGM, then pollen-based estimates of temperature decreases at the LGM would be overestimated-a bias opposite in direction to that asserted by Cowling and Sykes (1999).…”

“…Lakelevel records, which are regionally lower in several areas, including eastern North America, seem to provide support for palynological paleoclimatic reconstructions, but lake levels may also be influenced indirectly by atmospheric CO 2 through its control on plant transpiration. Partitioning the effect of CO 2 and low precipitation upon LGM community composition and pollen-based reconstructions requires modeling studies such as those performed by Cowling (1999) and Jolly and Haxeltine (1997). Pollen-based studies remain useful for showing past vegetation distributions (Jackson et al, 2000), and pollenderived climate estimates set a lower bound for the aridity of the LGM.…”