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African Savanna Trees Owe Their Increasing Abundance to Increases in the Air's CO2 Content
Volume 13, Number 37: 15 September 2010

In a paper recently published in Austral Ecology, Kgope et al. (2010) write that "over the last century, there has been a trend of increasing woody biomass in many savanna regions (Polley et al., 2002; Ward, 2005) [that] is generally attributed to changes in land use practice, particularly grazing and fire use, and to episodes of high or low rainfall," but they say that "the phenomenon may also have been influenced by increasing atmospheric CO2," citing the papers of Idso (1992), Polley (1997) and Polley et al. (1999). Thus, in an investigation into the strength of the latter hypothesis, the three South African researchers monitored "photosynthetic, growth and carbon allocation responses of African savanna trees (Acacia karroo and Acacia nilotica) and a C4 grass, Themeda triandra, exposed to a gradient of CO2 concentrations from 180 (typical of the Last Glacial Maximum) to 1000 ppm in open-top chambers in a glasshouse," as well as intermediate concentrations of 280, 370 (representing the present), 550 and 700 ppm.

The well watered and fertilized nitrogen-fixing Acacia trees they studied were started from seed and grown under the above conditions from the age of one week for a period of two years, at the midpoint of which time interval they were cut back and allowed to re-grow for a second season, while the T. triandra plants they evaluated -- which constitute "the dominant grass species in many frequently burnt grasslands and savannas in South Africa," as they describe them -- were established from tillers obtained from a C4 grassland in southern Kwa-Zulu-Natal, South Africa.

As a result of their efforts, Kgope et al. determined that "photosynthesis, total stem length, total stem diameter, shoot dry weight and root dry weight of the acacias increased significantly across the CO2 gradient, saturating at higher CO2 concentrations." And they say that "after clipping to simulate fire, plants showed an even greater response in total stem length, total stem diameter and shoot dry weight, signaling the importance of re-sprouting following disturbances such as fire or herbivory in savanna systems." However, and "in contrast to the strong response of tree seedlings to the CO2 gradient," in the words of the three researchers, "grass productivity showed little variation, even at low CO2 concentrations."

In terms of actual numbers, Kgope et al. report that "at the end of the first growing season, SDW [shoot dry weight] had increased by 529% in A. karroo and 110% on average in A. nilotica under ambient relative to sub-ambient CO2 treatments," and that "a further increase in CO2 from ambient to elevated CO2 significantly increased SDW of A. nilotica by 86%." As for the second season results, they found that the SDW of re-sprouted A. karoo shoot material increased by 366% from sub-ambient to ambient CO2, while and that of A. nilotica increased by 133% on average. In fact, the South African scientists say that "changes in CO2 from pre-industrial times to the present have effectively produced acacia 'super seedlings' in relation to their growth potential over the past several million years [italics added]."

In light of these findings, Kgope et al. conclude that "where fires once killed seedlings, they are unlikely to do so today, resulting in much higher seedling recruitment rates," and they write that "the rate of sapling release to adult height classes will also be greatly enhanced because they are able to grow out of the fire trap more rapidly." What is more, they state that the trees "should also be better defended against mammal browsers and insect herbivores." And citing yet-to-be-published results, they say that "both structural (spines) and chemical (tannins) defenses showed significant increases with increasing CO2."

As for the implications of these several observations, the three researchers write that they "provide experimental support for suggestions and simulation studies predicting that reductions in CO2 alone could have led to loss of tree cover in grassy environments in the last glacial (Bond et al., 2003; Harrison and Prentice, 2003)," and they say that "the large increases in CO2 from industrial emissions over the last century would now favor trees at the expense of grasses," which conclusion is supported by palaeo-records that indicate that "trees disappeared from current savanna sites in South Africa during the Last Glacial Maximum (Scott, 1999), re-appeared in the Holocene, and have rapidly increased over the last half century," the latter of which phenomena is an integral part of the great -- and much-to-be-desired -- CO2-induced Greening of the Earth that has occurred over the same time period.

So let's hear it for that wonderful life-giving molecule -- CO2 -- that we all supply to the air with every breath we exhale, which the U.S. Environmental "Protection" Agency -- in an absurd denial of reality -- has recently labeled a dangerous air pollutant.

Sherwood, Keith and Craig Idso

References
Bond, W.J., Midgley, G.F. and Woodward, F.I. 2003. The importance of low atmospheric CO2 and fire in promoting the spread of grasslands and savannas. Global Change Biology 9: 973-982.

Harrison, S.P. and Prentice, I.C. 2003. Climate and CO2 controls on global vegetation distribution at the last glacial maximum: analysis based on paleovegetation data, biome modeling and paleoclimate simulations. Global Change Biology 9: 983-1004.

Idso, S.B. 1992. Shrubland expansion in the American southwest. Climatic Change 22: 85-86.

Kgope, B.S., Bond, W.J. and Midgley, G.F. 2010. Growth responses of African savanna trees implicate atmospheric [CO2] as a driver of past and current changes in savanna tree cover. Austral Ecology 35: 451-463.

Polley, H.W. 1997. Implications of rising atmospheric carbon dioxide concentration for rangelands. Journal of Range Management 50: 562-577.

Polley, H.W., Tischler, C.R., Johnson, H.B. and Derner, J.D. 2002. Growth rate and survivorship of drought: CO2 effects on the presumed tradeoff in seedlings of five woody legumes. Tree Physiology 22: 383-391.

Polley, H.W., Tischler, C.R., Johnson, H.B. and Pennington, R.E. 1999. Growth, water relations, and survival of drought-exposed seedlings from six maternal families of honey mesquite (Prosopis glandulosa): responses to CO2 enrichment. Tree Physiology 19: 359-366.

Scott, L. 1999. Vegetation history and climate in the Savanna biome of South Africa since 190,000 ka: a comparison of pollen data from the Tswaing Crater (the Pretoria Saltpan) and Wonderkrater. Quaternary International 57/58: 517-544.

Ward, D. 2005. Do we understand the causes of bush encroachment in African savannas? African Journal of Range and Forage Science 22: 101-105.