By
J.I. House & D.O. Hall
Division of Life Sciences, King's College London

Changes in CO₂ concentration, rainfall and temperature will all affect NPP, fire frequency, etc. Few studies have been carried out on the effects of climate change on savanna ecosystems, there are no true FACE (Free Air Carbon Dioxide Enrichment) experiments and few CO₂ studies as yet incomplete (Scholes, pers comm.). In tropical savannas the woody biomass is C₃ and most of the herbaceous biomass is C₄. C₃ plants have a greater response to CO₂ enrichment in terms of increased productivity thus it has been postulated that trees will have an advantage, and in fact, woody species have been seen to increase in savanna areas, although more local changes may be the cause (Polley et al, 1996). However, this is only part of the story. Partial stomatal closure in CO₂-rich environments allows better water and nitrogen use efficiency which is very important in certain savanna areas constrained in either or both resources (Gifford et al, 1990); Owensby et al (1993) have shown an increase in production in C₄ grasslands due to this effect, and some preliminary results from trials in grasslands in the USA show C₄ grasses actually doing better than C₃ (Owensby et al, 1999). Increased temperatures will lead to increased respiration, however, CENTURY model simulations suggest that increased photosynthesis will exceed increased respiration resulting in a net carbon sink (Hall et al, 1995). All of the above changes will modify soil carbon storage which accounts for 80% of the total ecosystem organic carbon in tropical savannas (Scholes & Hall, 1996).

Savannas already have a great impact on carbon cycles due to fires, fuelwood and land-use changes. Amthor et al (1998) found the savanna biome to have the highest potential for carbon gain and among the highest potential for carbon loss. Scurlock & Hall (1998) cautiously propose that this biome may already constitute an annual sink of about 0.5 Pg C, as supported by recent eddy covariance measurements over Brazilian cerrado (Miranda et al, 1997, Table 5). Savanna burning releases bewteen 0.87 PgC/y (Scholes & Hall, 1996, extrapolation of Menaut et al, 1991) and 1.66 PgC/y (Scholes & Hall, 1996, extrapolation of Hao et al, 1990). Fire suppression and the resultant increase in tree density could store an extra 30 gC/m² annually in soils (Scholes & Van der Merwe, 1996).

Intoroducing deep-rooted African grasses and legumes to South American grasslands has increased soil carbon storage, mostly in the form of dead roots, which, if applied throughout Latin America's savanna areas could store 0.1 - 0.3 PgC per annum (Fisher et al, 1994). In Venezuela (San Jose et al, 1998) maximum aboveground yield was four times higher for African grasses than native savanna. However, the continuing pressure on these lands and their subsequent degradation is likely to lead to an increase of carbon loss in the future. These human induced changes will far outweigh any impacts due to climate change.

Many thanks to Bob Scholes (CSIR, South Africa), Xavier Le Roux (INRA, Clermont-Ferrand, France), Jonathan Scurlock (ORNL, USA) and Joe Scanlan (Department of Natural Resources, Queensland, Australia) for providing information and making corrections to the manuscript. Dale Kaiser & Sonja Jones (ORNL, USA) for calculating tropical % of the Olson et al (1983) "grasslands" category.

Sadly, David Hall passed away in August 1999 before this chapter was published. His knowledge and love of savannas was only surpassed by his eagerness to learn and teach.

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