Introduction

In the past few decades a global excess of photosynthesis over respiration has resulted in an annual accumulation of 0.21.4 Pg C in the terrestrial biosphere (Sabine et al., 2004), and slowed down the rate of rise in atmospheric CO2 (Ca). However, the processes driving this increase in productivity, with its spatial distribution and magnitude, remain uncertain. Specifically, it is not clear whether this fertilization effect has been driven primarily by global changes in climate and atmospheric composition, or is largely the consequence of historic changes in land use. This uncertainty has important implications for future carbon sequestration by terrestrial vegetation. If global change has been largely responsible for increased carbon sequestration, continued climatic and atmospheric changes could continue to increase carbon sequestration in terrestrial vegetation. However, published predictions of the effect of future global changes on carbon sequestration in terrestrial ecosystems vary enormously, ranging from ~150 to ~530Pg C by 2100 (Cramer et al., 2001; Gruber et al., 2004). Conversely, if past land-use change has driven recent increases in carbon sequestration, future sequestration may be limited. Managed carbon sequestration strategies, such as afforesta tion and reforestation projects permitted by the Kyoto Protocol, could provide for significant carbon sequestration in terrestrial biomass in addition to that which may occur due to global change. However, these projects are few in number and predictions of the extent to which they can be established globally are extremely uncertain. Estimates of potential global carbon storage range from a 'theoretical maximum' of 5.0 Pg C/year to an 'actually achievable' minimum of 0.2 Pg C/year (Cannell, 2003). If carbon sequestration in terrestrial biomass is to be used effectively as one mechanism among many by which the rise in Ca can be attenuated in the future, these uncertainties need resolving. Here we will provide an assessment of our current understanding of: (i) the drivers of past productivity of terrestrial vegetation;

(ii) current terrestrial productivity; and

(iii) the potential for natural and managed increases in carbon sequestration in terrestrial vegetation in the coming century.

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