Energy exchange and photosynthesis

Vegetation interacts with the atmosphere exchanging energy, carbon (via photosynthesis and respiration), and water (via evapotranspiration). Different vegetation types display distinct patterns of interaction with the atmosphere due to, in part, different ecophysiological properties.

Savannas almost always home to grasses that use the C4 photosynthetic pathway. Under equivalent conditions, the presence of such ecophysiologically distinct functional type would be expected to lead to lower ecosystem scale evaporation rates and higher losses of sensible heat. It has been shown that tropical forest and C4 pasture have similar CO2 uptake rates, but rates of water use are much lower for the pasture.

There are few species that exist both in rain forest and savanna. For example, “transitional forest” in Brazil is floristically similar to rainforest, but proximal woody savanna (cerradão) is quite different its canopy structure and species composition. Such dissimilarities also occur in other ZOTs. For example miombo woodlands in Southern Africa are structurally and genetically quite distinct from patches of rainforest growing in the same mosaic. Proximal savanna and rainforests plots in Venezuela have less than 15% of their species in common.

Grassy savannas and deciduous woodland have close to zero rates of photosynthesis and respiration towards the end of the dry season. But woodier savannas maintain some photosynthetic activity and active water use. This may be by accessing of water at depth by the evergreen woodyvegetation. During the wet season peak net ecosystem CO2 uptake rates are around 0.7 mol CO2 m-2 d-1 for tropical forest. This compares to about 0.4 mol CO2 m-2 d-1 for Cerrado denso in Brazil or mopane woodland in Africa. On the other hand, C4 dominated grasslands can achieve photosynthetic rates comparable or even greater than typical rainforests during the wet season and with a lower rate of water use. Despite these differences and their likely influence on global climate there has been no systematic study of differences in energy and CO2 exchange between these two biomes. TROBIT examines these differences and integrates the results with topic 3.

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