"From wet to dry" species in the tropics face increasing problems of water availability (Holbrook and Franco 2005). The dominating ecophysiological stress parameters in dry tropical forests are strong seasonal drought (H2O) and high irradiance (hv, see Chapter 4 for a detailed discussion) with strong interactions between them and with other environmental parameters, i.e.
1. h v ^ T: Absorption of radiation by leaves leads to heating.
2. H2O ^ h v: Heating and drying of the atmosphere, increases the leaf-air water-vapour pressure gradient and thus leads to increased transpirational water loss.
3. H2O ^ T: Water loss can be controlled by closure of stomata, but this then reduces transpirational cooling by evaporation, and leaves heat up further.
4. H2O ^N: Soil water deficit reduces the availability of N; the transpiration stream serves distribution of nutrients in the plant.
In their adaptations plants combine phenological, structural, physiological and biochemical responses.
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