7.2.3.6 Connecting Changing Vegetation to Changing Climate

Only large-scale patterns are assessed here. Analysis of satellite-sensed vegetation greenness and meteorological station data suggest an enhanced plant growth and lengthened growing season duration at northern high latitudes since the 1980s (Zhou et al., 2001, 2003c). This effect is further supported by modelling linked to observed climate data (Lucht et al., 2002). Nemani et al. (2002, 2003) suggest that increased rainfall and humidity spurred plant growth in the USA and that climate changes may have eased several critical climatic constraints to plant growth and thus increased terrestrial net primary production.

7.2.4 Modelling the Coupling of Vegetation, Moisture Availability, Precipitation and Surface Temperature

7.2.4.1 How do Models of Vegetation Control Surface Water Fluxes?

Box 7.1 provides a general description of water fluxes from surface to atmosphere. The most important factors affected by vegetation are soil water availability, leaf area and surface roughness. Whether water has been intercepted on the surface of the leaves or its loss is only from the leaf interior as controlled by stomata makes a large difference. Shorter vegetation with more leaves has the most latent heat flux and the least sensible flux. Replacement of forests with shorter vegetation together with the normally assumed higher albedo could then cool the surface. However, if the replacement vegetation has much less foliage or cannot access soil water as successfully, a warming may occur. Thus, deforestation can modify surface temperatures by up to several degrees celsius in either direction depending on what type of vegetation replaces the forest and the climate regime. Drier air can increase evapotranspiration, but leaves may decrease their stomatal conductance to counter this effect.