Vegetation: Transport Processes Inside and Outside of Plants - Transport in the Atmosphere-Vegetation-Soil Continuum - page 251

continuum and how plants cope with large negative pressure heads in their xylem
vessels. Plants change their stomatal resistance to control the transpiration loss, in
response to light intensity, leaf temperature, air humidity, internal CO 2 concentration
and leaf water potential. The water use eficiency or water productivity appears to be
affected mainly by daytime water vapour pressure deicit and plant type (C 3 vs. C 4
plants).
With respect to microclimate within the vegetation, important physical processes
are the extinction of radiation, the leaf temperature as function of radiation, the heat-
ing and cooling of air temperature by leaves and soil, and the wind speed proile and
its effect on turbulent eddies. During nighttime water vapour may condensate on the
canopy surface as dew, which may affect the water balance, plant pathogen develop-
ment and deposition of atmospheric trace gases. Rainfall interception may range from
10% to 40% of gross precipitation, depending on vegetation and climate. Rainfall
interception modelling concepts for forests and agricultural crops are explained.