Site preparation – fertilization

3.2. Site preparation – fertilization

In general, site preparation will restrict encroachment by clearcutting vegetation, reduce transpiration rates, and until canopy closure increase soil evaporation. Due to reduced competition and improved soil conditions, growth of forest saplings will be accelerated. Thus, net effects on water yield may be of minor importance. Fertilization may influence ET by altering leaf area and by changing stomatal behaviour. Additionally, indirect effects on water uptake by changing the shoot to root ratio, rooting pattern and conductivity of the xylem have been shown (Ewers et al., 2001). The authors compared effects of fertilization and irrigation on stomatal conductance of young Pinus taeda and Picea abies stands grown on nutrient poor soil. Fertilization doubled and tripled leaf area of P. taeda and P. abies respectively. In fertilized, non-irrigated P. taeda, canopy stomatal conductance decreased as a response to increasing leaf area index, while the combination of irrigation and fertilization caused a large increase in stomatal conductance. In P. abies fertilization caused no change in stomatal conductance unless irrigation was applied. Earlier reports by Guehl et al. (1995) gave evidence of reduced stomatal conductance after fertilization of Pinus pinaster. Ripulline et al. (2004) confirmed an increase of WUE after N fertilization for Pseudotsuga menziesii and Populus x euroamericana, while they found no evidence In general, site preparation will restrict encroachment by clearcutting vegetation, reduce transpiration rates, and until canopy closure increase soil evaporation. Due to reduced competition and improved soil conditions, growth of forest saplings will be accelerated. Thus, net effects on water yield may be of minor importance. Fertilization may influence ET by altering leaf area and by changing stomatal behaviour. Additionally, indirect effects on water uptake by changing the shoot to root ratio, rooting pattern and conductivity of the xylem have been shown (Ewers et al., 2001). The authors compared effects of fertilization and irrigation on stomatal conductance of young Pinus taeda and Picea abies stands grown on nutrient poor soil. Fertilization doubled and tripled leaf area of P. taeda and P. abies respectively. In fertilized, non-irrigated P. taeda, canopy stomatal conductance decreased as a response to increasing leaf area index, while the combination of irrigation and fertilization caused a large increase in stomatal conductance. In P. abies fertilization caused no change in stomatal conductance unless irrigation was applied. Earlier reports by Guehl et al. (1995) gave evidence of reduced stomatal conductance after fertilization of Pinus pinaster. Ripulline et al. (2004) confirmed an increase of WUE after N fertilization for Pseudotsuga menziesii and Populus x euroamericana, while they found no evidence

Hubbard et al. (2004) showed a tremendous increase in water use due to fertilization in Eucalyptus saligna stands, also an effect of increased leaf area, while canopy conductance did not change in the long run. In this case water use efficiency could not be improved by fertilization.

In a study by Powers and Reynolds (1999) Pinus ponderosa only showed increased WUE due to fertilization if drought stress was not too extreme – a clear indication of the dependence of fertilization effects on site conditions.

In summary, the increasing leaf area due to fertilization should lead to higher interception losses, even if transpiration is not increased due to improved stomatal control.