Choice of tree species

3.1. Choice of tree species

3.1.1. Evaporation

3.1.1.1. Intercepted rain evaporation

The evaporation of precipitation intercepted by a forest canopy is closely related to vegetation cover and vegetation type. The amount of intercepted and transpired water highly depends on tree species and their leaf-area index (LAI). Differences in the evaporation of intercepted rain between tree species are well studied, in particular for spruce and beech (Brechtel, 1970; Benecke, 1984) and their relevance concerning interception, water balance and microclimate has been proven in many studies (Augusto et al., 2002). The Solling study showed comparatively high evaporation of intercepted rain for spruce (29%) compared to beech (17%) (Benecke, 1984; Ellenberg et al., 1986). The higher interception rate of Norway spruce can be attributed to greater leaf area, both in summer time and particularly in winter. The study of (Kantor, 1981, 1984, 1995) also showed greater interception losses in mature coniferous stands (16%) compared to broadleaved forests (7 %), but these values are much lower than the Solling results. The difference may be explained by differences in precipitation (higher precipitation rates on Kantor’s sites). Of course there is a pronounced year to year variation depending on climatic conditions (Table 1). Species-specific differences are modified by stand age, stand structure, and forest management (Mitscherlich, 1981; Hager, 1988). Besides the impact upon total interception losses, tree species also modify spatial patterns of canopy throughfall and stemflow. While trees with a funnel like crown and smooth bark, like beech, guide a high share of precipitation as stemflow close to the trunk (up to 20% of the precipitation), conifers like spruce rather guide precipitation to the periphery of the crown (Beier, 1998; Peck, 2004). For spruce rarely more than 2% of precipitation reaches the forest floor as stemflow, except in very young stands. Pine shows stemflow values of approximately 3% and Douglas fir even up to 8%.

3.1.1.2. Soil evaporation

Soil evaporation in mature stands is usually of less importance than interception losses. In many studies intercepted rain evaporation and transpiration by forest floor vegetation is included in this term, therefore results of different studies are frequently not comparable. In the studies of Kantor (1984; Kantor, 1995) soil evaporation amounts to 6% for mature spruce and beech stands. During clearcut phases and early stages of stand development, however, this term will be pronounced. In a study by Katzensteiner (2000) soil evaporation during vegetation period at a clearcut amounted to 10- 20% of precipitation, compared to only 2% in the mature spruce stand. Müller et al. (2002) compared pine and beech stands in the NE-German lowlands under precipitation limited condition of forest growth. In the pine stand, soil (and ground vegetation) consumed as much as 35% of precipitation compared to only 12% in the beech stand. In summary total EIT was similar for all situations, and exceeded precipitation (Table 2).

3.1.2. Transpiration

Contrary to interception, plants are able to regulate transpiration in response to climate conditions and soil water availability. The main species specific property influencing the transpiration process is Contrary to interception, plants are able to regulate transpiration in response to climate conditions and soil water availability. The main species specific property influencing the transpiration process is

be a suitable predictor of canopy transpiration in European beech forests independent of stand age, canopy transpiration/LAI decreased with increasing stand age in Norway spruce (Köstner, 2001). Ryan et al. (2000) measured greater transpiration for 12 m tall, compared to 36 m tall, ponderosa pine trees – another hint at physiological changes in leaf conductance with tree age.

3.1.3 Total water use and water yield For European conditions a greater water yield can be assumed for broadleaved forests than for

coniferous forests. This difference has been shown for spruce vs. beech (see preceding chapters) or spruce vs. oak (van der Salm et al., 2006; Rosenqvist et al., 2009). In pine stands, high evaporation rates from ground vegetation and the forest floor may lead to a low water yield.

Care has to be taken with Eucalyptus replacing oak woodlands in Southwestern Europe. Even if water use efficiency is high for this genus (Stape et al., 2004; Whitehead and Beadle, 2004; Forrester et al., 2009), it has a fast growth reaction and thus increasing water consumption with increasing availability of water (Stape et al., 2004).