Forestry and water balance in Germany – consequences for choice of tree species and for management

Broad areas of north-east Germany are characterised by low precipitation, distinct periods of summer drought and sandy soils. In this region, forest hydrology research looks into the influence of differently structured forests on the landscape hydrology, and on the components of the hydrologic equation. The aim is to provide scientific guidance on the development of productive forests that enhance the quantity and quality of seepage water under the conditions of climate change. The main topics of the hydro-ecological investigations are: • The role of the forests in the landscape water balance? • Water consumption and tree growth with decreasing water resources? In unconsolidated substrates, lysimeters can be used to measure seepage and evaporation. Various types of lysimeters have been used in north-east Germany for more than a century. More than forty years ago, large-scale lysimeters with a depth of 5 m and a surface area of 100 m2 were planted with the tree species Scots pine, European beech, European larch and Douglas fir. To investigate the impact of intensified drought on tree growth, small-scale weighing lysimeters were also developed. Generating water in sufficient quality and quantity is the most important service of forests after timber production. The tree species and their ages, the vertical structure and mixture of the trees as well as their management can substantially influence the amount of the groundwater produced. Evaporation and groundwater production are functions of the tree structure and the tree growth. In beech stands, the lower interception of the crowns compared with pine stands, in combination with the stem flow, substantially influences the level of groundwater production over time and leads to a hydrological gain. In the longer term, the mixed pine-beech stands also make a positive contribution to the landscape water balance.