Methods for Measuring Water and Solute Balances in Forest Ecosystems

Abstract

The environmental monitoring of the state of forests throughout Europe can identify negative developments, allowing targeted countermeasures. In the Forest Monitoring Network, not only are regular representative grid surveys carried out but also ecosystem-relevant energy and solute fluxes are monitored over the whole year. A monitoring plot consists of an open-field plot and a plot in forest stands. An open-field automatic weather station records meteorological parameters. In the forest stands, soil water fluxes are monitored in measuring fields with tensiometers and soil moisture sensors. In addition, suction probes take soil water samples that are tested for quality. Tree increments are checked with dendrometers. The long-term measurement of meteorological, hydrological, and growth-related parameters provides information about the state of forests and also allows an intensive study of causes and effects in forest ecosystems. Some factors relating to the water balance in forests can be measured directly, while others are calculated using models. To validate the model results, it is necessary to investigate water fluxes in forest plots with various structures. Additional innovative instruments and methods are used for special environmental observations. Large-scale lysimeters are used to measure the influence of trees of various ages and species on the groundwater recharge and evapotranspiration (ET). Weighing lysimeters are used to determine the ET of ground vegetation and young trees. The monitoring indicates that the seepage water below forests is clean but affected by periods of summer drought, which also reduce tree increments. Groundwater recharge is influenced by the age and species of forest trees, the vertical structuring and heterogeneity of forests, and the way they are managed. Broad-leaved forests are found to have more groundwater recharge than coniferous forests due to the differences in the interception between the evergreen canopies of coniferous forests and broad-leaved forests which lose their leaves in the winter. Depending on the structure of the investigated stands, a redistribution of precipitation was found with effects on the proportions of the individual components of the ET so that the contribution of forests to the landscape water balance varies. The findings can be used to assess the future threats to today’s forests and to develop strategies for adapting to anticipated climate change.