The water budget of forest stands in relation to ozone, defoliation and climate change
Rationale
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Water budget of forest stands
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Since the mid 1980s, an increase of defoliation has been observed in forest trees in Switzerland, the specific reasons for which have been identified in only a few cases. There are suggestions that the water budget, and particularly drought, is a critical factor affecting needle and leaf loss. During persistent droughts, many indigenous trees react by shedding foliage in order to reduce evapotranspiration. Another important factor that can affect defoliation is enhanced ozone concentrations, which may often interact with the water budget. In addition, a change in the water budget may occur as an result of predicted global climate changes, which may lead to a critical water deficit. The extent to which the future combination of increased temperatures, reduced water availability and increased ozone will affect forest health is unknown.
Project aims
- Development of a regional water budget model, taking into account different site and vegetation characteristics
- Estimation of the local water budget of forest stands with the help of a new model, in particular the water amount available to trees and the drought conditions throughout Switzerland
- Statistical investigation of the relationships between water budget and defoliation on the sites of the Long-term Forest Ecosystem Research, the sites of the Sanasilva Inventory and in selected forest stands where forest damage is known and documented
- Statistical investigation of the correlation between water budget, ozone stress and needle and leaf loss; identification of critical factor combinations and endangered regions
- Performance of sensitivity studies on the effect of possible climate changes on the water budget and on forest health with the help of different scenarios
Methods
1. Water budget
To estimate drought, existing simple physical models of the water budget will be combined and further developed. For this, the following processes need to be taken into account:
- Meteorological parameters: air temperature and humidity, wind speed, cloud cover, and precipitation
- Interception
- Radiation budget
- Humidity exchange between the ground, vegetation and atmosphere, in particular potential and real evapotranspiration
- Water storage in the ground
- Vegetation characteristics dependent on the season
The measured quantities entering the water budget model are the meteorological parameters, the ground profiles (readily available field capacity), and the vegetation characteristics (tree species, stand height, leaf area index, ...). They determine, together with the radiation budget, the potential and actual evapotranspiration, which is calculated by the Penman-Monteith method. For the radiation budget, both measured values and simple models are available.
The interception is calculated using a box model, which considers the meteorological parameters and the vegetation characteristics. The water storage in the ground is also determined by a box model. The size of the ground water box is set equal to the readily available field capacity.
The data needed for the planned investigations are being made available by the following:
The data available only for variably-spaced points will be interpolated using suitable geostatistical methods. Special considerations are needed for the interpolation of meteorological and ground parameters, as these can vary strongly in space. In addition, the Soil Capability Map of Switzerland will be used as a source of information to interpolate the readily available field capacity from several hundred ground profiles.
2. Correlation between water budget, ozone and defoliation
Statistical and geostatistical methods will be used to investigate correlations between water budget, ozone and defoliation over time and space. In this connection, the time lag between the occurrence of stress due to drought or ozone and the appearance of the resulting symptoms, and also the dependence of the effect on the phenological state of the plants, require careful attention.
3. Climate change
Possible effects of future climate change on the water budget of forest stands are difficult to estimate because of the uncertain predictions of the future climate, especially the future distribution of precipitation. Therefore, in this project, the effect of different climate scenarios will be considered in order to investigate the sensitivity of forest stands to climate changes.
Work plan
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1997-1998
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Development of a water budget model
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1998-1999
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Correlation studies between water budget, ozone stress and
defoliation; performance of sensitivity studies regarding climate
change
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2000
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Writing up the results as a PhD-Thesis (Bärbel
Zierl)
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Publications related to the project
- Häsler R., Böhm J. P., 1996: Wald im Klimastress. Wald und Holz 9: 32-35.
- Innes J. L., 1997: Switzerland. In: Mueller-Edzards C., De Vries W., Erisman J.W. (eds), 10 years forest condition monitoring in Europe. Fed. Res. Centre Forestry & Forest Prod., Hamburg, in press.
- Webster R., Rigling A., Walthert L., 1996: An analysis of crown condition of Picea, Fagus and Abies in relation to environment in Switzerland. Forestry 69: 347-355.
Participants
- Martin Beniston
- Johannes Boehm
- Sucharita Ghosh
- Rudolf Häsler (Project leader)
- John Innes
- Bärbel Zierl
Contact
