Atmospheric deposition onto forests
Forests receive from the air a multitude of airborne substances, such as nitrogen and sulphur compounds, as well as basic salts and particulate matter. These substances may be deposited directly on the forest (dry deposition) or may enter the forest dissolved in rain (wet deposition). High deposition of certain substances, e.g. nitrogen, can seriously disturb the natural ecosystem of a forest. Because of the high human-caused emissions of air pollutants, a good knowledge of the atmospheric deposition of these pollutants is important. This knowledge can be used to set policy priorities for specific pollutants and to sharpen pollutant control strategies.
One can get an impression of atmospheric deposition on a forest in several ways. Roughly speaking, there are two methods: 1) a mathematical model and 2) a measurement of runoff water.
The first method consists of calculating deposition with mathematical models based on air quality data and meteorological data. The advantage of this method is that it is possible to calculate deposition for large areas (e.g. a whole continent), provided that sufficient air quality data are available. The disadvantage of this method is that the resolution is often necessarily limited and it is not possible to simulate specific situations. In addition, these models require measurements to validate the results.
The second method is the so-called throughfall/stemflow method. In this method, water is collected from the canopy and trunks of trees during rainfall and the concentration of relevant substances in that water is measured. In this method, the assumption is that this runoff water contains both the dry deposition that was present on the foliage and the wet deposition. For some substances, this is a rather crude approximation of deposition because the measurement may be disturbed by uptake or release of substances through the stomata of the tree leaves. Nevertheless, the method has been widely used in the past to obtain an indication of deposition and/or for validating deposition calculations with other methods (e.g., with atmospheric transport models).
The throughfall method has been applied in a variety of forests around the world over the past 70 years. Many scientific publications have appeared on this subject over the years (keyword combination of throughfall and deposition gives 2981 results in Sciencedirect). The large number of measurements and their great diversity (methodological set-ups, types of forest systems, geographical location) seem to provide a good basis for making statements about the suitability of the method for determining atmospheric deposition. Under which (ecological conditions, for which substances and under which measurement conditions is this method suitable?
A master's thesis research will basically consist of an extensive systematic literature review, which will analyze at least 50 years, possibly even 70 years of research with and into this method. The exact focus of the research can still be determined; for example, the research could be limited to a specific substance or group of substances, or it could specifically focus on studies in which throughfall measurements were used to validate atmospheric calculation models.
The research supervisor himself obtained his doctorate in 1990 with a thesis on "Atmospheric deposition onto forest", in which throughfall measurements were made and comparisons of throughfall data with model calculations were made. It is particularly interesting to see what progress has been made in science in this area over the past 30 years.
Contact: Dr. Wilfried Ivens