5.1 INTRODUCTION

Since the end of the last century forest ecosystems of central Europe have received large inputs of acidity, nutrients, and toxic substances via dry and wet deposition. Forests appear to have responded to these inputs by altering groundwater relations, soil chemistry, biomass production, species diversity, and by exhibiting forest decline. In order to relate these changes to the effect of 'acid rain', as a general expression for all these anthropogenic inputs, a number of deficiencies in the design of this unwanted 'experiment' have had to be accommodated:

1. The initial states of the ecosystems have had to be reconstructed from sparse historic sources.

2. Since all ecosystems are affected in mid-Europe, there is no untreated control.

3. Both the quantity and the quality of the acid rain input has changed through time--only rough estimates exist of the total deposition including dry deposition.

4. The treatment by acid rain concentrations has been low and within the range of other natural perturbations; therefore, ecosystem responses have been very complex and difficult to unravel. This difficulty has been amplified by the fact that only a few time sequences have been measured, and the variability in space of ecosystems is presently being analyzed instead, and the result is used to interpret the sequence of events in time which led to the observed change.

5. Although any experiment should be repeated, this has not been possible in acid rain research. Acid rain has occurred on a large geographic scale, and covers a time span of more than three generations of researchers.

6. At least research on acid rain has tested both a response and a recovery. These have been: irrigation with acidity of unaffected forest land (Tamm,1989), and liming of acid rain-affected habitats (e.g. Kaupenjohann et al., 1989).

In the following sections we present evidence of how acid rain may have affected forest ecosystems and tree health (Schulze, 1989; Ulrich, 1989), and draw conclusions for future experimental work in ecosystems. We are dealing with areas which exhibit features of forest decline, namely needle loss and needle yellowing, which is associated with various types of nutrient deficiencies and effects on root systems.