EXPERIMENTAL MANIPULATIONS OF WATER AND NUTRIENT INPUT TO A NORWAY SPRUCE PLANTATION AT KLOSTERHEDE, DENMARK .1. UNINTENDED PHYSICAL AND CHEMICAL-CHANGES BY ROOF EXPERIMENTS

At Klosterhede, Denmark experimental manipulations of water and nutrie nt inputs to a forest stand were carried out under a 1200 m(2) roof si mulating i) summer drought, ii) removal of N and S input, and iii) opt imal nutrition and water availability, In practise, manipulation of el ement fluxes in a complex system like a forest may cause unintended ch anges such as climatic effects from the roof which may interfere with the results of the intended manipulation. This paper illustrates the e xtent and effects of such unintended changes. The most important clima tic change under the roof was a 50% reduction of photosynthetic light which caused a decrease in moss cover on the ground floor. Soil temper atures under the roof were decreased 0.3 degrees C during summer and i ncreased 0.2 degrees C during winter. Air humidity was not changed. Th roughfall water amount decreased close to the stems whereas element co ncentrations in throughfall increased close to the stems and with tree size on the ambient plots. This distinct spatial pattern was changed under the roof to a more variable water distribution from the sprinkli ng system and to constant element concentrations in the sprinkling wat er. This loss/change of spatial variability in the input was reflected in the soil solution. The concentration patterns found in soil water (increasing close to the stem and with tree size) in the ambient plots were no longer observed under the roof. Exclusion of throughfall by t he roof disturbed the internal cycle of nutrients leached from the can opy. The removal of canopy leached Ca and K by the roof caused a decli ne in soil water concentrations, even on the drought plot where natura l throughfall was only cut off during two summer months. The internal cycle of Ca and K had to be restored by addition of Ca and K under the roof. Further, leachable elements and soluble compounds may be washed out from litterfall during the collection period on the roof, and thu s lost from the internal cycle by exclusion of throughfall. It is impo rtant to consider these problems in the design of new manipulation exp eriments and in the interpretation of the results. Some recommendation s are given.