EXPERIMENTAL MANIPULATION OF FOREST ECOSYSTEMS - LESSONS FROM LARGE ROOF EXPERIMENTS

Environmental impacts on forest ecosystems can be studied by manipulat ing energy, water, and element input or by changing the internal eleme nt cycling. In practice, the intended manipulations in a complex ecosy stem such as a forest are followed by unintended manipulations of othe r factors that may cause artifacts in the experiment, The character an d extent of such unintended changes were assessed in five major roof m anipulation studies in coniferous forests in Europe. In all five cases the roofs were placed beneath the canopy 2-5 m above the ground and d esigned to study the response to reduced N and S deposition and effect s of drought. Photosynthetic light was reduced 15-50% below the roofs and might have contributed to an observed decrease in forest floor mos s cover. Soil temperature differences were up to +/-0.5 degrees C, col der than outside during summer and warmer during winter, Climatic diff erences were least at the smallest roofs. The sprinkling system was th e most critical component in the experimental design, The sprinklers c ould not reproduce the temporal and spatial variability of natural rai n; event size and rain intensity increased, and the number of rain eve nts decreased, It proved particularly difficult to reproduce small rai n events, The stemflow proportion of the water input was increased by sprinkling. Observed decreases of litter decomposition and mineralisat ion under some of the roofs were probably caused by a reduced moisture content of the surface litter due to the differences from natural rai n, Exclusion of throughfall by the roof disturbed the internal cycle o f nutrients leached from the canopy (Ca, K, Mg) or present in suspende d material (N, P, Mg). The circulation of these elements had to be res tored by addition or recycling of suspended matter. The unintended cha nges probably delayed the soil response to reduced acidity input and a ccelerated the decline of nitrate leaching in response to reduced N in put. (C) 1998 Elsevier Science B.V.