Cyclic and progressive changes in short-rotation willow coppice systems

In a willow coppice system, harvest-related cyclic changes occurred with regard to annual production, yield and number of shoots. The system however was also subjected to long-term directional changes. The relative weight variation between stools increased while the number of stools per hectare decreased over several cutting cycles. The long-term trends are able to affect the short-term cycles due to the interaction of the plant population with local and temporal site productivity factors. Stool mortality occurred in such a way that local density, calculated on the presence of the eight initially nearest neighbours, varied by one order of magnitude in the same stand after three cutting cycles. Such within-stand density variation may give rise to local yield-density effects, implying that differences in individual stool growth are determined by differences in available growing space. However, there were high positive correlations between above-ground stool weight at harvest on the one side and number and mean weight of shoots resprouting the year after harvest. Consequently, below-ground storage can be identified as a major determinant for the maintenance of a once-established competitive hierarchy over harvest, while the local density variation enforces the competitive hierarchy between consecutive harvests.