DOI: 10.1111/j.1365-3180.2010.00763.x Weed-poplar competition dynamics and yield loss in Italian short-rotation forestry S OTTO*, D LODDO  & G ZANIN   *National Research Council (CNR), Institute of Agro-Environmental and Forest Biology, Legnaro (PD), Italy, and  Department of Environmental Agronomy and Crop Science, Padova University, Legnaro (PD), Italy Received 17 July 2009 Revised version accepted 23 November 2009 Summary In recent years there has been increasing interest in short-rotation poplar plantations for biomass produc- tion and cropping techniques and weed control strate- gies are being developed. Slow initial poplar growth can favour weed infestation and so there is a need for appropriate timing of weed control, based on an understanding of the competitive relationship and dynamics between poplar and weeds. A field experiment with annual and biennial crop cycles was conducted over two years. A 35 cm wide within-row weedy strip was allowed to develop as the basis for competition. Yield data were fitted with sigmoidal equations, in order to find the critical point (CP) and critical period of weed control (CPWC), using thermal time as the independent variable. Results showed that poplar is very sensitive to weed competition, especially in the first growing season after planting or cutting, and that competition can cause important yield losses. In the annual cycle, the CP corresponds to a yield loss of 26% and 8% in the first and second growing season respectively; in the biennial cycle the CP corresponds to a yield loss of 29%. Furthermore, the CPWC is long. For example, in the first year, based on a 5% yield loss, it lasts from 30 to 84 days after planting. This makes effective weed control difficult to achieve only using a pre-emergence herbicide, and thus necessitates a post-emergence treat- ment, either with herbicides or inter-row cultivation. These results, together with the inability of the crop to recover from damage, mean that resources must be invested in weed control soon after the beginning of the cropping cycle. Keywords: weed competition, biomass, critical period, Populus, short-rotation coppice, bioenergy. OTTO S, LODDO D&ZANIN G (2010). Weed-poplar competition dynamics and yield loss in Italian short-rotation forestry. Weed Research 50, 153–162. Introduction In Italy, short-rotation forestry (also called short- rotation coppice or short-rotation plantation) for biomass production has become very popular in recent years, with over 4000 ha already planted almost exclusively with hybrid poplar clones (Populus sp.) (Spinelli et al., 2008). Yield potential of such a crop is variable, but in some conditions the chipped wood can be profitably used in small-scale combined heat and power plants, thus meeting the increasing demand for self-sufficiency in bioenergy in decentralised rural areas (Gruenewald et al., 2007). There are no well-established cropping protocols for Italian conditions, just some recently-defined recom- mendations by the main hybrid clone producers. It is known that the success of short-rotation forestry depends on optimal site preparation and effective chemical or mechanical weed control during crop establishment (Buhler et al., 1998; Rohle et al., 2008), because the crops do not recover from damage due to weed competition in that period (Clay & Dixon, 1997). Correspondence: Dr S Otto, National Research Council (CNR) – Institute of Agro-Environmental and Forest Biology, Via dellÕUniversita`, 16 – 35020 Legnaro (PD), Italy. Tel: (+39) 49 827 2884; Fax: (+39) 49 827 2818; E-mail: stefan.otto@ibaf.cnr.it Ó 2010 The Authors Journal Compilation Ó 2010 European Weed Research Society Weed Research 50, 153–162