Phytotoxic effect of barley, oat, and wheat-straw mulches in eastern Quebec forest plantations 1. Effects on red raspberry ( Rubus idaeus)

Abstract

Under laboratory conditions, cold-water extracts of barley, oat, and wheat straws strongly inhibited red raspberry ( Rubus idaeus L.) propagule growth. Shoot dry weights were 10, 44 and 68% of the control for oat, wheat, and barley straw extracts, respectively. Field experiments were carried out from 1985 to 1987 to verify the allelopathic effect of the straws. Balsam-fir/birch stands (5 ha each), located in three sites of eastern Quebec, were clearcut-harvested in 1984 and 1985 and site-prepared in 1985. In fall 1985, the created furrows were covered with straw mulches of barley, oat, or wheat straws, or a mixture of the three. The use of Populus wood shavings as a control mulch was used to separate physical and chemical effects of the residues. Vegetation survey was carried out in the summers 1986 and 1987 with special attention to red-raspberry ( Rubus idaeus L.) seedling growth. Cover and frequency of all plant species were visually assessed along transects. Rubus idaeus seed germination and growth were inhibited due to the treatments. The reduction in R. idaeus establishment on the treated plots did not help establishment of other opportunistic species. In summer 1987, an overall reduction in the total cover percent was noted on the treated plots (mean cover 20%), as compared to the control plots (mean cover 61%). In summer 1986, the mean frequency of red raspberry on the treated plots was 24% and 52% for the controls, and reached 76% and 86% respectively in summer 1987. Chemical analyses of aqueous extracts of straw were performed in June 1986, which revealed the presence of at least five phenolic acids, which had been previously identified as the toxic principles involved in the phytotoxicity of the straws. Nitrogen content of R. idaeus foliage from the treated plots (mean N content 1.49%) was significantly lower than in seedlings from the control plots (mean N content 2.24%). The results suggest the potential for using allelopatic cover mulches to reduce competing vegetation. However, more work is needed to assess the usefulness of the technique for forest vegetation management.