Toward an integrated weed management strategy for bareroot forest nurseries
Nursery experiments were conducted with eastern white pine (' Pinus strobus' L.) at Kemptville, Orono and St. Williams, Ontario from 1992 to 1994. The critical period of weed control (defined as the period during which the crop must be kept weed free if yield is not to be reduced) was 700 to 1500 growing degree days (GDD) after sowing in first-year seedlings and from 700 to 1500 GDD after five days of a positive GDD value in second-year seedlings. These values were experimentally determined by fitting the Gompertz growth model to seedling morphological variables. Leaf dry weight had the best combination of sensitivity and low coefficient of variation when compared with eleven other response variables. First-year seedlings were more sensitive than second-year seedlings to the presence of weeds. The critical period was a product of the weed biomass on the site, and weed height was an important determinant of weed competitive ability. Herbaceous weed competition reduced seedling dry weight and diameter but height was only slightly reduced. The allocation of carbon to roots, stems and leaves in response to the presence of weeds varied from experiment to experiment, but the allometric coefficient of total biomass to leaf biomass was relatively consistent across experiments. Mean relative growth rates declined: (1) through the season in both first and second years; (2) from first to second year; (3) with increasing seedling size; (4) with increasing weed competition. Seedlings suppressed by weed growth in year one grew in year two at the same rate as seedlings that were weed free in year one. Height growth in year two was positively correlated with weight at the end of year one. Spring wheat ('Triticum aestivum' L.), grown as a companion crop, was evaluated as a replacement for wooden lath shades. Although weed biomass was reduced, the weed control provided was not acceptable. When weeds were not controlled, growth was increased in the presence of the companion crop, but when weeds were controlled, the companion crop reduced tree growth. Neither lath shades nor a cereal companion crop were of value in the culture of eastern white pine seedlings. Development of an integrated weed management strategy requires that the competitive relationship between the prop and its weed community be understood, and that all aspects of the culture of the crop be considered in terms of their impact on weed-crop dynamics.