Effet des sites et des traitements sylvicoles sur la croissance, l'allocation en biomasse et l'utilisation de l'azote de semis de quatre espèces feuillues en plantations dans le sud-ouest du Québec

To identify conditions leading to the successful establishment of hardwoods in plantations, several parameters related to growth, biomass allocation, and nitrogen nutrition were analyzed in relation with variations in soil resources, among four plantation sites, and different weed control treatments. White ash (Fraxinusamericana L.), butternut (Juglanscinerea L.), bur oak (Quercusmacrocarpa Michx.), and red oak (Quercusrubra L.) seedlings were planted in 1988 on four sites typical of glacial tills, littoral sands, and marine clays found in southwestern Quebec. Weed control treatments consisted in the use of a low persistence herbicide (glyphosate) and that of an organic mulch composed of chipped hardwood branches. A control treatment was included in the design. Weed control was best with the herbicide, resulting in an increase in soil temperature, water content, and nitrate concentration. Seedlings from the herbicide treatment reached a greater height and their leaf biomass was 25% that of the total biomass, and twice as much that of the control. Their root biomass accounted for 35% as compared to 50% for the control. The total amount of leaf nitrogen was 10 times greater in the herbicide treatment. Mortality rates were highest for white ash, bur oak, and butternut on sites where water availability was lowest, as well as in the absence of weed control. Red oak mortality was lowest on the littoral sand site. Mortality was the same with either weed control treatments although clearly lower than that of the control. The relative growth rate and net assimilation rate, as well as nitrogen use efficiency, were lower on the marine clay site, a high agricultural potential site. Because of a reduction of weed competition in the herbicide treatment, white ash had similar height growth on all sites, whereas bur oak, butternut, and red oak, in particular, grew best on nonexposed sites with light-textured soils. Successful establishment of hardwood seedlings depends strongly on the control of competing herbaceous weeds during the first years of growth in plantation. In southwestern Quebec, on glacial tills where topography increases water supply through seepage, wind-sheltered sites show a high potential for valuable hardwood reforestation. Such sites have a low agricultural potential because of their very high stonyness.