Growth of planted lodgepole pine and hybrid spruce following chemical and manual vegetation control on a frost-prone site

Abstract

The 5-year height and diameter growth response of a frost-tolerant species, (lodgepole pine, Pinuscontorta var. latifolia Dougl. ex Loud.) and a frost-susceptible species (hybrid spruce, Piceaglauca (Moench) Voss × Piceasitchensis (Bong.) Carr.) to different vegetation-control site-preparation treatment and timing combinations was examined using a factorial experiment. The site chosen for the experiment is subject to frequent summer growing season frosts. The vegetation control treatments were: an untreated control, manual cutting, and glyphosate applied at the rates of 1.4 and 2.1 kg active ingredient (a.i.) per hectare. Each vegetation-control treatment was applied at four dates (timing) that covered the active growing season of the vegetation on the study area. Vegetation-control method had a major impact on the growth of the planted seedlings. The influence of timing of control treatments on growth was found to be minor. Fifth-year height and diameter in the manual cutting and control treatments were equally poor. Growth was significantly improved by both levels of chemical vegetation control and after 5 years, no differences were apparent between the two. Pine height and diameter and spruce diameter continually improved as the level of thimbleberry (Rubusparviflorus Nutt.) was reduced. Below 5% thimbleberry cover, this growth increased dramatically, suggesting a response threshold. As vegetation cover was lowered, however, spruce seedlings were damaged by frost, resulting in reduced total height. In this case, better height growth was associated with either very low or moderately high thimbleberry cover than with moderate to low levels.