Abstract
In conifer plantations, competing vegetation and variation in planting site at the microsite level can influence growth and survival of seedlings. Site preparation and subsequent release treatments are often necessary to ensure successful regeneration on planting sites in the northwest United States. Two experiments were designed and implemented in northern Idaho, examining Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. glauca) and western larch (Larix occidentalis Nutt.) seedlings. The first explored the influence of competing vegetation and logging debris at the microsite level on soil moisture, soil temperature, and seedling growth. The second tested the effects of a post-planting herbicide application targeting competitive forbs on the growth and survival of Douglas-fir and western larch plantations.The first research objective was to examine the effects of incremental competition removal around individual seedlings at the microsite scale. In 2019, two sites were chosen on PotlatchDeltic Corporation land—one site characterized as high productivity and one as low productivity based on Douglas-fir site index. Thirty seedlings of each species were identified at each site, to which five different levels of vegetation control treatments were randomly applied. The treatments were 0%, 25%, 50%, 75% and 100% vegetation control, achieved through directed applications of glyphosate. The treatments were maintained through two growing seasons (2019 and 2020). Seedling morphological characteristics such as height and diameter were tracked in each year. Soil moisture was measured monthly throughout the growing season around each seedling, and soil temperature was measured every hour using iButton Thermochrons (Maxim Integrated, San Jose, CA) buried in mineral soil. Soil moisture was higher in the presence of increasing levels of coarse woody debris around Douglas-fir seedlings at the high productivity site, and around western larch seedlings at the low productivity site (p ≤.013). Root collar diameters of both Douglas fir and western larch at the high productivity site were smaller in response to increasing levels of competing vegetation (p ≤.003). Coarse woody debris also contributed to larger seedling diameters for Douglas-fir at the low productivity site and western larch at the high productivity site (p ≤.002 ). Controlling competing vegetation, retaining logging debris on a site after harvesting, and intentionally planting seedlings on the north side of stumps and other debris all contribute to more favorable microsite conditions, improving seedling growth in the formative years.
The second research objective examined the efficacy of a clopyralid herbicide treatment applied one year after planting on the growth and survival of Douglas-fir and western larch in pure plantations, one and two years after treatment. Eleven sites were chosen on PotlatchDeltic Corporation land, representing a gradient of site productivities defined by site index, with three sites each classified as either low productivity, moderate, or high productivity. Six plots per species of thirty-six trees were established at each site, three plots of which were randomly assigned to be released with clopyralid. Competing vegetation, seedling height and diameter, and seedling survival were monitored over three growing seasons; one season prior to treatment, and two seasons afterwards. The release treatments were effective at reducing forb cover in the year after treatment, and in some cases into the second year after treatment. The herbicide application did not affect survival of either Douglas-fir or western larch seedlings. On average, Douglas fir seedlings showed growth increases in response to treatment in the following years, with seedlings achieving 46% and 71% higher stem volumes in treated vs. untreated plots by the end of the study. No noticeable growth effect was observed for western larch. Variability in growth of western larch may have been caused by grazing of both domestic livestock and free-roaming ungulates, masking any response to herbicide treatment. This study provides an example of improved growth of Douglas-fir seedlings following a springtime clopyralid application in the Inland Northwest. The limitations and possibly confounding factors that could be restraining a response in western larch suggest additional research with grazing exclusion could determine if clopyralid improves growth of western larch.