Forest restoration provides the opportunity to increase biological diversity, enhance ecosystem services, test ecological concepts, serve as an educational resource, and development management guidelines. The aim of this project was to study coniferous forest restoration in former agricultural fields, now part of the St. Olaf College Natural Lands in southeastern Minnesota, USA. Little information is available on early growth patterns of conifers. Eight conifer species commonly found in northern Minnesota were planted as two-year old seedlings in 1993 and 1999 on two sites.. The species were balsam fir (Abies balsamea), black spruce (Picea mariana), jack pine (Pinus banksiana), red pine (Pinus resinosa), tamarack (Larix laricina), white cedar (Thuja occidentalis), white pine (Pinus stobus), white spruce (Picea glauca). A total of 899 trees were measured every four-five years for twenty-six years. Colonizing woody species were also recorded in 2019. The objectives of the study were to 1) compare tree size, growth, and mortality patterns among species over time, 2) measure density of colonizing woody species, 3) measure soil physical and chemical characteristics, and 4) make projections about the impact of climate change on future conifer growth patterns.
Results/Conclusions
Measurements of tree growth patterns over 26 years showed strong initial growth by jack pine, but after 10 years red pine and white pines were the largest species followed by jack pine and white spruce. Mortality rates since planting were highest for white spruce (30%) and jack pine (19%). Successful reproduction, indicated by new conifer trees (diameter > 2.5 cm), was found in white pine, jack pine, white spruce, and balsam fir. In 2019 only balsam fir seedlings were found, along with a number of deciduous tree seedlings of white ash (Fraxinus americana), American elm (Ulmus americana) and boxelder (Acer negundo). Soil tests found adequate levels of percent organic matter, between 6.8 and 7.6 percent. Nitrate levels between 4.8 and 5.1 mg/kg soil and orthophosphate levels from 3.1 to 5.2 mg/kg soil also indicate adequate nutrient levels. Although the conifer species in this study were planted south of their normal range, our data shows that they can grow and reproduce at warmer temperatures that are likely coming to northern Minnesota. However, colonization of the understory by deciduous trees suggests that management to remove deciduous species will likely be required in the future.