The effects of soil compaction on the germination and growth of Picea mariana and Pinus banksiana

Soil compaction has long been an issue for the forestry industry. Soil compaction can reduce soil aeration, porosity and drainage, and restrict the growth and success of trees and other plants. The objective of this study is to determine how the degree of soil compaction affects the seed germination and seedling growth of jack pine (Pinus banksiana) and black spruce (Picea mariana). The study was carried out in a greenhouse at the Lakehead University Thunder Bay campus. Silty clay loam soil was used for the study and the treatment consisted of four levels of compaction with the bulk densities of 0.9 g/cm3 (C), 1.1 g/cm3 (C1), 1.2 g/cm3 (C2), and 1.3 g/cm3 (C3). The soil compaction was implemented in buckets of 36 cm in height and 30.2 cm in diameter. Each treatment level was replicated five times. Seed germination was tallied daily. Seedling height and root collar diameter were measured after 18 weeks of seedling growth. Then the seedlings were harvested. The foliage and root systems were scanned and analyzed using the Regen WinSeedle and WinRhizo systems, respectively, and the following parameters were subject to statistical analysis: root length and diameter, total root length, and projected and surface areas of roots, average needle length, average needle width, total leaf area and the number of needles per seedling. The biomass of roots, total biomass, as well as root to shoot ratios, were determined after the samples were dried in a drying oven at 80 C for 48h. It was found that root biomass, total biomass, root to shoot ratios, main root lengths, seedling height, total root length, average root diameter, root surface area, and the number of root tips were significantly affected by soil compaction and were generally significantly different between the two species, except for the root to shoot ratios. While root diameters increased under increased compaction, every other growth parameter faced general downward trends in growth. Under the most severely compacted soil (C3), total root lengths and number of root tips decreased by 64.6% and 76.2%, respectively in Picea mariana. Large reductions in the main root lengths and the number of root tips were found for P. banksiana under the most severely compacted soil as well, with reductions of 46.4% and 46.7%, respectively. The reductions by soil compaction of all the measured growth parameters were greater in P. mariana than in P. banksiana. Overall, this study illustrates that while the growth of both species was negatively affected by all levels of soil compaction, P. banksiana was less sensitive than P. mariana, suggesting that jack pine may have a competitive advantage over black spruce on sites with soil compaction. Therefore, all efforts should be made to avoid or minimize soil compaction in forestry operations to maintain the forest productivity of the site