Effects of experimental crude oil spills on subarctic boreal forest vegetation near Norman Wells, N.W.T., Canada

Data are presented on the effects of experimental crude oil spills made on two subarctic boreal forest plant communities near Norman Wells, N.W.T. Spray spills of fresh unweathered crude oil at an intensity of 9.1 ℓ/m2 had a general herbicidal effect and caused the death of any green tissue coming in direct contact with the oil. Death of lichens and mosses was rapid and complete. For some higher plants, a considerable lag period occurred between the time of the spill and the time of death (up to 4 years for some individuals of Picea mariana). For others, death occurred during the first winter, with marked effects on cover values in the spring. These effects resulted in large decreases in total plant cover and frequency at spill sites. However, within a few weeks, and in subsequent years, some species developed regrowth shoots. Other species survived as underground rhizomes for a number of years prior to their reappearance above ground (i.e., Equisetum scirpoides). Limited seedling establishment by vascular plants was first observed in the fourth postspill growing season, when some sporeling establishment was also noted for several bryophyte species. No Picea mariana regeneration has occurred in the spill plots in the six postspill growing seasons monitored thus far.Crude oil spills made in winter were found to be less damaging than equivalent summer spills in their short-term biological effects and on rates of recovery and species affected. Initial observations indicate that a summer diesel oil spill shows roughly equivalent toxicity to a summer crude oil spill of the same intensity. Comparisons between an intensive spill (8500 ℓ) made at one point and dispersed spray spills indicate that the former are far less damaging per unit of oil applied to the plant community, with severe detrimental effects being largely limited to areas of direct surface contamination. In the point spill examined, most of the oil percolated downwards and then laterally. Surface vegetation growing above areas with subsurface horizons contaminated by oil was not greatly affected in the first 2 years. An increased area of damage appeared in postspill years 5 and 6, including death of Picea mariana. Oil also appeared to move laterally in 1976 when severe rains occurred, and the oiled area increased somewhat.Limited short-term effects of the spill treatments on depth of active layer thaw have been noted in this study, but these initial effects were not maintained after the first postspill growing season. The low rates of oil application make the conclusions about the effects of large spills on active layer stability conjectural. Potential effects on vegetation are much more firmly based. Oil in the boreal forest soil appeared to retain toxic properties throughout the 5-year study period.