While the certification process differs between these groups, all of them include some mention of undefined "forest stewardship", "respect for aboriginal peoples", and compliance with local, provincial or national environmental laws, forest worker safety, education and training, and other issues. The certification is largely about tracking, to ensure traceability, and does not de-certify lumber obtained from clearcuts, or taken without the consent of aboriginal peoples. Insects Recent years have seen outbreaks of insect pests in forest-destroying plagues: the spruce-bark beetle (Dendroctonus rufipennis) in Yukon and Alaska; the mountain pine beetle in British Columbia; the aspen-leaf miner; the larch sawfly; the spruce budworm (Choristoneura fumiferana); the spruce coneworm. Pollution The effect of sulphur dioxide on woody boreal forest species was investigated by Addison et al. (1984), who exposed plants growing on native soils and tailings to 15.2 μmol/m3 (0.34 ppm) of SO2 on CO2 assimilation rate (NAR). The Canadian maximum acceptable limit for atmospheric SO2 is 0.34 ppm. Fumigation with SO2 significantly reduced NAR in all species and produced visible symptoms of injury in 2–20 days. The decrease in NAR of deciduous species (trembling aspen [Populus tremuloides], willow [Salix], green alder [Alnus viridis], and white birch [Betula papyrifera]) was significantly more rapid than of conifers (white spruce, black spruce [Picea mariana], and jack pine [Pinus banksiana]) or an evergreen angiosperm (Labrador tea) growing on a fertilized Brunisol. These metabolic and visible injury responses seemed to be related to the differences in S uptake owing in part to higher gas exchange rates for deciduous species than for conifers. Conifers growing in oil sands tailings responded to SO2 with a significantly more rapid decrease in NAR compared with those growing in the Brunisol, perhaps because of predisposing toxic material in the tailings. However, sulphur uptake and visible symptom development did not differ between conifers growing on the 2 substrates. Acidification of precipitation by anthropogenic, acid-forming emissions has been associated with damage to vegetation and reduced forest productivity, but 2-year-old white spruce that were subjected to simulated acid rain (at pH 4.6, 3.6, and 2.6) applied weekly for 7 weeks incurred no statistically significant (P 0.05) reduction in growth during the experiment compared with the background control (pH 5.6) (Abouguendia and Baschak 1987). However, symptoms of injury were observed in all treatments, the number of plants and the number of needles affected increased with increasing rain acidity and with time. Scherbatskoy and Klein (1983)