Impact of Climate Change on Tree Pollen Levels in Southern Ontario

Abstract

Scientists have predicted longer, more intense pollen seasons due to climate change. Our NAB station in SW Ontario, Canada has reported an opposite trend in ragweed and no measurable changes in grass pollen. Trees may be subject to the cumulative impacts of environmental stressors. Annual pollen counts of 14 different tree varieties covering 20 years, from 1998 to 2017, were analyzed. Tree pollen dissemination in this area begins in March or early April, with Mulberry typically constituting one-quarter of the annual counts. Maple, Juniper, Poplar and Oak each represent about 10% of the counts. We employed anomaly detection procedures to identify when tree pollen counts were extraordinarily high or inordinately low, outside of the expected range. Data analysis showed outliers when annual tree pollen levels were either extremely high or low, far beyond the 1st or 99th percentile. The number of outliers was greater than expected. Distribution of counts was highly skewed at the lower end, often tailing to the right where pollen counts were excessive. Climate change may be affecting tree pollen production in an unexpected manner. Years of drought, excess precipitation, pestilence, high temperatures, bitter cold winters, etc., sometimes successively, will have cumulative impacts on tree health. Temporal adjustments to the prevailing seasonal conditions, as well as the cumulative stress, manifest in extreme pollen seasons. The effects of climate change on tree pollen production are complicated and not as well understood as current science has led us to believe.