Halifax Tree Project

TREE ALLERGIES

Jordan Haughn, Halifax Tree Project  

2021-05-26  

After many months of leaf-less trees, the warm spring air has finally arrived to restore the streetscapes of Halifax to their fully foliaged glory. However, springtime also means tree-pollen allergy season, a dreaded downside for many folks hoping to enjoy the outdoors. While other properties of trees - such as tree nuts - are also allergens, pollen is among the most common causes of allergies in Canada and can be difficult to avoid. Around the world, the intensity and duration of allergy season is increasing as well as the number of people impacted. Furthermore, approximately 80% of people with asthma also have pollen allergies, causing further difficulties. Although most tree pollen allergies are short-lived each year and treatable, it is important for managers to consider these health impacts when designing the urban streetscape. Ideally, we want tree species that maximize ecosystem services while minimizing public health impacts.

First of all, what is pollen? In a nutshell, pollen appears as a fine, yellow, powdery substance (although individual grains can only be seen with a microscope due to their small size) produced by male flowers to fertilize female flowers of the same species. Pollen can be transported by insects and water, but wind is of particular concern during allergy season in the streetscape as plants can release billions of pollen grains at once. Once inhaled, individuals with pollen allergies experience adverse reactions including watery eyes, sneezing, and a stuffy nose. Warm, dry, and windy conditions are ideal for pollen transport. Wet and rainy days usually result in lower pollen concentration, although rain can initially exacerbate allergies by busting large pollen particles, spreading them further.

The largest concentrations of pollen are released between mid-March and mid-June; however, seasons can fluctuate by as much as two to four weeks in Nova Scotia. Tree pollen and pollen concentration also vary from tree to tree. Birch (of moderate allergenicity), for example, can deliver high pollen counts with the season ranging from mid-April to early June while ash (of high allergenicity) usually demonstrates low or moderate counts with a season ranging from mid-May to mid-June. When selecting trees to plant in the streetscape, it is important to consider these variations. The pollen in the HRM comes from a mixture of many genera including alder, ash, beech, birch, elm, hemlock, maple, oak, pine, fir, spruce, and aspen. Of these species, maple, oak, and elm are most common on urban streetsides with only oak demonstrating consistently high counts of high-allergenicity pollen during its season. While we don’t generally have coniferous species on the streets of Halifax, it is interesting to note that nine of the ten native coniferous tree species in Nova Scotia (save eastern white cedar) have no allergenicity issues (according to the website pollenlibrary.com).

So, how can the urban forest be managed to alleviate those downsides? When it comes to pollen, many alleviation strategies must be weighed against potential downsides. One strategy would be plant as many female-flowering dioecious (trees that will have either male or female flowers, not both) species as possible. However, female flowering trees can also produce seeds, pods, and fruit which, when they drop, create a mess, exude foul odors, and attract pests. In fact, only male specimens of the Ginkgo species are planted in Halifax due to the foul-smelling and messy fruit produced by female individuals.

Another management strategy would be to prioritize trees that produce pollen which is heavy and coarse – including crabapple, dogwood, magnolia, callery pear, and tulip trees. This strategy also has similar problems of unwanted mess and pest attraction. Many of these trees are also typically smaller in size, reducing ecosystem services like shade. Planting a mix of these trees among high-pollen-producing trees that provide other desired services may be a compromise that prevents the downsides of both from magnifying into more serious issues.

Considering that the tree pollen season is relatively short and is not currently presenting a large public health risk in Halifax, an adaptive management strategy is my recommendation. The pollen concentrations and rates of tree allergies in the population must be monitored over time to determine if more drastic action is needed to tackle tree allergies. Currently, Halifax’s urban forest presents a mix of species – some produce pollen in high concentrations, some low, and some do not produce any. This, combined with health management such as allergy treatment on the individual level (such as medications, checking the pollen counts before going outdoors, etc.) further reduces risk.  I believe this mix will create a robust system which hopefully minimizes the nuisance to those with tree allergies.