Halifax and the Tree-Pollen Issue
Peter Duinker, Halifax Tree Project, 2024-07-13
Each spring, many people in Halifax suffer from pollen allergies. While the whole array of plants in Halifax produces pollen throughout the growing season, trees are responsible for most of the spring pollen production. I wrote this article to try to dispel some myths circulating in the news and social media and to emphasize how precious little we know, with confidence, about tree pollen, its production and distribution, and how it afflicts residents and visitors. I will use a Q&A format for this presentation.
1_ What is the role of pollen in tree reproduction?
Pollen grains are the male gametophytes (in animals, these are sperm cells) needed to fertilize the female gametophytes (in animals, these are eggs) to produce viable seeds. Pollen is produced in either male-only or perfect (bisexual) flowers.
2_ How does pollen move from male flowers on one tree to female flowers on another tree (or from perfect flowers on one tree to perfect flowers on another tree)?
There are two main ways that pollen moves from flower to flower. One is by pollinator insects like bees. The other is by wind. Some tree species depend exclusively on wind, some on insects, and a few on both. Less frequently, pollen is moved by birds and mammals (including humans; for instance, on our footwear).
3_ What is the relative production level of tree pollen in wind-pollinated plants compared to insect-pollinated ones?
Pretty much all sexual reproduction in plants and animals alike is characterized by huge quantities of male gametophytes (pollen and sperm, respectively) compared to the quantities of female gametophytes (ovules). However, in general terms, wind-pollinated plants produce relatively more pollen of smaller size compared to insect-pollinated plants. When you think of the need for the female flowers (or flower parts) to receive pollen of the same species through the air via wind, the process is pretty much random and there have to be thousands upon thousands of appropriate pollen grains out there for a female flower to have a chance to be fertilized. So, no wonder the air is full of pollen during the reproductive phases of the trees.
4_ How far can tree pollen move by wind?
This article is focused on pollen in the air and its allergenicity to humans, so while pollen is dispersed by other means (as noted in question 2 above), we will limit our attention to
wind dispersal. The answer depends on the size of pollen grains and the wind regime. Since most trees do not breed themselves (meaning that pollen from a tree does not fertilize female/perfect flowers on the same tree), we can imagine that pollen may have to travel some considerable distance for female flowers on one tree to have a chance to become fertilized. Pollen cannot just be released from male flowers and fall quickly to the ground – the grains have to be carried on the wind. My consultations with pollen experts confirm my suspicion that pollen can easily travel, especially if winds are vigorous (as they are frequently in Halifax), distances measured in kilometres, even tens and hundreds of kilometres for some species. An implication of this is that pollen produced in rural woodlands around Halifax would be easily found, in abundance, in the city. Our air is a veritable pollen smorgasbord in spring.
5_ What is the relative pollen allergenicity across tree species?
The answer to this question is full of uncertainties. Several research groups in Canada, including in Halifax, have canvassed a wide range of information sources to try to find reputable data about relative allergenicity values for our tree species. Allergenicity is the ability of an allergen, like pollen, to induce a reaction like sneezing. Sometimes the data don’t even exist, and sometimes – often, even – there are different opinions (yes, opinions) by authors about species-specific pollen allergenicity. This means that we have very flimsy knowledge on this, so I will not even attempt a generalized list of species or genera suspected to be highly allergenic and those not. If we knew more about allergenicity of tree pollen by species, then some helpful actions might be meaningful in terms of urban forest management. Thus, if we can shift the species distribution of the entire urban forest away from high-allergenicity species, especially trees producing high quantities of pollen (like male-only trees), we might be able to reduce people’s suffering from pollen allergies in the spring. I will speak later about the possibilities of doing this.
6_ What is the relative contribution of trees planted by HRM Urban Forestry to the tree pollen load in the city
Since the HRM Urban Forest Master Plan was implemented starting in 2013, HRM Urban Forestry has planted something like twenty thousand new street trees. Other trees have been planted in the city by other agencies and landowners, but let’s focus on the street trees. The first thing we can say is that few, if any, of these trees are producing pollen because they are too young for that. Some tree species produce pollen early in their lives (say, in the first decade or two), and many start pollen production only after several decades of life. The second thing to look at is the potential pollen production from the rest of the trees in Halifax. For the sake of argument, let us imagine drawing a line five kilometres past the urban boundary used in the 2013 UFMP. This would outline a huge territory of woodland at the periphery of the city but also include all the mature trees growing throughout the city’s built-up areas. How many sexually mature trees might we estimate to be within that expanded boundary, most of which would be producing pollen that could, wind conditions permitting, be wafting over the city’s neighbourhoods? That number is impossible to know with any useful precision or accuracy, but suffice it to say here that many millions is NOT an overestimate. A population of twenty thousand young trees in a context of many millions of mature trees leads me to the conclusion that the contribution of the recently planted street trees to the allergenic pollen load in the city is, at present, essentially nil. This may change, of course, in future decades as the young trees mature.
Another factor here is the balance of planted vs naturally regenerated trees in Halifax. From my observations and research over the past two decades, I am firmly convinced that the vast majority of trees within that area I sketched above (i.e., five kilometres past the boundary of the urban core of the 2013 UFMP) are naturally regenerated. This is true for both woodlands and built-up areas as well. Even in my neighbourhood in the centre of the Halifax peninsula, I estimate that roughly half of the trees have been planted (mostly in the streets) and half are naturally regenerated (mostly in backyards). This then begs the question of how much control Halifax actually has over the tree populations that are producing pollen year after year.
7_ What factors contribute to the purported increase in pollen-allergy problems in HRM?
I don’t challenge the observation made by many experts that pollen allergy problems in HRM are increasing. This may mean that the tree pollen load is increasing. There is growing evidence that trees will produce more pollen as they live through a changing – read: warming – climate. The overall pollen production season is lengthening too. Other factors that can contribute to the problem is the increasing level of carbon dioxide in the atmosphere, which many experts see as a kind of tree fertilizer, and situations where urban air pollution (e.g., ozone and particulate matter) is relatively high.
8_ What can pollen-allergy sufferers do to reduce their suffering, and what can others do to help?
People who suffer from allergies to tree pollen have a few options to try to obtain some relief. Most are aware of the allergy-relief medications. Following the pollen alerts produced daily by various agencies, including the weather forecasters, can be helpful so that sufferers might avoid vigorous outdoor activities on days predicted to be high for allergenic pollen load in the air. Or they can stay indoors, if possible.
What can others do? Most of us who are not allergic to tree pollen can do little. Urban foresters and planners might do what they can to shift the urban forest toward species and cultivars that produce lower amounts of allergenic pollen. Increasing tree-species diversity – which would mean fewer trees of any specific species – could lower the air burden of pollen of individual tree species. However, as stated above, this may have little effect if natural and semi-natural woodlands are abundant in and around the city.
One myth I must address is the notion that planting a higher proportion of female trees (or trees with perfect flowers) can reduce the pollen load in the air because more of the pollen would be taken up by the female flowers. I suggest that because the rates of production of wind-disseminated pollen grains is so massively high compared to the number of female flowers depending on pollen to intersect with them, increasing the density of female flowers in the treescape is unlikely to have any effect on the density of pollen grains in the air.
Given the discussion at number 7 above, one could imagine that success in arresting climate change would be helpful to allergen sufferers. An optimist about present and future climate-action programs like HalifACT might conclude that the climate-change aggravation of the pollen allergies should be reduced. On the other hand, a pessimist might conclude that more climate change will be a global reality despite best intentions to arrest it. At this time, I lean toward the pessimistic view.
9_ What measures of urban and rural forest management can help reduce the atmospheric load of allergenic tree pollen?
Given that, in Halifax, so much of the urban pollen load is generated in natural and semi-natural woodlands at the periphery of the city, and given that so much of both the rural and urban forest is owned privately by individuals and corporations and not the municipal or provincial governments, it seems to me that any attempts to shift meaningfully the relevant tree populations to species and cultivars that produce low amounts of allergenic pollen will be infeasible. Unless we dramatically reduce the actual tree populations, a move that would be totally unacceptable to most people in Halifax because of the enormous and diverse benefits we get from trees (for articles on these benefits, see the posts on this website), allergenic tree pollen is here to stay.
Conclusion
Despite the tremendous uncertainties in assessing the nature and gravity of the tree-pollen issue in Halifax, I feel confident to say that the contribution of HRM’s urban-forest management program is miniscule. People who live in HRM live in a thriving urban forest surrounded by a thriving rural forest. Trees are pretty much everywhere we go in HRM – most are there naturally, and in the built-up areas, many are there by design. This means that tree pollen is everywhere, in abundance, during the spring. Much of it may be allergenic, and because the tree pollen load is so strongly influenced by natural and semi-natural woodland, urban-forest managers have little influence, through their actions, on the overall tree pollen issue. However, that said, they should still do what they can to try to keep the problem from getting worse. Probably the most promising approach to this is to ensure that species and cultivar choices for tree planting around the city are guided, in part, by pollen production and allergenicity.