Unsurprisingly, a lot of it is the males’ fault — but not the human males, the tree males. Seriously, planting a lot of male trees is a good part of what’s causing your allergies to flare. The other culprit may be climate change.
You may have noticed that, in recent years, allergies (and pollen allergies in particular) have gotten much worse. Especially in urban areas, they affect more people and tend to produce stronger effects than they used to. It’s easy to speculate on why this is happening: it could be the polluted air in cities, our immune systems growing weaker, or even the fact that trees have a harder life in urban areas. However, one researcher believes there’s something else behind this issue: botanical sexism.
Just like in humans, gender isn’t exactly a simple thing for trees. Some trees are dioecious — they are distinctly either male or female. Others, like the oak and pine, are monoecious — they have both male and female flowers on the same plant. Both groups of trees produce pollen which can trigger allergies, but horticulturalist Tom Ogren believes the bulk of the blame is carried by the former group; specifically, the male trees.
Ogren, whose wife suffers from asthma and allergies, had the bad luck of moving into a pollen-rich area. This proved to be quite a problem, but as he looked at neighboring areas, he found more of the same. It seems that everywhere he looked, there was more pollen than there used to be in past decades. He also noticed another weird thing: all the trees were males.
Decided to dig deeper, Ogren started to figure out what was happening when he came across a 1949 USDA Yearbook of Agriculture. The guideline noted:
“When used for street plantings, only male trees should be selected, to avoid the nuisance from the seed.”
Female tree parts are the only ones that produce seeds, and it seems that for city planners, this was a mild inconvenience — a nuisance, as the USDA put it. This nuisance was enough to dramatically reduce the percentage of female (and monoecious) trees used in city planning. But there’s another reason why things have gotten worse, at least in the US.
America loves its elm trees, and coincidentally, elms don’t really produce all that much pollen. In the first half of the 20th century, elm trees towered over many of the country’s major cities. However, an unfortunate shipment from Britain brought along a virulent strain of Dutch elm disease, a fungal illness spread by the bark beetle. The disease spread like wildfire. By 1989, some 75% of America’s 77 million elms had been killed.
Eager to replace the fallen trees as quickly as possible, municipalities repopulated the streets according to the USDA recommendations — with male trees. As the trees matured, they shed more and more pollen, producing the increase in allergy symptoms we see today. Nurseries also prefer male trees because they are easier to clone than waiting for the male and female parts to fertilize naturally, further perpetuating the issue.
“Botanical sexism runs deep,” Ogren says.
Ironically, this was all for naught, says Ogden. If city planners had taken the diametrically opposite approach, planting only female trees, they would have eliminated the problem of seeds as well as that of pollen. Female trees only produce seeds if there are males nearby, so a female-only population wouldn’t really produce seeds.
The worsening of allergenic potential is quantified by something called the Index of Urban Green Zones Allergenicity (IUGZA).
“The index was created with the aim of assessing the risk of allergic symptomatology in a green area as a function of the plant species that grow in them,” explains Paloma Cariñanos, a botanist at the University of Granada who, along with colleagues, developed the index in 2014.
The index starts from an exhaustive inventory of species, assigning values for a series of biological attributes related to allergenicity — things like pollination strategy, the duration of flowering, and the capacity of a plant’s pollen grains to trigger an allergy.
Recently, Cariñanos applied the index to greens spaces in 23 cities located in six Mediterranean countries, finding that a handful of native ornamental species are the main culprits for allergy. Cariñanos also says that ending the botanical sexism bias can help reduce cities’ allergy potential. Other measures that can work are diversifying local fauna and monitoring the plant inventory.
But there’s one thing that also worsens allergies, and will be much harder to tackle: climate change.
Climate change also makes allergies worse
In 2020, Texas had its worst allergy season in years, while states like Georgia and New York had near-record levels of pollen. In Europe, things are also becoming worse, with climate change playing an important role.
The prevalence of allergies among US citizens has skyrocketed in just a few decades. In 1970, about one in ten Americans suffered from hay fever, which is caused by airborne allergens, such as pollen. By 2000, three in ten did, which represents 20 million on the adult population, according to research by the Asthma and Allergy Foundation of America. Two separate studies found that climate change is, at least in part, to blame.
It’s making pollination start sooner and last longer. According to one of the studies, pollen season currently starts 20 days earlier than in 1990, and there’s also 21% more pollen in the US than there was three decades ago. Climate heating is increasing the freeze-free period giving plants more time to grow, flower, and produce pollen.
More than simply higher temperatures, increasing carbon dioxide in the air can also contribute to longer and more intense pollen seasons, which can worsen allergy and asthma symptoms in children (and to a lesser extent, in adults as well). Lab tests show that plants growing with more carbon produce more pollen.
Another aspect that may be contributing to the problem is the diversity of plant pollen we are exposed to nowadays. Exotic and invasive plants are much more common in many parts of the world than they were a few decades ago, and in some instances, these can also contribute to the problem.
So yes, climate allergies are getting worse, and you can thank a mixture of climate change and unfortunate policy for it.