The story starts with horticultural epidemiologist Thomas Ogren. His major citation is that of the 1949 USDA Yearbook of Agriculture where it was recommended that “when used for street plantings, only male trees should be selected, to avoid the nuisance from the seed”. He claims this led to a major skew toward planting male trees in urban cities in the following years in attempts to reduce the work needed to pick up the petals, berries, and fruits produced by female trees. Then, in the 1960s, after many of the oldest elms in the United States fell victim to the Dutch elm disease, there was a major increase in the planting of clonal male trees. Ogren claims that once all of these male trees matured, there was a major increase in city pollen. Thus, according to Ogren, preference for the planting of male trees and the subsequent rise in pollen levels is responsible for the drastic rise in pollen allergies and asthma.
Taking a step back, what constitutes a male versus female tree? Let’s take a quick refresher on basic plant biology. Plants have two “equivalent” male and female sex organs. The male sex organ is known as the stamen and is responsible for producing pollen which contains male gametes. The female sex organ is known as the pistil and has an ovary where the ovules are located. Once the pollen fertilizes the ovule, seeds can start developing! There are also two types of plants, monoecious and dioecious plants. Monoecious plants have both male and female sex organs whereas dioecious plants have only one or the other. The majority of plants are monecious.
Analyzing Orgen’s argument, the case for the overabundance of male trees causing allergies in urban cities becomes highly unlikely. Orgen’s first claim is that the skew in male trees initially arose following the USDA recommendation to plant male trees. However, he failed to cite the whole passage. The USDA recommendation was that “when used for street plantings, only male trees should be selected, to avoid the nuisance from cottony seed”. Therefore, while there may have been more male cotton trees planted as a result of this recommendation, other trees were likely unaffected. Secondly, the cause-and-effect relationship between increased pollen from an increase in the number of planted male trees and increased allergies in urban cities is very difficult to establish. Up until now, no empirical and objective data exist to provide support for this causal claim. While pollen allergies are on the rise, there are a series of factors that could be involved in this phenomenon and none of them are easy to isolate as the culprit. For example, it is believed that air pollution plays a major role in the increased impact of pollen on the allergies of individuals. While the interaction between air pollution and pollen is hard to examine, it is proposed that air pollutants make pollen more allergenic by changing the elemental composition of pollen, increasing total pollen count, and causing pollen to release sub-pollen particles into the air. Furthermore, warmer temperatures brought about by climate change are also thought to play a role as plants can start pollinating earlier in the season and continue pollinating later into the season. Finally, there are three major types of pollen allergies: tree, grass, and weed. Therefore, even if we were to just plant just female trees, people can still suffer from pollen allergies.
While botanical sexism may not be the cause of increased pollen allergies, pollen allergies and exposure to allergenic pollen is on the rise, particularly in urban environments. This draws attention to the potential use of urban forests in mitigating the arising health consequences. One such example of this is the use of hypoallergenic trees to reduce allergy and asthma symptoms. However, while this solution may seem promising it is not without its challenges. There is very little knowledge about the allergenic potential of many common tress and what is known tends to be overgeneralized from a few studies. Furthermore, the systems available for reporting intrinsic allergenicity of pollen grains, like the Ogren Plant Allergy Scale (OPALS) system are not published in peer-reviewed journals. Therefore, without a valid scale based on objective, it is difficult to assess the allergenic risk of different pollen grains. Some suggest that in the absence of valid data on tree allergenicity, it is best to avoid planting monodominant species; greater diversity seems to be the safer choice.
@DanielaPadres
Daniela is a recent B.Sc. graduate from the program of Physiology at McGill. She is very passionate about understanding the human body and how we can all individually adapt our daily lifestyles to improve its functioning.
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