Spring arrives, and with him, the fateful moment of the sexual reproduction of plants. But how is it going? As often the living is as diverse as it is astonishing, and its observation has sometimes been able to give rise to questions about possible sex wars, even to accusations of “botanical sexism”. What about?
Monoic or dioic?
In most species of trees you know, each individual has both male and female reproductive organs. They say they are bisexual.
Within these bisexua, there are two main categories.
In all conifers (pines, fir trees, cedars, etc.) and in many forest species, there are on the same tree, flowers (or cones in conifers) male and female flowers (or cones). We then speak of monoic species.
Visually, these male and female flowers or cones are different. In oak or hazelnut, for example, male flowers are grouped on kinds of ears that are called kittens, while female flowers are tiny, similar to small buds from which the stigma that will capture the pollen grains emerge.
In pines, bright yellow male cones consisting of many stamens are smaller than the female cones which will transform into “pine cones” after fertilization.
But sometimes, too, both sexes find themselves in the same flower. We then speak of hermaphrodite flower trees. It is less common for the trees of our forests (even if we can see such flowers on the linden or on the Sorbiers), but it is the rule among fruit trees (apple, cherry, cognass …) and in many pleasure trees (magnolias, chestnut trees).
In other trees, the situation is still different. There are some individuals who wear only male organs producing pollen and other individuals who only wear female organs, producing eggs, then that will produce fruit. So there are male trees and female trees. These species are said to be dioic.
If such a sexual distribution, with male individuals and female individuals is the rule in humans and in many animals, it is rather rare in the plant kingdom. Only 6 % of the 300,000 species of plants identified present this peculiarity. We can cite the Ginkgo BilobaIF (Baccata taxus), the thuriferous juniper (Juniperus Thurifera), the poplars (POPULUS SP) or even the date palm (Phoenix Dactylifera). For the latter species, as for a herbaceous species, the white companion (Silene Latifolia), we could even identify the presence of XY sex chromosomes for males and XX for females.
This distribution of the sexual organs is therefore not very common, but it however has a major advantage. That of avoiding any consanguinity. The pollen produced by a male tree must, transported by the wind or by animals, find a female tree to reproduce. And this female tree will be genetically different from the male. Hence an important genetic brewing in perspective.
Bisexual species, which therefore relate to the same tree of the sexual organs of both sexes, had to develop other strategies to limit this consanguinity, such as the temporal gap in the flourishing of male and female organs. Thus, the stamens do not mature at the same time as the pistil of the same flower, the pollen of this flower will have to fertilize flowers of other individuals with mature and genetically different pistil.
But the distribution of the sexes is far from being always so clear and fixed. In common ash populations (Fraxinus excelsior), we can thus meet male, female or bisexual trees.
The mystery of the old juniper
Still stranger, the extraordinary old juniper (Juniperus Phoenicea) Gorges from the Ardèche or Verdon are deemed bisexual, but actually have hesitant sexuality. They can in fact change sex; Bisexua a year, males or female another. Does this correspond to an adaptation to the particularly difficult environmental conditions of these vertical cliffs, on the almost non-existent ground, allowing these trees to reach ages which can exceed a thousand years? The ecologist Jean-Paul Mandin, a great specialist in these amazing cliff trees could show that, in their vast majority, bisexual individuals then go through a male state, that is to say by a year when they do not express their female sex.
He hypothesizes that everything happens as if an individual who made female flowers and therefore who then invested a lot of energy in the development of the fruit, “rested” the following year by being much more male or by not blooming at all.
Sex-ratio: males or female equally?
If we now focus on the species in which male trees and female trees are systematically found, many questions happen. First, are there as many male individuals as female individuals? This question, essential from an ecological point of view is that of sex-ratio. If this proportion is unbalanced, with for example, more females than males, what is the meaning? ; Are female trees more resistant than males? What consequences is it on the dynamics of this population?
For many species, it has been suggested that the females would be fewer, as less competitive. They in fact invest more resources than males, because they produce eggs which then give fruit, often large, when the males produce them pollen, certainly in large quantities but smaller. This investment of females could therefore be made to the detriment of their growth or defense against predators, which would make them more vulnerable than males, would limit their longevity and ultimately lead to a sex-ratio in favor of the latter.
But this is not a general rule.
A sex-ratio where females predominates, for example, has been found at the level of the Thuriferous juniper populations of Moroccan Atlas who have benefited from a very precise analysis in order to better understand the dynamics of these populations threatened by the ax of the shepherd, the cattle tooth and climate change.
Male thuriferous juniers produce pollen in very large quantities because its pollination is done by the wind. This produced pollen is therefore mainly exported and represents a gross loss for the tree. Male juniers thus invest a lot in reproduction and could then be less competitive than females.
But for their part, the production of female cones, which is called galbuals, can also be considerable (up to 4 kg of galbules per year). However, unlike pollen, these galbubbles remain on the spot, falling under the crown of trees, and thus enrich the ground in nutrients when they decompose. A phenomenon while for the benefit of the tree which sees that its significant investment in the production of female cones is then compensated by the improvement of the soil where its roots grow.
This different investment in reproduction at the expense of growth is undoubtedly not the only factor to explain this sex-ratio of juniper populations. Currently, however, there are no other elements showing that a sex is, for example, more requested than another by the herd or the inhabitants of the surrounding villages.
Because sex-ratio can also be linked directly to human action. This is the case for Moroccan palm groves. In “natural” palm groves, there can be as many males as females. On the other hand, in orchards with reasoned exploitation, female palm trees have been favored compared to males which should not represent more than 4 % of the trees, because only female palm trees produce dates. We then use traditional or semi-mechanized artificial pollination.
Male trees, responsible for allergies?
Recently, it was the sex-ratio of trees in town that has been the subject of many questions.
Many allergies being due to the grains of pollens produced only by the male reproductive system of flowers, a controversy has developed trying to accuse urban landscapers and their “botanical sexism” which would be responsible for the increase in allergies.
Abundantly shared on Tiktok, from 2021, this theory defends the idea that we started to plant male trees, producers of allergenic pollens in cities to avoid the inconvenience that their female counterparts can cause which, by producing fruits and seeds, can generate detritus, slippery sidewalks, obstruct the sewers …
However, going up the thread of this idea that has become viral on social networks, we realize that this theory is taken from a work mentioning this reality for a single dioecious species: the deltoid poplars of which only males are planted in certain cities of the United States.
However, the main officials of respiratory allergies remain bisexual trees (cypress, hazelnut, birch, alder, plane tree, oak, ash, linden, etc.), and poplar that generated this crazy online theory, on the other hand, is not so present in our cities. So let's not throw stone into male trees. Science obviously considers that one cannot establish hierarchy between male and female trees within a dioic species, the two being essential for the sustainability of the species. Moreover, as we have seen, each of the sexes must make significant efforts to produce either pollen or fruits and seeds.
Finally, allergies can also be contact allergies or in relation to the fruit or the seed ingested … And there, regardless of the sex of the tree knowing that if we speak of allergies linked to the fruit, it is the female tree which will be responsible if the species is dioic.
So let's not be mistaken for enemy. The increase in allergies to pollen in town is actually due to climate change, which leads to pollen production earlier in season and in larger quantities, and atmospheric pollution that contribute to the intensification of allergy symptoms.
