Sowing the seeds of love
A forester and author describes the remarkable complexities of arboreal procreation
The leisurely pace at which trees live their lives is apparent when it comes to procreation. Reproduction is planned at least a year in advance. Whether tree love happens every spring depends on the species.
Whereas conifers send their seeds out into the world at least once a year, deciduous trees have a completely different strategy. Before they bloom, they agree among themselves. Should they go for it next spring, or would it be better to wait a year or two?
Trees in a forest prefer to bloom at the same time so that the genes of many individual trees can be well mixed. Conifers and deciduous trees agree on this, but deciduous trees have one other factor to consider: browsers such as wild boar and deer.
Boar and deer are extremely partial to beechnuts and acorns, both of which help them put on a protective layer of fat for winter. They seek out these nuts because they contain up to 50 per cent oil and starch — more than any other food.
Often whole areas of forest are picked clean down to the last morsel in the fall so that, come spring, hardly any beech and oak seedlings sprout. And that’s why the trees agree in advance.
If they don’t bloom every year, then the herbivores cannot count on them. The next generation is kept in check because over the winter the pregnant animals must endure a long stretch with little food, and many of them will not survive. When the beeches or oaks finally all bloom at the same time and set fruit, then it is not possible for the few herbivores left to demolish everything, so there are always enough undiscovered seeds left over to sprout.
“Mast years” is an old term used to describe years when beeches and oaks set seed. In these years of plenty, wild boar can triple their birth rate because they find enough to eat in the forests over the winter.
In earlier times, European peasants used the windfall for the wild boar’s tame relatives, domestic pigs, which they herded into the woods. The idea was that the herds of domestic pigs would gorge on the wild nuts and fatten up nicely before they were slaughtered. The year following a mast year, wild boar numbers usually crash because the beeches and oaks are taking a timeout and the forest floor is bare once again.
When beeches and oaks put blooming on hold for a number of years, this has grave consequences for insects, as well — especially for bees. It’s the same for bees as it is for wild boar: a multi-year hiatus causes their populations to collapse. Or, more accurately, could cause them to collapse, because bees never build up large populations in deciduous forests in the first place.
The reason is that true forest trees couldn’t care less about these little helpers. What use are the few pollinators left after barren years when you then unfurl millions upon millions of blossoms over hundreds of square miles?
If you are a beech or an oak, you have to come up with a more reliable method of pollination, perhaps even one that doesn’t exact payment. And what could be more natural than using the wind? Wind blows the powdery pollen out of the blossoms and carries it over to neighbouring trees.
The wind has a further advantage. It still blows when temperatures fall, even when they drop below 53 F (11.6 C), which is when it gets too chilly for bees and they stay home.
Conifers bloom almost every year, which means bees are an option for pollination because they would always find food. However, conifers are native to northern forests, which are too chilly for bees to be out and about while the trees are blooming, and that is probably why conifers, like beeches and oaks, prefer to rely on the wind. Conifers don’t need to worry about taking breaks from blooming, like beeches or oaks, because they have no reason to fear deer and wild boar. The small seeds inside the cones of Spruce & Co. just don’t offer an attractive source of nutrition. True, there are birds such as red crossbills, which pick off cones with the tips of their powerful crossed bills and eat the seeds inside, but in general, birds don’t seem to be a big problem. And because there is almost no animal that likes to store conifer seeds for winter food, the trees release their potential heirs into the world on tiny wings. Thus equipped, their seeds float slowly down from the tips of their branches and can easily be carried away on a breath of wind.
Spruce & Co. produce huge quantities of pollen, almost as though they wanted to outdo deciduous trees in the mating department. They produce such huge quantities that even in a light breeze, enormous dusty clouds billow over coniferous forests in bloom, giving the impression of a fire smouldering beneath the treetops.
This raises the inevitable question about how inbreeding can be avoided in such chaotic conditions. Trees have survived until today only because there is a great deal of genetic diversity within each species.
If they all release their pollen at the same time, then the tiny grains of pollen from all the trees mix together and drift through the canopy. And because a tree’s own pollen is particularly concentrated around its own branches, there’s a real danger its pollen will end up fertilizing its own female flowers. But, as I just mentioned, that is precisely what the trees want to avoid. To reduce this possibility, trees have come up with a number of different strategies.
Some species — like spruce — rely on timing. Male and female blossoms open a few days apart so that, most of the time, the latter will be dusted with the foreign pollen of other spruce. This is not an option for trees like bird cherries, which rely on insects.
Bird cherries produce male and female sex organs in the same blossom, and they are one of the few species of true forest trees that allow themselves to be pollinated by bees. As the bees make their way through the whole crown, they cannot help but spread the tree’s own pollen. But the bird cherry is alert and senses when the danger of inbreeding looms. When a pollen grain lands on a stigma, its genes are activated and it grows a delicate tube down to the ovary in search of an egg.
As it is doing this, the tree tests the genetic makeup of the pollen and, if it matches its own, blocks the tube, which then dries up. Only foreign genes, that is to say, genes that promise future success, are allowed entry to form seeds and fruit. How does the bird cherry distinguish between “mine” and “yours”? We don’t know exactly. What we do know is that the genes must be activated, and they must pass the tree’s test. You could say, the tree can “feel” them. You might say that we, too, experience the physical act of love as more than just the secretions of neurotransmitters that activate our bodies’ secrets, though what mating feels like for trees is something that will remain in the realm of speculation for a long time to come.
Some species have a particularly effective way of avoiding inbreeding: each individual has only one gender.
For example, there are both male and female willows, which means they can never mate with themselves but only procreate with other willows. But willows, it must be said, aren’t true forest trees. They colonize pioneer sites, areas that are not yet forested. Because there are thousands of wild flowers and shrubs blooming in such places, and they attract bees, willows, like bird cherries, also rely on insects for pollination.
But here a problem arises. The bees must first fly to the male willows, collect pollen there, and then transport the pollen to the female trees. If it was the other way around, there would be no fertilization. How does a tree manage this if both sexes have to bloom at the same time? Scientists have discovered that all willows secrete an alluring scent to attract bees. Once the insects arrive in the target area, the willows switch to visual signals. With this in mind, male willows put a lot of effort into their catkins and make them bright yellow. This attracts the bees to them first. Once the bees have had their first meal of sugary nectar, they leave and visit the inconspicuous greenish flowers of the female trees.
Inbreeding as we know it in mammals — that is to say, breeding between populations that are related to one another — is, of course, still possible in all three cases I have mentioned. And here, wind and bees come into play equally. As both bridge large distances, they ensure that at least some of the trees receive pollen from distant relations, and so the local gene pool is constantly refreshed. However, completely isolated stands of rare species of trees, where only a few trees grow, can lose their genetic diversity. When they do, they weaken and, after a few centuries, they disappear altogether.
Conifer trees, such as spruce, produce huge quantities of pollen, almost as though they wanted to outdo deciduous trees in the mating department