Spring in the Kawarthas is synonymous with a ubiquitous yellow dust that descends upon everything from cars and patio furniture to rivers and lakes. Even the edges of puddles become marked with what looks like yellow chalk. For cottagers, the strange powder is most visible in June, when it piles up on shorelines and beaches.
What you are seeing is pollen – a manifestation of the sex lives of our trees as copious amounts of the magical dust are released to the wind. When the weather is hot and dry, you will sometimes even notice what looks like a yellow cloud around conifers when the wind jostles the branches.
In May, most of the pollen comes courtesy of the flowers of deciduous trees like Sugar Maple and White Birch. In June, however, the main culprits are the male cones of conifers such as pine, spruce and fir. Cones – named after their shape – are the reproductive parts of an ancient branch of plants known as gymnosperms. In this respect, they are akin to flowers. Conifers form the largest group of living gymnosperms, but Ginkgo trees also belong to this class of plants. About 300 million years ago, the gymnosperms became the dominant trees on the planet. They continued their dominance throughout the Triassic and Jurassic periods – the age of the dinosaurs. Their cones were even a favourite food of species like duckbill dinosaurs. The gymnosperms reigned supreme until the rise of the angiosperms – the flowering plants – during the Cretaceous period.
As is the case with many flowers, cones can be either male or female. Except for junipers, both occur on the same tree. Let’s look at the female cone first. These are the typical hard, brown, woody cones. They consist of a central stalk surrounded by stiff, overlapping scales, reminiscent of wooden shingles. The ovules, which when pollinated become seeds, are located at the base on the inner surface of the scale. If you pry open the scales of a mature cone before it falls from the tree, you can often see the seeds inside. In White Pine and Balsam Fir, the female cones are located high up in the tree at the tips of the branches. In most other species, they can also be found lower down.
The male or pollen cones are much smaller – often only a centimetre or two in size – softer and less conspicuous. Usually located on the lower branches, they are usually light brown or reddish in colour and resemble little spikes or buttons. They have a central axis, which bears pollen-producing structures. You’ve probably brushed up against them, causing a smoke-like puff of yellow dust. Soon after the pollen is released, the male cones whither and drop from the tree.
All conifers are wind-pollinated. Unlike deciduous trees like cherries, basswoods and, to some extent, maples, conifers do not rely on insects to spread their pollen. Cones therefore lack bright colours, nectar rewards, or tantalizing perfumes to attract pollinators.
The reproduction story of the White Pine is typical of many conifers. In the spring, before the female cones develop, pale yellow-brown pollen cones appear in clusters at the base of new shoots. They are usually located in the lower part of the crown, although some appear even on the bottom branches.
The green-coloured, seed-producing female cones are larger and tend to be in the upper part of the crown. Female cones become receptive to the wind-blown pollen at precisely the same time as the pollen grains are being released. At this time, they are soft, pliable, and their scales are partially separated.
As pollen grains are carried off by the wind, some inevitably encounter female cones and sift down between the open scales. With luck, a pollen grain will come to rest on one of the two ovules attached to the bottom inside of each cone scale. The egg cell within the ovule thereby becomes fertilized by the male gamete (sperm cell) contained within the pollen grain.
After their pollen is released, the male cones soon wither and fall away, often dropping from the trees in a veritable shower. Dry and shriveled male cones are a common sight anywhere pine trees occur and often cover the ground under the trees. We sweep them up, muttering “dirty tree” – often with no idea what they even are.
Following pollination, the scales on the female cones fuse together, and a pitch-like material seals the outside. Over the next two years, the cone gradually grows to full size. In White Pines, the seeds are ripe by August or September of their second summer. At this point, the cone scales open again, and the seeds are released to the wind.
White Pines may start to bear female cones when 5 to 10 years old. Large numbers of cones do not usually appear, however, until the trees are about 6 m (20 ft) tall. The abundance of cones varies greatly from one year to the next. Their relative abundance has a major impact on the populations of birds and mammals that eat the seeds.
Pollen grains are fascinating structures. First, they are extremely small, which means that a scanning electron microscope is often needed to make out their details. In the case of conifer pollen, they are also uniquely designed for wind travel. Two air bladders give the grains buoyancy and enable them to take what amounts to a balloon ride.
When pollen grains land on a lake, they form a temporary film but soon sink to the bottom. That is not the end of the story, however. Because they are protected by a tough outer wall, they are highly resistant to decay. The grains therefore become microfossils that remain unchanged in the bottom sediment for thousands of years. Because the wall is often sculptured and can even bear spines, the markings can be used to identify which genus or species of plant the pollen came from. This allows paleobotanists to describe with great accuracy the history of the vegetation of an area. And, by knowing what kind of vegetation existed, scientists can also theorize what the climate was like. For example, by examining the pollen grains found in deep peat bogs, scientists have been able to piece together the story of the changes in climate and vegetation that occurred during and since the last Ice Age. As the glaciers retreated, vegetation followed. The pollen grains in these peat bogs show that the first trees to repopulate the land were firs and spruces. Later, pines and tamaracks came along, followed by birches and elms. Finally, oaks and maples appeared on the scene. You can see the northward advance of spruce forests since the last ice age by Googling “spruce pollen viewer”. There is a similar video for maples.
Pine pollen often gets blamed for allergy symptoms. However, these symptoms are usually caused by the much lighter wind-borne pollens of birch, ragweed and various grasses that are often present at the same time. Also, the chemical composition of pine pollen makes it less likely to produce allergic symptoms. People with tree pollen allergies sometimes assume that trees with colorful flowers – like apple or cherry trees – will trigger their symptoms. Flowering trees usually have bigger, stickier pollen that doesn’t blow in the wind or cause symptoms. The same is true for goldenrod pollen.
Because the climate crisis is extending the frost-free season, trees and other plants have more time to grow, flower, and produce pollen. Some plants, too, like ragweed and many grasses, benefit immensely from the higher levels of CO2 in the atmosphere. This allows them to grow faster and bigger and to produce even more pollen than before. Unfortunately, allergy sufferers can expect longer and more intense pollen seasons as we continue to dump more carbon into the atmosphere. This is just one more example of how greenhouse gas emissions are already damaging Canadians’ health.
Take the time to go out and closely examine the male and female cones of our conifers this month. Their colour, shape, texture and location vary widely from one species to another, but they all share a special beauty. The deep red female cones of the tamarack (larch) are particularly attractive and almost look like scrumptious little fruits decorating the branches.
Climate Crisis News
A growing number of local groups and citizens want the City of Peterborough to declare a Climate Emergency. Several hundred Canadian municipalities have already done so, including Kingston, London, Burlington, Halton Hills, and Ottawa. There are still too many Peterborough citizens who are not engaged with this issue. Some people still think, “If things were that serious, our elected officials would be saying so.” This is the essence of why a declaration is so necessary. It would be a call to action on the part of the entire community. A Climate Emergency declaration would also support the Greater Peterborough Climate Action Plan, which city council has endorsed. Strategies in the plan include the need to “foster a culture of climate change awareness” and to “encourage civic engagement around climate change.” Citizens need to be informed in they’re to act in their own best interests, such as preparing our homes for the coming severe weather events. A declaration might also inspire people to get involved in the upcoming election and provide the “social license” to share their concerns about the climate crisis with others. Right now, it’s a no-go zone for many people. In many ways, the quickest path to meaningful action on climate change is at the municipal level.