Male alder catkin. Photo by Jerry A. Payne USDA Agricultural Research Service.

Pollen is to trees what sperm is to animals. It carries the tree’s male genes; it is the male fertilizing unit of woody plant reproduction. Typically, pollen appears in spring as a powdery cloud of fine, yellowish grains. Each tiny grain is a single cell, encased in a tough, ridged, or spiked coating. When inhaled by susceptible people, these grains can cause nasty allergic reactions. But they are absolutely essential for the continuing life cycle and evolution of trees.

Pollination is the movement of these pollen grains from the male reproductive part of a plant to the female reproductive part. It is a prerequisite to fertilization, which occurs when the sperm-making pollen is united with the egg-containing ovary of a plant of the same species. If all goes well, the making of pollen, the subsequent act of pollination, and the biological marvel of fertilization all lead to the growth of fruits and seeds. This is a very good deal. Not only is it the basis for agricultural food production for human needs, it also perpetuates trees and determines the composition of forests. Pollen is key.

While fortuitous accidents do happen, and some trees get lucky despite being rooted and unable to move about in search of a mate, most trees need a bit of help – from wind or animals – to get their pollen from one flower to another. Globally, the incidence of wind pollination increases with both latitude and elevation. It is most common in our temperate deciduous and in boreal forests but extremely uncommon in tropical rain forests. There, animal-aided pollination – especially by insects – is much more common.

Wind pollination is most effective in open habitats and in early successional ecosystems, where wind is likely to be an advantage. Indeed, it is common among forest trees that reach canopy height and whose flowers or cones are exposed to winds, but almost nonexistent among understory plants, which live in less wind-prone conditions. Nearly all of our common conifers – including pines, spruces, and firs – rely on wind pollination, and so do many broadleaved trees, including aspens, cottonwoods, oaks, ashes, elms, birches, and walnuts. These hardwoods all share a common male flower form: they tend to occur in elongate, drooping catkins well-positioned for wind dissemination. Many of them – most famously the aspens, oaks, and ashes – avoid interference from leaves by forming flowers and shedding their pollen well before leaf-out. But relying on the vagaries of wind and weather to deliver pollen is arguably something of a crapshoot. Consequently, trees gambling on such a risky mode of pollination seem to hedge their bets by producing enormous amounts of pollen – ten million grains from one cluster of birch catkins, for example. That’s billions of pollen grains from a single tree. And while at times it may seem as if most of them are in your lungs and the rest are on your car, at least some of that pollen actually finds the intended female flower parts.

While this throw-pollen-to-the-wind strategy may be effective in stands with many individuals of the same species growing near each other, it does have its shortcomings – particularly in mixed-species stands and in fragmented landscapes. Although some pollen can travel great distances, it doesn’t remain viable for very long, and most airborne pollen comes to rest close to the tree that produced it. Thus, it is not as effective at delivering pollen to distant trees. Moreover, it is very expensive, energetically, for the parent tree to produce such large quantities, and seems wasteful when so much pollen never reaches its intended target.

By contrast, trees that rely on pollination by animals (chiefly insects, but also birds and bats) tend to produce far less pollen in any given flowering period, because pollination is more direct and efficient. The animal-transported pollen also tends to be slightly sticky, less dust-like, and produced later, when the leaves are forming. This approach seems to work better among widely-spaced individuals. There are fewer examples of native non-windpollinated species. Among the best known insect-pollinated trees are apples, basswood, cherries, black locust, catalpa, horse chestnut, tulip tree, and the willows. As logic would have it, species that rely on insects (mostly bees, wasps, flies, beetles, butterflies, and moths), birds, and bats, tend to have fragrant (sometimes stinky), large, or otherwise showy flowers.

Attracting insects that might carry pollen from one tree to another tends to facilitate an out-crossing mode of reproduction. This carries weighty implications for genetic variation and flexibility, which in turn allow species to cope with varied habitats and changing growing conditions over time. It is also good reason to remember that most insect pollinators do this important work when they are adults, even if some of them can be voracious defoliators during their earlier, immature life stages.