Trees and Bees

Wind pollinated trees are abundant in temperate forests. Male flowers cast pollen into the wind in random search of a mate. In early spring, it is not uncommon to see bees and other insects visiting the male flowers in search of pollen, but they are foragers, not pollinators.

I regularly visit a number of trees to observe their behavior thorough the seasons. There is a small stand of bur oaks, Quercus macrocarpa, in a park near my home. One tree is ancient, the others are about 80 years old. These trees predate the park and were not planted.

In ten years of closely watching the trees, I have observed one tree that is heavily visited by a variety of native bees, though not by honeybees. The bees are collecting pollen and packing yellow pollen into their scopae, the pollen carrying apparatus on their legs and abdomens.

This is the only oak in the stand that attracts a lot of bees, and has done so every spring. The other trees flower at about the same time. Bees rarely visit the other trees. It seems unlikely that these are random visits. I have seen this phenomenon every year for ten years.

Either bees have become conditioned to visit this particular tree and avoid the others or there is something that makes this tree attractive. There is a bit of variation in flower maturity dates among the trees, but not enough to account for the observations.

Here is what makes this observation exceptional: the catkins on this tree have a slight sweet fragrance. None of the other trees in this stand have any discernible fragrance, nor do bur oaks in other stands that I have examined.

There are no nectaries in these catkins, at least as far as I can tell with a hand lens. A literature search shows no accounts of insect pollination of Quercus, though other trees in the Fagaceae are insect pollinated.

Oaks are self-incompatible, incapable of pollinating themselves to produce viable acorns. The bees are not pollinators unless they carry their collected pollen to female flowers on another tree. I don’t know whether the bees are visiting female flowers.

A single tree may not tell us much about evolution, but a wind-pollinated tree that becomes attractive to insect pollinators may have a reproductive advantage over trees that have no attractive capability. Indeed, this is probably the evolutionary origin of insect pollination.

Whether this means anything in a larger context I can’t say. It does tell us that observational natural history, especially over long periods of time, can reveal unexpected insights into Nature.

Oak flowers
Male oak flowers in spring

Bees on oak flowers
Bees visiting oak flowers with their scopae packed with oak pollen

Bees visitng oak flowers
These bees are abundant on this tree, but not on nearby trees.

Bees visiting oak flowers
Bees have been visiting flowers on this oak for many years, but nearby bur oaks get no bee visitors

Bees on oak flowers
These are not honeybees, but several species of Andrena, or miner, bees

Bur oak acorn
The production of an acorn requires a complex interaction of male and female flowers