This is a blog about the native conifers of the Pacific Northwest. It is a companion to the Northwest Conifers site. The blog will focus on timely and interesting details about our conifers, their connections to the rest of the environment, and our connection to them.

Friday, April 25, 2014

Conifer Pollination

Pacific Silver Fir pollen cones
It is that time of year, when the cherry blossoms bloom and everyone with pollen allergies begins to sneeze. These flowering plants get a lot of attention this time of year. After all, the strategies they have developed to attract insects and birds to assist in pollination are also appealing to us. By our normal standards of floral beauty, conifers don't produce beautiful flowers as such, but they have developed their own amazing, complex strategies that accomplish what flowers and bees do without the help of either. And they have been doing it for millions of years, before there were either bees or flowers. And what is the primary agent of conifer pollination? Wind. Conifers produce prodigious amounts of pollen, which is carried by the wind, often straight to the noses of susceptible humans. But the wind also delivers conifer pollen to the small, immature cones that develop into seed cones.

Douglas Fir pollen cones
Conifers produce two separate types of cones: male pollen cones and female seed cones. We are all familiar with the large seed cones, but it is easy to overlook the tiny pollen cones. If you look closely, you can see them in great numbers in the spring. After the pollen cones release their pollen, they often fall to the ground. You can see large numbers of them under Douglas Fir (Pseudotsuga menziesii) trees in late spring.

Most conifers are monoecious. That is, they produce both male and female cones on the same tree. This presents some problems, because it is usually not good to allow a tree to pollinate its own seed cones. Self-pollination seldom produces viable seeds, and when it does, the resulting trees are often inferior to those produced by cross-pollination from other trees. With this in mind, it is no surprise that conifers have developed various strategies to minimize self-pollination:
  • Conifers often produce pollen cones lower in the tree than the seed cones. The wind is more likely to blow the pollen to another tree than up to the seed cones above on the same tree. This is particularly true of the firs, which produce seed cones near the treetop. Their pollen cones are typically lower in the tree.
  • Western Hemlock pollen cones
  • Some trees stagger the development of pollen cones and seed cones. The seed cones will be receptive to the pollen before or after the pollen cones release their pollen. Due to variability in the timing of the development of different trees, pollination will happen between trees. As you can see in the photo here, Western Hemlock (Tsuga heterophylla) produces both seed cones and pollen cones on the same branches. (The seed cone is older, but there is an old pollen cone near its top.) The timing of pollen release is different than the time that the seed cones are receptive to the pollen.
  • Some conifers are dioecious. That is, they produce pollen cones and seed cones on separate trees. Self-pollination is impossible on these trees. The Pacific Yew (Taxus brevifolia) is the only dioecious native conifer in the Northwest. You can find these yews growing throughout the Northwest, usually a solitary tree growing in the shade of other conifers. In fact, they are so widely dispersed that they often live a solitary, celibate life and don't reproduce at all.
  • Ponderosa Pine pollen cones
  • Ponderosa Pine (Pinus ponderosa) has an interesting growth pattern that enhances cross-pollination. The young trees start producing pollen cones before they produce seed cones. These young trees can pollinate the cones on larger trees.
The pollen cones on conifers often go unnoticed, but you may have seen the the tiny pollen cones on Incense Cedar (Calocedrus decurrens). Because they appear just before Christmas, you will often see Christmas wreaths made of Incense Cedar with bright yellow pollen cones on the ends of the branchlets.

Incense Cedar pollen cones
Conifer Reproductive Biology by Claire G. Williams, pp. 126-128.
Lives of Conifers by Graham R. Powell, pp. 155-164.