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Sunday - March 06, 2011

From: Washington, DC
Region: Mid-Atlantic
Topic: Trees
Title: Are baldcypress trees (Taxodium distichum) self-fertile
Answered by: Nan Hampton

QUESTION:

We are considering planting a bald cypress in a grassy children's play area that has fair amount of clay in the soil and receives a good amount of rain water from an adjacent slope. This seems a good choice for a shade tree since we would like to choose native plants that offer wildlife benefits, but we don't have room for TWO baldcypress trees. Does this monoecious plant produce more seed when cross pollinated with other baldcypress plants (like corn does) or are they self fertile?

ANSWER:

This Mr. Smart Plants did considerable database and Internet searching and consulted with several other Mr. Smarty Plants, but none of us has been able to find any specific information about whether Taxodium distichum (Bald cypress) is self-fertile or not. Here is what I didn learn, however:

In angiosperms (flowering plants) there are several ways to insure that plants don't self-fertilize since self-fertilization decreases genetic diversity. (You can read an outline of a lecture from Missouri State University about "Plant Sex" with information about angiosperm barriers to self-fertilization.) For instance, the plants may be dioecious (male flowers occurring on a different plant from the female flowers). This mechanism occurs in some gymnoperms [e.g., Juniperus ashei (Ashe juniper) and Juniperus virginiana (Eastern red cedar)]. Another way is that the male and female reproductive parts on the same plant may mature at different times. I wasn't able to find any reference to this being the case in Taxodium sp. or any other gymnosperm. The most common way that angiosperms avoid self-fertilization, however, is by being self-incompatible. When a pollen grain from a plant lands on a stigma of the same plant, biochemical signals allow the plant to recognize the pollen as its own pollen and prevent it from fertilizing the ovule by inhibiting the growth of pollen tube. This system, however, doesn't appear to function in gymnosperms. A 1986 paper by Michael S. Zavada and Thomas N. Taylor ("The role of self-incompatibility and sexual selection in the Gymnosperm-Angiosperm Transition: a hypothesis." THE AMERICAN NATURALIST, Vol. 128, no. 4, October 1986) states: "The origin of intraspecific self-incompatibility (SI) is regarded as a significant event in the evolution of flowering plants. ... There are no documented cases of SI in gymnosperms."

So, the upshot is that I don't see any reason why you can't plant just one baldcypress and still expect to get an optimal production of seeds for wildlife. Gymnosperms are wind pollinated and this aids in self-fertilization as well as cross-fertilization. If there are other Taxodium distichum trees in the general vicinity, your tree might even be pollinated by the pollen brought in by the wind from those.