Tree Sex: Gender & Reproductive Strategies
Trees become sexually mature, depending upon species and the individual, anywhere between 1 and 50 years of age. Tree sexual classification depends upon both the flower parts present and their function. The proportion of male and female parts in flowers, and the proportion of cosexual, male and female flowers on a tree, begin to determine potential gender. The functional sexual class of a tree is based upon perfor- mance as a viable pollen provider or successful ovule parent in passing genes onto another generation. Functional estimates of gender are made by counting the number of flowers producing pollen grains and the number of flowers generating fruit and viable seeds. Individual trees which produce proportionally much more pollen within a species can be considered functionally male. Individual trees which produce proportionally much more seed within a species can be considered functionally female.
Male and female flower parts are functional if they preform reproductive services. Appearances can be misleading. Functional flower parts in the same species can look quite different from individual to individual. Nonfunctional male and female parts can be fully developed, modified, or stunted in appear- ance, or missing altogether. Functional male and female tree parts on the same tree, may be present but may not successfully generate viable seed.
There is a strong trend in trees to be self-incapable. Many trees, through flower timing, pollen identification markers, and female part physiology, minimize self-pollination and maximize genetic advan- tages of cross-fertilization (called allogamy). Self incapability systems in trees tend to be concentrated at the point where pollen attaches to a pistil. To prevent selfing, female parts kill pollen from the same tree upon arrival, or slows pollen grain growth and the fertilization process. Trees can also undergo early seed abortion if selfed. Each selfing event can gener- ate a seed and seedling which at the start or even into middle age, can suffer from inbreeding depression and less efficient growth. For example in longleaf pine (Pinus palustris), selfing was found to cause a 60% decline in seed yields, a 53% increase in young tree mortality up to 8 years of age, and 33% less height growth through age 8.