Seed Dispersal in Conifers

Overview

The usual mode of seed dispersal in conifers is wind dispersal of a winged seed. This is the prevalent dispersal mode in Pinaceae, Cupressaceae, Araucariaceae, and Sciadopityaceae. Note that nearly all conifers also disperse their pollen by wind, and this reliance upon wind for seed and pollen dispersal is commonly seen as a primitive trait of conifers, indicative of their evolutionary antiquity. This view may also be seen as poetic nonsense, though, since many of the cycads, commonly viewed as an even more "primitive" group, are insect-pollinated; whereas many conifers rely on animals for seed dispersal; this is the usual state in the Podocarpaceae and Taxaceae. In this, as in most respects of conifer anatomy, physiology, and ecology, traits seemingly preserved out of antiquity can in fact be seen to confer superior competitive advantages in the modern world; they may be old, but they are still effective.

Seed dispersal essentially occurs via three mechanisms, involving seed transport by movement of air, water, or animals. Seed transport by air, as noted above, is essentially the default condition, and is used by a broad ecological spectrum of conifers. Seed transport by water or animals, though, has profoundly influenced the ecology of species using these mechanisms.

Dispersal by wind

Most conifers produce a seed cone comprised of bracts spirally arranged about a central axis, with two ovules on each bract, each ovule developing a single seed with attached seedwing. When the seed cones mature and open, these seeds are shed and dispersed by wind. This description is accurate for most species of the Araucariaceae, Pinaceae, and Sciadopityaceae; the Cupressaceae differ primarily in having variant cone geometries, sometimes with variant numbers of ovules, or only one ovule of a pair developing to maturity.

Many studies have examined the effectiveness of wind dispersal. Examination of these studies suggests that it is most effective when there is a lot of dry wind, and when the species is highly adapted to wind dispersal. Generally, closed-canopy forests, have a less wind than more open stands, and still more wind is available in open woodlands or at sites that have burned recently and thus have very restricted forest cover. Wind also tends to be more abundant in the mountains. Moisture is also important, because a wet seed is heavier than a dry seed and cannot be expected to be carried as far by the wind. The aerodynamics of the dispersing seed are also important; light seeds with relatively large wings will disperse farther than seeds with relatively smaller wings, or relatively heavier seeds. There are trade-offs at work here, though; for instance, there are varied reasons why a seed may have to be relatively large in order to have a reasonable chance of survival after germination. The effectiveness of dispersal is also a function of the number of seeds produced; a large number of small seeds may each have a poorer chance of survival, but be more likely to travel a long distance and find a suitable seedbed, compared to a smaller number of large seeds.

Dispersal by water

Dispersal by animals

Citations