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Seeds of Fortune

Natural History, Oct, 2000 by Peter J. Marchand

How does a fir tree cross a desert? By winning a lottery.

The place is Wind Whistle in southeastern Utah, just a stone's throw from Canyonlands National Park. Tucked into the deep shadow of a north-facing cliff grow three Douglas fir trees, the largest more than two feet in diameter and nearly a hundred feet tall. The trees might not have attracted my attention but for one thing: Wind Whistle is in sagebrush desert, and these firs are a long way from their nearest kin. While the site obviously suits the trees, their presence here tells much about the sweepstakes character of long-distance seed dispersal, one of the great gambles in the life of a plant.

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Seed dispersal is no less important to the success of a plant than are its germination, growth, and flowering. It is the sole means by which plants track shifting resources in an ever changing landscape. And autumn is the best time to view plants' strategies for getting around. A walk through the woods and tall meadows at this time of year reveals an abundance of architectural innovations for spreading seeds: the fluttering samaras of ashes and maples; the winged nutlets of birches, alders, and conifers; the sticky coating of mistletoe; the sweet berries of heaths; the wind-borne plumes of asters and milkweed; the explosive pods of jewelweed; the clinging spikes of beggar-ticks; the minute, dustlike seeds of bellflowers and orchids; and on and on. It's a display of extraordinary beauty and purpose, but despite their elaborate structures and their adaptations to specific agents of transport, seeds are often ill fated. Long-distance dispersal, it turns out, is much like a lottery.

Though I can see where my Douglas firs might have come from--the hazy blue slopes of the La Sal Mountains rise sharply from the plateau on the distant horizon--getting seeds from there to here would have presented a problem. The valley between is broad, about twenty-five miles across, and the seed of a Douglas fir is not well equipped to travel that distance. It is winged enough to be carried beyond the shadow of its parent tree, but experimental evidence suggests that drift farther than two or three tree lengths is improbable. But that's always the rub for wind-transported seeds. Most of them alight on the ground close to their origin or get snagged in nearby growth. Even for some of the most buoyant plumed or dust seeds (in some species weights of the latter approach one-millionth of an ounce), a half mile can be a long haul. Thus the real pioneering excursions by plants, including those with winged seeds, usually require outside help.

The best opportunity for long-distance transport to suitable new territory seems to lie in getting picked up and carried by an animal, partly because animals tend to move between similar habitats and the plant has already proved successful in at least one of these places. A good way for a plant to get itself transported by an animal is to barter a nutritional reward, such as the juicy pulp of a fruit, in exchange for a lift--an evolutionary tactic that works for a great number of plant families. In most cases, however, this mode of transport seldom carries seeds very far. Except during migratory flight, the combination of an animal's limited foraging range and the usual rate of passage for food moving through its gut generally results in a seed's being defecated within a few hundred feet of where it was consumed. (Surprisingly, some carnivores provide an exception. Bears, coyotes, and pine martens, for example, frequently ingest berries when available and then move the seeds considerable distances by virtue of their large territories. Food-caching specialists, such as jays and nutcrackers, may also transport seeds a few miles, where they are often buried and not recovered.) For most plants, then, the best bet may be to sidestep the expense of nutritional enticement and just attach their seeds directly to an animal. Although propagules such as the familiar burdock and beggar-tick must still await a chance encounter with a passing animal, once affixed, the clinging seeds may be carried, undiscovered, quite far by a bird or large mammal, especially a long-distance migrant, before finally being groomed off.

Dispersal by attachment is not always a matter of design, however. While a number of seeds possess hooks, barbs, or viscid substances for a quick stick, many small seeds without special anatomical adaptations can get caught up in fur, feathers, and feet. In Origin of Species, Charles Darwin recounts having been sent a specimen of a red-legged partridge with considerable dried mud on its feet. He removed the mud, moistened it, placed it under a bell jar, and subsequently germinated no fewer than eighty-two seeds. The most telling tale, though, comes from Macquarie Island, some 600 miles out to sea between New Zealand and Antarctica. All of the thirty-five terrestrial plant species on Macquarie are animal dispersed, mostly via adhesive propagules. The floras of the Cocos, Galapagos, and Hawaiian Islands, too, are mostly the result of animal dispersal, although ocean drift has contributed significantly to seed arrival, with wind only a minor player.

Seeds of Fortune

Natural History, Oct, 2000 by Peter J. Marchand

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