Fall Harvest: the Gift That Keeps on Giving

Year after year, domestic and wild plants give us—and local wildlife—”free food,” as it were. But some years are special. Trees bent under the weight of fruit they carried; now pantry shelves bend under the weight of jams and jellies as we try to cope with overwhelming abundance.

In the nut and seed world, when plants overwhelm the critters waiting to gobble their seeds, we call it a “mast year.” Production becomes so impressive we can’t help noticing. For pinons and perhaps acorns, this seems to be one of those years. Seed-eaters will never be able to keep up, and the trees will have a chance to produce seedlings that survive. Of course, it may take them a few years to recover from the effort. Mast years often recur on a semi-regular basis, with famine years in between, as if the trees were indeed exhausted. In fact, that’s part of the strategy of masting. Populations of mice, birds, and other seed predators can’t depend on the high production; they have to survive those lean years as well. By being undependable, trees improve their chance of reproductive success. The trees may seem to give freely, but feast is sure to be followed by famine.

The unfamiliar word “mast” literally means food. From the Old English and High German, it traditionally referred to the beech nuts and acorns that dramatically littered the forests of Europe some years and provided quantities of food, mostly for hogs. In temperate areas, however, many woody species have mast years, even our own native conifers and oak. What’s dramatic is that trees of a given species often synchronize, so that their mast years coincide, and woe, in the form of increased seed loss, befalls the misfits. Environmental conditions help control the timing of mast years, but not necessarily as we would expect. For ponderosa pine, for example, this fall’s seed crops were determined by prevailing conditions back in 1996.

Each fall, as seeds of all kinds embark on adventures beyond imagining, a vast harvest begins. Although we may see squirrels busily cutting cones, most of the harvest activity will go unnoticed. For some seeds, getting far away from the parent—where they are easy targets for predators—is crucial to survival. But there’s no guarantee life will be any easier after they get away. [In recent decades, ecologists have shown us that what happens to seeds helps determine what our landscapes look like. Once seeds leave the parent tree, they become invisible and we tend to forget about them—at least until we see new plants coming up. But the seeds are everywhere among the fallen needles and in the soil, as the critters that depend on them never forget.]

Although winged seeds have distance potential, most seeds will fall near their parent plant. For heavy seeds like acorns, travel is limited mostly to places the squirrels take them. Acorns don’t always stay where they fall either. If they land near the route of a foraging wild turkey, for example, their days are up; they become part of his daily calories. A Douglas-fir tree can deposit more than 300 seeds per square meter—that’s about 30 per square foot—directly below its canopy. A large Engelmann spruce puts down thousands of seeds per square meter close to home, but some, if they get into the wind, are carried off. There will still be hundreds of spruce seeds per square meter 150 meters out from the parent tree. If the seed crop is reduced 50% by a poor season or by seed-eaters, the number of seeds getting any distance away is also halved. That leaves plenty for colonization, which is what dispersal is for, after all. Once they land, they’re still vulnerable: in one study of Douglas-fir, 69% of the seeds were eaten or otherwise lost, but the remaining seeds still produced 3.7 seedlings per square meter.

Birds and small mammals collect and bury seeds, especially large nutritious tree seeds, after dispersal. The catch is that caching often works better for these predators than it does for the seeds. In a study of 840 pine seeds in 35 caches made by deer mice, the fate of seeds appears grim indeed. Ten caches were dug up and eaten before the end of autumn; nine more were destroyed the following May; and the others were used through the winter. Forty-nine plants developed from only six of the caches, but the mice uprooted and killed seedlings in three of them. At the end of the first growing season, only one cache still had live seedlings. Next time you see clumps of pine and Douglas-fir seedlings germinating from caches during a wet spring, you might want to go back later and see how many survivors you can find.

Does predation matter? Are forests endangered by mice and squirrels? Studies show that when one cause of mortality is eliminated, others often increase to compensate. Imagine all those cachebased seedlings again. In each clump, not all can survive as trees. Many more will die young, even if the mice don’t get them. Despite fears that forests will be decimated, seed predation only becomes relevant when it reduces the number of seeds below the number of seedlings that can survive in the environment.

The good news is that a plant, even a large tree,needs to reproduce successfully only once in its lifetime to replace itself; its odds are good despite massive losses. You might say that plants pay, sometimes dearly, for the dispersal services they receive. Caching may determine which seed, among the millions produced in the lifetime of a tree, will survive to replace its parent. But because predators cache more seeds than they need, most years improve the odds even further, as more caches are left uneaten. Those uneaten seeds are future forests, gifts to future generations of birds, mice, squirrels, and to all of us.

Copyright © 2011 Sally L. White