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Western hemlock no athlete when building up nutrients

High-performance athletes spend years preparing for competitions in the hopes that their efforts will give them enough of an edge over rivals to bring home a title. In a recent study, Canadian Forest Service researchers found that matching fertilization to growth did not similarly condition nursery-grown western hemlock seedlings to consistently outperform competitors when field planted. It did, however, induce greater growth in the seedlings while in the nursery, which led to greater performance—compared to control seedlings—in the field.

Scientists planted 1,500 western hemlock seedlings near Jordan River, British Columbia, to test effects of nutrient loading on subsequent growth. It turns out, western hemlock doesn’t load nutrients; it immediately uses them to grow bigger, which confers its own headstart in subsequent growth.

“Our reasoning had been that the higher the concentration of nutrients stored in foliage before field planting, the greater the head start the seedlings would have,” says Pacific Forestry Centre Research Scientist Al Mitchell. “But it didn’t turn out that way.”

He and his team compared nutrient concentrations in seedling needles and seedling growth after two seasons of fertilization in the nursery, and after one and two years in the field. Some seedlings received doses of fertilizer in regular, unvarying amounts; other seedlings were fed regular, but ever-increasing doses of fertilizer to match increasing seedling growth—a feeding system called exponential fertilization.

“By the end of the growing season, when seedlings go dormant, exponentially fed seedlings receive very high doses of nutrients, which we thought would be taken up and stored in seedling tissues. Nutrient loading is a way to condition seedlings for greater performance.”

Instead, however, seedlings that underwent exponential fertilization grew more in the nursery, but the fertilization regime had little effect once they were field planted. When scientists measured nutrient accumulation in needles after planting, they found concentrations less than one-half percent higher in exponentially fed seedlings than in constant-rate–fed seedlings—a difference that quickly dissipated.“

It’s the same with people training to run a marathon,” says Mitchell. “Some people become conditioned really easily, and others don’t. And some people, once they stop training, lose conditioning quickly. Western hemlock loses conditioning quickly.”

Instead of loading nutrients, says Mitchell, western hemlock immediately uses them to grow bigger. This confers its own competitive advantage out in the field.“

Plants that start big get bigger faster; plants that start small never catch up.”

From the April 2006 edition of Information Forestry

Information Forestry is a newsletter of research and development activities at the Pacific Forestry Centre. This newsletter contains information about research in various aspects of forestry, such as silviculture, remote sensing, biological control, and insect and disease management.