The Long-Awaited Greening of the Hills

One of the most dramatic changes of spring is the transformation of Alaska's aspen-and-birch covered hillsides from rusty brown to neon green. Greenup, the time when tree buds burst into leaves, happens so suddenly that all the leaves seem to pop out on the same day. The greening of the hills is noticed right away by most people, but few with the fervor of Jim Anderson.

For years, Anderson, biosciences librarian for the University of Alaska Fairbanks, has marked his calendar on the date a hillside across from his workplace turns green. He recently shared his observations with Rick Thoman and Ted Fathauer of the National Weather Service in Fairbanks, who came up with a formula for predicting the day when millions of tiny green solar panels will begin to unfold on birch and aspen trees.

Since 1974, Anderson has noted the day when Chena Ridge shows the sign of greenup, which he defines as when "leaf buds in birch and aspen open just enough to produce a faint but distinct green flush through the forest canopy." Fathauer has made the same observations for slopes near his home, farther south on Chena Ridge, since 1986. He and Thoman came up with a method to predict greenup that recently was put to the test by students at Pearl Creek Elementary School in Fairbanks.

Warmth is the main factor in determining when birch and aspen leaves pop, Fathauer said. He and Thoman figured that the daily high temperature is a factor from which the greenup date can be predicted. By trial and error, they found that the greenup clock starts when the average high temperature for three consecutive days is 50 degrees or more. Beginning on the day the greenup clock starts, they subtract 40 degrees from the high temperature to get the number of growing points for each day. When the points reach a sum of about 400, the hills should turn green.

Fathauer visited Pearl Creek School and asked groups of fourth, fifth, and sixth graders to predict greenup. Because the students had to guess at the temperatures in the coming weeks, their estimates were different. Two groups chose May 5, one May 8, and another May 11 for the day Fairbanks will turn green. Fathauer said Anchorage typically greens up about a week to ten days earlier than Fairbanks.

Since Anderson began marking his calendar in 1974, greenup date has fluctuated by about a month, with a slight trend toward earlier greenups in the past decade. The earliest greenup was in 1993, when the hills changed on April 30. The latest greenup occurred on May 25, 1992, when the average temperature for the month was 41.8 degrees, more than six degrees colder than normal.

Anderson thinks one of the more noticeable aspects of greenup is the corresponding rise in the amount of pollen in the air. Anderson has for years measured pollen concentrations in the air with a device mounted on top of the building in which he works. He's created charts for people who suffer from pollen allergies. Birch pollen gives people the most problems, Anderson said, and to a lesser degree alder and willow.

Using pollen and greenup data for the past few decades, Anderson determined that birch trees start to release pollen about two days before greenup. More importantly, he can show that birch pollen reaches its maximum concentration--3,000 pollen grains per cubic meter of air--about three days after greenup.

Since greenup happens so suddenly, maybe we should start a lottery, like the Nenana Ice Classic. This year's contest will probably go to some of the students of Pearl Creek. Fathauer, famous for past predictions of cold or warm winters, has his own guess for Fairbanks. He thinks the green will arrive on May 7 or 8.