Cold weather this April is delaying two cherished milestones in Interior Alaska:
the start of the birch sap run and green-up.
Below-normal temperatures are
likely to continue for a week or more, according to Rick Thoman, a climate
specialist with the Alaska Center for Climate Assessment and Policy. “We’ve got
green-up pushed back to quite late,” he said in mid-April. “We’re now looking at
the 50% likelihood for green-up is now May 11-15, and the long-term average is
May 8.”
That also means the birch sap isn’t likely to start running until late
in April, Thoman said.
“The weather models continue pushing back any big
warm-up, and it now looks nearly certain that we’re looking at very late sap
flow/green-up,” he said, adding that April’s temperatures are “historic-level
cold.”
Knowing when the sap is likely to start running is important for people
who tap birch trees for the sweet liquid, which can be distilled into an even
sweeter syrup, said Jan Dawe, who leads OneTree Alaska, part of the University
of Alaska Fairbanks Institute of Agriculture, Natural Resources and Extension.
Tapping too early results in the tree sealing over the tap hole, blocking sap
flow, and tapping too late misses the first days of sweet, clear sap.
Thoman and
Dawe have been working together to find a formula that will allow them to
predict sap flow and green-up — which also heralds the start of the pollen
season.
Starting March 1, Dawe starts to track the temperature. She adds up the
number of degrees by which the maximum temperature exceeds 32 degrees Fahrenheit
each day, called the Accumulated Heat Sum (AHS). Sap usually starts running by
the time the tally reaches 225° AHS, depending on the location of the tree.
OneTree Alaska issues a “sap watch” by 175 degrees AHS and recommends people on
south-facing slopes tap up then. So far this season, the tally reached 70
degrees AHS on April 4 and stalled as cold weather moved in and refused to
budge, Dawe said. On April 17, it had inched up to 85.
That cold weather is also
delaying green-up. Since 1974, observers on the University of Alaska Fairbanks
West Ridge have observed the trees on Chena Ridge to judge when green-up —
defined by former UAF biosciences librarian Jim Anderson as the day birch and
aspen leaves open just enough to produce “a faint but distinct green flush
through the forest canopy,” occurs. Thoman notes that, as in forecasting
weather, there’s a good deal of uncertainty weeks out and he’ll have a better
idea in early May of when green-up is likely to happen.
“Of course, the other
thing that is coming after green-up, within a day or two, is we have a pollen
explosion,” Thoman said. “For people with sensitivities, that timing is
certainly important to know.”
