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NatureSpeak Articles

Winners and Losers: A Forest Shell Game


Working as a Mountain Host/Naturalist on Blackcomb Mountain during the summer of 1995 I was often asked about the clusters of plump, purple cones standing erect on the upper branches of trees beside the lift. The trees, of course, are subalpine fir, a species superbly adapted to survival in regions of high snowfall. Their tall narrow crests and symmetrical wedge-shaped profiles are designed to shed snow without damage to the trees, allowing them to thrive up to the very limit of treeline.

1995 was a "mast year" for the Subalpine Fir on our local mountains; a year when they bore a bumper crop of cones. By the spring of 1996 most of the cones had disappeared; either stripped of their scales by seed-seeking birds or, like the cones of many fir species, disintegrating on the tree, leaving only their slender central spines standing vertically like tiny long-stemmed mushrooms. These remnants of the 1995 cone crop clung to the upper branches of the trees for the next four years, but until this year few new cones were produced. Now, after five years, we have come full circle to what appears to be another mast year.

The time between mast years is extremely variable. It appears to be controlled by both external factors such as weather conditions during the preceding year, and internal factors such as the recovery time needed for trees to restore nutrients and expanded root systems between cone crops. But regardless of the triggering environmental cues, the tendency toward irregular, unpredictable fruiting appears to be etched into the genetic programs of many tree species that are subject to seed predation by birds and rodents. A feast-and-famine cycle of seed production has the effect of starving out predators during lean years and then swamping their reduced population with an over-abundance during mast years. Thus ensuring that large numbers of seeds survive.

For our local subalpine fir the principle targets of this game of subterfuge are the huge winter flocks of pine siskins and crossbills, both highly irruptive species, that periodically check out the local cone crop. In the 1995 mast year the birds got it right. They arrived in record numbers and settled in for a winter-long feast. Returning in 1999, when over 1,500 siskins were recorded in the Christmas Bird Count, they found a dearth of cones and were gone within days. Now, in the mast year of 2000 the cones have developed, ripened, and with each wind their scales, bracts and seeds are spread across the snow untouched by a sharply reduced bird population. So the score is even – one for the birds, and one for the trees.

Upcoming Events :

March 3 — Monthly Bird Walk . Meet at the base of Lorimer Road at 8 a.m.

Written by: Jack Souther


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