EXPERIMENTS ON LICHEN GROWTH .2. EFFECTS OF A SEASONAL SNOW COVER

Seasonal growth of the foliose lichens Xanthoparmelia lineola and X. subdecipiens was monitored for 26 mo at two transplant localities in a montane meadow on the East Slope of the Colorado Front Range. One of the transplant sites is exposed to strong westerly winds and is periodically scoured free of snow in winter. The other is sheltered from the wind and remains covered with snow for almost half the year. Differences in snow cover and lichen-drying rates at the two sites caused seasonal differences in radial growth: lichens grew faster at the snow-accumulation site than at the snow-free site in winter and summer, but faster at the snow-free site in spring. Long-term growth rates at the two localities were statistically identical; except for two thalli of X. subdecipiens whose threshold of tolerance for snow were exceeded, the costs and benefits of growing in a snowbed environment counterbalanced each other. To determine how snow affects long-term lichen growth rates at higher elevations, Rhizocarpon subgenus Rhizocarpon thalli were measured along an isochronous surface that traverses snow-free and snow-accumulation areas above timberline. No systematic differences in maximum diameter were recorded, despite large differences in duration of snow. The studies indicate that zoned lichen communities in the Colorado Front Range are more likely to have resulted from episodic snowkill than from slow growth beneath a seasonal snow cover.