Secondary Dispersal of Seed Across Snow in Betula Lenta, a Gap-Colonizing Tree Species

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Abstract
Betula lenta is a gap-requiring forest tree depending on wind-borne seed dispersal for discovery of suitable regeneration sites. Modes of winter seed movement and snow conditions required for seed deposition in this species were examined usig a variety of selective seed traps. Seed fall was monitored during winter and autumn of 1985, and compared with aspects of weather on a daily basis. Conditions required for seed release were identified by subjecting catkins to experimental temperature and humidity treatments. In natural populations, seed was released preferentially on days with high winds and below-freezing temperatures. Catkins held in experimental environments released seed at low humidity, regardless of temperature. Through secondary dispersal across a snow surface, seeds in a natural population were distributed over an area 3.3 times greater than the area covered by aerial dispersal alone. Large numbers of seeds accumulated in experimental indentations in the snow, and in shallow depressions corresponding to the bases of uprooted trees. By contrast, emergent shurb and herb stems had a relatively minor impact on seed deposition, and very few seeds were deposited on the smooth snow surface. Daily control of seed release in Betula lenta during an autumn-winter dispersal period appears to promote long-range secondary dispersal by wind across a snow surface. Through this mechanism, ''passive'' dispersal by wind produces non-random, aggregated seed distributions, and increases the chances of seeds arriving in suitable regeneration sites.