Yellow birch (Betula alleghaniensis) gets its name from its golden yellow bark. It produces tiny seeds with an average weight of only 1.6 mg. This is four orders of magnitude less than red oak with an average of 2475 mg. While the oak parent provides a lot of energy for its offspring, there are almost no such reserves in the birch seed for germination and establishment. As soon as a yellow birch seed has germinated, the tiny seedling needs to make contact with the soil quickly, before it runs out of energy. Try picturing an average forest floor and likely you will conjure up a picture of half-rotten leaves and a carpet of living vegetation, with the soil hidden underneath. If a birch seed lands and germinates on top of this organic layer, it will almost certainly not be able to penetrate and establish contact with the soil before it dies. This is why yellow birch relies heavily on specific locations to regenerate: tip-up mounds and nurse logs. A tip-up mound is formed when a mature tree falls over, ripping up its roots from the ground and hence exposing open soil connected to the roots. A nurse log is a fallen tree which, as it decays, turns spongy and also holds a lot of moisture. Because they are naturally elevated, tip-up mounds and nurse logs are less commonly covered with litter and hence are prime sites for yellow birch to establish. A yellow birch seed arrives to these locations just by chance, as it has two tiny wings inserted along its sides for wind dispersal. Because yellow birch seeds are exceedingly cheap to produce, a parent birch produces so many of them to literally cover the whole forest floor with seeds. This is especially visible in the winter when many of the seeds are released and blanket the snow. Once the snow melts, a few lucky winners will end up on top of tip-up mounds or nurse logs to establish the next yellow birch generation.
Text written by Ivana Stehlik
Caspersen J. P. and Saprunoff M. 2005. Seedling recruitment in a northern temperate forest: the relative importance of supply and establishment limitation. Canadian Journal of Forest Research 35: 978-989.
Greene D. F. and Johnson E. A. 1994. Estimating the mean annual seed production of trees. Ecology 75: 642-647.
Yellow birch. https://www.ontario.ca/page/yellow-birch#
Photo credit: Krzysztof Ziarnek, Kenraiz, CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0), via Wikimedia Commons