Thursday, May 14, 2015

Paper birch - Betula papyrifera - Bouleau à papier

Betula papyrifera is a species native to North America, particularly Canada and parts of the northern US. A native range map for the species can be found here; note that although this map excludes Nunavut from the native range of the species, the Plants of Canada database indicates that the species' native range also includes this territory. This species is well-adapted to cold weather, and tends to grow poorly where summer temperatures are too high [1,2,3].

B. papyrifera catkins & leaves
B. papirifera, like many of the maples including A. saccharinum, is wind-pollinated. But, its flowers look quite different: B. papyrifera produces a catkin, a tubular inflorescence (inflorescence: a structure which contains flowers) with no petals or inconspicuous ones, composed of a large number of flowers, usually all of the same sex. The catkin is an inflorescence shape which lends itself well to anemophily (wind-pollination), as the tubular cluster of flowers helps ensure the distribution or sifting of pollen from the air with high efficiency regardless of wind direction. Catkins also tend to be quite unremarkable to look at, as they are not (generally) adapted to attract pollinators.

B. papyrifera female catkin
B. papyrifera male catkin
B. papyrifera is monoecious, meaning that each individual produces both male and female gametes. In the case of B. papyrifera, the individual inflorescences are imperfect, being either staminate (male) or pistillate (female) [4]. Both male and female inflorescences occur on the same tree [4]. B. papyrifera has both male and female catkins. The male catkins are formed in the fall, creating dense clusters of three on branch-tips and overwintering on the branches [5].

One possible reason that the male catkins are set in the fall but the female catkins are not is related to the timing restriction on the use of the types of gamete. The pollination season is relatively short, so male flowers must produce as much pollen as possible as soon as possible in order to ensure that their pollen fertilizes the female flower. In this situation, it makes sense for the male flowers to be set in the fall, permitting the male flowers a head-start on opening in the spring.

The female flowers have to be ready quickly for pollination, but this is not where the most energy is expended for female flowers; the female flowers, once pollinated, must develop and produce the fruit or seed which will then germinate into a new individual. In principle the female flower has the whole growing season to set fruit or seed, so it is not so urgent to have a fully developed female flower as early as possible (indeed, the female flower can't really mature until after pollination has taken place), just one that is receptive to pollen. This can be achieved without setting the flowers in the fall.

The study of timing of floral development and opening is a whole field unto itself, with a wide range of plant strategies for a vast array of reasons, which I will not get into here but may discuss in a dedicated blog post.

B. papyrifera has some interesting characteristics. The bark of this tree is exceptionally weather-resistant, often remaining essentially intact on the forest floor long after the wood has rotted away [6]. This species is sometimes grown as an ornamental, specifically for its attractive white bark [1,2] which is visible in the background of the photo below:

B. papyrifera, white bark in background
B. papyrifera's bark is extremely flammable, making this tree very vulnerable to forest fires [1]. This trait also makes the bark excellent tinder for starting fires, as it will burn hot even when wet [1,5]. That said, please consider the health of the tree and if using this bark to start a fire don't go stripping it off the tree; the tree needs its bark as protection against weather, insects, bacteria, and fungi. Collect it from the ground instead, and make sure to follow all reasonable safety precautions with your fire.

This species is listed as vulnerable in Indiana; it is imperiled in Illinois, Virginia, West Virginia, and Wyoming; and it is critically imperiled in Colorado and Tennessee [6]. In Canada, it is listed as secure in all provinces and territories except Nunavut, where it is sensitive [7]. Please bear the species status in mind when interacting with B. papyrifera and interfere as little as possible in places where the species is at risk.

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