Thursday, April 25, 2013

The Boys are in Bloom

Spring wildflowers prepare to release pollen ... but why bother?
Hort Log is soliciting posts for April’s Berry Go Round, a blog carnival for plant-minded folks.  The theme is Smelly and/or Ugly Plants.  With winter just now ending, we don't have many candidates aside from leafless trees and the remains of last year’s plants.  So my contribution is about lack-of-beauty -- the drab, inconspicuous, beauty-less male flowers of quaking aspen.
Not a lovely spring bouquet.
Quaking aspen -- Populus tremuloides -- is the most widespread tree species in North America.  It's known for its beautiful white bark and the slender-stemmed leaves that quake in the wind.  Every year many of us head into the mountains to enjoy gold, orange and red aspen leaves glowing in the low light of autumn.  But few people notice the flowers.
Aspen are dioecious -- male and female flowers are borne on different individuals.  Because quaking aspen propagate vegetatively (from root sprouts), an “individual” often is an entire stand, with trees connected underground by a single root system.  Stands can be quite old, on the order of 8000 years or more -- relics of the last glacial retreat (NRCS USDA).
Aspen stands often are giant clones, hence the claim to be the largest plants.
Image from USDA Forest Service.
Male flowers appear early in the season, before the leaves.  They’re borne in catkins that elongate over time.  The catkins in the photos below are from the branch in the photo at the top of the post ... but three weeks later.
Quaking aspen:  2 - male catkins; 3 - female catkins in flower, 4 - in fruit;
5, 6 - male, female flowers.  Image from USDA Forest Service.
Aspen flowers are highly reduced, i.e. many of the normal flower parts are small or missing (click on illustration above for more detail).  There are no sepals nor petals -- just a disc with stamens in the case of the males.  That's enough though.  These guys have everything they need to produce and cast their pollen to the wind.   
A male aspen flower, 1 mm in width (0.04 in).
But there's a problem ... actually several.  Because aspen usually grow in stands of a single sex, pollen has to travel far to land on female flowers.  Even if fertilization takes place and seeds are produced, the likelihood of successful establishment is small.  Aspen seeds are tiny (three million per pound) with no protective coat nor stored food.  If a seed manages to germinate, survival still is iffy because aspen seedlings are so intolerant of drought, requiring constant moisture (the site can't be too wet either).

There was a time when ecology students were taught that aspen rarely if ever reproduce from seed, that most of today’s stands were established during the late Pleistocene and have persisted by vegetative propagation via root sprouts (aka sucker shoots).  But this story -- like so many -- has turned out to be not so simple.  In fact aspen often reproduce by seed in Alaska, northern Canada and eastern North America, where there’s more likely to be habitat with sufficient moisture (USDA NRCS).  Vegetative propagation is the mode of choice in the drier West, but even here aspen will reproduce from seed in the right circumstances.  For example there was widespread establishment of aspen seedlings following the extensive Yellowstone fires of 1988 (Romme et al. 2005).
Above, female aspen flowers (USDA Forest Service).  The gals aren’t any more showy than the guys until the fruit mature and split to release silky-tailed seeds ... little Pollyannas taking flight.

In celebration of national Poem in your Pocket Day last week, Anne Buchanan of The Mermaid’s Tale shared a thoughtful poem about aspen by Edward Thomas.


  1. Fascinating. And the idea that aspens might not reproduce by seed, or, as with most trees, at least that most seeds a tree produces don't ever grow into mature seed-bearing trees themselves, certainly makes me think that evolution is less about 'survival of the fittest' than it is about luck!