Aspen Announce Spring

What causes trees to recognize spring and break bud differs by species, but is typically a combination of a long winter chill, warming spring temperatures, and increased day length, or photoperiod. One particular tree that was leafless just a week ago, but now has small green leaves emerging is the quaking aspen (Populus tremuloides). Before the leaves emerge, the aspen trees flower. Many people may not recognize the hanging aspen catkins as flowers, but the fuzzy pendants are made up of tens of tiny flowers attached to a central stem. Most flowers with which we are familiar are called “perfect” flowers, because they have both male and female parts (stamen and pistil, respectively), but aspen flowers are “imperfect,” containing only male or female parts. In fact, aspen trees are dioecious, meaning they are either male or female trees. Male trees flower first, and dropped their male catkins around mid-April. The female trees flowered after the males and, here in the Tetons, developed their catkins in late April and early May. Pollen from the male flowers is blown by the wind to pollinate the female flowers, which then develop seeds in small green capsules that will mature and release very small seeds encased in cottony threads to aid in wind dispersal.

Most reproduction in aspen is asexual, in which new stems sprout from underground lateral roots from neighboring trees. In this way, aspen stands can be a single clone; what look above ground like individual trees are actually stems, or ramets, of the same organism, all connected by underground roots. These clones can be differentiated definitively by genetic techniques, but the keen observer may be able to tell one clone from another by subtle differences in leaf shape or synchronized foliage changes in the fall.

Historically, most scientists assumed that regeneration from seed in aspen trees was exceedingly rare and happened only every few thousand years when conditions were just right. But recent research has shown that aspen seedlings (plants that come from the germination of a seed rather than sprouting from a pre-existing root) are more common than has been assumed. Interestingly, naturally occurring aspen seedlings have only been observed after severe disturbance like a severe fire or forest clearcut, as these areas can have little competition from other plants, ample sunlight, and sustained soil moisture, all of which are essential for successful aspen seedling survival. In fact, thousands of aspen seedlings emerged after the severe 1988 Yellowstone fires, many of which are still thriving today. Though not common, these disturbance events could be very important for the future of aspen trees in the Greater Yellowstone Ecosystem. Any newly established seedlings will have a unique genetic makeup that could make them more resilient to a warming climate.

With spring comes a new green to Jackson Hole, and with it, opportunities for new beginnings.

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