By Sarah Bisbing and Kristen Pelz
Following a summer of record high temperatures and extreme drought, we bet you’ll have a hard time imagining the feel of a cool, autumn day. For just a minute, though, let’s go there. Imagine: cool, crisp mornings; piping hot soups and apple pie; thick, wool sweaters; and (our favorite) the true mark of fall’s arrival – the turning of leaves and the coloring of our forests.
Although the eastern U.S. is renowned for its fall color, the western U.S. has its own colorful gem. The quintessential player in fall coloring of the western U.S. is quaking aspen (Populus tremuloides). Despite the many ecological roles this species serves, aspen is best known and most loved for its beauty and its stunning fall transformation. Its unique bark texture, quaking leaves, and beautiful fall color are so enthralling that people travel from near and far to witness its spectacular fall color. Aspen’s yellow to orange leaves stand in stark contrast to the deep green color of western conifers. It is simply captivating.
Can you imagine a fall without these vibrant pops of yellow, orange, and red leaves?
In the last decade, there have been signs of trouble for some of Colorado’s aspen. Sudden Aspen Decline – known as SAD – was first spotted in Colorado in 2004 (Worrall et al., 2008) and is to-date responsible for aspen mortality on nearly 100,000 hectares (imagine 100,000 baseball fields!).
This decline is not a specific disease but rather a stress-related dieback – a dieback driven by a complex set of interacting factors, including: drought, high winter temperatures, insect and disease damage, old age of aspen trees on the landscape, and a lack of regeneration due to intensive grazing/browsing.
The highest levels of decline are occurring in the state’s hottest, driest areas (typically lower elevation, south-facing slopes) and initiated by recent extreme-drought conditions (Worrall et al., 2010). This suggests that the main driver of this decline is climate, with drought and high temperatures stressing the individuals already growing under the harshest of conditions.
But, warm, dry conditions alone do not kill these trees.
We just spent a week cruising around SAD-affected forests in southwestern Colorado. The multiple factors stressing the trees were apparent everywhere we looked. When we had to mark the primary damage agent affecting a tree, a group discussion was often necessary. So . . . the tree has insect damage and diseased leaves, elk stripped its bark, and it appears to be drought stressed. Oh, and the surrounding vegetation has been browsed and trampled by cattle. Hmmm. Which one of these factors is really stressing this individual?
Decline of these trees is ultimately a result of the interacting effects of a complex suite of factors.
One could argue the following: Aspen trees are initially stressed as a function of increasing temperatures and severe drought. There is a fine balance under which trees (well, all plants) can photosynthesize (take in CO2 and make sugars/energy for growth). This is because plants and their generation of energy are limited by water and temperature. Each time a tree takes in CO2, it loses some water. This is part of the exchange. If water is limited, though, there reaches a point when the plant must slow or stop its consumption of CO2. When this happens, the plant does not have a way to make energy. Thus, plant stress and reduced growth.
Damage by wild (deer, elk) and domestic (cows, sheep) animals further increases tree stress. Elk browse can lead to significant damage of bark on mature stems, leaving an open ‘wound.’ Just like when your scrapes or cuts are exposed, the tree’s new wound is more susceptible to additional harm. This wound acts as an entry point for insects and disease.
Another important factor in SAD is the older average age of aspen forests and the lack of regeneration at the landscape scale. Older trees are less able to cope with these combined stresses. They are especially susceptible to damage by insects or disease, which opportunistically attack weak trees and ultimately cause tree death. With the death of the aging forest, a new cohort of aspen should be waiting in the wings – ready to take over the canopy. Fire suppression and increased animal browsing over the last 100 years have dramatically altered this process by reducing the presence of younger aspen. Were younger, more vigorous aspen present in greater numbers, these trees would have the potential to grow into the canopy. This ‘new’ canopy would then allow aspen forests to persist following death of SAD-affected older trees.
As climatic conditions are predicted to become even more severe, it is possible that some of our current aspen-dominated landscapes may become nothing more than a memory. But, this is far from certain. Forest resilience could be restored in many areas by reducing browsing pressure or by reintroducing fire. The increasing probability and frequency of fires predicted with climate change will be good for aspen in the long run (Moritz et al., 2012; See Kelly’s post). The species sprouts prolifically following fire, and a young stand is not as likely to succumb to stressors as easily as an older forest. Also, there is evidence that aspen is successfully moving into new territory – at higher elevations (Elliot and Baker, 2004) and latitudes (Hawkes et al., 2004) – as climate changes.
So, what is the fate of Colorado’s quaking aspen?
This adaptable species will likely thrive in many areas, but these areas may not be the landscapes or scenic areas of the past. As with most things in ecology, though, the answer is complex and depends on the interactions of many factors. Guess we’ll just have to wait and see . . .