For long-lived organisms like trees, climate change is not something that will only affect future generations. Climate change is happening so rapidly that trees that are alive today will experience profound changes in their environment. There are many unknowns about what will happen to trees and forests around the world as climate change accelerates. There is also confusion, both in the popular and technical literature, over terms like “adaptation.” This is the first of several columns that tries to clarify the trajectory of trees in a warming world and bring some clarity to the terminology.
The pawpaw trees grow in deep shade beneath the pine, oak and spruce in my back yard. They seem to like it there. My family gets a couple of pecks of fruit every year, though I seem to be the only one who likes the fruit – my kids prefer my pawpaw cake.
We decided to remove a spruce tree to give my wife’s garden more sun. The sunlight on the pawpaw increased by about 30% with the spruce gone.
The garden liked it, the pawpaws did not. Within hours, the leaves began to wilt. By the next day the leaves were brown and falling off. The increased light caused UV damage to the thin leaves, the increased temperature caused heat stress and the increased water stress caused the leaves to wilt. For a few days, the trees looked dead.
They were not. The buds began to open within a couple of days and soon a new crop of leaves formed to support the developing fruit. The new leaves, though, were different from the old ones smaller, thicker, with a stiffer, waxy feel.
This is what makes trees so resilient – their ability to acclimate to change. Acclimation is a physiologic process involving changes in metabolism and in gene expression but not in gene frequency. Acclimation can sometimes take seconds, such as in the production of protective heat shock proteins. It can take hours or days, as in my pawpaw’s replacement of leaves. Or it can take years, as in the reconstruction of a new crown in a lightning-damaged tree.
Adaptation is different. It is the genetic response of populations of trees to environmental change. It is the result of sexual reproduction in a population, the natural selection of individuals better suited to the new environment, and a change in gene frequency that is passed on from generation to generation.
Why is this important? Because the climate change discussion in the popular press and some of the science press does not distinguish between acclimation and adaptation. Adaptation is mentioned in the media about 20 times as often as acclimation. For trees and forests, acclimation is every bit as important as adaptation.
Think of it this way – adaptation is required for the next generation of trees and forests to grow. Acclimation is necessary for the current generation of trees to survive. Climate change is happening so swiftly that its most profound impacts will be on trees that are currently alive and must acclimate to survive.
Many biologists are worried that climate change will be too rapid for plants and animals to either adapt (by producing new progeny) or migrate (by moving to another place). For trees, which are both very long lived and fixed in place, both adaptation and migration mechanisms may be overcome by the speed of change. But more worrisome, climate change may overwhelm the ability of our current forests to acclimate.
Now, imagine what our forests will be like as the worst impacts of climate change take hold. Louis Iverson and Anand Prasad and their colleagues at the USDA Forest Service have created the Climate Change Tree Atlas as a first attempt to model the future habitat of tree species in Eastern North America. Their work is not trying to predict the future range of tree, but the future suitable habitat for trees. Using the high-climate change model (HADLEY CM3, which now looks quite conservative), many of the trees in our current forests will not find suitable habitat within 80 years, a very short time in the lives of trees.
Black walnut, for example, is abundant here in Kentucky, a very important timber tree throughout the midwest. However, by 2100 (using the Hadley CM3 model), suitable habitat for black walnut will have moved far to the north. This is a very short period in the life of a long-lived tree like black walnut. which can live more than 200 years. That means that there are trees alive today that will find themselves in habitat not suitable for their growth.
And that is where acclimation becomes critical. To survive in its current range, black walnut trees will have to adjust their physiology to tolerate higher temperatures, warmer winters and greater drought. We do not really know the limits to the ability of black walnut to acclimate to a rapidly changing environment. The great experiment is under way, and we will soon find out.
The subject of acclimation, adaption, mitigation and migration in response to climate change will be the subject of a number of future columns. Your comments are welcome.