The Race North

January 3, 2018

An aerial view of the Boreas Ponds property in the Adirondacks. Photo © Blake Gordon for The Nature Conservancy

Most plants respond to warm and cold temperatures. Give them a few warm days in spring and they will begin to grow. Give them a few cool days in autumn, and many will begin to turn colors, degrading their chlorophyll, which is green, in favor of other pigments, which can be yellow, red or brown.

These are seasonal changes, but long-term conditions of temperature make a difference as well. The USDA recognizes “plant hardiness zones,” which show what will grow best in different regions of the country. In the desert Southwest, creosotebush is not found where the temperatures fall below -4 F (-20 C) on any night of the year. In the Northeast, sugar maple is not found north of an average wintertime temperature of -40 F (also -40 C).

So, with the ongoing warming of our climate, it makes sense that plants will respond. One might expect that many species might be found in more northerly locales, or higher on mountain slopes, in warmer conditions. Indeed, in Oregon, many plant species have already expanded in the hotter, drier slopes of the Siskiyou Mountains.

The pattern of change can be messy, and these natural history observations do not lead directly to a mechanistic understanding of what causes the changes in plant range. Changes in temperature are often accompanied by changes in soil moisture. Some plants cannot move unless essential root fungi also move under the same conditions. And, some plants cannot reproduce in new habitats if their pollinators are not present.

For forests, the response to higher temperatures is not immediate. A big, old tree can hang on for many years after its optimal conditions are lost, simply because it’s there, it’s well established, and it can shade the saplings of species that might be expected to outcompete it. Of course, in the face of windstorms, fire, lightning, and insect attacks, no tree lasts forever, and eventually even the giants of the forest will succumb. Attack by exotic insects can exacerbate the onset of change. With climate change, the new trees that replace them are likely to be different species, and eventually the entire forest composition may shift.

Ecologists question whether the dispersal of seeds from existing populations to new areas that become favorable habitat will be fast enough to keep up with climate change. The warming at the end of the last continental glaciation and the resulting changes in forest distribution provide some natural history data that pertain to dispersal rates.

Aerial view of the Thelon River and forest landscape of Canada’s far northern Thelon Game Sanctuary. The refuge is the largest and most remote game sanctuary in North America. Photo © Ami Vitale

Most forest species can disperse northward at rates of about 300 feet (0.1 km) per year. Trees with seeds dispersed by birds show some of the highest rates; seeds spread by wind fall close to home. Most projections of future climate indicate that dispersal rates in excess of 3000 feet (1.0 km) per year will be needed for trees to keep up. There will be winners and losers in the race to colonize new habitat. Those that fall behind are likely to be lost as a component of future forests and perhaps entirely.

The future distribution of trees is of more than passing interest to a few weekend naturalists. The forest-products industry should be deeply concerned about what is growing in forests today and what will be the optimal forest composition in the future. They are long-term investors in trees.

Unlike the father giving his naturalist son advice in the famous New Yorker cartoon of a few years ago, there may be big money for those who know about trees.

This post originally appeared on William H. Schlesinger’s blog Citizen Scientist, published by Duke’s Nicholas School of the Environment.

William H. Schlesinger

William H. Schlesinger is one of the nation’s leading ecologists and earth scientists and a passionate advocate for translating science for lay audiences. A member of the National Academy of Sciences, he has served as dean of the Nicholas School of the Environment at Duke and president of the Cary Institute of Ecosystem Studies. More from William H.

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  1. Interesting article. We do forestry in southern Virginia. Already, we are planting more southern species and types. For example, we are on the northern edge of the longleaf pine range. Presumably, if it gets warmer, we will be nearer the middle of the new range. We are in the northern part of the loblolly range. Already, our lobolly genetics come more from the southern part of the range, since a lot of their genetic material comes from Georgia and South Carolina. I have also planted some bald cypress that are from significantly farther south.

    Generally, we have mixed species, sometimes planting southern trees farther north and sometimes northern trees farther south. I have seen bald cypress and tulip poplars in Minnesota and spruce in Florida. They are now in second and third generations and have made some epigenetics adaptations.

    That means that in many cases, the trees need not move. They are already there. What we will see is some odd associations. I think a prototype of this might be those sky islands in the Southwest, where you might find Douglas fir growing next to agave cactus. The concern is that the natural forests we have come to love will be changed.

    I am less worried about forest industry in this respect. Researchers are always developing new genetic varieties of the major timber trees and since our forests are mostly regenerated by planting seedlings. This can be adapted on the level of decades, if not years.

  2. I have seen several articles like this,discussing the subject in general. However ,I have not seen anything that gives specific advice to the various growing areas. These areas may vary not just in temperature, but also in moisture and soil types. In addition to other considerations such as purpose,uses and interaction with existing flora and fauna. There would presumably be a transition period . So, if we are looking ahead, what specific plants do we cultivate today? For example , what tree would replace the Hemlock in northern Pa.? How will blueberries be affected?

    1. There are obviously many unknowns associated with this issue and much research to be done. To start, it is best to look at what grows today south of your specific location on the soils and moisture regimes that concern you.

  3. Trees are the Lungs of the Planet. Without Healthy Lungs, the Planet Struggles to Breath and Thrive. If We Diminish the Number of ( Healthy ) Trees on Our Planet, Soon we will All Notice the Dwindling Oxygen until it fades into a Dense Fog ,that Chockes out All Life . To Strengthen Our Planets Health We must All do our part to Protect, Plant and Save The Trees .

    1. See my posts on the parent blog site:
      on the subject of “Running Out of Oxygen” (14 June 2016) and “Trees and Air pollution” (10 March 2017)