Trees can run?! Ensuring healthy, productive future forests in Minnesota

Are you stressed this holiday season? You’re not alone. In 2011, Minnesota experienced one of the driest Novembers on record. Last week’s low temperature of -5 degrees F in my Carlton backyard occurred with little snow on the ground. Cold winters with low snow-pack are tough on plants and trees that benefit from the insulating qualities of deep snow.

The winter of 2002-2003 was also one of northern Minnesota’s driest winters in recent memory. I remember it well. Along with the members of the Minnesota Tree Improvement Cooperative I worked to establish a seedling trial of white spruce in spring 2003. The seedlings overwintered outside the former Potlatch nursery in Cloquet. With almost no snow to protect them, many were chlorotic and dead by the planting date. In forests, conditions like the winter of ‘02-’03, along with invasive species, high deer and insect populations are pushing trees to the edge of survival. The local forest industry has good reason to worry: stressed trees don’t grow as fast as healthy, vigorous trees.

Stressful weather is far from a novelty in Minnesota. On the edge of the prairie in central North America, this area has witnessed repeated cycles of drought and floods during recent geologic history. Populations of insects and diseases, animals, and plants ebb and flow with changes in the environment. Over periods of several centuries, Minnesota’s boreal forests have tracked southward and then migrated northward in response to previous climate extremes, and scientists predict that these patterns of migration and expansion will continue into the future as well. Facing stress, trees – and the species with which they coexist – have only a few options: adapt and adjust, migrate or become locally extinct. Their fate is predetermined in large part by the encoding of their DNA.

What’s the difference between adapting and adjusting? Trees can adjust their form and function in response to changes in their local environment. For example, after drought, their crowns might die back to smaller size that is better equipped to handle future droughts. After defoliation, oaks sprout new leaves from axillary buds to resume photosynthesis. Plants and trees, despite their sessile nature, can make adjustments so they are better equipped at capturing resources and surviving until conditions improve.

Adaptation is a very different process: Trees adapt through their seeds. As the next generation is established, natural selection removes seedlings that are ill-suited and unable to tolerate local conditions. A species that is “well-adapted” throws out many seeds that establish successfully. Trees are especially good at adapting because their offspring contain tremendous genetic diversity. Chances are favorable that at least some will be well-suited to a novel climate or site, even if the mother tree no longer has characteristics that are well-suited to the current conditions.

How do trees migrate? Trees lack legs to run from unfavorable conditions, but they possess two vital modes of mobility. For one, tree pollen can travel long distances and still remain viable. A pollen grain in Minnesota can travel hundreds of miles, pollinate a tree in Wisconsin or Ontario and produce viable seed. In addition, tiny conifer seeds have wings that allow them to pick up wind currents and travel. So trees possess specialized mechanisms to spread even when they are threatened by local extinction.

Have you seen a bumper crop of cones on a dying tree? This “stress” crop produced is the botanical version of a tree’s “last breath.” So while trees are stationary creatures they are, in some respects, good runners. Over centuries and millennia they’ve migrated south, forced by glaciers, and back northward as glaciers retreated. They will likely migrate again in the future. Pretty good for an organism with a butt and no legs, eh?
Genes provide the hardware for organisms to evolve and adapt to change. Trees, especially conifers, possess high levels of genetic variation that exceed than any other terrestrial plant, as reported in a landmark paper by researchers at the University of Georgia (Hamrick et al 1992, New Forests). Tree breeders are well aware of the challenges they face: conifers are wild creatures and as variant as the landscape they inhabit. Two seeds collected from the same tree with the same two parents (accomplished through controlled breeding) are vastly different from each other. Genetic diversity is an especially important attribute that will allow our trees to adjust and adapt to either different locations or novel climates in the future. Conifers are well-buffered from global extinction, but they could become locally scarce if the environment in Minnesota become unsuitable for their survival.

Healthy, productive family woodlands

Family woodland owners can take measures to help forests thrive even in an uncertain climate. This “five-step program” will help your forest thrive.

  1. Monitor your woods carefully for new forest health or invasive species threats. When you see them, take prompt action to control them. Insect and disease outbreaks may travel through stands more quickly due to tree stress. The more quickly you can identify and control outbreaks, the more damage you can prevent. Your neighbors will thank you!
  2. Talk to a local forester about opportunities to increase the diversity of your woods. Trees with large, full crowns receive more energy from the sun and are more resilient than crowded, spindly trees. Thinnings and woodland stand improvement treatments may help to improve the diversity and vigor of your woodlands, increasing their natural capacity to adapt to change.
  3. Plant a tree. Plant a bunch of them. Natural regeneration may be a viable option for regeneration of species like aspen, but planting is recommended to increase the presence of conifers, oaks and other species that lack a nearby seed source. Before planting, pay attention to site preparation after a site is logged or thinned. Proper site preparation improves survival and growth of young trees. Sites with some exposed bare-mineral benefit most tree species. Sometimes herbicides are helpful, sometimes not: killing native grasses might favor the entrance of exotic, sod-forming grasses that are far greater competitors for soil resources than native grasses. Consult a forester before herbicides are applied, to get the most up to date recommendations for you area.
  4. Increase genetic diversity by planting a variety of species that are appropriate to your site. Avoid planting only a single species across a large area. By planting the right species on the right site, you can shorten the time required for a tree or stand to become firmly established.
  5. Visit the trees you planted, and monitor closely for signs of stress. Unfortunately, in many instances, trees will require protection from browse, especially young pines and oaks. The use of fencing (in small areas), bud-capping, hunting or trapping when herbivores get too numerous are recommended.
  6. Use local or genetically-improved seed if available to purchase in your area or seed zone. Request local seed sources from private nurseries. If improved seedlings are available to purchase plant them on a prime site, or combine improved sources with local, unselected sources.

Make sure to plan ahead: contact a grower in advance so they have time to obtain seed for your planting. With the coming closure of the Minnesota state tree nurseries, Extension is working with MTIC and other partners to provide a list of private growers who are willing to provide local sources of seed for reforestation.

Most importantly, get outside and enjoy the woods and the fruits of your labor. By taking these and similar measures to protect young trees, we can help future forests endure, and perhaps even prevent them from running away.

There’s more on Minnesota woodlands and climate change here.

Carrie Pike directs the Minnesota Tree Improvement Cooperative, based at the University of Minnesota's Cloquet Forestry Center.

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