People usually do not think of trees being able to migrate. But this is untrue. If you just think back 12,000 years ago, trees migrated just ahead of the crushing forces of advancing glaciers in Canada. Trees would sexually mature, send out seed and pollen ahead of the glaciers and create an advancing line of trees, moving southward. As a matter of fact, DNA testing found that some white pine tested in the Mississippi valley had come from the Algonquin park area. Of course, migrating birds and wildlife help move seeds of trees by ingesting seeds and later fertilizing another area with droppings. The native people of various regions would plant favored shrubs and trees, such as yellow wood, on their travels. In this way, they were always assured of having this plant material available.
So what I have just described is the process of migration for plants and trees. You can imagine my level of confusion when I happened upon the new term, ‘ assisted migration ‘. So were they just renaming an old term? Turns out, No. To truly understand this terminology you must believe in climate change and its consequences.
Through industrial activity and tropical deforestation, we have set the planet on a course to warm to temperatures not seen in the past 100,000 years. Earth has seen similar warmings in the distant past but the RATE of these warmings were at a substantially slower rate. For Ontario, warming is projected to be greater in the North than the South. These are the weather pattern scenarios that are being predicted by climatologists. Thunder Bay will experience weather more likely found in Peterborough. Barrie and Parry Sound will experience weather more typically found in Windsor and the Golden Horseshoe area will witness weather similar to Southern Ohio and Indiana. Precipitation wise, Northwestern Ontario and most of Southern Ontario will see a 10% decline along with an accompanying increase in temperatures. By 2100, we should see our annual temperatures rise by 6C. Overall, at a forest level, soils are going to be drier.
So what has been the immediate impact on our forests and tree species? Basically, as the climate changes, some individual trees or even whole local populations of a species may not prove adaptable to the new conditions, nor have the capacity or time to become adapted. These trees may not be able to migrate to more favorable conditions, given that climate is changing faster than natural migration via wind, water and animals, has occurred in the past. Another negative effect, a barrier to migration, is temperature and other weather extremes affecting flowering and seed production.
In the Spring of 2007 and 2012 temperature variabilities caused freezing damage to boreal forests in Northern Ontario. Above average spring temperatures led to evergreen trees losing their cold hardiness early, resulting in needle loss and bud injury when weather turned cold again. In the 6 years after the 2007 cold snap, 30% of the evergreens did not survive. Another consideration is that milder winters and hotter summers and moisture stresses creates favorable conditions for insect and plant diseases to spread.
So it is clear that forests are vulnerable to climate change. But how will our forests respond? The consensus is that we will see changes of species compositions and age – class redistributions. A higher percentage of our forests will be in younger age classes and there will be an increase of frequency of early successional species. Species adapted to disturbances will increase. Of course, then local laws of nature will apply where species best suited for the local conditions and climate will mature and set seed. And all of this take TIME. Lots of time is needed for the most fit to survive, colonize and seed a forest stand. Unfortunately, the rate of migration imposed by climate change will not allow for this best fit scenario. In the end, trees will be less adapted for local areas as the march northward continues.
When you hard boil all the scientific evidence down, the real problem is the tree migration rate needed. Historically, trees can migrate at less than 10 kilometers per 100 years. With climate change and warming happening even faster than predicted, trees will have to move 150 – 200 kilometers in the next 100 years to ensure continuation of tree species within favorable climatic conditions. Obviously, the evolutionary migration rate of trees cannot keep pace with the anticipated rates. Another impediment to the migration is the fragmented landscape of Ontario, especially southern Ontario. Seed dispersal is hindered by human developments.
What to do? The choice seems to be assisted migration. It is the intentional migration or relocation of southern trees species north, by man. It is a planned relocation of species, still within its natural growth range, but at the northern extent of its current habitat. In my next article, we will look at the mechanics and controversy of assisted migration of trees.