Urban Forestry Part 5: In Which We Appreciate Mixed Blessings
“I hate the cold.”
I can’t really say that that statement is wrong at all, but it worries me that winter is unappreciated in our society. The absence of seasonal cold in our Canadian climate would actually wreak havoc on our forests and lakes, and since climate change is supposed to result in greater warming at higher latitudes, warmer winters may very well be in our future. You can play around with some models here, but try to think about the assumptions behind the models while you are playing with them!
Historically, cold winters have been responsible for maintaining balance between forest pests and their hosts, such as the Mountain Pine Beetle and several species of Pine (including Lodgepole, White, Scotch, and Ponderosa. It seems to be true for our local pest of the day, the Emerald Ash Borer (EAB). Apparently, populations of EAB larvae suffer about three-quarters mortality at about -30 degrees Celsius and almost complete mortality around -35 degrees Celsius (you can read more about it here!).
Specific information about EAB life cycles is a little bit hard to come by, but according to an article by inect enthusiast Debbie Hadley , adult female Emerald Ash Borers seem to lay about ninety eggs per cycle, which is either every year or every two years. The larvae remain in larval form over the course of the winter and survive by eating the phloem of the tree (which is the pathway for nutrients to travel between the roots and other parts of the tree). If one assumed that half of those larvae were female, and each of those larvae survived to lay ninety more larvae, and half of those went on to lay ninety larvae each, there would be 182,250 offspring in the third generation, and if you follow it further that way, there would be 8,201,250 in the fourth. However, if three-quarters of the populations were killed every year (assuming one generation is one year), there would only be 2,848 offspring in the third generation and 32,040 in the fourth. These numbers look a little bit more manageable, right? It’s still exponential growth, though, so let’s make it a little colder.
If we start with one female who lays ninety eggs and 98% of them die in the -35 degree Celsius weather, and half of the surviving eggs are female, we will have 0.9 females. Let’s give her the benefit of the doubt and say that she’s a full beetle. It is obvious now that if she lays ninety eggs and the same weather happens next winter the population will stay the same.
But it would have to be -35 degrees Celsius for a while, and speaking as someone who recently got frostbite on her ear, I don’t think I’d like that very much. Add the fact that EAB larvae live under the bark where they are insulated and it has to be colder for even longer to reduce that effect.
I am cold just thinking about it.
When you think about it, though, it might be worth bundling up for. The Canadian Forest Service estimates that the direct costs incurred by EAB may add up to $2 billion by the late 2020’s or so. The indirect costs include lost habitat for species dependent on Fraxinus (the Ash genus), lost air and water filtration, less shade, degraded soil health, lower real estate values and sadness at losing beautiful trees.
I was going to touch on some of the other ways that cold temperatures are important for our forests but this seems to be enough for now. If you are in forestry at U of T there is a great class called “Forest Fire Behaviour and Management” that will teach you the physics behind some of the impacts of climate change.
It is big stuff, but I encourage you to think about it! There are a lot of questions that are still unanswered about many organisms and the impact of climate change on their reproductive cycles, and with EAB being such a hot topic right now, maybe some NSERC awards are out there just waiting to be snapped up for EAB life-cycle research.
Stay warm!