Trees are interesting creatures. We often think of them as part of the scenery, but they are really vital and dynamic organisms in their own right. Looking at the forest through my office window as we’ve been hit with wave upon wave of snow, ice, and cold temperatures, I started thinking how amazing it is that they can so easily make it through the winter, even in our relatively temperate climate.
How can trees survive when they seem to have so few options for coping with the cold? Animals can make their own heat to keep themselves warm, and they can burrow and hibernate to escape harsh conditions. Annual plants die and leave their seeds to overwinter, usually under the snow or in the soil where conditions are much more mild. Many perennial plants die back completely to the roots, and small woody perennials can also overwinter under the snowpack, protected and insulated. But trees are large, tall, and immovable. They have no choice but to face everything winter can throw at them.
And yet, as you travel north throughout the world one thing is ubiquitous: forests. Primarily (but not exclusively) coniferous. Trees are nearly the last things to give up the ghost as we approach the north pole, and the dense, boreal forests of Canada and Russia (which survive some of the coldest, snowiest conditions on the planet) are an iconic image of winter in our minds.
There are several hardships trees (and all other organisms who weren’t smart enough to stay in the tropics) have to deal with in the winter, but the first and most obvious is the cold. Trees in arctic climates sometimes have to withstand temperatures of up (or rather, down) to -60 C. It’s this challenge of staying alive and frost-free in cold temperature with no way to generate heat that interests me primarily today.
Trees can be incredibly cold hardy if introduced to low temperatures properly. Some trees (even non-hardy ones) can survive down to the temperatures of liquid nitrogen. That’s -196 C/-320 F! Obviously that’s under lab conditions, since I’m pretty sure Earth hasn’t seen those temperatures since seed bearing plants evolved. Pretty impressive, nonetheless.
How can they manage such a feat? Well, not everything about plant cold-tolerance is fully understood, but we do know that what actually kills cells dead is not so much the temperature itself but the formation of ice – the freezing of the water inside the cell itself. Once ice crystals form within a cell it’s game over for man, beast, or tree. Trees, even evergreen trees, go pretty much dormant in the winter. The cells don’t really need to do much during this time, but if they get damaged, that’s it – they’re done for. Enough dead cells and you have a dead tree.
Ice is a problem because, as you may have noticed, ice is pointy. When water freezes normally it “expands” because its molecules reorient themselves into geometric shapes as they freeze. When the water inside a tree (either within the cells or between the cells) freezes, all of a sudden there are hundreds of tiny ice crystals, all taking up more space than they used to and just ready to rip through cell walls and gut them.
So how do trees prevent this? They acclimate! Even the hardiest trees aren’t frost-ready all year round. The tree that can survive the coldest arctic winter might still be killed off or damaged by a cold snap in July. Growth and heavy photosynthesis aren’t terribly compatible with being ready for a freeze (which is why frosts in the spring are especially deadly).
Most of the same hormones that trigger dormancy are responsible for cold acclimation. Based on a combination of slowly lowering temperatures and shortening photoperiod (shorter days as we head towards winter), a chemical chain reaction occurs that tells the plant that it’s time to stop growing, hunker down, and get ready for a big chill.
Of course not every tree can handle the cold, even with acclimation. All plants can “harden off” a bit from summer highs, but some species have found their warm temperature niche and have no interest in venturing north. They don’t even care for an occasional dip below 0 C. And it’s not just differences between species. Members of the same species that are from different climates have shown a widely ranging ability to get used to cold temperatures, both in speed and, ahem, degree. The white cedar (Thuja occidentalis) from South Jersey won’t have the chops to deal with a Minnesota winter, even if white cedar populations outside St. Paul get by just fine.
And acclimation can be even more finely tuned than that. Aspen (Populus tremuloides) is one of the most cold-tolerant deciduous species and grows high up in the mountains, in places mostly reserved for conifers (why conifers are the main and sometimes sole survivors in the very coldest regions of the arctic is the subject of a whole other post).
Green trees at lower elevation are not as cold-acclimated as their genetic twins above.
Aspen also grows in these huge stands of clonal trees, all of which are identical genetically and share a root system. In fact, one of the oldest and heaviest known organisms in the world is a stand of aspen trees. Despite this, an aspen colony on a mountainside may exhibit different stages of cold acclimation at the same time, depending on elevation. You can even watch the leaves turn at different times, as higher up trees go yellow while the lower down trees are still green.
So what is this amazing process called acclimation? Well, the chemical mechanisms for it are only now beginning to be understood, but as it gets colder and darker various hormones are produced which put the plant into dormancy and start to physically prepare it for the cold. Among other things, these hormones will cause leaves to fall, growth to stop, stomates to close, sap to stop flowing, and cause physical changes to the cells to allow them to withstand freezing, such as causing the plasma membrane to become more flexible and possibly more permeable (we’ll get to why that’s important in a minute) and even replacing water molecules with sugar molecules in some structures. It’s only recently we’ve begun to understand at all what role each chemical plays in cold-protection, and we still have a lot to learn, so excuse any vagueness on my part.
So, let’s look at what happens when it starts to get cold. Like most organisms, trees contain a lot of water. They have water that is inside their living cells, and water that exists between the cells. When it gets cold the first thing to freeze is the water between the cells. This might not happen right at 0 C, as you might think. Due to funky physics involving water tension in tight spaces, this water can often “supercool” (get below freezing without, well, freezing) for a bit before ice forms. But eventually it does form an ice crystal.
Luckily two things happen at this point. Even though the ice crystal gets all pointy and big, it doesn’t kill the cells around it because of one of the mjor things that makes trees different from humans. The cell wall. Plants have it, animals don’t. The cell wall can take a beating and being punctured by ice doesn’t bother it that much. It surrounds the plasma membrane, which is the sac that holds all the cell’s organs and fluids. The plasma membrane is flexible and can pull away from the cell wall when ice crystals burst through the cell wall, thusly:
(figure taken from Marchand 1996, page 48)
Part of what enables this is that when the ice crystal forms between the cells, the energetics of the frozen water molecules draw more water to themselves as it gets colder, so water starts to travel through the plasma membrane towards the ice crystal and freezes out there, instead of inside the cell itself.
This does two big things. One, it decreases the total volume inside the cell membrane, which allows more room for it to shrink away from ice crystals poking through the wall. Two, it increases the concentration of solutes inside the cell. Remember, it’s only pure water that freezes at 0 C, and inside of the cells wasn’t pure water to start with. It’s an organic soup containing sugars and lipids and proteins too. The water moves out, the other stuff stays in, raising the freezing temperature.
Another thing that happens during the acclimation process seems to be major changes of the contents of the cell itself. The large vacuole that contains mainly water is replaced with many smaller compartments which store starch, proteins, and fats – much more freeze resistant. The composition of the intracellular fluid is altered to contain a lot more sugars, as well. It appears that sugars are particularly important to the process of cold acclimation for a number of reasons, including the the lowered freezing temperature of sugar solutions.
These two strategies (less intracellular water to freeze and more concentrated solution within the cell) are the main ways most plants seem to survive really, really cold temperatures. Despite these measures, though, cell death can still occur because a cell can only lose so much water and survive (eventually the concentration of solutes will get high enough to poison the cell or it will just dehydrate too much to function). And of course, if it gets cold enough, even the highly concentrated solutions, fats, and starches inside the cell will freeze.
But there’s one last strategy that trees have to survive super-cold temperatures too. You see, not all frozen water becomes ice as we know it. There are way more states of water and of frozen water than the ones we’re taught in elementary school or even college. This goes into physics and chemistry and I’m not even going to try explain how it works, but under certain conditions when water freezes it just…freezes. It doesn’t expand, its molecules don’t realign and get pointy. It just turns to glass (it’s called vitrification).
If this happens inside a cell, the cell is usually fine. It’s like suspended animation. The main problem is until recently, the only way these non-crystalline states of frozen water (known as amorphous states) were known to occur was by flash-freezing water at -196 C or lower, before it had a chance to know what hit it. This is how even non-hardy plant tissue is able to survive those temperatures. But it has to be incredibly cold and happen super fast. Unfortunately, those conditions don’t occur in nature much (or ever).
However, fairly recently it was discovered that there are other conditions under which vitrification can occur that don’t require quite such extreme temperatures, like -26 C. This doesn’t happen in many species, but in the ones that can do this, the sugars and carbohydrates that fill up the cytoplasmic solution with the onset of winter play a key roll. Certain forms of sugar (particular tetrasaccharides) can help water vitrify at temperates way above liquid nitrogen. This how the trees at the most extreme latitudes manage to handle the chill.
As you can see, even though trees have relatively few options for dealing with cold temperatures, they’ve evolved some elegant and pretty awesome answers. And we still don’t understand all the intricacies of their survival mechanisms. The more I read about plants in winter and winter ecology in general the more fascinated I am with the way various organisms handle extreme conditions. I hope to write more about how trees (and other plants and animals) handle winter over the next few months, as we’re immersed in what looks to be one of the snowiest winters New Jersey has seen in a long time!