Why Did All The Trees Fall In The December Storm?

The “snowmageddon” storm of December 2021 was one to remember. Just ask the trees. Some lost branches. Some were bent over—sometimes seemingly permanently—by the weight of the snow. Many many more were uprooted, leading to the power and transportation issues we lived with for days or weeks after the storm. What caused these responses? Why did some trees only lose some foliage, while others were permanently reshaped or killed? The answer lay deep within the tissues of the trees; the roots, the stems, and the anatomy of wood. Here we’ll review what controls tree response to stress like storm damage, and how the previous stresses a tree experienced may influence its response to future stress. 

As we all know, trees are made of wood. But what is wood? Wood is actually a complex arrangement of plumbing and scaffolding that serves to give a tree both structure as well as food and water. The arrangement of this plumbing and scaffolding is what gives trees in temperate climates (defined by moderate temperature and moisture, between a desert and the tropics) their definitive annual rings, as well as their structural resistance to things like wind and snow. Further, the arrangement of these cellular components of wood are what allow trees to grow to such great heights, conferring both rigidity and flexibility. So what are these structures and how do they tell us about why some trees responded to the great snowstorm of 2021 the way they did?

The structure of wood: who is who and what is what?

Wood is primarily formed of two components, but of various types: xylem and phloem. Xylem is the water transport system of wood, while phloem is what transports sugars (sap) down the tree from the photosynthesizing leaf. Phloem is present entirely on the outer layer of a tree as a thin layer of cells under the bark. Xylem, meanwhile, makes up the majority of wood. 

Xylem comes in two flavors: vessels and tracheids. Vessels are the large (gigantic, relatively speaking) water transport cells in hardwoods like oak trees. Tracheids, meanwhile, are the only xylem cells present in softwood conifers like cedars, pines, and firs. Tracheids and vessels serve the same function: to move water from the roots through the tree up to the leaves and out into the atmosphere. Trees that are taller have to move water to often extreme heights. Think of a Coast Redwood (Sequoia sempervirens); these trees have to move water from underground to over 300 feet up in the air! Trees grow to such great heights to out-compete the vegetation around them (in the world of plants, that which grows tall gets the most light!). So how does a tree deal with having to “pump” water to such a great height while also not just falling over in the slightest breeze? The answer is a substance called lignin. 

Xylem cells are made up entirely of two primary substances: cellulose and lignin. Cellulose is the primary component of plant cells, forming the inner portion of the cell. Lignin is the material that forms the walls of wood cells (Figure 3), and is the substance that gives plants structure and flexibility. Have you ever wondered why celery is so flexible without breaking? Lignin! Lignin gives structural integrity while allowing flexibility. The more lignin, the more stiff but also flexible. 

Great, but how does this tell me why my tree fell over or is bent?

Essentially, the wood you see when you look at a cross-section of a tree is a bundle of xylem straws, arranged vertically. The dimensions of these straws can tell us a lot about how a tree has responded to prior stress, and how it may respond to future stress. Larger cells (bigger straws) move more water than smaller straws, but this comes at a cost. Lignin (the structure!) is very carbon-expensive, costing 1.3-1.7 times more “units of carbon” per unit volume of wood than cellulose. Tall trees need more lignin for structural support, but also have to spend more carbon to make that wood, but there is one more extremely important component of the tree physiology – snowmageddon tree issue: drought. Drought plays an extremely important role in tree response to future stress, often directly influencing tree physiology and resulting response to conditions years after droughts occur. 

If wood is a bundle of straws, soil water is the milkshake a tree is drinking. Water movement through trees is “passive”—trees don’t “move” water from the soil to the atmosphere. Instead, water is “pulled” through the tree by the gradient in water vapor pressure applied by dry air blowing over a leaf surface; dry air blows over a leaf, and the difference in water content between the air and the leaf causes water to leave the leaf through pores in the leaf surface. This movement of water creates a tension (called “water potential”) by which water is drawn from the leaf to the air, the xylem to the leaf, the roots to the stem xylem, and the soil matrix to the roots. So how does drought change this? 

Imagine putting a thin-walled straw into a thick milkshake and taking a sip. What happens? The straw collapses under the extreme pressure, or the shake comes through the straw in “chunks”, with air gaps between those pulses. This is called “cavitation”, and forms emboli (plural of embolism), much like in human blood vessels. Trees that have thicker xylem cell walls and smaller cell diameters (smaller straws with thick walls) are often resistant to this drought stress. Some trees have a genetic predisposition to growing these kinds of cells, while others actually respond to drought by growing them after the stress. But…think back to the carbon cost of building thick-walled cells. There’s a cost! Carbon! 

Carbon isn’t just used to build wood in trees. It is also used to resist disease and pests. Bark beetles, gall rust, and (this one’s important here!) fungal pathogens like sources of root rot resistance are all carbon-intensive—trees use available resources to form the resin, wound closures, and volatile compounds that allow trees to repel invaders (Figure 4). So what happens when trees experience multiple droughts, back to back, with limited recovery time? They make “choices” of where to allocate those resources (often determined by genetics and response to the environment). Build new wood? Build thick-walled cells? “Bet” on drought resistance being the best strategy at the expense of disease resistance? 

Out of the sauna, into the snow

So why did so many trees fall or bend in the 2021 winter snow storm? First off, let me say that after what I’m sure was an engrossing venture into the world of tree physiology, the answer is what it always has been: “it depends”. Our forests are heavily over-stocked, so trees are competing for limited resources, regardless of stress like drought and storms, and thus are often weakened. But how and why they are weakened often comes back entirely to physiology, for the following primary reasons:

1. Trees in our area have not experienced a storm like that in decades, leading to physiology that was not “prepared” for such a storm (i.e. they were not “acclimated” to such weight because they haven’t grown wood resistant to it in response to a storm).
2. Persistent drought weakens trees, due to tradeoffs in carbon allocation throughout the tree, resulting in decreased resistance to things like root rot and other pathogens.
3. This type of storm, an “atmospheric river” (a topic for another discussion!) that dumps that much snow in one location for such a prolonged period, overwhelms the physiological defenses of trees.

But what about the trees that just look permanently bent? Lignin, again! Those trees have a high concentration of lignin, conferring just enough flexibility to bend under the weight of snow without snapping. Have you ever seen trees that have large right-angle bends in them out in the forest? My money would be on these bent trees—if they survive—turning into those lovable screwball trees that stand out among their neighbors once they correct their growth (with lots of lignin on the lower side of the tree!) and once again reach for the sun. 

So the next time you walk around and look at obvious storm damage leftover from December, remember that you are witnessing the wonders of variation in tree growth strategies. Bending, snapping, dropping branches, or uprooting; all of these responses depend on growth strategies these trees employed before the storm. Some strategies doomed them to failure in such a storm, while others allowed them to bend, but not break. 

Now if only we could get a few more storms…

References:

Lauder, J.D. From the Cell to the Stand: Trait-based Approaches to Understanding Forest Response to Climate Change. UC Merced. ProQuest ID: Lauder_ucmerced_1660D_10559. Merritt ID: ark:/13030/m5ff91jw. 

Lauder, J. D., E. V. Moran, S. C. Hart. Tree Physiology, Volume 39, Issue 7, July 2019, Pages 1071–1085. https://doi.org/10.1093/treephys/tpz031

Stokes, M. A., T. L. Smiley. 1968. An Introduction to Tree-Ring Dating. University of Chicago Press, Chicago, Illinois.