Trees: How Do They Work?
November 25, 2014 12:19 PM   Subscribe

Tell me about the energy dynamics of non-deciduous alpine trees. How do saplings stay alive when continuously covered by snow all winter?

I have an advanced degree in the life sciences but have never taken a botany class. My knowledge of plant physiology is basically limited to the steps of photosynthesis, the composition of a chloroplast, that xylem and phloem are a thing, and what little I've learned from trying not to kill succulents indoors. But I've been skiing up here in Colorado and looking at the conifers (subalpine firs, I think) up above 9-10,000 feet. Already this season the smaller ones are starting to be covered with snow, and in many places I assume trees under a few feet tall will be completely submerged from now until March or April.

I want to know about how that works, so I guess I want to know about how trees work! What are the energetics of this, on a single-organism level? How dependent are conifers on photosynthesis during the winter? Could they live in the dark for months? What's the 101 on tree metabolism, anyhow? I feel shockingly ignorant and I'm not sure how to start finding answers to my questions.
posted by deludingmyself to Science & Nature (8 answers total) 12 users marked this as a favorite

Plants have genes that code for proteins — basically, little machines — that protect them from extremes of temperatures, among other environmental stresses.

Heat shock proteins get made when the temperature goes above what the plant can normally tolerate. Heat is bad, because it changes the chemistry of how the usual operating set of proteins fold and behave, like slowly scrambling an egg. The heat shock proteins help these other proteins to fold normally, so that the cell can continue to live.

In the other direction of extreme cold, there are proteins that help lower the freezing temperature of water inside the cell, usually by binding to water molecules. This inhibits ice crystal formation, which like a knife can cut and kill the plant cell from within. Also, ice is less dense than liquid water and takes up greater volume, which can cause the cell to effectively explode, like putting a soda bottle in the freezer.

Other cold shock proteins make changes to cell membranes, the boundary that protects a cell from the outside world, become less permeable to the environment outside — like shutting a barn door to keep the draft out, but on a microscopic scale.
posted by a lungful of dragon at 12:53 PM on November 25, 2014 [1 favorite]

Best answer: Great question! This is right in my research wheelhouse - I study tree seedling microclimate and forest ecology. Snow actually insulates the seedlings to a large extent, protecting them from bright radiation (which can cause chronic photoinhibition), harsh drying winds that could otherwise dry out the soil and thus jeopardize the tree's turgor pressure, and frigid minimum temperatures which can also cause freeze-induced cavitation in really poor conditions. Conifers are generally photosynthetically inactive over the winter and draw energy from their roots (energy sinks become sources and vice versa) but some species can take advantage of scattered nice days and photosynthesize a little - but be aware that this varies a lot from species to species, so just looking at some random tree species isn't going to help a lot.

For a start, this link has a good simple rundown of the many adaptations conifers have to winter conditions, including some detail about their tracheid development and effects on water relations. More squarely answering your question, this paper has a really good overview of the photosynthetic behavior of overwintering conifers (and it also has a great list of further references for low-temperature effects on photosynthesis at the end of the introduction). This paper has a nice summary of how subalpine firs transition from winter dormancy to the growing season, which might help to understand the metabolic shifts trees undergo from season to season.

Basically, trees don't photosynthesize much over the winter, but they do have to find a way to dissipate all that extra radiation energy they aren't using to prevent injury to its photosynthetic apparatus. Photosystem II is largely responsible for the dissipation of that energy in a process called photoinhibition. Often this is an adaptive process, but if it goes on too long, PSII gets damaged.
posted by dialetheia at 1:01 PM on November 25, 2014 [16 favorites]

Response by poster: some species can take advantage of scattered nice days and photosynthesize a little - but be aware that this varies a lot from species to species

We have so many full and partial sun days here in the winter, even in the mountains. It's great for skiing, and (I assumed) for trees that stick out enough to catch those rays. If I understand right, you're saying photosynthesis activity is really rate-limited by temperature in addition to sunlight? I don't know why I'd never considered that.

Really digging these responses, keep 'em coming.
posted by deludingmyself at 1:43 PM on November 25, 2014

The summer/winter metabolic rate differential is the mechanism that results in tree rings.
posted by HiroProtagonist at 3:48 PM on November 25, 2014 [1 favorite]

Best answer: If I understand right, you're saying photosynthesis activity is really rate-limited by temperature in addition to sunlight?

Exactly. Photosynthesis rates can be directly limited by light, temperature, and CO2 concentrations, and indirectly limited by water availability (especially for C3 plants, which have to close their stomata more to save water in dry conditions). This link has great simple figures showing photosynthesis rate as a function of light, temperature, and CO2. Once the plant has enough light to reach saturation, CO2 becomes limiting. Light and carbon dioxide are limiting because they are the "ingredients" to photosynthesis - but you still need the RuBisCO enzyme (the most abundant protein on Earth!) to fix carbon with that light energy. Like any other enzyme, it only performs well under a certain range of optimum temperatures (limited by low activity at the low end and enzyme denaturation at the high end), so the rate at which those "ingredients" can be processed is ultimately temperature-limited.
posted by dialetheia at 4:35 PM on November 25, 2014 [2 favorites]

Response by poster: Oh yeah, C3 and C4 plants and RuBisCO! That rings a bell from high school biology, but I still think we've just doubled my awareness of plant related science basics in this thread.

Follow-up question for dialetheia or others: are there any good books that go into plant biology with decent treatment of things happening at a molecular level? I don't want pages and pages of derivations of Michaelis Menten kinetics for plant enzymes, but something more rigorous than those figures you linked would be nice. I'm limited to open access journal articles for the next 10 months, but if there's a good review-based publication (Expert Reviews doesn't seem to have a Botany version, heh), I might be interested in seeking that out, too.
posted by deludingmyself at 5:01 PM on November 25, 2014

keep in mind that over half the tree is underground and the soil is full of bacteria and fungi that keep cranking out heat all winter (breaking down leaf litter and bug poop and what-not) and all this activity is basically insulated from the cold by the blanket of snow (like eskimos in an igloo stay warm even though their house is made of ice.)
posted by sexyrobot at 10:52 PM on November 25, 2014 [1 favorite]

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