My world has no axial tilt. Therefore instead of having seasons from axial tilt it gets its seasons from actually moving farther away from its parent star. In the mid latitudes the temperature fluctuates from a balmy summer in the mid-high 70s (Fahrenheit of course) to a brutal -20 or -50 degree winter in the worst areas (average temperature). My question is what kind of mechanisms would plants, namely trees, develop to cope with this extreme swing? A full day is fairly normal at 25 hours.
Truly extreme cold resistant flora exists on earth.
Have a look over at the article written here:
This is a study compiling data from experiments to discern the low temperature tolerance of woody tree species. It was found that a significant amount of cold-tolerant species can survive even liquid nitrogen (at -196 °C / -321 °F): Their seeds or bark was found to be capable of reproducing after being slowly cooled to freezer temperatures, then quenched in liquid nitrogen.
This isn't too surprising. Temperatures in the siberian taiga can drop down to -70 °C / -94 °F with -50°C / -58 °F being average for the coldest months (climate change has upped this to -40 to -45°C / -40 to -50°F in modern times), and the difference between that and -196°C / -321 °F, to a living organism, is not much.
In other words, plants can potentially have absolute cold tolerance. I.e. only absolute zero is enough to guarantee that you can wipe out a plant within a short amount of time.
Cold, in general, has two main deleterious effects on plants.
- Freezing of the liquid contained within the cells.
- Dehydration as water is extracted from living plant cells into surrounding ice.
How does a plant survive winter?
Plants can combat the effects of cold thorugh a variety of ways:
- Changing the chemical composition of the liquid within their cells1
- Reducing the permeability of cell membranes to prevent/reduce dehydration.
- Burning sugars to remain warm.
- Having a cell or root structure that is more isolated form the environment.
- Allow part of the plant to die to reduce its surface area.
1: Sugars and alcohols can have lower freeze temperatures of cellular lipid (fats and oils) by up to 57°C / 103 °F , see https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1300786/, and thus reduce the energy a plant needs to expend. The rate of osmosis can be reduced this way too.
Plants on alien worlds can have additional adaptations. For example, you could have a different atmosphere. Or the plants may have deep roots with access to abundant geothermal energy.
Not all species have all cold adaptations. There's some cost associated with them. There appears to be significant groups of plants that die off after temperatures go below -40°C to -50°C / -40 to -58 °F, and another big group that tolerates down to about -70°C to-80°C or -94 to -112 °F. (Coinciding with typical minimums possible in Northern hemisphere boreal climates with and without oceanic influence)
How does a plant then grow?
If a climate has monthly temperatures below -3 °C / 25 °F year-round, there's not enough liquid water, hence no plants. The local surroundings are either a glacier, or (when very dry) a rocky wasteland.
In order for plants to grow, they mainly need three things. Liquid water, carbon, and sunlight. Then, they need enough of this during the short summer to not lose more in the long winter (I.e. to produce enough sugar to last through winter and have a surplus in an average year). Liquid water forms at temperatures above 0 °C / 32 °F, the higher above zero, the more quickly snow will melt.
In these extreme climates, average (yearly) temperatures are sometimes far below 0 °C / 32 °F. Underground temperatures (isolated by the earth) at low depth are usually equal to the average yearly temperature, excepting volcanic hotspots. This creates a problem: the ground is frozen and hard for roots to permeate or extract nutrients from. Thus don't expect to see the largest and tallest trees when there is permafrost. The size of root systems is restricted to how much of the permafrost melts each year (I.e. the amount of degree-days above 0 °C / 32 °F is correlated with how tall trees can get).
Look up https://en.wikipedia.org/wiki/Trewartha_climate_classification, especially the sections on group E and group F.
We can see how much trees need by observing where the Tree-Line is on tall mountains and in the arctic north, the separation between a boreal and alpine or tundra climate. This is the limit at which the above condition is met for a plant that actually survives through the winter itself (a perennial plant). A quick rule of thumb for an earth-like climate is that you need about 50 out of 360 days with average temperatures above 0 °C / 32 °F during which the top layer of permafrost melts, or about a 10 °C / 50 °F maximum in the warmest month. This is what common climate classifications are based on. They're fairly accurate, though exceptions may exist due to specific local conditions.
There's also a second category, plants that die off, but their seeds survive through winter. This strategy is even hardier than the above (as a plant can invest a large amount of energy in keeping a very small amount of cells alive), and is the only way plants survive in the Tundra climate. A quick rule of thumb for this climate is that you need about 2 weeks of frost-free days (0 °C or above average daytime temperature).
On the border between these two climates, you'll find Krummholz, where some local spots (like rock formations or valleys) provide some cover against freezing wind, but the landscape as a whole is in the Tundra category.
Length and Depth of winter
These figures are written with respect to Earth. On your alien world, if the year lasts longer (typically, your year will be longer if your orbit is more eiliptical, unless you make your star lighter than the sun is to compensate), then you might need a bit more time above 10 °C / 50°F before you hit the Ec Trewartha climate (read, the ability to have Trees). Similarly, if winters are even colder than in siberia, you might need a bit more energy during summer to compensate (say, 40 days above 10 °C / 50°F instead of 30 days). These rules of thumb work for Earth, the more alien your custom planet, the more you may need to adjust them.
Thermogenic plants generate metabolic heat. These are plants on our world!
On your world, the deep roots of your trees make heat - enough to prevent themselves from freezing. Insulated underground, the heat does not escape. The roots are big with mighty sugar reserves. Thus they overwinter. When things warm up, the roots and cells wintering there flow back up and reconstitute the tree above.
That deep zone in the forest is considerably warmer than the surface, with all the root warmth. The animal life takes advantage of this, hunkering down deep for their hibernations where the trees will keep them warm.
You are describing most of the Boreal forest that covers the northern hemisphere. Look up temperatures for Cities in the region. Yellow Knife has recorded extreme low -50C/-60F recorded high +35C/80F. Yellow Knife is below the tree line, further north can get colder.
Look into Boreal, alpine, and tundra biomes.