Understanding the Winter Hardiness

[Ayxowpat]

Hello everyone,

I am trying to understand the physiology of trees' metabolism during winter. Do they store their sugars as starches in Fall and get into the dormancy at some certain point, and stay fully inactive? Or do they keep increasing their sugar storages as winter goes?

I think they increase their winter hardiness as it gets cold. So that, long and cold winters (not as cold as freaking cold ) make the trees get stronger and they grow better in following spring. This also explains that hard pruning or repotting made in Fall makes the tree vulnerable to harsh winter conditions.

If you suggest any books or scientific papers, I would be happy to hear.

 

[Wires_Guy_wires]

In general, and this is very general, trees store their unused sugar throughout the year as starches. When dormancy hits, they enzymatically (no energy needed) reduce these starches to sugars because sugars in water can prevent ice crystals from forming. Sugars also decrease the temperature at which water freezes.
Some processes continue, which require sugars to be burned. So the overal sugar and starch content drops when moving towards spring. Most of it though, is used for the spring flush and this is why we let most weak trees grow strong for the first part of spring and summer: they need to restock their depleted sugars through photosynthesis.

Repotting and trimming trees in winter damages stuff because wounds allow rain water to both seep out and seep in. Because their metabolism is slowed, healing hardly takes place and most plants resort to compartmentalization: shut down everything damaged from a node below the wound. This stops the sugar processes and allows ice crystals to damage the interior structures. If you freeze and thaw non-flowing water with high amounts of sugar, you get a watery part and a sugary part. The watery part crystalizes, the sugary part does it too, but way slower and shows less expansion.
Sugar isn't the only solute. Glycerole, salts, sap composition (hydrocarbon content) and so on, all play a part in this.

The cooler it gets, the slower everything goes. Since most biological processes are water bound and require liquid water to complete, winter hardiness is more or less stable after temperatures drop below -5°C but can continue at a super slow rate (depending on the solutes) to -20°C after which everything stops completely. I apply a general rule that the stickier the sap, the longer they stay active.

A good read is Plant Physiology by Taiz. Should be available more or less legally, for free if you google around a little. I know there are PDF's circulating in public sources.

 

[Ayxowpat]

In general, and this is very general, trees store their unused sugar throughout the year as starches. When dormancy hits, they enzymatically (no energy needed) reduce these starches to sugars because sugars in water can prevent ice crystals from forming. Sugars also decrease the temperature at which water freezes.
Some processes continue, which require sugars to be burned. So the overal sugar and starch content drops when moving towards spring. Most of it though, is used for the spring flush and this is why we let most weak trees grow strong for the first part of spring and summer: they need to restock their depleted sugars through photosynthesis.

Repotting and trimming trees in winter damages stuff because wounds allow rain water to both seep out and seep in. Because their metabolism is slowed, healing hardly takes place and most plants resort to compartmentalization: shut down everything damaged from a node below the wound. This stops the sugar processes and allows ice crystals to damage the interior structures. If you freeze and thaw non-flowing water with high amounts of sugar, you get a watery part and a sugary part. The watery part crystalizes, the sugary part does it too, but way slower and shows less expansion.
Sugar isn't the only solute. Glycerole, salts, sap composition (hydrocarbon content) and so on, all play a part in this.

The cooler it gets, the slower everything goes. Since most biological processes are water bound and require liquid water to complete, winter hardiness is more or less stable after temperatures drop below -5°C but can continue at a super slow rate (depending on the solutes) to -20°C after which everything stops completely. I apply a general rule that the stickier the sap, the longer they stay active.

A good read is Plant Physiology by Taiz. Should be available more or less legally, for free if you google around a little. I know there are PDF's circulating in public sources.

Thank you for the detailed answer.

Can we say 'longer and cooler (not below -20°C) winter lets a stronger tree in Spring.' ?

As I understood, starches are being stored during the growing season and summer. When the tree senses the photoperiod and temperature are decreasing, it starts to prepare for dormancy. It sheds the leaves, blocks all veins they have exit to outside, turns starches into sugars in cells to lower the freezing point. As long as winter goes between -20°C and -5°C degrees, more cells are filling with sugary water, which leads stronger cells after Winter. Is this correct?

 

[Wires_Guy_wires]

As always with plants, not entirely and not definetively.
The strength of a plant relies on a lot of things. A good solid frost stops a lot of things.. Some plants can manage and gain a bit of vigor if temperatures stay stable at below freezing. Others perform better with soft frosts, and others do better with no frosts at all.
The sugars in this case act mainly as a protectant and not as a growth booster. If temperatures drop too low, too rapidly, some biochemical processes might stall until spring.

I think you could look at it as a frozen highway; in some areas (like in some plants) it doesn't matter that everything is frozen, people keep driving and resources get where they need to go. In other places, people stop driving and everyday life stalls for a while. Their survival relies on the preparation.
In other places, people move resources when they can and stop doing so when the weather gets too bad. Aaaand then there's some fur traders that don't even go out if it doesn't freeze, but they'd go hungry if they can't stockpile money to buy them food and goods in the hunting season.
Who will come out of winter the strongest?
You'd think the people that keep moving over that highway, but they crash, they spend resources, while the rest is chilling at home saving their resources.
So all in all, it depends on what these people do, what they need, what they spend and whether they crash or not.

A solid dormancy with a few warmer days to complete biochemical pathways would probably result in the best spring physical condition, but again, it relies on the plant and its origin or natural range.

 

[Igor. T. Ljubek]

Can we say 'longer and cooler (not below -20°C) winter lets a stronger tree in Spring.' ?

Do you have in mind trees or bonsai trees? Because it's a huge difference between trees and bonsai trees when we talk about winter hardiness. Frozen pots (for longer period) usually cause more harm than benefits to bonsai trees.

 

[Ayxowpat]

Do you have in mind trees or bonsai trees? Because it's a huge difference between trees and bonsai trees when we talk about winter hardiness. Frozen pots (for longer period) usually cause more harm than benefits to bonsai trees.

Trees in nature. When I understand the physiology of trees in nature, I can convert these knowledge for bonsai better. This is why I am trying to understand the mechanism.

 

[Wires_Guy_wires]

I like your way of thinking @Ayxowpat
Please note that although I posted this as an answer, it's always more complex when you dive in deeper. There will be parts I'll never understand, simply because I refuse to learn the valence of certain ions, and how carbon chemistry works exactly.

To add to that, plant science is slowly evolving. Way slower than mycology or microbiology. So when new theories are proven, they can take decades to be adopted by the general population. One of those theories is that auxin can be produced locally by almost every structure in a plant that needs it. This shifts the apolar auxin transport mechanism to.. Not being as important as it was made by thousands of plant experts, or even Darwin himself.

So do take things with a grain of salt and absolutely do make conclusions that sound logical if you find something; they might be righter than ancient hippie theory.

 

[Ayxowpat]

I like your way of thinking @Ayxowpat
Please note that although I posted this as an answer, it's always more complex when you dive in deeper. There will be parts I'll never understand, simply because I refuse to learn the valence of certain ions, and how carbon chemistry works exactly.

To add to that, plant science is slowly evolving. Way slower than mycology or microbiology. So when new theories are proven, they can take decades to be adopted by the general population. One of those theories is that auxin can be produced locally by almost every structure in a plant that needs it. This shifts the apolar auxin transport mechanism to.. Not being as important as it was made by thousands of plant experts, or even Darwin himself.

So do take things with a grain of salt and absolutely do make conclusions that sound logical if you find something; they might be righter than ancient hippie theory.

Thank you. I am trying my best to understand what has been proven so far. With that little amount of knowledge, I can link the dots and create a story to teach bonsai in a better way. I can also try new things by using this knowledge. I was focused on soil, fertilizers, roots' communication with other metabolisms, etc. That helped me a lot to understand the fertilizer regime, soil mix details, repotting and keeping a plant strong by keeping the soil microbes active. This year I am inclined to understand the dormancy and trees' attributes in seasons. I am also writing a book for newbies. I don't like to talk about some 'solid rock' knowledge that has been transferred through generations, I'd rather use scientific info which is actually solid rock (until they are proven to be wrong, however it is another good knowledge). By putting the stones one on one with a strong basis, structure becomes rigid. This approach lets me to achieve too many things in bonsai in a very short time, yet I still have millions of miles to walk.