U of M Professor Peter Reich: How Plants Evolved to Weather Cold

The basic thrust of this project was to understand
a little bit more about how plants came to have evolved the traits they have that allow
them to withstand cold. Plants are mostly water and when it gets below 32 Fahrenheit
water freezes and if the water freezes in the cells of plants that’s not good for
the so they’ve evolved a wide variety of mechanisms to deal with that. We know quite
a bit about those mechanisms but we don’t know how did different plants – how did
maples and oaks come to have those particular traits that allow them to persist? We all, especially those of us who live in
cold climates, we take for granted that we’re surrounded by plants that actually have this
magical capacity – in my view—to be outside with no clothing at subfreezing temperatures
and they’re made out of water. How do you do that? They have very clever adaptations
first of all by freezing in the spaces between cells rather than in cells, which actually
draw water out of the cells and the cells shrink and they also become very salty so
they’re less likely to form ice within their cells, which protects them. That’s the kind
of base trait that plants are going to have to have if they are going to have tissue that
is exposed to below 0 Celsius or 32 Fahrenheit temperatures. They’ve de-evolved other traits
that are also amazing. The capacity to lose all your leaves each year and put new ones
on is a very creative way of coping with long, cold winters. If it is too expensive to carry
those leaves all winter if you can build leaves that have the appropriate characteristics
so that having a sixth month live span – it’s kind of like engineering a certain longevity
of leaves that has the proper lifespan. If it’s used up it’s usefulness you can drop
it in the fall and then you store enough energy in your stems and root system to build a new
set of leaves in the spring when conditions become approp for photosynthesis again. That’s
a very clever strategy. 
And so what we tried to do here is understand
these strategies plants for coping with the cold and then try to gain more insight into
how these strategies evolved. They didn’t just pop up one day. And whether or not these
were strategies that all kind of existed for other reasons in the tropics because the original
woody plants and flowering plants were mostly shrubs and trees in the tropics — the warm
wet tropics — and they were evergreen. So, how this came to be is an important part of
our heritage so to speak or our botanical heritage in the cold parts of the world. Then separately we’ve got data on is a plant
a woody plant or not does it have a woody stem or not. Is it a tree or flowering herb?
We also separate databases on if it’s a woody plant is it deciduous meaning like a
maple or oak that drops its leaves each year. Or is it a more like an evergreen such as
our pines or spruces, which are not flowering plants here though whereas in the tropics
where all the flowering plants are the vast majority in forest, for instance, are evergreen.
If you go down south let’s say in Georgia a live oak is an evergreen because it keeps
its leaves all year whereas our oaks drop them. So we have this big database on whether
or not trees are deciduous or evergreen and finally a smaller database – but still several
thousand species – we’ve got data on the size of pipes — the conduits inside the tree
that carry water because the size of the pipes — so to speak — influences whether or not
it gets filled with air bubbles in freeze thaw times times and then blocks flow and
creates major catastrophic dysfunction for the plant during winter. So you can either be an herb so you don’t
keep leaves, don’t keep above ground stems, you basically escape the winter above ground.
That’s one way of keeping your stems from freezing just build new ones next year. Secondly,
you can be an oak or a maple and drop your leaves but you still have the wood there that
has some water in it, so you still need to have some degree of cold tolerance in those
and having narrower pipes so to speak helps in that respect. We’re able to estimate whether or not when
plants as they first occupied cold climates whether or not they had already had those
traits that made them well adapted to the cold or did they evolve them after they arrived
in the cold. What we found that the main figures from the paper is that being deciduous plants
evolved after they arrived in the cold. They were evergreens like the live oak I mentioned.
When oaks moved further and further north after they got to the cold they evolved this
ability to lose their leaves to avoid being cold. Now you can’t take a live oak from
Georgia and plant it in Canada and expect it to survive. But Georgia freezes a little
bit so there are gradations in just how often it freezes and how badly it freezes so this
evolution happens when you’re at the margins of the cold. It’s not like a Georgia tree
jumping someway with a jay carrying the acorn thousands of miles into northern Canada where
it’s colder like it is today in Minnesota. But it’s that when you get slightly colder
places where you have more serious cold temperatures — then you have the opportunity to evolve
the deciduousness without being killed by weeks and weeks of minus 20-degree temperatures. Now when you look down the road that knows
what people will be doing in 50 to 100 years. It may be possible that in 50 to 100 years
that you can actually breed cold tolerance in a different way than we do today into certain
plants to make them more cold hardy. We already do this and have for some time. There’s
programs with apples and grapes in Minnesota that in the past had challenges with our cold
temperatures. But by understanding the evolutionary pathways and advantages of different traits,
it might be possible to think about using that information at some future date to better
improve the kind vegetation we have here. Probably much more likely to be of use in
near term — in 10 to 20 years — this kind of info can help us with models of what’s
going to have with vegetation the earth as climate changes. It’s probably going to
get warmer rather than colder but also drier. These traits and the ability to deal with
cold temperatures aren’t free. Like everything it comes at a cost. So those plants might
become increasingly at a disadvantage as it gets a little warmer so if you don’t need
to adapt to the cold why make the expensive investments of in throw away leaves every
year for instance — that deciduous trees do — if you can have leaves that last for
several years. So trying to understand the nature of these traits, how they evolved and
how they are distributed among lineages of flowering plants may help us build better
models of what’s going to happen in the future as climate changes.

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