By Michael Snyder
posted
Jan 3, 2013
Illustration by Adelaide Tyrol
Trees are about half water, maybe a little less in winter. And
if the temperature drops low enough, the water in even the most
cold-hardy tree will freeze.
So how do trees survive below-freezing temperatures? They can't
move south or generate heat like a mammal. Sure, the below-ground
parts of a tree are kept insulated by a layer of snow, and that is
important to winter survival, but the exposed parts of a tree are
not so protected.
To survive winter cold, a tree begins its preparations in late
summer as day length shortens. Cold acclimation occurs gradually
and includes a number of physiological changes in leaves, stems,
and roots. And while fall color seems to get all the attention,
it's what trees do later in autumn that is the most stunning, if
harder to see.
Some of these later changes really do seem to border on magic,
and while some of the details remain a mystery to science, general
mechanisms have been explained.
Paul Schaberg, a research plant physiologist with the USDA
Forest Service's Aiken Forestry Sciences Laboratory in Burlington,
Vt., has led many investigations of cold tolerance in trees,
particularly in the foliage of montane spruce and fir in New
England.
Schaberg's work suggests three basic ways in which living tree
cells prevent freezing. One is to change their membranes during
cold acclimation so that the membranes become more pliable; this
allows water to migrate out of the cells and into the spaces
between the cells. The relocated water exerts pressure against the
cell walls, but this pressure is offset as cells shrink and occupy
less space.
The second way a tree staves off freezing is to sweeten the
fluids within the living cells. Come autumn, a tree converts starch
to sugars, which act as something of an antifreeze. The cellular
fluid within the living cells becomes concentrated with these
natural sugars, which lowers the freezing point inside the cells,
while the sugar-free water between the cells is allowed to freeze.
Because the cell membranes are more pliable in winter, they're
squeezed but not punctured by the expanding ice crystals.
The third coping mechanism is altogether different. It involves
what Schaberg describes as a "glass phase," where the liquid cell
contents become so viscous that they appear to be solid, a kind of
"molecular suspended animation" that mimics the way silica remains
liquid as it is supercooled into glass. This third mechanism is
triggered by the progressive cellular dehydration that results from
the first two mechanisms and allows the supercooled contents of the
tree's cells to avoid crystallizing.
All three cellular mechanisms are intended to keep living cells
from freezing. That's the key for the tree; don't allow living
cells to freeze.
A tree doesn't have to keep all of its cells from freezing, just
the living ones. This is significant, since much of a tree's living
trunk is made up of cells that are dead (though it's strange to
think of these cells as dead, because they're still involved in
functions, such as sap flow, that keep the tree alive). Dead cells
can and do freeze; even the lowest temperature can't kill an
already dead cell! And that's the magic: while the overwhelming
majority of a tree's above-ground cells do indeed freeze regularly
when exposed to subfreezing temperatures, the small percentage of
living ones don't. There are living cells in the trunk that remain
unfrozen even though they are right next to - and at the same
temperature as - dead cells that are frozen solid.
This cellular magic of pliable membranes, sweet antifreeze, and
glasslike supercooling, with frost on the outside and viscous
dehydration on the inside, helps trees avoid freezing injury to
living cells, but it is not without consequence. According to
Schaberg, the freezing of those dead cells does have implications
for the tree's health. For example, gas bubbles can form among them
upon thawing, and these can prevent sap flow in spring. But
Schaberg says that trees have other means to overcome those
temporary problems and that it is far better for the tree to deal
with these than to allow the water-based contents of nearby living
cells to freeze and possibly kill the tree altogether.
Michael Snyder, a forester, is Commissioner
of the Vermont Department of Forests, Parks and Recreation. The
Outside Story is assigned and edited by Northern Woodlands magazine
and sponsored by the Wellborn Ecology Fund of New Hampshire
Charitable Foundation: wellborn@nhcf.org
Tagged:
The Outside Story, trees, winter
