What would you do if you were a tree?

What's wrong with this picture? (Laramie, Wyoming; click on image to view).

Like most plants, trees are powered by the sun.  Leaves are their solar collectors, and inside them are little powerhouses and factories (chloroplasts) where sunshine, carbon dioxide and nutrients are made into carbohydrates for life and growth.  Obviously leaves are hugely important to a tree, but they aren’t always a good thing.  In fact where winters are cold, they're a liability.  They could freeze, or trap snow and break branches.  So what to do?

Deciduous trees toss their leaves each fall but then must produce a new set in the spring.  This seems wasteful and costly.

Every fall, quaking aspen (Populus tremuloides) kills it leaves by starving them, a pretty but brutal process.

Evergreens keep their leaves all winter.  They’re ready to go first thing in the spring, but isn’t this a bit risky where winters are long, cold and snowy?

Western red cedar, an evergreen conifer, keeps its beautiful needles year round.

What would you do if you were a tree?  Here are a couple of tools to help you decide, produced by botanists in collaboration with Mother Nature.  First, a table of terms and definitions:

*Gymnosperms and angiosperms are two evolutionary branches within seed-producing plants (spermatophyes).

Next, a map showing forest types of the world (source).  Trees can help us pick a strategy because there’s a pattern to their distribution ... a gross over-simplification of course, but close enough to reality to let us ponder why different types of trees grow where they do (“needleleaf” is equivalent to “conifer”).

In a sense, the ideal place to live is where it never freezes and there’s always adequate moisture, for example the tropics.  In these places a tree can grow year-round, so it’s not surprising that most tropical trees are evergreen.  Furthermore, they’re broadleaved.  It’s worth investing in large thin leaves that maximize sunlight and carbon dioxide uptake.  Highly-productive leaves mean faster growth to compete for sunshine.  Evergreen leaves do die and fall off but not all at once.  There’s always plenty to feed the tree.

Left, Shorea sp., a canopy species of tropical forests in Borneo, from Landmark Trees.

Of course evergreen broad leaves would be a poor choice where the climate is seasonal.  They aren’t hardy enough.  Deciduous is a better strategy -- drought-deciduous in deserts, cold-deciduous where winters are harsh.

The cold-deciduous weeping birch (Betula pendula, right) produces a new set of thin highly-productive leaves for the growing season, tosses them in the fall, then goes dormant and rests over the winter.

Limber pine (Pinus flexilis) in the Laramie Mountains, Wyoming.

Where growing seasons even shorter and winters are long and cold, evergreens again dominate but these are conifers, with needles or scales instead of broad leaves. They’re smaller, hardier, more frost resistant, but not nearly as productive. Still, being evergreen they’re a better choice overall. There’s no need to grow new leaves every spring. A tree can get on with the business of photosynthesis right away.

Juniper scales and berry-like seed cones (source).

Then at the very northern limits of forests we find ... a big surprise! Mother Nature has thrown a twist into our nice sensible pattern of forests types. Often it’s deciduous trees that dominate the northernmost forests (note yellow and orange patches in the far north in map above). In Asia, these are mainly deciduous conifers (larches). In North America, they’re more often broadleaved species -- poplars, willows and birches.

Dahurian larch, Larix gmelinii, makes up extensive forests in eastern Siberia (source).  UPDATE (source):  Dahurian larch grows further north than any other tree, and probably is the most abundant conifer on earth.

What kinds of adaptations allow trees to live where growing seasons are so short, and winters so frigid? Some can export water from cells into inter-cellular spaces, where it then freezes. Once in this state, tissues can survive temperatures at least as low as -196º C! (Thomas 2000).

So evergreen or deciduous, broad leaves or needles ... which would you choose? I would be an evergreen conifer so I wouldn’t have to go do the intensive-growth-thing. Summers are too short ... I want as much leisure time as possible!

Tree impersonation, a popular plant-geek pastime.

This is the final post in a short series about autumn tree strategies.


Sources (in addition to links in post)

Havranek, WM and Tranquillini, W. 1995. Physiological processes during winter dormancy. in Smith, WK and Hinckley, TM, eds. Ecophysiology of coniferous forests. Academic Press.

Thomas, P. 2000. Trees: their natural history. Cambridge University Press.

Autumn Tree Strategies II

Note leaning limber pine emerging from large fissure in granite outcrop, near center of lower left quarter of photo (click on image for better view, or see photo at end of post).

I’m following two trees.  One is a weeping birch that grows in my yard, subject of a recent post.  All summer long its leaves collected sunshine and carbon dioxide, cranking out carbohydrates to keep the birch healthy and growing.  Then about a month ago, after turning a pretty gold, they died and fell off.  Just as well ... thin flimsy birch leaves would freeze in winter.  Now the tree is dormant.

Weeping birch, Betula pendula, native to Europe and Asia.

My other tree is a limber pine (Pinus flexilis) in the Laramie Mountains east of town.  It grows out of a deep fissure in a large outcrop of 1.4-billion-year-old Sherman granite.  I went up there just last week to see what it’s doing.  These are the perks of tree-following, going to nice places and seeing and learning things I probably wouldn’t otherwise.

All the quaking aspen along the trail were bare.  They got rid of their worthless leaves over a month ago.

Below, approaching the limber pine.

This tree appeals to me because of where it grows, and because it’s big, rugged, gnarled and scarred.  Unfortunately, the location makes it difficult to photograph.  The photo below was shot from the best place to see most of it.  I’m always impressed by the large half-dead tree growing out of the rocks.

The tree got its start in a fissure 4-5 feet deep.  How did it survive down there in the shade for so long?  Did it take off once it was tall enough to bask in sunshine?

Some of the larger branches are dead and others are missing bark, but no bother.  The pine keeps growing, a little bit each year.

Below is the tree from above.  I walked along the top of a large granite fin, peeking over the edge until I saw it.

It's November, so what has the limber pine been doing to get ready for winter?  Not much it appears.  The long flexible branchlets still have green needles and in fact they will all winter, for limber pines are evergreen.

This is one of several common but confusing tree terms:

Evergreen -- with leaves year-round; most conifers and many broadleaved trees.

Deciduous -- leafless part of the year when conditions are inhospitable to growth; a few conifers and many broadleaved trees.

Conifers -- Gymnosperms, with naked exposed seeds frequently in cones (no flowers); e.g. pine, spruce, Douglas fir, redwood.  Leaves usually needle-shaped or scale-like, but sometimes broad.  A more primitive group evolutionarily.  Also called “softwoods”.

Broadleaved trees -- Angiosperms, with developing seeds housed inside ovaries in flowers; e.g. magnolia, maple, eucalyptus, beech, weeping birch.  Leaves usually broader than needles but not always.  A more advanced group evolutionarily.  Also called “hardwoods”.

Below, a compact and durable conifer needle, followed by part of a thin flimsy but more productive broad leaf.  Source (modified).  Photosynthesis takes place in the mesophyll.

Fortunately the limber pine has needles instead of broad leaves, a much better choice for winter.  They shed snow more readily and have less sap that could freeze.  They lose less water (big issue in winter) because the ratio of surface area to volume is smaller.  Their thick epidermis covered with a waxy cuticle also protects from desiccation.  These are all very good adaptations but ... they aren’t enough for frigid winters in the mountains.

Limber pine needles are five to a cluster.

Turns out the limber pine has been getting ready for winter, for some time now.  It went through a pre-dormancy stage as it does every year.  Resting buds were formed, growth stopped, metabolism slowed, and various chemical changes took place -- for example increasing salts and sugars in cells to lower the freezing temperature.  Now it can rest through the winter.

Dormancy ends in spring after a sufficiently long period of sufficiently cold weather.  It’s then that the benefit of keeping needles all winter becomes apparent.  The limber pine won’t have to grow a whole new set of leaves, it can get to work photosynthesizing and making tree food right away!

Some conifers even photosynthesize at low rates during warm spells in winter, another advantage of keeping needles on.  Can a limber pine do this?  I don’t know.

I'm following a tree ... are you?

This is the second post in a short series about autumn tree strategies.

Sources  (in addition to links in post)

Havranek, WM and Tranquillini, W.  1995.  Physiological processes during winter dormancy in Smith, WK and Hinckley, TM, eds.  Ecophysiology of coniferous forests.  Academic Press.

Thomas, P.  2000.  Trees:  their natural history.  Cambridge University Press.

Autumn Tree Strategies, I

Lucy of Loose and Leafy has returned to blogging after a summer sabbatical, reminding me that I haven’t posted any news of my two trees this year.  Lucy kindly hosts and encourages a loose-knit group of tree-followers.  She explains, “I've been observing several trees over the last few years, following their progress through the seasons ... an affection grows, an awareness of small things - the presence of lichen, the sudden absence of familiar twigs and branches.”

I like following trees for the same reasons.  I look more closely, point my camera, always learn something.  I joined the group a year ago October, when I introduced a weeping birch (Betula pendula) in my yard.  It’s often very photogenic but that was not its finest moment.

Last October the birch's leaves were few and ragged.

A month later I decided to follow a second tree, a mighty limber pine (Pinus flexilis) growing among huge granite outcrops in the Laramie Mountains.   Its size and rugged form are impressive.

Looks like this limber pine has been through a lot, given all its scars and dead branches.

What have these trees been doing since then?  Let’s start with the birch.

Winter

All winter the birch stood dormant, resting and waiting as it always does.  The branchlets were covered with buds grown last summer, each containing tiny shoots and leaves ready to expand when conditions were right.

The birch's white peeling bark is attractive even in winter.  Note the prominent lenticels, pores for gas exchange.  All trees have them but usually they're not so showy.

Spring

By mid-May, young leaves had emerged from the buds and were expanding, ready to go to work.

What kind of work do leaves do?  They collect sunshine and carbon dioxide, and make carbohydrates for the growing tree.

Leaves are green not for beauty but to capture sunshine via chlorophyll, a green pigment.

Solar-powered tree ramping up production.

Summer

Weeping birch is native to Europe and Asia, and is popular in the USA for landscaping.  It’s very attractive with its white bark and elegant serrated leaves on long drooping branchlets.  Unfortunately this makes it hard to photograph here in windy Wyoming.  Too often it looks like this:

Otto Wilhelm Thomé’s illustration below shows what’s on the flying windblown branchlets:  serrated leaves and young female catkin (flower cluster) top center; male catkin lower right; tiny female flowers with reddish stigmas lower left; male flower with yellow anthers to right of female flowers.  Click on illustration to view details.

Flora von Deutschland, Österreich und der Schweiz, 1885. 

Autumn

Birch prepares to rid itself of potentially-dangerous leaves.

Deciduous trees are famous for their gorgeous leaf colors in fall, but there’s a sordid tale behind the beauty.  In winter a dormant tree won’t need leaves so resources shouldn’t be wasted on them.  Furthermore they’re a liability.  They could trap heavy wet snow and cause branches to break.  So it’s best to get rid of the leaves ... by starving them until they die and fall off!

When nights get sufficiently short, a change comes about in specialized cells at the base of leaf petioles (leaf stems).  Cells in the abscission layer start to thicken and block the flow of nutrients to the leaf.  Chlorophyll can’t be replenished and the green color fades, revealing yellow and orange pigments that have been there all along but obscured by showy chlorophyll.

Green chlorophyll beaks down, revealing yellow xanthophylls and orange carotenoids.

Red fall leaves are puzzling (quaking aspen).

Some fall leaves are red, due to production of anthocyanins.  Why a tree preparing for dormancy would convert valuable carbohydrates to showy red pigments is not clear.  For more, see the US National Arboretum’s excellent webpage on what we know and guess about fall colors:  Science of Color in Autumn Leaves.

Eventually the yellow and orange pigments fade as well, leaving birch leaves in various shades of dull brown.  By now the thickened cells of the abscission layer have lost their cohesiveness.  The leaves break off in wind, rain and snow.  Fortunately there’s also a protective layer in the abscission zone that has developed into a seal against invasion of pests or disease (see diagram above).

By mid-November the leaves are dead, waiting to fall to the ground or blow away.

Dead birch leaf but also buds -- promise of another spring.

Meanwhile, up in the Laramie Mountains where winter will be even colder and snowier than here in town, the mighty limber pine appears unconcerned.  What's its strategy? We'll look into that in the next post.

I'm following a tree ... are you?

This is the first post in a short series about autumn tree strategies.