Thursday, November 14, 2013

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.


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.


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.

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. 

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.

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