Saturday, October 17, 2015

Everlasting Pine Needles

My wife tells stories of her Catholic education.  One that I enjoy is the high school science teacher who insisted that pine trees don't drop their needles.  Asked by observant girls how the pine needles they found on the ground got there, the nun opined that squirrels nibbled them off.

White pines of my acquaintance beg to differ.

Needles turning yellow alter the appearance of the whole tree.

I had been taught that pine needles lasted two seasons, so that each fall the previous year's needles fell,* leaving the present year's still on the tree.  That seems true of the twig above.

But the reality can be more complicated.  I have seen needles yellow and fall in spring as well.  Are these needles that lived through a second winter? or are they last year's needles falling prematurely?  I think it's the first, but I didn't mention it in my journal last spring.  I'll have to watch closely over the coming months.

I think I have also noticed these out-of-season needles yellowing and falling individually--sometimes leaving others in the same cluster alive.

Whatever the case, the needles fall.  And from a big tree, the result is drifts big enough to be obvious even to the most obtuse.  

But how are any pine needles retained through the harsh winter months, when broad-leaved trees in this climate nearly all lose theirs?  Cold is only one of the stresses, and not the chief.  Carrying leaves over the winter means losing water when liquid water is scarce.  I also suspect that carrying broad leaves risks wind damage at a season winds are strongest.  With days so short and sun angles so low, there simply isn't enough benefit from photosynthesis to be worth the cost.  

Pines, though, have needles that are adapted to drought.  Needles are thickly coated in water-proof wax.  Their stomata (pores) can be closed more tightly to limit water loss.  Also, the stiff needles don't depend on internal water pressure (as most broad leaves do)* to maintain their shape.  And needles present less resistance to wind than broad leaves.  These adaptations, plus the ability to do photosynthesis year-round whenever conditions permit, plus the huge benefit of not having to grow a full set of leaves every spring, all tip the balance in favor of pines being evergreen.

Here is a concise but wide-ranging discussion of winter adaptations of trees.

*Actually, autumn leave don't just die and fall off, they are actively shut down, some of their most valuable nutrients recycled, and then they are "deliberately" cut loose.
**Wilt is the condition of having too little internal water pressure to maintain shape; a leaf is like a football: nearly rigid when fully inflated, very floppy when not.

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