Why evergreens have needles
Q: Why do evergreen trees, such as pine trees, have needles, while other
trees, such
as maples, have flat leaves, which are shed in the fall?
Douglas fir needles. Photo courtesy of Susan Sweeny and
Wikipedia.
A: Evergreen trees have needles, instead of flat leaves, to survive winter
hardship. Needles cut evaporation so trees can save water — dear in the winter.
The narrow leaves manage this with three adaptations:
- thicker skin, to retain water
- a thicker coating of water-proof wax
- simpler needle-like shape. The long, slender shape reduces leaf area,
which, in turn, reduces the amount of water vapor escaping the leaf.
Trees grow wherever the rainfall exceeds water loss due to evaporation during
the growing season and temperatures are above 50 degrees Fahrenheit much of the
time. About 20 inches of annual rain does the trick although trees can get by
with less rainfall where it is cooler and evaporation rates are less. Evergreens
can cope with much less water than broadleaf trees. In the mountains, you can
identify zones where the different trees can live.
Consider the Sandia Mountains in New Mexico. At 10,000 feet, the rainfall is
about 30 inches and drops to less than 15 inches at the 14,000-foot summits.
Let's wander up the slopes and look at trees. We'll start off about 8000 feet
in the Transitional Life Zone. Pines, firs, oaks, cottonwoods, birches, and
maples abound. Climbing into the Canadian Zone around 10,000 feet, we can still
find maple, alder, mountain ash and the like but also dense stands of spruce,
firs, and pines.
We clamber into the Hudsonian Zone around 12,000 feet. Brrrr! It's cold. We
see small deformed Engelmann spruce and stunted alpine fir. An occasional
foxtail pine and alpine larch. No broadleaves. Not enough water. They can no
longer survive; only hardy conifers with narrow needle-like leaves can make it.
Scrambling another thousand feet higher into the Alpine Zone, we survey land
as bleak as the arctic barren grounds. No trees at all. Not enough water even
for the evergreens.
That's why pine needles are narrow: to conserve meager water found up high or
far north. By the way, the evergreens are the earliest seed plants to survive
until modern times — virtually unchanged for more than 300 million years.
Further Surfing:
Evergreen needles don't last forever by Rosie Lerner, Purdue University
Extension
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