Karen Snyder, age 13, of Williamsport, Pa., for her question:
WHY DON'T PINE TREES LOSE THEIR LEAVES?
Right now the northern woods are knee deep in crunchy snow. It drifts around the trunks of stately pines and sometimes piles heavy white cushions on their boughs. Sparkling icicles often hang from the tips of their furry greenery. Yet somehow the sturdy pines manage to keep their leafy needles through all the hardships of winter.
Every part of a handsome pine tree is designed to cope with wintry weather. Its protections include waterproof waxes and gummy resins, plus specially shaped leaves and branches. These same protections also help to withstand droughts and summer's heat, serious wounds and greedy insects. Surely the evergreen conifer is a tree for all seasons.
Maples and other deciduous trees must shed their leaves in the fall because they are not designed to cope with the winter. Their wide, papery leaves have thin skins and numerous pores through which gallons of moisture evaporate into the summer air. In winter, this moisture cannot be replaced from the frozen ground. Frost would shatter their fragile cell walls with tiny daggers of ice. Back breaking snows would pile up and break off their wide spreading arms.
The pine tree and other evergreen conifers are built to cope with these problems. Their skinny needles have thick skins and waxy coats to seal out the frost. These smaller leaves have a smaller surface and fewer pores, so as to retard evaporation. All the conifer cells are re enforced with tangy resins. These tacky gums ooze out to seal wounds from broken branches and also discourage attacking insects.
The stately tree tapers to the top and fans out toward the ground. Its springy boughs slope downward. Blizzards may load them with piles of snow. But the heavy loads soon slide down the slopes without breaking the branches.
Come summer, the same skinny pine needles are less likely to wilt during a hot dry spell. So the all season pine tree can keep its furry greenery throughout the year.
We tend to think of evergreens as the piney conifers of our northern forest. But many evergreens also live in the tropics. Usually, they have large wide leaves with thick skins and waxy surfaces that retard evaporation. A few of these tropical leaves are shed every year. But a spiky little pine needle may stay on its tree through six or seven years.