Why Do Pine Trees Stay Green All Year Long?
A pine tree remains green throughout the year because it doesn’t lose all its leaves at one time. Trees that remain green all year long are known as evergreens. Pines, spruces, and firs are among the most familiar evergreens. These trees have tough, needle shaped leaves that withstand frost and drying winds.
Evergreens do shed their leaves, but because the trees grow new leaves before shedding the old ones, the leaf drop usually goes unnoticed. If you walk through an evergreen forest, you will notice many dried up leaves scattered about the ground.
The biggest problem for plants in winter is desiccation or drying out. When the ground freezes, plant roots are unable to draw liquid water from the soil. When the plant becomes dehydrated, the plant cell membranes lose their shape and rupture, killing portions of the plant. Cell tissues can also die if ice crystals form inside cell membranes.
Evergreen plants have many different ways to survive the cold and dry winters. Many evergreens have tough, leathery leaves or needles that are resistant to drying out. Some plants can actively concentrate chemicals that work like antifreeze in their cell fluids. For other plants, simply being buried in a snowbank can help reduce water loss.
Snow provides moisture as well as shelter from drying winds. However, being buried under a thick layer of ice for too long may kill a plant because toxic concentrations of compounds like carbon dioxide and ethanol build up in the closed air space beneath the ice.