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Why do pine trees stay green?

Asked by: Laura Robertson
School: 0wego-Apalachin Middle School
Grade: 5
Hobbies/Interests: Reading, writing, acting and animals 
Career Interest: Actress, priest, dairy farmer

Answer from Stanley N. Salthe

Visiting Scientist

Research Area:
Natural philosophy
Ecology, evolutionary biology, semiotics, systems science, and thermodynamics.  Woodland gardening, nature walks, all of the arts
Columbia University
wife Barbara, two children Becky and Eric

Pine trees are green because, like all plants, they contain chlorophyll, a green chemical that allows them to capture some of the energy in sunshine. This energy is used to make sugars allowing for growth, blossoming and making fruits.

All kinds of plants in most parts of the world stay green all the time.  Many plants that live in places where it gets cold drop their leaves as the cold season approaches.  But not the pines, or their relatives the spruces, firs, cedars and yews (conifers) that also live in these colder regions.  Because these plants stay green, they can carry on some of their chemical activities in the warmer time of day on sunnier days.

So, then, why do conifers stay green?  Well, why not?  Actually, the question that scientists would ask is not ‘why’, but ‘how’.  Why questions have no definite answer because different viewpoints would give different answers.  For example, a plant might like to ‘say’ “I’m green so that I can make sugar”, or, “I drop my leaves because I can’t make sugar when it’s very cold.”

How, then, are these evergreen conifer plants able to keep their leaves (called needles) all during the winter?  The answer involves water, where all the chemistry takes place in leaves and needles, where water is the basis of the cell sap. Water gets to be in short supply in winter because it freezes.  Conifer needles have a thick waxy coat, which helps to prevent water loss. Conifer needles, like all leaves, have holes in them that allow the intake of carbon dioxide, which is used in making sugar. When it is very dry or very cold these stay closed, preventing water loss. Still, water gets to be in short supply because it freezes in the ground and in the tubes that carry it to the needles. This causes the concentration of chemicals in the needles to increase, lowering the freezing point of the sap -- creating an antifreeze solution.  A winter hardening process may also produce special antifreeze molecules.  So that’s how pines and all conifers stay green in winter.

Last Updated: 9/18/13