Nitrogen is Expensive, Carbon is Cheap

Bradford pear (Pyrus calleryana) showing red color
from anthocyanin,and yellow from xanthophyll.

Every fall, trees put on a spectacular show.  As the days get shorter and the nights cooler, trees undergo a transformation.  From the maple forests of New England to the aspen groves of the American west  trees show a fantastic range of colors; red, orange, yellow, purple.

This extravagance is the basis of an important tourist industry, catering to “leaf peepers”.  Vermont’s hotels and restaurants bring in more than $100 million dollars in October much of that from people coming to see the colors. 

From spring to later summer, most trees are green.  The shade of green in trees shows a wide variation between species but all those shades are due to the photosynthetic pigment chlorophyll.  This biochemical workhorse, found in the chloroplasts of leaves, participates in food production and oxygen generation from spring to fall.  Chlorophyll is not the only photosynthetic pigment in leaves.  Carotene and xanthophyll are present in the chloroplasts too and work in concert with chlorophyll to carry out photosynthesis.  Carotene is orange (it gives carrots their color) and xanthophyll is yellow.  Plants have another type of pigment called anthocyanin that comes in many colors including red and purple.  Anthocyanin is not involved in photosynthesis but protects plants from environmental stress.  In summer carotene, xanthophyll and anthocyanin are invisible because there is so much chlorophyll in the leaves. 

Leaves of sugar maple (Acer saccharum) with a
range of fall colors 

The reason we have fall color is chlorophyll contains nitrogen.  Nitrogen is usually in short supply for plants.  They have to take up this scarce resource from the soil, sometimes with the aid of symbiotic fungi associated with plant roots.  Plants then transport the nitrogen up to the leaves where it is used to make chlorophyll. 

Deep red anthocyanin in the leaves of
Euonymus americana

                                                                                      Carotene, xanthophyll and anthocyanin are made only of carbon, hydrogen and oxygen.  These elements are available in abundance for the plants coming from CO₂ in the air and H₂O taken up by the roots.  For plants, nitrogen is expensive and carbon is cheap.  In the fall, trees do not want to waste the expensive nitrogen in chlorophyll by dropping it with the leaves, so the chlorophyll is broken down and the nitrogen stored in twigs for use in next year’s chloroplasts.  As chlorophyll disappears from leaves carotene, xanthophyll and anthocyanin can shine through.  These pigments are not recycled because they are made of cheap resources and the trees drop them to the ground with the leaves. 

As we move from the bright green of summer to the brown of winter let’s thank cheap carbon and expensive nitrogen for the dazzling fall show.