This time of year the beautiful colors of the trees are very noticeable. Why do trees “change” their summer green leaves to the brilliant reds, yellows, purples, and oranges which we associate with autumn? The broadleaved trees of the Temperate Deciduous Biome are known as deciduous plants which mean they change seasonal. Broadleaves require a lot of water to keep the leaves growing and healthy. Therefore the sap of these trees is composed primarily of water, almost 99%. When the cooler and cold temperatures approach in fall and the sun’s angle decreases, trees prepare for the winter by moving water and valuable sugars to their roots. Water when reaching temperatures of 4 C (39 F) starts to expand. So if trees left large amounts of water in their stems and trunks they would split apart thus killing the tree. When trees move the water to the roots the leaves are cut off without a water supply. The leaves will soon die but before dying they move the valuable green chlorophyll pigment out of their leaves and into their stems and twigs.
With the green pigment gone from the leaves, the other pigments (which have always been in the leaves) now become visible. So technically the leaves are not changing colors they are revealing their colors. We now see the yellows (xanthophyll pigments), and the oranges and purples (carotenoid pigments). Some plants that have high sugar content to their sap can form a secondary pigment under sunny and cold conditions called anthocyanin. This is the pigment which gives us the deep brilliant oranges visible in trees such as sugar maples. Eventually, even these pigments get moved to the twigs and stems and the leaves turn brown from the existing tannic acids and other phenolic compounds. Curious if this really happens? Gently scratch the tree’s twigs and the green chlorophyll will be visible. Also careful observation of a leaf as it “changes” its colors will show that the green color moves towards the leaves’ veins just before disappearing completely.