By Wendy Leonard,
Natural Preserve Officer
Every fall I am inspired by the beautiful color change in our Natural Areas. The red of the Spanish oaks (Quercus texana), dappled in the bright green of the Ashe juniper (Juniperus ashei), sprinkled with a little gold from the cherries (Prunus serotina), and topped with a fiery red from the Virginia creeper (Parthenocissus quinquefolia), paint a beautiful picture along hillsides. It is amazing to think that we get so much color change this far south. As I walk through the Natural Areas, I wonder, “How do trees and other woody plants deal with the cold and sudden changes in temperature?”
For most vegetation, dealing with the extreme winter temperatures starts in the leaves. As the days get shorter and the nights become cooler, the synthesis of new chlorophyll seizes, photosynthesis declines, and the green in leaves begin to disappear. Trees can become the pinnacle of efficiency, pulling nutrients in to twigs and branches where thick bark can act as insulation. As the chlorophyll disappears, the other pigments like carotenoids and anthocyanins become apparent. The anthocyanins give the Spanish oaks and other vegetation a bright red hue. It can be affected by high sunlight, generating a reaction between the pigment and the excess sugar the tree produced on warm, sunny days. Sharp changes in the weather can cause the veins in leaves to close, holding sugars in and preserving the spectacular red color in the leaves. Dry summers, like the one we just struggled through, can delay fall color change for weeks. The most ideal conditions for a spectacular fall color show is warm, moist, sunny days, followed by cool, crisp nights.
Trees can also deal with cold winter temperatures by breaking large molecules (starch) into smaller ones (sugar). This reduces ice crystal formation inside the cell. Trees can “harden” roots and shoots by cell dehydration and ice crystal formation inside the cell walls. However, drastic changes in temperatures can reverse hardening and leave a tree exposed and vulnerable to freeze damage. When the abscission layer starts to form in mid-October and chlorophyll production stops, the tree is almost dormant and is usually not affected by freezes in this area. The only possible damage sustained by native trees occurs with new growth late in the season that has not had a chance to harden off.
Fall color is hard to predict since weather plays such a variable role. The native vegetation is well adapted to the extreme central Texas environment and only the strongest, most well adapted native plants will survive to reproduce another year.