Fall is here in Central Texas, announcing its arrival through color. This time of seasonal change is a particularly vibrant time at Brackenridge Field Lab. Splashes of vivid yellow and red appear when Flame Sumac, Spanish Oak and Cedar Elm trees prepare to shed their leaves. But why do trees do this?
These new hues arise as trees begin breaking down chlorophyll to sequester hard-won nutrients. Before fall during summer and spring, these green leaves have cells packed with chlorophyll, allowing the trees to convert ample sunlight into food. Because the days are shorter during fall, and the temperatures are cooler in the Northern Hemisphere, these leaves stop making food. The breakdown of chorophyll leaves behind yellow and red carotene, xanthophyll (orange) and anthocyanin (red) pigments. Not only are these new colors a sign that the trees are reprogramming their nutrients, but some hypothesize that the colors may signal unpalatability to insects.
Additionally, the tree develops a layer of cells at the point where the leaf's stem is attached to the tree. This layer severs tissues that support the leaf, thus causing the leaf to fall. Some trees, especially in the south where it is warmer, are evergreen, maintaining the normal photosynthetic cycle. Leaf color is also affected by light, temperature, and water availability. Low temperatures just above freezing will support bright red and yellows, but an early frost will weaken the intensity of this color.
Looking out at the usually green woodland canopy along the Balcones Escarpment, it's easy to spot colorful patterns of tree diversity during the Fall. Once the red and yellow leaves have fallen, the evergreen Junipers and late-senescing Live Oaks still provide green cover for birds and animals. At BFL, we watch these seasonal dynamics and wonder how the timing of Fall colors will change as climate itself is changing.
Dr. Rob Plowes is the Operations, Research, and Teaching Coordinator at the Brackenridge Field Lab
 |
| Flame Sumac (Rhus lanceolata) |
 |
| Cedar Elm (Ulmus crassifolia) |
