Woodland Walk to Enjoy Fall ColorsSUNDAY, OCTOBER 16, 2011
From last weeks “Indian Summer” weather with above normal temperatures, highs in the 70’s, winds from the southwest, sunny days and bright fall colors, especially in the Longenecker Gardens, we are now experiencing cooler weather with highs in the 50s, occasional rain, and strong northwest winds. With the change in weather the earlier many of the more colorful leaves have fallen. We have returned to more typical October fall weather it appears our peak of fall colors was before mid-October this year.
There are still some reds in the sumacs, maples, virginia creepers and woody trees, shrubs and vines. But many of the yellows visible last week in the birches, ashes, hickories, aspen, black cherries, cottonwoods, etc. are more visible underfoot than up in the trees. With rain, strong north and west winds many of these leaves have fallen.
Why do the leaves fall this time of year and why do they turn so many different colors? The advantage of leaves falling is reduction of leaf surface during the dry season of frozen temperatures when water is scarce because it is frozen. Cold air is often dry and frozen soils yields little water to roots. Leaf fall occurs with colder weather. It is triggered partly by shortening day length in fall that causes a layer of special cork cells called the abscission layer to form at the base of each leaf stem. As the layer forms, it slows down the supply of water from roots to leaf and the leaf begins to dry up before falling off the tree. The drying out interferes with the making and repairing of chlorophyll. During this period, with the supply of chlorophyll gradually dwindling, its masking effect slowly fades away, revealing the presence of essential yellow pigments. These pigments are carotenoids, which give yellow and orange and other hues. Carotenoid pigments contributed to the brilliant yellow and orange color we saw last week in the birches, aspens, hickories, ash, black cherries, cottonwoods, and some of the red and sugar maples.
The anthocyanins are a group of pigment molecules that also occur in cooler weather with shorter days and longer cool nights that contribute to the red, purple and blended darker brown colors in fall leaves. Sugar made by photosynthesis in the chlorophyll molecules by day normally is transported down through the leaf veins and stalk into branches, bark and roots, where it is stored as food for next year. But cool weather and shorter days slows down the flow, while bright sunny days in fall enable the leaves to make sugar. In the last few days before the leaf stem waterproof abscission layer is complete, while chlorophyll remains, sugar accumulates in such quantity that it interferes with the living cells activities and is converted into other compound molecules, including anthocyanin. The development of anthocyanins depends on the breakdown of sugars in the presence of bright light as the level of phosphate in the leaf is reduced. The brighter the sun’s light, the greater the production of anthocyanins and the more brilliant the fall color display. When fall days are bright and cool and the nights chilly, but not freezing, the brightest deep fall colors develop.
This year the early “Indian Summer” days brought out more brilliant colors than usual, in part due to the above normal temperatures and in part due to the stress of about two weeks without rain. When trees are stressed, either by drying from lack of rain or by a quick frost or from other factors, perhaps related to soil or things yet to be discovered, fall colors are usually more brilliant.
On our walk today we could see as many colorful leaves under foot on the trails as still up in the trees. We could also see many woody plants with green leaves, most of which are nonnative invasives. Many of these plants do not change leaf color in response to our changing seasons. Instead they remain green until the leaf cells are killed by freezing and then fall during the winds of November or during winter.
UW Arboretum Naturalist