Monday, September 27, 2010

Fall Colors!

The Rocky Mountains seem to be redeeming themselves this year. Last year we reported on the total disaster in terms of fall colors due to an intense cold snap that occurred early in the season causing all the leaves to simply die and turn brown. This year here in Colorado we are having the perfect combination of weather conditions which has led to a spectacular start on the colors here.

The foothills and mountains are a dazzling gold and yellow, mostly from trembling aspen (Populus tremuloides), willows (Salix spp.) and plains cottonwood (Populus deltoides). So how does weather help bring out these colors? In general you need an abrupt transition from hot summer days to crisp fall days with lots of sun, combined with cool but not freezing night temperatures. The cool nights and the shortening days accelerate the hardening off process for these plants where they turn off production of the green chlorophyll pigments, and the warm sunny days allow the other types of pigments (yellows and reds mostly, from carotenoides and anthocyanins) to produce more sugars and bring out maximum colors. In coastal climates the transition from summer to fall is too subtle or you do not have the crisp night temperatures to bring out the full colors that you see in places like the Rocky Mountains or New England. Even in coastal locales you can still find beautiful colors at higher elevations where you again get those cooler night temperatures. In the Northwest and Alaska this can lead to bright scarlet colors in huckleberries and blueberries in particular.

Soon the fall colors will be coming to the lower foothills and plains here in Colorado and should be coming to many northern climates in the next few weeks. We are now getting reports of peak colors starting on trees in northern New England, and from the northern Midwest (especially Minnesota). Other northern and eastern regions are very close to starting to turn. So if you have not yet selected some deciduous trees to monitor this fall now is a good time to get started so you can catch them when they first hit the peak color stage in the next few weeks (when about 50% of the branches and or the leaves start to turn color). This does not mean this budburst stage is when the trees reach their most brilliant colors, that stage often comes a few days or weeks later. After that the last budburst stage to watch for in these trees or shrubs is peak leaf fall, when half or more of the branches loose their leaves.

Now is also a great time to see the effect of site conditions on the development of fall colors. In our neighborhood for example native green ash (Fraxinus pennsylvanica) and western hackberry (Celtis occidentalis) are common street trees. Starting on last Saturday the first green ash trees reached budburst’s full color stage. These trees were generally near street intersections where you have the maximum heating effect from the concrete in the streets and curbs. Trees in parks that are surrounded by grass by contrast are still 100% green. This is why last spring we added more features to site descriptions for budburst so that we can get more information on your observation sites, so that scientists can help sort out these local site effects from broader regional climatic effects on plant phenology, or the timing of these events.

Here is a good example of a tree first reaching budburst's full color stage. Notice there are still plenty of green leaves, but clearly more than half the branches have at least some leaves turning color (Green ash, Fraxinus pennsylvanica, right on the intersection of two streets, near the Colorado Front Range in Berthoud, Colorado).

Let budburst and the advancement of scientific knowledge about fall be your excuse to more fully enjoy the colors of fall!

For more information about planning travel to see fall colors in your area see:

The weather channel

USDA Forest Service

More on why leaves change color

Paul Alaback

Note: All photos by Paul Alaback. First two from Independence Pass, near Aspen, showing trembling aspen in full color, the others from Berthoud, Colorado featuring western hackberry, and green ash.

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