Wednesday, November 1, 2017

Fall Leaf Color Fail

Naming this season "fall" came about in Britain in the sixteenth century, and refers to the observation that this is the time of year that leaves fall off trees. Really. Prior to that the season was autumn, borrowed from France (automne) and stemming from the Latin, autumus. Or else just referred to as "harvest." Of the season names, summer and winter go back more than 1,000 years, whereas spring and fall date to half that.

New England is renowned for great leaf peeping – driving about on weekends to see the leaves turn colors. This year, however, is a particularly bad year for fall color for a number of reasons. Firstly, the very wet spring and early summer that led to the end of drought status across much of eastern Massachusetts also promoted tar leaf spot disease. This fungal disease affects all species of maple trees, but is most damaging to Norway maples (an invasive species) which is prominent in urban and suburban neighborhoods. 

Maple leaf showing browning and tar leaf spot fungus
Tar leaf spot infection begins in early spring. Long periods of moist to wet weather conditions promote growth. By summer, leaves are showing yellow spots that over time enlarge and turn black in the center. Severely afflicted leaves will wither and turn brown from the edges inward and then prematurely fall from the trees.

The good news is that tree health is not compromised. The following spring the trees will bud new leaves, and if weather patterns are closer to our norm, be entirely healthy. Homeowners may want to consider either having the fallen leaves hauled away, or if composting on the property, add a layer of soil over the bins or piles of leaves. This will reduce the risk of the fungus spreading next spring. 
 
A second reason for our poor leaf color season is unseasonably warm weather. Sunny days followed by crisp nights helps maximize color. Instead it has been so warm that in some parts of the country people were calling for renaming October as “Hotober.” Our normal late October weather for this area – ignoring for the moment any thoughts of global warming – should have been averaging highs of 61 degrees and lows of 40 degrees. Instead what we had was September temperatures – averaging highs of 72 degrees and lows of 52 degrees. With only one night of frost. Combined this warm weather with the late summer and early fall drought was the last nail in the coffin for leaf peeping season this year. It was both late and mediocre.

Two maple leaves from a better year
As to why leaves fall off, it’s a matter of winterizing and recycling. By shedding leaves, trees avoid the branch-breaking burden of heavy snow accumulation. And, as the leaves were tattered and hole-ridden by fall, combined with photosynthesis being diminished by cold temperatures and closer-to-the-horizon sunlight, no great loss. The fallen leaves create an insulating layer on the ground that protects roots from freezing. And in time, the leaves will decay, returning nutrients to the soil.

Shedding leaves is an active process. As days become shorter, the tree grows a specialized type of cell between the stem of the leaf and the twig to which it is attached. These cells weaken the connection. In time, the stems detach, leaving behind a dry leaf scar. If, instead, a branch is almost entirely broken in mid-summer, the leaves will die and turn brown, but in winter, when the leaves have fallen off the live branches, those leaves on the dead branch will still be attached.

Beech tree leaves in winter. Click on photos to enlarge.
As to the business with the color changes, chlorophyll is responsible for the green color. All spring and summer, chlorophyll is continually being metabolized and replaced. Come fall, replacement stops. Families of molecules called carotenoids and anthocyanins, which were present all summer but are less susceptible to metabolic breakdown, provide a wide range of yellows, oranges and reds once the green is gone. As to why these other molecules are there, one theory is that they protect leaf cells from ultraviolet light damage, in effect, anti-sunburn. A second is that they (not always successfully) make the leaves taste bad to insects and herbivores.  

For reasons not entirely clear, a few trees retain dead leaves until spring. These include several species of oak trees, and also American beech trees. The latter appear a very light brown in color, easily seen during a winter walk when all other trees are naked to the world. 

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