Better red than dead

In my yard, this year’s fall foliage was pretty much a dud. In particular we didn’t get the bright reds we usually see on our Japanese maples, 

Most years a maple in the front yard turns bright red

neither on a purple threadleaf variety in the backyard (Acer palmatum var. dissectum) nor on a broader-leafed version in the front. Most years they see the foliage season out with glowing scarlet leaves. This year the leaves just turned brown, shriveled, and hung on. Last weekend’s snow hung dishearteningly on the brown leaves.

This year leaves turned brown and hung on

    I feel cheated. Why did this happen? I thought this had been a favorable growing season. Some Internet research convinced me that it’s hard to know exactly why fall foliage is dramatic one year and drab another, because several factors interact to produce the autumn display. But it’s clear that events conspired this fall to prevent formation of the red pigments, anthocyanins, that give red maples and Japanese maples their gorgeous red fall color.

Late October 2015, same tree as the one above

    We were relieved this spring when rainfall stayed ahead of the annual average. By early summer, New England precipitation was well ahead of expectations, making up for last year’s drought. As of December 15, we were still ahead, with 42.30 inches of precipitation for the year, compared to the normal year-to-date total of 41.79 inches. 

    But the fall months demonstrated the swings between extremes we can expect with climate change. Around Labor Day it was unusually cold, then we had a heat wave later in September with temperatures up to 90. Heat and humidity kept the leaves on the trees. Although the tail end of Hurricane Philippe swept through our region late in October, bringing flooding and high winds, this fall season was short on rainfall. That already meant that foliage color was likely to be dull.

This fall's conditions weren't favorable for bright leaf color

    What signals trees to prepare to drop their leaves is a combination of longer, cooler nights and less intense sunlight. They get ready for winter by stopping production of chlorophyll, the pigment that enables photosynthesis and makes leaves look green. With only their yellow xanthophyll and orange carotenoid pigments remaining, leaves look yellow.

 

Witch hazel leaves turn yellow without chlorphyll

    For red and purple color, leaves need sunny days and cool but not freezing nights. These conditions trigger production of anthocyanins from sugars left behind in the foliage when trees cut off transport of materials in and out of the soon-to-be-jettisoned leaves. 

Red color made from stored sugars

My conclusion is that this November’s cold snaps with temperatures in the twenties cut off this process before our Japanese maples could turn red.

    Every gardening year is different, of course, and we can’t attribute all weather variations to global warming. There does seem to be a trend at work here, though. Researchers in phenology, the study of changes that occur seasonally, say that warmer temperatures lasting later into the fall are causing delayed leaf senescence and therefore later appearance of bright fall foliage. 

     We can only hope that next year Japanese maples will have time to turn red before winter cold intervenes. 

It's nice when each plant shows its fall leaf color