Episodic Leaf Curling

as observed by Gerry Hawkes

near Woodstock, Vermont

report dated June 10, 1998
updated October 3, 2001

I first saw episodic leaf curling in the upper 1/3 of the crowns of sugar maples in July of 1996. This upward curling of the leaf edges occurred several times during that summer and would last from a few minutes to an hour or two in the early morning or late afternoon. A light leaf drop also began at about the same time, but the leaves were not necessarily the same ones that were curling and many of the fallen leaves appeared healthy.

In 1997 the episodic leaf curling began in June, extending at times down to the mid-point of the tree crown. Curling of leaves around our home and office was observed approximately 20 times during the growing season both in the early morning and early evening. Leaves in the upper canopy curled as if they were in moderate drought stress, but would uncurl after 1-3 hours. Once during 1997 I was able to observe leaf curling on a hillside from a distance of about 1 mile. While sugar maple exhibited the most severe curling, simultaneous curling was also observed to varying degrees in the upper crowns of white ash, black cherry, red maple, yellow birch, white birch, basswood and red oak (virtually all the hardwood species I can observe near my office).

In 1998, the episodic leaf curling began in May and occurred almost on a daily basis with curling sometimes encompassing the entire canopy of the trees, although the curling was always the most severe in the upper crown. Some trees exhibited more severe curling than others, but they always showed the symptoms simultaneously. The leaf curling became more pronounced and longer in duration, with some episodes lasting through the entire day although curling was at its worst in the early morning and early evening. There were three occasions where I could observe up to 70% of the hardwood leaves curling on hillsides a mile or more away (in one case about 7 miles with binoculars).

During the 1999 and 2000 growing seasons episodic leaf curling was infrequently observed.

Weather seems to make no difference in the occurrence or severity of the curling and its episodic nature defies logic. This is not the most serious of the host of symptoms I am seeing in a forest ecosystem under severe stress by an all-pervasive influence that can only be explained by the multi-faceted effects of air pollution, but it is certainly the most perplexing.

July 25, 2001 Update: Moderate episodic leaf curling has been observed nearly every morning and evening for the past week. On the evening of the 24th episodic leaf curling could be seen on hillsides as far as 4 miles away. Caution must be used when looking at distant hillsides not to confuse the turning up of the leaves by a breeze with the effects of episodic leaf curling.

October 3, 2001 Update: Moderate episodic leaf curling has been observed nearly every morning and some evenings since mid-July.

Questions & Responses

Question:
I am interesting in getting some information on this leaf curling
phenomenon. Where is it? How widespread? Is it isolated to a single
stand or side of a hill, or does it cross valleys and hills? Are all
species of trees in an area affected, or just a single species? Are all
the individuals of a species in the area affected; are they affected to
approx the same degree? Is the defoliation severe (what percent of
foliage is lost by any one tree)? Is there any reflushing of latent buds
or do the defoliated branches stay bare?

Response:
The symptoms certainly do cross valleys and hills. I have seen
several different hillsides with simultaneous leaf curling when
viewed from a good vantage point. From a distance it looks much
like the underside of leaves exposed by a breeze before a storm,
but it will occur when there is no breeze. Also the effect is
rather static, not variable as with a breeze, requiring a period
of many minutes or hours for change to become noticeable.

The leaf and twig loss I have been observing does not appear to
be directly associated with the episodic leaf curling since
healthy, uncurled leaves have been falling along with leaves that
are curled, chlorotic and undersized. There appears to be no
reflushing of latent buds and some twigs die and fall, although
leaves toward the apical bud often remain to keep most twigs
alive. This results in increased crown transparency. As you
have no doubt seen, there is a wide variation in foliage loss in
many species of trees ranging from complete tree mortality to
only slight thinning of the crowns, although the number of trees
with significant crown thinning and dieback is increasing
exponentially. Again I must emphasize, that while I think
episodic leaf curling is an alarming symptom, it is not directly
associated leaf and twig drop.

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Contact Gerry Hawkes: ghawkes@eco-systems.org