Warm Dry Days/Cold Nights or Change by Species?

“Curious” posted the following comment on The Mighty Sparrow today in response to something I’d posted previously… “I’m still wondering about “the wet season…warm dry days and cold nights” part. Have the introduced trees and native poison oak in the Bay Area not gotten the memo? Every fall the liquidambars turn every color between yellow and burgundy, the dogwoods turn persimmon, the ashes send down flurries of gold, the pistachios go fire-engine red, many varieties of maple catch fire, all without regard to summer drought and mild nights. This fall was quite mild yet the poison oak (starting before the two early rains) was particularly spectacular in the Peninsula hills. Could it be that the species of plant is the more important variable, so that even without the boost of a rainy summer and cold autumn nights many varieties of plants can have deep, intense color? Or is it that the wet season in question can happen season-before-last and the cold in “cold nights” just needs to be equivalent to a typical Bay Area night in early October? (PS Love your web site!)”

I responded, “You’re right, foliage turns by species. That is, poison oak tend to change collectively and that can be on a different schedule than liquidambars or other deciduous foliage. The triggering mechanism, however, is a shortening of the days which results in lower chlorophyll production, thereby revealing the color. Rain in advance of autumn serves the purpose of making color last longer. In a drought, the leaves tend to dry up faster which shortens the time they show color. As for temperature, the ideal conditions are warm days and cold nights. That combination serves to intensify the color. Rainy nights are not good for the fall color as cloud cover retains heat. Clear, cold night skies and warm days are best.

In our reports, you’ll see that some species turn earlier than others, but also note that color has been descending since mid September at a rate of about 1,000′ per week. Right now, the color has dropped in elevation to be appearing all over Northern California. I found so little seasonal color in Southern California this past weekend, that it’s hard to say whether there’s just none to be seen or it hasn’t gotten to that latitude and elevation, as yet.”

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