Monday, April 13, 2015

What makes things spring in spring?

The blooming and leafing-out of trees and shrubs just beginning is pretty predictable: silver maples bloom before red maples, which bloom before Norway maples, for example.  But they do not bloom on a particular date, nor do they necessarily seem to wait for warm weather.  What is it that sets them off?

The buds formed the previous summer have been dormant all winter, tiny flowers and leaves protected from further drying-out (in many species) by waxy bud scales.  This dormancy, triggered in part by the lengthening nights of fall, is mediated by accumulation of a hormone called abscisic acid.  (--so called, because it is also involved in leaf abscission--cutting-off--in fall.)  At high levels this hormone inhibits growth, but it gradually breaks down in temperatures close to freezing.  Temperate zone trees need between about 500 and 2000 hours of such temperatures for the abscisic acid levels to fall low enough to allow growth.  Once this time is accomplished, warm weather triggers bud-break. 

Why would a tree need such a cold period?  How did this system arise?  Consider a tree that enters dormancy in late fall, but is ready to grow as soon as the weather warms.  Mid-winter thaws are common in temperate regions: a tree could easily leaf out during a January thaw, and then have its leaves and flowers killed by the freezing weather that follows.  This more complex system probably evolved because it gave trees an edge by preventing them from leafing out dangerously early.  The required cold period varies from species to species: more northern species will generally require longer cold periods than more southern species. 

On your mark, get set, GO!
Red maple, pignut hickory, Norway maple, and quaking aspen.

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