Ah, my favorite topic: leaf color change, abscission and dormancy - favorite because it is so beautiful and dramatic and our knowledge is so incomplete!
A couple of observations: 1 - dropping leaves is an active process and takes time; that is, trees form an abscission zone of dead corky cells at the base of the petiole, sealing off the xylem and phloem, then the leaves are able to fall, sometimes in unison! I have come upon a ginkgo that has gone golden yellow, and then one morning, all leaves begin floating to the ground, in the absence of any wind, gently carpeting the space around the tree, leaving a stately naked skeleton with a golden skirt - as good as any Goldsworthy scuplture! 2 - trees that are frost nipped before they complete dormancy hang on to their leaves, until the petiole is frayed by the wind, or is pushed off in the spring by new leaves emerging - oaks often do this up here in MA, possibly because the day-length sensor is out of phase since they are on the northern edge of their distribution 3 - leaves that fall on their own, e.g. through dormancy and abscission, have very different nutrient contents than those that stay - so resorption of nutrients may be one selective pressure for getting the job done before heavy frost shuts the whole process down.....the counter pressure is off course getting that last little bit of carbon fixed into starch - so trees gamble! 4 - color change is even more wonderful and mysterious - hypotheses abound! but the reds (anthocyanins) definitely develop during the dormancy process and are not there before - at least in a colored form - they may take on color by being combined with sugars; and the pattern of color change is so different one species to another - sugar maples show a gradation in color that correlates with light intensity and/or duration - I tell the students that trees are the first photographers; frequently you can pull away one very red/golden leaf to reveal an outline of the that leaf in green on a leaf below it 5 - black gum leaves turn one at a time; green next to red, apparently NOT influenced by light - poplars keep a tuft of green leaves at the very top of the tree until the very end 6 - and, sadly, flagging, or branches turning all red or yellow before the entire tree goes, can be a sign of disease or distress in that section of the tree. What fun. Larry Winship Edward Frank wrote: > Lee and ENTS, > > I guess my thoughts were ill-formed when I first posed the question. > What I am thinking is that there are two mechanisms that cause the > leaves to change colors and fall. One is they are nipped by frost and > drop. The other is that they drop because the process is triggered by > the change in day length. One sort of backs up the other. If the > cold doesn't get them, then the leaf change goes to its fail-safe > option of change in day length. Why would the trees not simply wait > until cold caused the leaves to die off? Why have this day-length > mechanism at all? > > There must be some disadvantage to allowing the leaves change and drop > to be triggered by temperature change, or some advantage to the leaves > being triggered by day length changes. What is it? On the face of > it, it would seem it would be best overall for the trees to hang on as > long as possible, to extend the growing season, but the day-length > trigger must have some advantage that outweighs the gain from > extending the growing season at the cost of the leaves freezing. > This must be some specific ratio or formula balancing the benefits and > costs of both options. Why is the change at a specific day-length? How > specifically is this leaf change formula from day length change > determined? It may be different for different species. Like now the > black gum has long since fallen (thin leaves), the maples are mostly > down, but the oaks and tuliptree leaves are still hanging on and to a > large degree green. Why is there this pattern, and why is color > triggered by these day length changes? > > It is a genetic mechanism. Clonal colonies all seem to change color > at the same time. Trees transplanted from the north change colors > sooner than southern trees. Is the time of nut production or fruit > production triggered by the period since leaf-out or also by a change > in day length? That would apply to blossoming in the spring - is it a > certain time after leaf out or set by temperature or set by day > length? I know blossoming can be delayed by weather, but what if the > weather is good? I am just trying to understand the processes happening. > > Ed > > > "Two roads diverged in a yellow wood, And sorry I could not travel both. " > Robert Frost (1874–1963). Mountain Interval. 1920. > > ----- Original Message ----- > *From:* Lee Frelich <mailto:[EMAIL PROTECTED]> > *To:* [email protected] <mailto:[email protected]> > *Sent:* Tuesday, October 28, 2008 7:02 AM > *Subject:* [ENTS] Re: Fall colors and leaf drop > > Ed: > > Because both temperature and day length control formation of fall > colors. Up here temperature used to be the primary controlling > factor (it got cold enough to cause leaves to change color and > drop before the days were short enough to do so), but now it > doesn't get as cold as early, so day length has more chance to > cause the leaves to change. > > Lee > > > > -- Lawrence J. Winship, Professor of Botany School of Natural Science, Hampshire College, Amherst, MA 01002 [EMAIL PROTECTED] --~--~---------~--~----~------------~-------~--~----~ Eastern Native Tree Society http://www.nativetreesociety.org You are subscribed to the Google Groups "ENTSTrees" group. To post to this group, send email to [email protected] To unsubscribe send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/entstrees?hl=en -~----------~----~----~----~------~----~------~--~---
