Is it just my imagination or do old trees of a given species tend to turn before the young ones?
On Oct 28, 2008, at 3:02 PM, Lawrence J. Winship wrote: > > 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 -~----------~----~----~----~------~----~------~--~---
