Ed, ENTS- Perhaps the timing of leaf coloring and drop can be thought of as dependent on two main factors: 1). Day length, and 2). Environmental stress.
Color due to reduced day length is obvious, but color due to stress might be caused by: cold temperatures; increased daily temperature fluctuation; drought; physical injury to tree; or insect/disease infestations, and other factors I'm sure. I think we have all seen the occasional tree in full fall color, but displaying the colors in July or August, not October. This could not be caused by cold temperatures, but rather indicates the tree is in some sort of stress, and also shows that reduced day length alone is not the only cause of fall color. I think in the area where I live the weather patterns 20 to 30 years ago were such that the forest in general was in a more stressed state in early autumn, and that combined with day length created "earlier" falls, by about two weeks. As a separate comment, concerning intensity of fall colors independent of timing, it seems like young trees typically display brighter colors that mature trees, especially among oaks. Pin oak is the commonest oak in my area, and mature trees, 2' or more in diameter, usually turn from green to buff or just right to brown, while saplings in the woods are often brilliant crimson red. The white oaks and red oaks show the same pattern, with young, vigorously growing trees producing the best fall colors. Steve On Tue, Oct 28, 2008 at 1:54 PM, Edward Frank <[EMAIL PROTECTED]> 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 <[EMAIL PROTECTED]> > *To:* [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 > > > > > --~--~---------~--~----~------------~-------~--~----~ 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 -~----------~----~----~----~------~----~------~--~---
