Thursday, October 29, 2009

Early Frost Shows Effects of Extreme Weather Events

Sudden hard frost turned all the leaves of these shrubs brown and
“freeze dried” them to branches, so that leaves will drop weeks
late this year. Mt Sentinel, near Missoula, MT.


This has not been a great year for fall colors in many parts of the country. In some places like Minnesota nice fall colors were cut short by an early intense frost. In much of the Rocky Mountain region an epic frost hit in early October before most species had even begun to come into color. All of this points out how sensitive plants can be to changes in the timing of seasons, and how much you can learn by keeping careful records of phenological changes as we do in Project Budburst. When we think of frost damage we usually think of early frosts in spring and how they can have devastating effects on fruit trees for example. It is generally true that plants are particularly sensitive to frost when tender new leaves and shoots have just formed. But unusual fall frosts can have damaging effects too. Some data suggests that plants can become more sensitive to frost damage with climate change, just because they are not as well prepared (not hardened off) for these temperatures when mild or shorter winters prevail.

How trees prepare for winter varies by species, and for ornamentals where they were bred. What generally happens is the process starts when night length exceeds 12 hours, or after the fall equinox. As cool temperatures, normally in the 40 degree range occur at night this can hastens the process of hardening, where plants change their chemistry, in particular the balance between starches and soluble sugars making them capable of withstanding progressively colder and colder temperatures without tissue damage. They continue to harden off from the beginning of fall all the way into the dead of winter, when some trees can tolerate temperatures of -30 F or more without visible damage. The ability of trees to tolerate these extreme temperatures in fact explains the northern distribution of many tree species and also why conifers dominate the northern or highest elevation forest regions (they can withdraw more water from outer areas of trunk than hardwoods, preventing damage from ice crystals).

So while here in Montana temperatures of 5 or 10 degrees are something that all our plants should be tolerant of in the dead of winter, when these temperatures occur all of the sudden, and in early October they can cause a lot of damage! The 30 year normal first hard frost is in late October or early November, but we had our first frost, which was a hard frost (28 degrees or colder) on October 6th. We had hard frosts every day over a week, including the worst frost on Oct 13th at 3°! Three more days with temperatures below 10° continued to hit our plants over the past two weeks.

In our area the main effects of several days of early severe frost were easiest to see on ornamental plants that appeared to be the least hardened off when the Arctic Front hit us. Leaves went from lush green to gray, brown or black within a day or two, and many also had leaves that curled up, but still held firmly onto their twigs and branches.


Interestingly in our local paper the City of Missoula announced that their leaf pickup dates for street cleaning were delayed until further notice. Another great example of how we try to plan so many things way ahead of time for efficiency, but sometimes mother nature throws us for a loop.

So why are are the leaves not falling when they normally should in the fall? As I mentioned in the budburst blog a few weeks ago most plants use day length to determine when to start hardening off, and when to start cutting off the flow of water to leaves, and the flow of sugars from leaves to the stem. Cool temperatures (but not freezing) can accelerate this process as well. The timing of leaf colors is more consistent in terms of a calendar date than the timing of spring flowers. So if a severe frost occurs before the leaves have begun to turn colors then it is likely that they have not completed growing an “abscission layer” or a corky growth that cuts of the flow of liquids between the base of the leaf and the stem. So in essence the leaves were “freeze dried” to the stems. Now it has been several weeks since this series of killing frosts so with this warmer weather some leaves have hints of yellow or orange from the small remaining sugars that still occurred in the leaves, and have begun to finish developing an abscission layer.

A few individual paper birches, and silver maples turned colors before the frost, and they are losing their leaves close to the normal calendar date. Western larch a conifer that is deciduous is also turning colors close to its normal date, presumably because it was more hardened off when the frost occurred. But for most of our native budburst species, we now have stiff brown leaves. They will come off when we get our first hard rain or windstorm. For many of our plants in Missoula this occurred last Friday (Oct 23rd). Look for the effects of these weather events on the plants in your area, it often makes a difference in determining when your plants get to the 50% leaf fall stage. For many of us up here in Montana the Budburst Phenology season of 2009 is virtually over. It will be interesting to see how the season unfolds in more southerly places now.


In the meantime now is a good time to enter the last of your budburst observations for 2009. We are anxious to start analyzing all your data from this year. Now with three years of budburst data for many places we should have enough data to begin to see some trends. Stay tuned for further news on our results…


Paul Alaback


Photo Captions


Photo 2. When the first heavy frost first hit on Oct 6th, a light snow occurred as well. Only the most northerly of ornamental trees were in color. Here we see sugar maple and paper birch giving the only color we saw this fall.


Photo 3. Black locust an east coast and Midwest native that is commonly planted as an ornamental in the Rocky Mountain region is one of the last trees to turn color and shed its leaves in the fall. This year most leaves turned an odd gray color, with leaves curling up in response to the severe cold. Few of them have yet lost leaves from their branches.

Photo 4. Common snowberry, usually has yellow leaves that turn in early September and persist for a month or more into the fall season. This year many have turned brown or even close to black where the frost was particularly severe and long-lasting. Notice that the leaf here that was starting to turn yellow in the center, but then had frost kill the margins of the leaf.


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