Fall Color Science Essay on Daylength, Temperature and Precipatation

One of the aspects of fall color that I have tried to explain in past posts is that trees cue in on three major factors that determine when their leaves will turn color and senesce (meaning to die): these are daylength, temperature, and precipitation. Regarding daylength, once we get past the summer solstice on June 21, where we have the longest day of the year, daylength continually shortens until we get to the winter solstice, the shortest day of the year. In Boone, the winter solstice will occur at 10:59 am on Tuesday, December 21. After that date, the days will continually lengthen until we get to the summer solstice.

A digression: while I state that trees cue in on daylength, they may in fact be sensing nightlength instead. We all know that certain plants flower when the days are short (i.e., nights are long), like poinsettias and mums, while others flower in the middle of the summer when days are long (i.e., nights are short), like coneflowers and lettuce. For trees, the initiation of bud formation and leaf senescence processes, including turning colors, may also be tied to the length of the night rather than that of the day. That’s an entirely different essay though.

The reason for the change in daylength, of course, is that the Earth is tilted 23.4 degrees from vertical, caused most likely by a major impact event about 4.5 billion years ago (Figure 1). That impact also created our moon. However, if you want to get into the nitty gritty of this, the axial tilt also varies between 22.1 and 24.5 degrees due to wobbling of the axis around itself, known as the precession of the equinoxes. However, we do not have to worry too much about this, because a complete precession takes around 26,000 years to occur.

However, it is this tilt that causes our seasons. When we are angled away from the sun, the days are shorter than 12 hours, and when we are angled toward it, they are longer than 12 hours (Figure 2). Seasons do not result from the elliptical shape of Earth’s orbit, as many think, because winter occurs when we are nearest the sun, and summer when we are farthest from it (Figure 3), just the opposite of what one would expect if this were true.

Trees cue in on shorter days (i.e., longer nights) because they are a signal that colder weather is coming. However, daylength and temperature co-vary, because as the days get shorter, temperatures also begin to decrease. However, this is highly dependent on latitude. It gets colder sooner at higher latitudes, so trees there have to be prepared for freezing temperatures to occur earlier in the season than in more southerly climes, and consequently when daylengths are longer than in the south.

On the same date, days will be longer (nights shorter) at higher latitudes than southern ones during the summer and early fall (Figure 4). Remember, during the summer, the sun never sets above the Arctic Circle, and in winter, you have 24 hrs of darkness. People living in northerly locations have daylight well into the evening. I was once in Finland walking around Helsinki when my friend and I decided to get dinner at a restaurant. We were surprised to find most restaurants closed, even though it seemed to be early in the evening, and the sky was quite bright out. However, when we looked at our watches, we saw that it was 10 pm, even though outside it appeared more like 6 pm! Luckily, we did find an open restaurant and managed to get a very nice, but late dinner!

Thus, if a tree cues in on daylength up north, they will begin senescing early compared to trees in the south, because in the south, that same daylength means colder temperatures are still a ways off. This is why if you transplant a northern tree to the south, it will begin coloring up and dropping leaves in late September (or even August!) while native southern trees remain green. The northern trees “think” cold weather is just around the corner and are winterizing themselves, even though in reality they could continue photosynthesizing for a few more weeks.

Conversely, if you transplant a southern tree up north, at that same daylength it will “think” that it still has several weeks of warm weather left and won’t begin turning color and senescing leaves, thereby making it susceptible to frost damage. This is yet another reason to worry about climate change, because as it gets warmer, trees will begin migrating north, and if they don’t adjust to the different daylengths at more northerly latitudes, they could be subject to freeze damage. Of course, trees did migrate north after the glaciations, but that occurred over thousands of years, which gave evolution enough time to accommodate such changes. But with the current rate of warming being so rapid, it is possible that trees won’t have time to genetically adapt to their more northerly habitats, and thus could become damaged by freezing temperatures.

Back to fall color cues. Temperature is one other big kahuna. As days get shorter, some trees follow daily temperature minima and maxima to determine when to change leaf colors. If evening and morning temperatures start to drop as days get shorter, the trees “assume” freezing temperatures will not be far behind, and they will start to turn color and begin the senescence process. If the temperatures are warmer, they will take advantage of this and continue to photosynthesize and gain extra carbon, as well as retranslocate nutrients back into their twigs. Some trees are more dependent on daylength than others are, and some combine daylength and temperature to determine when to turn color. This variation in cueing signals among trees is why changes in temperature can upset the traditional fall color display.

Trees cueing in primarily on daylength will turn color the same time each year, with little influence by temperature, while those cuing in on the combination of daylength and temperature will vary the timing of leaf color in a more temperature-dependent manner. This can cause what I term “desynchronization” of the fall color display, where it is spread out over a longer period of time, especially during warm falls. This usually results in a sub-par display, as the usual explosion of colors is muted and diluted. For example, in warm years, by the time late turning trees color up, the early ones have already dropped their leaves.

The third factor, precipitation, exerts its effects in a variety of ways. When there is abundant precipitation, especially in the summer, trees produce a full complement of leaves which can turn brilliant colors in the fall. If, however, there is significant drought in mid- to late-summer, then trees sensitive to water stress may drop their leaves. Tulip poplars are known for doing this, and many of them are now showing yellow leaves because it hasn’t rained much over the past three weeks. Alternatively, if it is sunny and cool in the fall, with little cloud cover or rain, then this sets up the appropriate conditions for a good fall color season, for reasons explained in my earlier science essay on the pigments that cause fall colors (click here to read).

Which trees cue in on daylength, and which cue in on the combination of daylength and temperature, is still in need of study. Two researchers, Keenan and Richardson, using satellite imagery, found that in years where trees leafed out early in the spring, they also lost their leaves early in the fall of that same year. However, other studies have shown contradictory patterns whereby warm falls delay the onset of color and senescence. For example, Matsumoto and Ohta reported that a 1 degree C increase in fall temperature (about 2.5 degrees F), resulted in a delay in leaf fall of four days.

Marco Archetti and other researchers at the Harvard Forest used an extensive multl-decadal dataset to elaborate on the environmental controls over autumn phenology. They showed that for most species, warming in September delayed the onset of fall colors, while precipitation in certain months only had minor effects. Citing other studies, they noted that with current warming predictions, researchers expect delays in the onset of fall leaf color of between 1.4 – 2.3 days per decade, or, up to 13 days by the year 2100.

In this same study of New England trees, Archetti et al. also looked at how individual species responded to variations in the weather. Peak color occurred earliest in ash, red maple, and black cherry (much as occurs here in the southern Appalachians), later for sugar maples, and latest for oaks (as also occurs here, although sugar maples seem to color earlier in our area than in New England). In fact, red maples peaked about 23 days before the oaks did. Red maple responded strongly to autumn temperatures, delaying leaf fall and color more in warm years than cool ones. I’ve seen that happen here too. Interestingly, though, even warm springs can result in earlier coloration in this species. However, ash responded quite differently. Warm years resulted in earlier leaf coloring, but this also delayed when trees reached their peak color, while at the same time it hastened leaf fall. Putting this altogether, warming reduced the duration of leaf color for ash trees. For black oak, warming delayed both coloring and leaf fall, so that they peaked later in the season, again, as we see here in our area.

In conclusion, the effects of weather and climate on fall leaf color timing, duration, and intensity are quite complex, and require additional research. The more we learn, the better we will be able to predict fall color displays and how trees will respond to future climate change.