Want fall foliage color, don't blame the trees, blame the temperature!

The first thing we learn about fall foliage color is that it doesn't just happen. Just like making chicken soup, making fall foliage color requires the right ingredients. The right ingredients in the latter case are the proper species, shortening day length and warm days and cool nights. Let's take a look at each of these.

Proper Species- The seasonal color change of tree foliage in the fall occurs primarily with broad-leaf deciduous trees in temperate regions. These are the maples, oaks, dogwoods and other shade trees that are found in abundance in Rhode Island and throughout New England. Inside the leaves of the vast majority of these species are found the pigments so essential for the foliage color we see in fall. There are four of these pigments. Their names are chlorophyll, carotenoids, anthocyanins and tannins. The chart below identifies the major colors and hues produced by these leaf pigments.

The interesting thing about these pigments is that their manufacture is linked to the food production and food storage functions of the plant. Both of these functions are under the control of photosynthesis. In brief, as photosynthesis occurs over the spring and summer, Chlorophyll production is very high and helps it to mask the other pigments, especially the carotenoids and tannins which, at this time, are also being produced in the leaf cells. The tree's leaves will remain green as long as chlorophyll production exceeds chlorophyll destruction. Nothing lasts forever, however, and the decline of the chlorophyll molecule soon begins with the arrival of fall. It is triggered by shortening day-length and cooling temperatures. This opens the door for the other pigments that, up to this point, were missing in action. The carotenoids, as we learn, are the first to come forward. They show up in the leaves of the birches, ashes and hickories. As the chart shows, they fill the leaves with colors of yellow, golden- yellow and orange. The next pigment to become activated is the anthocyanins. Unlike the other pigments, anthocyanin is not present in the leaves during the summer months but only appears when the fall season is well underway. That's because huge daily temperature swings are essential to spark its production from complex reactions among sugars in the plant's leaf cells. These sugar reactions are well worth the wait, however, as the anthocyanins unleash some of the grandest leaf colors we observe in trees. These are the brilliant hues of red, crimson, and pink and, also, the deeper purples and magenta rarely seen in nature. But you won't find these colors in the leaves of many species. Unfortunately, they are found only in certain species of deciduous trees, such as Red maple, Scarlet oak, White oak and sumac.

The last leaf pigment to assert itself is the tannins. They are found in the cell wall of the leaves of some species of trees like the oaks. These species show brown coloration rather than yellow or orange in the fall. Finally, the tannins come late to the fall foliage dance as they are only activated when the chlorophyll and the carotenoids disappear.

Shortening Day-Length- This is a no brainer. We all know that the amount of sun-light in a day begins to decline after reaching its peak at the start of summer. In 2011, the peak (15 hours 13 minutes) was reached on 8 straight days from June 17 through 25. Then, the days grew shorter. The decline continued over the summer and fall and by November 17 the actual day length was reduced to 9 hours and 46 minutes. The chart below shows this month to month reduction between June 25 and November 17. This chart shows that the trees were suitably conditioned to have a good color season.

Warm Days/Cool Nights- Here is where things get interesting. Look at the temperature charts for the months September through mid-November 2011 that I obtained from www.wunderground.com.

September 2011

October 2011

November 2011

The charts show that while we did get moderating temperatures over this period, the trend was actually toward warmer days and less cool nights during this 2 ½ month period. This trend is evidenced by the daily spikes above and below the normal daily high/low temperature band ( two black lines) on the graph. Matter of fact, when I examined the actual daily temperature and compared it to the historical average daily temperature for the same period, it revealed that there were 45 higher-highs as compared to 31 lower-lows over this 78 day period. When I compared 2011 to the two previous years (2009, 2010), I found the 2010 results to almost mimic 2011. However, 2009 was totally different. In 2009, there were far fewer higher-highs and many more lower-lows than in either 2010 or 2011 (see Table 1).

Table 1. Number of days when the actual high or low daily temperature was either greater than or less than the historical average high and low daily temperature for the period September 1 through November 17 (2009, 2010, 2011).

                                2009                           2010                          2011                 

Based on the Table 1 data, we probably could be tempted to take an educated guess that 2009 was "less warm" than either 2010 an 2011.  Well, when we look at the average daily temperature for these years (see table 2), we find that our intuition was right on target!

AVERAGE DAILY TEMPERATURE (Maximum, Mean, & Low)

Table 2

The table 2 results show that 2009 was cooler than the other years. In fact, average mean temperature in the month of September 2009 was 5°F lower (63°F) than in 2010 (68°F) and 2011 (68°F). The same could also be said for October 2009. Its average mean temperature was 4° cooler (52°) than in 2010 (56°F) and 2011 (56°). By the way, the leaves turned early in 2009 and the landscape was ablaze with vivid foliage color by mid-­‐October. I felt it was one of the best years for fall foliage color in a long time.

Summary-­‐ Finally, from all the data presented here, a case can be made that the 2011color performance from our leafy friends was pretty normal. They did about the best they could when given the environmental "challenges" they were up against. The fall season started out warm and stayed warm until the last week in October when cooler temperatures were recorded. As a result, most broadleaf deciduous species continued to produce and store food through the process of photosynthesis. This delayed the decline of the chlorophyll molecule for the longest time. With the other leaf pigments held in check by the chlorophyll dominance, there really wasn't much color change, and the leaves of most plants remained green. Foliage color change eventually did arrive around October 27th following a week of lower daily temperatures. The color yellow soon was everywhere. Within days the color red and its derivative hues of bright reds, crimson and magenta were also observed. The carotenoids and anthocyanins had begun to assert themselves as the production of the pigment chlorophyll initially declined and, then, ceased altogether. Finally, in early November, the leaves of the Pin, Red, and Black oaks turned brown. This is nature's signal that the foliage color season is nearing its end. In terms of tree biology, it tells us that the chlorophyll and carotenoid pigments were done for the season and the tannins were now ascendant in the leaf. Last, but not least, fall foliage color performance is a tricky thing. Some years the color change is more uniform and of greater duration(2009). Other times like this past year, it comes and goes quickly. As the data in this article indicates, the key to what we get is usually linked to temperature. And historically, it's the cooler temperatures that produce those crisp autumn days that give us the best color performance by leaves. In 2011, we didn't get those daily temperatures. So, in summing up this article, I leave you to ponder the wisdom of this old RITree proverb: "You want fall foliage color, don't blame the trees, blame the temperature!"