This weekend was the Canadian Thanksgiving. I was at my parents’ house for the holiday, gone for two days. I had fully intended to put together a post-dated post that would go up while I was away, but a last-minute change of plans regarding the weekend arrangements meant I had less time than originally planned. It was a nice weekend, I don’t get to see my family very often now that we’re spread out all over the province. Also, with my parents taking possession of their new house at the end of the month, and listing their current place soon, it may be the last family event we’ll have there. It’s a little sad to think about; my parents have been there some 30ish years and it’s been the only real home I’ve known, until this move to what will hopefully be a longer-term residence here in Frontenac. I still catch myself saying “I’m going home for Thanksgiving” even though I haven’t resided at my parents’ in a number of years. To me, that place will always feel like home. Look at me, I’m starting to get a little choked up already.
So it was a nice weekend. The Niagara Escarpment is always gorgeous in the fall. The area has a higher percentage of forest cover compared to the surrounding regions, and the trees, a lot of maple and other species that tend to turn bright colours, form brilliant blankets over the rolling hills. While living in Toronto I would always look forward to return trips to my parents’, to enjoy the fall colour. Thanksgiving weekend always seemed to be right around the peak, which was great timing.
Dan* and I had great hopes for the region surrounding our house when we moved in in the summer. It’s nearly completely deciduous here, except for a few evergreens scattered along the lake’s shoreline, with maples making up a large part of the species composition, so we anticipated a spectacular autumn show. Up to this point this fall we had so far been rather underwhelmed. The maples, it seemed, were turning yellow or brown and then dropping their leaves before they got a chance to form any sort of bright displays. There were a handful of trees that were behaving in a respectable manner, producing flame-orange and red foliage, but they were just scattered individuals, here and there.
*Now that Blackburnian has his own webpage and blog up and operational, he’s requested that I use his real name, Dan, rather than his codename, Blackburnian. I’m just as happy to oblige, since inevitably I start to type the former as I’m writing these posts anyway.
The Frontenac region seems to have reached its peak this weekend, while I was away. Driving back up our road, all the trees seemed to be afire. Perhaps it was simply being away for a little while that skewed my perception on things, or perhaps it was the particular weather patterns recently, cooler weather last week followed by a couple of gorgeously warm days this weekend, that prompted the rapid colour change. Either way, it was a beautiful drive up our road. It’s a beautiful drive anyway, but the eyecatching colours just made it that much more scenic.
The brilliant colours of autumn is a phenomenon unique, for the most part, to deciduous trees of temperate regions. We often tend to think of it being a feature of northeastern North America, but fall colours can be observed through much of North America and northern Eurasia, from Scandinavia to Japan. Many of these regions, including New England and eastern Canada, have developed an autumn tourism industry built around a pasttime sometimes called “leaf peeping”, driving the countryside enjoying the show. The Weather Channel in North America even has a segment in the fall where they report on the state of change for different regions, and they maintain a similar page on their website.
The changing colours is the result of the tree pulling its nutrients back into its trunk and branches prior to dropping its leaves. There are a lot of valuable compounds and nutrients invested into a leaf during the summer that would represent a huge loss to the tree if it had to replace them again in the spring. By removing these important compounds from the leaves before they fall, the tree saves a lot of time and energy. But why does a tree shed its leaves? It is believed that the costs associated with maintaining leaves during the sub-freezing winter months far outweigh any benefits the tree may gain through photosynthesis during that relatively dark period, and so it is more energy-efficient, in the long-term, to drop the leaves for the winter and grow new ones in the spring.
Leaves are green during the summer because they contain a compound called chlorophyll. This is what the tree uses in photosynthesis, which is the process of converting carbon dioxide and water to sugars using the energy in sunlight. During the summer the chlorophyll is constantly being replaced as it wears out in the leaf, maintaining the rich green colour. Come fall, the tree stops replacing the burnt-out chlorophyll, and gradually the tree loses its green. The extremities of the leaves are the first to go, with the veins often remaining green well after the rest of the leaf has changed colour.
The reason it turns various colours, though, instead of just going brown, is because of the presence of other pigments within the leaf that are masked as long as the chlorophyll is present. Once the chlorophyll starts disappearing, those other colours begin to show. The yellows and oranges are the result of carotenoids, the pigment that also gives carrots, egg yolks and Baltimore Orioles their characteristic colours. Many trees, such as aspen, birch, black cherry, sycamore, and others, contain these pigments and will generally turn primarily yellow in the fall.
Some trees, though, turn reds, purples, or fire-orange. These colours are produced by different pigments, called anthocyanins. Cherries, blueberries, red apples, and other similarly-coloured fruits contain these pigments. Unlike the carotenoids, the anthocyanins aren’t present in the leaf during the summer growing season. Rather, they are actively produced by the tree in the fall as the chlorophyll is starting to disappear. Why would the tree invest so much energy into producing pigments when it’s just going to drop the leaf anyway? There are a few hypotheses, the most frequently proposed being that these pigments protect the leaf from scorching and dessication from the sun, extending the lifespan of the leaf and allowing the tree to get the last of the sugars, nitrogen and other nutrients out of it before it falls. It also helps to make this process more efficient. Maple, oak, dogwood, and others, are among the trees that actively produce these pigments.
The cues that trees use to know when to start reabsorbing nutrients aren’t clear, but may have something to do with changing light levels. As the days get shorter and the tree starts receiving less light, it will trigger the tree into beginning its fall process. I recall hearing on CBC Radio One back in late August that the reason some trees were starting to change already is because we’d had such a wet summer, and all that cloud cover had triggered the low-light mechanism in some individuals. That could also explain some of the lack-lustre colour this fall, since the brilliance of the fall display is proportional to the amount of anthocyanin produced, and the amount of anthocyanin produced is proportional to the amount of light received during the reabsorption process. Bright, cool days with chilly (but not freezing) overnight temperatures produce the best colours. Interestingly, recent research suggests that rising carbon dioxide levels delay the onset of fall colours, though I’m not sure exactly how the extra CO2 (the “breath” of trees) helps to extend the life of the chlorophyll and green leaves.
The fall colours are fleeting, and before we know it winter will be upon us, but in the meantime I enjoy the bright displays and that indescribable smell of fallen leaves that indicates autumn has arrived.