Oh, those beautiful fall colours are starting to appear on some of our trees and I can’t wait to rake all those leaves up for this year’s compost pile!
Canada has had some weird weather this year and that has had an impact on the fall colours. As well, historic patterns of when trees are at their most colourful, and how colourful, may be shifting due to climate change.
Before autumn, the healthy leaves on deciduous trees get their green colour from the chlorophyll molecules in their microscopic factories, or chloroplasts, that convert water and carbon dioxide into sugars, other carbohydrates and oxygen. The chlorophyll hides the yellow, orange and red pigments present in the leaves because it is so intense. In autumn, trees stop replacing the chlorophyll in their leaves, which then breaks down, allowing the other pigments to become more and more visible. The proportion of pigments varies by species. Maples have lots of anthocyanin, for example, which gives a red colour.
Scientists call this process senescence. In fall, a tree needs to recover some of the huge store of resources it put into growing leaves. A long senescence is good for the tree because it is recapturing more resources from its leaves. Large amounts of nutritive reserves must be recovered for winter storage so they can boost the array of activity which begins with new growth each spring.
Trees get their cues about when to start their shutdown process from the interaction of day length and overnight temperatures. Light influences the timing, magnitude and degree of leaf colour change and the temperature threshold to initiate change can differ among tree species. Leaves at the top and outer edges of a canopy usually have far brighter colour than the leaves that get less intense sunlight.
The early spring in some parts of Canada can have an impact on the process and the drought that took hold later in parts of Ontario and Quebec should lead to earlier fall colour and may be not so brilliant because during a hot, dry spell, some of that senescence would have been initiated earlier. The conditions were so hard on the trees that they began an early recapture of resources from their leaves.
Once senescence is initiated, the best weather for fall colours is nice, crisp nights and warm, sunny days. The combination is a cue to the tree to get on with the transition, but that it can do it slowly. A sudden plunge in overnight temperatures can result in trees all changing colour at once, since their different temperature thresholds for shutting down will all be crossed. Although day length, light, temperature, ground moisture and insects all affect fall colours, scientists say that climate change might also be having an impact. They cite a trend of earlier springs and warmer autumns. Evidence shows that the growing season of the trees is being influenced each year by the climates that they experience, but scientists don’t have a good handle on whether there is a disconnect happening between trees’ sensing of conditions and the climate. Trees are genetically programmed to respond to both day length, a pattern that is not changing, and temperature, for which the pattern appears to be changing.
A volunteer organization called PlantWatch is looking at how plants are responding to climate change and identify ecological changes that may be affecting our environment.