There are few signs more indicative of autumn than the pleasant hues that crown treetops when leaves change colour. From September to November, this dramatic change sweeps its way southward across Canada.
These brilliant colours are due in part to the breakdown of green chlorophyll, the pigment responsible for capturing sunlight and converting it to energy through photosynthesis. As it performs this vital job all summer long, chlorophyll is continuously broken down and replaced. As cooler temperatures and shorter days arrive, it is more energetically efficient for trees to enter a state of dormancy. Tests with radioactive tracer elements have shown that nutrients in leaves are reabsorbed just before they fall. Leaf veins are then closed by a corky layer that seals off water and nutrients, ultimately leading to abscission, or shedding. Recently, University of Missouri researchers uncovered the genetic basis of abscission. Through experiments using Arabidopsis thalia, commonly known as thale cress, they found a signalling pathway of genes regulate the programmed shedding of floral parts. This pathway is involved in not only leaf shedding, but abscission of fruit and damaged plant parts as well.
While the abscission layer forms, lack of decayed chlorophyll replenishment allows pigments previously cloaked by chlorophyll to appear, such as orange carotenes and yellow xanthophylls. Anthocyanins—pigments which cause red and purple colouration—start to emerge as well. Unlike carotenes and xanthophylls which were merely masked, anthocyanins are synthesized by the plant as autumn approaches. Experts are still unsure why a plant would expend energy producing extra red pigmentation at a time when it is most in need of conserving it. One theory suggests anthocyanins are produced to ease the process of reabsorbing nutrients before leaf-fall by blocking sunlight, a hindrance at low temperatures. Other hypotheses speculate red leaf colour may warn harmful insect pests of chemical defences or help attract migrating birds to eat trees’ fruits, thus dispersing their seeds.
Of course, this tide of colour is not just happening here in Southern Ontario. North American deciduous trees, such as oaks, maples and beeches, have close relatives overseas. Outside the Western Hemisphere, there are two ecological regions that boast showy autumn foliage akin to ours.
One is the European temperate forest, which occupies an area from the British Isles to the steppe grassland in Hungary, Ukraine. A small belt of area even stretches along the coast of the Black sea of Turkey to the Caucasus Mountains. The other hotspot of fall colour ranges from the Russian Far East to northeast China, Korea and the Japanese islands, where leaf-peeping for autumn maples has been cultural fashion for around a thousand years. However, due to their history of civilization, these regions have had their cover reduced far more than our own.
Paleontologists have long theorized that ancestral trees of these three separate but remarkably similar zones were once part of a single giant forest that covered the Northern Hemisphere. This assemblage, dubbed the Arcto-Tertiary flora, existed around 50 million years ago, when climates were warmer and current ice-covered tundra was forested. Shifting continents and global cooling led to the evolution of deciduous habit, and cut back the Arcto-Tertiary flora descendents into the separate cold-temperate North American, European, and East Asian forests we know today.
