Saturday, 24 November 2012

The Autumn Colours. What are They?

One of the wonders of autumn, along with the flowering of the mushrooms, is the spectacle of the autumn colours. You do not need to be a naturalist to appreciate this. Unlike many natural processes there is nothing subtle about this change. Virtually everybody notices these brilliant colours, even though few know anything about their causes and chemistry. In eastern North America there is a tourist industry based on this phenomenon. Rooms in the eastern deciduous forest may be booked up a year in advance.

Why do leaves show these striking reds, oranges, and yellows before they fall? Like may other natural processes, the basics are known, but many of the details have yet to be discovered. 

The shedding of leaves by deciduous trees and shrubs is  a complicated procedure. Deciduous species are not able to grow and photosynthesize under winter conditions. The conifers, in contrast, are the trees of cold areas. They have turned their leaves into narrow needles with minimum surface area, and protected them with a thick waxy surface. One of the problems for plants in winter is desiccation. When the ground is frozen, water is not able to enter the roots. If too much evaporation takes place such a situation can prove fatal. Conifer needles are an adaptation to this. They still photosynthesize through the winter, but decrease the surface area where water loss can take place. Larches (Larix spp.) are an exception to this rule. They are deciduous conifers that shed their needles in the fall. It is believed that this is an adaptation to extreme winter cold, as many larches grow in the high north or at high altitude, where the ground is completely frozen for much of the year.

Broad leaved trees are able to survive in colder climates, by growing in the summer, and going into dormancy in the winter. Before going into this dormancy they take some of the nutrients from the old leaves, and store them over the winter. These are then used next spring to supply a short spurt of food to the newly developing buds. The base of the leaf also forms a weak corky layer of cells, called the abscission layer. This is weaker than the original leaf base. The old leaf is now supported by the vein of woody tissue which at one time brought water into the leaf, and transported glucose from the leaf to the rest of the plant. The weakened vein eventually breaks, and the old leaf falls to the ground.

The predominant colours in autumn leaves are yellows and reds, and the pigments that make these colours are produced in quite different ways. The yellows are most often due to pigments called xanthophylls. They are present in the leaf right through the growing season, but you cannot see them because they have been hidden by the green of the chlorophyll. Chlorophyll continually breaks down and is replaced by new chlorophyll. When the old leaf is cut off from the tree’s nutrient supply, no new chlorophyll is made. Xanthophylls are more stable than chlorophyll and take longer to break down. They are  light gathering compounds and collect light in frequencies where chlorophyll is less efficient. They also protect chlorophyll from excess light. Chlorophyll can be degraded if the light intensity is above that which is needed for photosynthesis. Xanthophylls absorb some of this excess. One place you probably see xanthophylls is at the breakfast table every morning. They give the yellow color to egg yolks, and this originates in the leaves eaten by the chicken.

The reds are quite different from the yellows. Autumn reds are not there during the growing season. They are usually due to anthocyanins, the same compounds that make blueberries blue. As some leaves begin to die, sugars accumulate. Normally these would be exported to the rest of the tree. Sunlight reacting with these sugars produces the anthocyanins. One tree that illustrates this relation to sunlight is the vine maple (Acer circinatum). In shady forests its leaves turn yellow, but in sunny sites they become a bright red. Young emerging leaves in the spring may also have a red tinge. This is also due to anthocyanins. They protect the newly developing leaves from damaging ultraviolet light. There is also evidence that they may also serve a protective role in senescing leaves. 

Our most common deciduous tree, red alder (Alnus rubra) does not change color at all. It just dumps its leaves when they are still green! Alders have nitrogen fixing bacteria (Frankia alni) in their roots. These take nitrogen from the air to produce ammonium compounds as fertilizer for their host tree. The alder does not need to hold on to its leaves until they deliver some of their contents back to the tree. For most trees availability of nitrogen is a limiting factor for growth. It is like money for most people. Alders have all the currency they need to do whatever they want. When its leaves fall this currency is delivered to all of its neighbors.

In eastern North America there is a tourist industry based on the autumn leaves. This is the area of the eastern deciduous forest. Not only are there more deciduous trees to create this spectacle, but the cooler temperatures, and bright sunny days favor the development of bright red leaves. In Vancouver most of our fall colouration is produced by street trees, some of which are native to the eastern forest. Two frequently planted ones which give a good show are the red maple (Acer rubrum) and the sweet gum (Liquidambar styraciflua).

                                                               Red Maple Leaves
                                                          Photo by Rosemary Taylor

There are, however, places to go to see local native species with beautiful fall displays. Although our forests are coniferous, our shrubs are deciduous. In the mountains where shrubs of the heather family dominate the subalpine slopes and ridges, there can be magnificent panoramas of yellows, oranges, and reds. The closest of these displays for Vancouverites are the ski slopes of Hollyburn Ridge, along the trail to Hollyburn Peak. During an October with sunny days these slopes can be stunning.

The most striking colours are from the blueberries. Since these are the dominant shrubs of many subalpine slopes, these slopes can be a painted a brilliant red, visible from kilometres away. The most common species at Hollyburn is the Alaska blueberry (Vaccinium alaskaense). It tints the ski runs a purple red. Mixed with it is the black huckleberry (Vaccinium membranaceum). Ordinarily you need to look at them up close to distinguish one from the other. But at this time of year that is not the case. Its leaves turn a brilliant red and the two species can be identified from a distance. Scattered among these reds are splashes of yellow from copperbush (Cladothamnus pyroliflorus), and white rhododendron (Rhododendron albiflorum). The white rhododendron has a way of changing color that is unlike that of any others of our native shrubs. The leaves do not turn completely yellow. They develop a yellow spot on a completely green leaf. This spot gradually increases in size until the entire leaf yellows. 

                                                                    Hollyburn Colours
                                                             Photo by Rosemary Taylor

Scattered among the major players of this display are some lesser ones that fill in the gaps. Fireweed (Epilobium angustifolium) patches present their red withering leaves. Devil’s club (Oplopanax horridus) is yellow. The occasional Sitka mountain-ash (Sorbus sitchensis), even though it is in low numbers, stands out because its leaves often turn orange. 

A trip up to Cypress Provincial Park on a sunny October day is well worth the effort, and if you are looking for some exercise you can continue past the little lakes, and up to the peak for its panoramic view across Burrard Inlet to Vancouver.

Autumn displays with all their beauty are still shrouded to a great extent with mystery. Although the basic processes are now understood, the details have yet to be unravelled. Modern tools of molecular biology and biochemistry, however, are rapidly filling these details.