In summer, we seldom pay much attention to the colour of leaves. It is only in autumn, when yellows, oranges and reds begin to replace the normal summertime greens that we suddenly take note of leaf colour. Still, whether they are yellow, red, purple or green, all leaves are coloured by the various pigments they contain.
Chlorophyll is probably the best known of all plant pigments. It is the one that colours plants green, and plays an essential role in photosynthesis. Put in the simplest possible terms, photosynthesis is the process where sunlight is captured by green plants and used as an energy source in the construction of sugar molecules. The sugar molecules formed during photosynthesis serve as the plant's primary source of food. Since plants, in turn, serve as the primary source of food for all other living things, chlorophyll is at least indirectly responsible for supplying food to all things living on earth. As if that were not a bold enough claim, the process of photosynthesis is also responsible for providing oxygen. When we consider these two roles played by chlorophyll, it becomes almost impossible to overstate the importance of this common plant pigment. Certainly, chlorophyll does a lot more than simply colouring a plant green.
Still, chlorophyll is a lot like good health -- we only tend to notice it once it's gone. We tend to think about leaf colour in fall simply because this is when teh leaves are no longer green. A few plants have non-green leaves throughout the summer, however. For example, the leaves of a `Schubert' chokecherry are green in spring, but distinctly red-purple throughout most of the summer. In this case, chlorophyll is present and functional in the leaves throughout the summer, but as each leaf ages, another type of plant pigment known as an anthocyanin begins to accumulate, tinting the normal green to a red-purple. Anthocyanins pigments range in colour from red to blue, and are responsible for most plant parts that are coloured red blue and purple. In a large number of plants, the level of anthocyanins tend to build if the plants are exposed to bright light when the temperatures are low. This is also why we see an increaseing reddening of some plants in fall.
Carotenoids are the third major type of plant pigments. These pigments normally range in colour from yellow and orange to red. It's easy to remember the name of these orange pigments once you know that they were first identified as the colouring agents in carrots. Yellow carotenoid pigments are present in most leaves during summer, but because chlorophyll is also present at this time, their yellow colour is concealed. When a plant suffers from a lack of nitrogen, however, the level of chlorophyll decreases, and the yellow colour of the carotenoid pigments become more obvious. Very often, when a plant turns yellow, it is because the chlorophyll level has decreased, and the carotenoid pigments have simply become more obvious.
So how do all of these various pigments affect the development of fall colour in trees? Most healthy plants have a high level of chlorophyll, so they appear bright green. As temperatures begin to decline, and days begin to shorten in late summer, the chlorophyll in the leaves begins to break down. The same conditions which lead to chlorophyll breakdown, also lead to increases in anthocyanin and carotenoid pigments. The leaves are therefore losing the green colour as the yellow and red colours are increasing. The most dramatic show of colours is therefore seen on those autumns where there is a well-defined shift away from warm summer temperatures, to cool autumn temperatures. These conditions speed the breakdown of chlorophyll, and increase the productions of anthocyanins and carotenoids, to produce bright clear colours. While early cool temperatures bring about the best colours, an early frost can diminish the show of fall colours as it kills the leaves outright, before the gradual shift in colour can develop.
In addition to autumn weather conditions, colour development is highly dependent upon the species of plant. The majority of plants suitable for prairie landscapes tend to show yellow fall colour. For this reason, it is often desirable to include a few landscape plants that will provide some red to the mostly yellow fall landscape. Nannyberry, or Viburnum lentago is perhaps our most reliable shrub for supplying red fall colour. This large shrub is usually the first to turn red in fall, and will reliably provide a deep red wine colour to the landscape for several weeks. While most of the red-stemmed dogwoods also provide red leaf colour in fall, plants which originated in more northerly locations tend to colour up first, with the brightest colour development. This is why `Siberian Coral' dogwood is among the most popular of dogwoods for fall colour.