For us in the Southern hemisphere the leaves of our deciduous Bonsai trees are starting to change color and they will soon be on the ground. In the Northern hemisphere Spring growth is to be seen everywhere and I have noticed a proliferation of photos indicating just that on Social Media. The difference is that the daylight hours are getting less in the South and the opposite is happening in the North. Suffice to say then that it must be the availability and intensity of light that triggers these phenomena. Today I would like to explore the color changes in leaves during Autumn.
We know that there is a pigment in leaves called chlorophyll. It is the site where photosynthesis takes place in leaves. This is the process where water and carbon dioxide gas with the aid of light energy is converted into sugars for plants to use as food or to be stored.
As the days (light) get shorter, there is not enough light energy for photosynthesis to take place at optimum levels. The trees must rest and basically shut down and the only food available to get through winter, is what is stored. As this progresses, chlorophyll disappears from the leaves and the bright green fades away. Chlorophyll is not the only color pigment in leaves. As the green fades, the other color pigments become visible. These are the yellows and orange colors (carotenoids). In some trees, like maples, the glucose produced during photosynthesis gets trapped in the leaves and the sunlight causes the leaves to turn this molecule into a red color. It is known that the cooler temperatures have a role to play in this. There are other pigments present as well. Red anthocyanin pigments can also be produced during this time.
Currently, my trees, and especially the Maples, are not showing the bright red colors yet. This is late for this time of year. Our night time temperatures have not dropped sufficiently for this to happen. Other climatic factors play a role as well. What is needed for all the splendor of autumn to show in our Bonsai trees is a warm, wet Spring, followed by a Summer with average temperatures and an average rainfall as well as an Autumn with many sunny days and relatively cold nights. There is nothing we can do about any of these except for controlling water, but it might be possible to shift trees in one’s garden during Summer and Autumn to make use of micro-climates caused by other plants and the layout of your garden.
The old debate of whether Bonsai is Art or Horticulture is actually a non-event as we all by now know that a dead tree cannot be designed as a Bonsai and be admired as a living sculpture. We need both. The tree must be kept alive and sculpted to enable the Bonsai within to be shown.
I am in the process of re-reading a lot of my quite sizable Bonsai library. The majority of these books and magazines focus on the Art side of Bonsai cultivation. It is rare to find a good solid article or chapter in a book, dedicated to the Horticulture involved in Bonsai development. A quick search on the Internet shows more of the same. There is a bit more to be found on the Internet, but a lot of this is not necessarily linked to Bonsai specifically. A few examples are of importance here.
The first to mention is the practice of defoliation. When we remove the leaves from a tree, we drastically reduce the level of transpiration (loss of water through leaves), which is an important process supporting the flow of water through a plant. Partial defoliation obviously does the same, but just less. Then there is the practice of leave cutting. This will reduce the surface area of the leaves, which in turn will also have a reduced transpiration level as an outcome. Photosynthesis levels will also drop. There are very good reasons why we do this in the cultivation of Bonsai, but it is a process rarely seen in the cultivation of other plants.
Another of our Bonsai specific activities is the process of deadwood carving or carving in general. Deadwood is dead and nothing much needs to be taken into consideration except for aesthetic principles. When we carve into living wood, things change a bit. Tree tissues are laid down in layers. On the outside we have bark and under this we have the phloem, the tissue that conduct the nutrients produced in the leaves through photosynthesis. The next layer is the cambium. This tissue is responsible for the secondary growth in stems as well as roots. The next layer is the xylem. This tissue transports water from the roots to the leaves. Then we get to the lignin. These are dead cells and make up what we refer to as “wood”. This is the stuff we carve. To get to the lignin, we have to go through the living layers. This needs to be done carefully as we are interfering with some very important processes when we start carving. Not only do we interrupt the flow of nutrients and water, we directly impact on growth. This works well for us as well, as the cambium layer is the one that will help with the healing on the edges of the carving.
My advice is for Bonsai enthusiasts to read and learn about the science behind our activities as well. Not only will this knowledge help to keep more trees alive, it will help us understand the design process and principles better as well.