Top left: Melia volkensii (mukau): from Kenya’s dryland to a glass vessel and back Top right: Jatropha clones produced in-vitro
By Stefaan Werbrouck*
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Seeds versus clones ost trees propagate through seeds. Unlike crops like pea, bean or lettuce, trees are heterozygous1. Trees preferentially cross-pollinate and their offspring vary greatly. We all experience what it means to be heterozygous. Every child is unique and has a different mix of traits that can be recognised in its parents and grandparents. Some trees prefer to propagate themselves vegetatively. For instance, in the American Rocky Mountains, nearly all aspen reproduce asexually through root suckering - new shoots sprout from horizontal roots and grow into trees that are genetically identical to their neighbours. The largest known aspen clone - nicknamed Pando - contains 47,000 trees on about 40 hectares.. Other trees, like willow, initiate roots in every branch or twig. When the branch breaks off, it can easily and readily make roots. 1
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Heterozygous: when an individual has two different alleles (or coding sequences of a gene) for one single treat, inherited from the organism’s two parents.
Ficus bengalensis, a remarkable tree found in India and tropical Africa, sends down from its branches a great number of shoots, which take root and become new trunks. A single tree thus may spread over a large area and look like a small forest. The largest specimen grows in Sri Lanka. It has 350 large trunks and over 3,000 small ones. Ages ago, men recognised the benefit of vegetative propagating plants - a clone preserves the selected characteristics of the mother plant. Nowadays, layering, cutting and grafting are common practices in horticulture practice. A number of fruit crops, vegetables and ornamentals can be cloned easily this way. Nevertheless, this is not so evident in forestry. Forest productivity can be increased by planting tree farms with fast-growing high-value trees that produce more wood, fruits, energy or chemicals. But a number of problems arise when a forester wants to plant clones of a super tree, meaning an exceptionally good individual from a certain species, either because of vigour, shape or other characteristics.
Which super tree? When there is no breeding programme in a particular tree species, super trees have to be selected by mass selection. By walking through a wood and by judging good and bad trees, a good forester can select a super tree. But what he sees is the phenotype of the plant: the result of its genes and its environment. A good-looking tree just could have a lot of luck, by growing on a very fertile soil with good water supplies. Clones from super trees have to be tested in different places. After several years, only a few of them will prove to be outstanding. Old trees’ cuttings root badly A tree that is too young to bloom is called “juvenile”. When a tree is recognised and selected as a super tree, it is mostly an old, flowering, “adult” tree. By every cell division in their growing points, trees gradually lose a number of juvenile characteristics. They may lose thorns or change leaf shape. What is more important, they become very recalcitrant to vegetative propagation by cuttings. These tree species that Miti January-March 2009