Some trunks form a unique shape and architecture. Trunks of Ponderosa pine grow twisted. If the bark is removed, the spiral architecture of the trunk’s wood can be observed. There were many theories regarding reasons of such growth behavior. Seubpong Leelavanichkul and Andrej Cherkaev suggest, that the reasons could be mechanical, showing the strength of twisted and normal wood structures (Leelavanichkul and Cherkaev 2004). Regardless of the reason of such unusual wood architecture, it has a strong implications to be considered in the creation of visually interesting trees.
There are many insects, which feed or live in bark and tree trunks. Mainly these are insects from the beetle, to the moth and horntail wasp groups. Usually only weak or dead trees are attacked. The insects which attack healthy trees are called “primary invaders”. “Secondary invaders” are insects that start living in a tree only after the tree has already been strongly damaged by other hazards or diseases. Secondary invaders are of the previously mentioned species, but also include termites, carpenter bees and carpenter ants. Insects can not only harm a tree but also kill it. Usually the presence of such invaders is only noticeable after parts of tree start to tear off the main structure and tree starts to die. Many of such insects produce sawdust like particles which are excrements (Buss and Foltz 1993).
Fig 1. Wood-boring insect’s damage to Flat Headed Apple tree. Picture taken from:
Frank Hale. 2007. Drought and Wood-Boring Insects, Ornamental Pest and Desease Update, UT Extention.
It has been noted that weakened trees are sometimes heavily covered with moss. The reason for this is that such trees often loose big amounts of leaves, which allow for more light to reach the trunk and branches’ surfaces. The increased light makes moss grow very fast (Gilman 1991). While it is not a tree defect, it should be addressed when wanting to model visually appealing trees.
Trees can be severely injured by fire which could be a normal forest fire or a fire resulting from lightning. Bark serves as a protection layer. It is a low conductor of heat, and saves inner layers of wood from fire. Bark protection is different amongst different tree species. The Bark’s texture and shape also has an influence on its’ fire resistance. Cross sections show that trees after a fire with thicker bark, haven’t been badly injured, while thin barked Red Maples were more frequently and severely damaged. The majority of the trees survive fire damage, and only small fire wounds are left. Usually trees manage to grow new wood layers, and therefore hide burned wood. Well defined injury boundaries usually result from a successfully stopped decay. Small and well defined wounds usually are surrounded by healthy new layers of wood (Smith and Sutherland 2006).