To persist and survive in any environment, a tree must be mechanically reliable and achieve a reasonable factor of safety. This safety factor, which must be adjusted over the course of plant growth, can be computed for stems and roots based on the quotient of the working load and load-bearing capacity of each organ. This method is illustrated for the stems of cherry trees (Prunus serotina) and for the root systems of an arborescent columnar cactus (Pachycereus pringlei) differing in size. The factor of safety of these organs decreases with increasing plant size. The susceptibility of stems and roots to mechanical failure thus increases as plants grow in mass or height. However, the risk of wind-induced tree failure is reduced in the case of cherrytrees by the selective wind-failure of small peripheral branches, which reduces the overall drag forces and bending moments acting on trunks. In the case of the columnar cactus, the demand for root water absorption/storage may take precedence over anchorage as plants increase in overall size. These two case studies illustrate that manifold factors of safety must be considered and ranked simultaneously in terms of the probability of damage or death for different environmental risk factors.
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