Coping with external dynamic loads such as wind, rain, snow, etc. is essential for branches to maintain its mechanical safety. Among these loads, wind load is regarded as a particularly important element. The maximum stress that occurs in a branch due to a load gets smaller as the diameter increases. Therefore, the form of branches has a close relationship with branch safety against loads. The impact of wind load on the tree stems, and the safety of stems against mechanical failure caused by wind are estimated by the calculation with several assumptions about wind profile (Niklas 2000). However, direct measurement in the field is also necessary for clarifying the reality of the stresses caused by wind in nature.

To clarify the influence of actual loads on branches, we performed direct measurement of the strains of several branches. The data of the strains that actually occurred in horizontal branches of Fagus crenata in the field were collected using strain gauges for a year, and the corresponding stresses were calculated from this data and the diameter of the branches. We also collected the data of wind speed and atmospheric temperature around the branches. From these data, we will compute how much the influence of the wind element to the branch, and will present the results.