MIYAZAKI - The silent eyes of the forest

In 1974 the Japanese Ministry of Agriculture, Forestry and Fisheries launched an experimental forestry program in Southern Japan, near Nichinan City, Miyazaki prefecture, to study the influence of concentration of trees on their growth. The experiment consisted in planting a total of 360 Obi cedar trees along 10 concentric circles with various radii centrifugally increasing and resulted in the very peculiar eye-like patterns that can be observed today, after 50 years of growth (see Figure ). The experiment was designed in order to calculate the optimum distance for tree planting so to create the biggest yield of timber in the smallest possible space. The cedars were planted with a density ranging from 400/ha (in the center) to 10/ha (on the outside), and their heights now range from 15 m in the center to 20 m on the outer circle, with chest height diameters from 14 cm in the center to 40 cm on the outer circle. This experiment demonstrated clearly that higher concentration of trees (in the central part) resulted in smaller trees and closer planting limit their growth. Moreover, trees also grew by fanning out into the outer forest. The Miyazaki forestry compound represents an enchanted mix of regular eyes-like layout, surrounded by a naturally random tree distribution. In light of the studies that led Giorgio Parisi to win the 2021 Nobel Prize for Physics, one can argue that the “eyes” of Miyazaki forest represent a source of “frustration” for the surrounding forest. Although this forestry experiment is now considered over, we advocate in this project that this unique tree pattern should be further studied from a mechanical standpoint. Does the peculiar “frustrated” geometry impact the fingerprint dynamic behaviour of this fascinating forest pattern? There are huge chances that the Miyazaki forest remarkably alters the surface and seismic wave propagation at a sub-wavelength regime (Colombi, Roux et al., 2016a). Recent studies (Ungureanu et al., 2019, among others) proved that similar configurations of vertical resonators are extremely effective for seismic isolation and vibration control, in order to design the metacities of the future. Can the hidden features of such Miyazaki forestry compound be exploited for innovative vibration control strategies in civil engineering, in a 10-100 Hz frequency range? Once planted, the Miyazaki forestry compound needed fifty years to naturally grow as it is at the present day. However, despite the impossibility of controlling the design of experiment, no other experimental set-up of this kind has ever been studied at real scale so far. For sure, such unique contingency could not be met by any means, during a three-year project. This testifies the interest in retro-engineering the Miyazaki forestry compound to understand its modal behaviour and its possible interaction with seismic waves. Following a semi-analytical, numerical and experimental approach, this project aims at unveiling the complex dynamic behaviour of the Miyazaki forest to inspire innovative and eco-friendly vibration-control solutions.