Maintaining the soundness of multi-storied plantations is extremely difficult

Evaluating the soundness of growth performance of planted trees in old-aged multi-storied plantations of Japanese cedar (Cryptomeria japonica) under intensive forest management

Kazuki Miyamoto (a), Tatsuya Otani (b), Atsushi Sakai (b)(c), Takeshi Sakai (a), Shiro Okuda (d)(e)

(a) Department of Forest Vegetation, FFPRI, Tsukuba, Ibaraki, Japan.

(b) Shikoku Research Center, FFPRI, Kochi, Japan.

(c) Tohoku Research Center, FFPRI, Morioka, Iwate, Japan.

(d) Kansai Research Center, FFPRI, Kyoto, Japan.

(e) Department of Plant Ecology, FFPRI, Tsukuba, Ibaraki, Japan.

Bulletin of the Forestry and Forest Products Research Institute, 19(1)(No.453), 45-53, March 2020, FFPRI,
URL: https://www.ffpri.affrc.go.jp/pubs/bulletin/453/453toc-en.html

We can see some artificial forests of Japanese cedar and cypress that are thinned and planted with young Japanese cedar and cypress trees underneath to form multiple stories such as lower, middle and upper stories. These kinds of artificial forests are called "multi-storied plantations". One of the advantages of multi-storied plantations is that there are no large areas of bare ground that are created when trees are harvested. However, lower story trees can be damaged when upper story trees are harvested, and inadequate light conditions might hamper the growth of lower story trees. Furthermore, little is known about the growth process of planted trees in old-aged multi-storied plantations - with their upper layer being more than 100 years old.

For the past 16 years, we have studied the growth process of lower and mid story trees in a multi-storied plantation of Japanese cedar that was more than 110 years old and which has a planting history of at least 30 years. As a result, we found that upper story trees, and mid story trees immediately beneath them, had a high diameter growth rate of 8mm/year. However, because the diameter growth rate of lower story trees was suppressed at about 2.5mm/year, their height/diameter ratio exceeded 100¹ (that is, they were extremely slender), and there are concerns that they will be vulnerable to weather-related phenomena such as snow damage.

Over a period of many years, the owner of this multi-storied plantation has meticulously observed, cultivated, and managed each tree. However, even in such well-tended multi-storied plantations, it became clear that growing healthy lower story trees is extremely difficult. This finding can be used as new scientific knowledge for considering long-term management of old-aged plantations.

Note1. This is an index that indicates the shape of a tree's trunk by the proportion of the tree's height (cm) to its diameter (cm). A value of 70 or higher indicates particular vulnerability to weather-related damage.

Photo 1. Scene from the interior of an old-aged multi-storied plantation. The planted trees in the lower story are very slender, and there are concerns that they may be damaged in the future by weather-related phenomena.