Deciduous tree - Rakuyoju

A tree that loses its leaves during the unsuitable growing season. There are rain-green trees that lose their leaves during the dry season and summer-green trees that lose their leaves during the cold season. Rain-green forests develop in tropical to subtropical regions with a dry season of 4 to 6 months and an annual rainfall of 1000 to 2000 mm, and are distributed around rain-green forests at the transitional area between tropical rain forests and savannas. Teak (Tectona grandis LF) and sal ( Shorea robusta Gaertn.) are components of rain-green forests in South Asia and are also well known as afforestation species. The forests are used for grazing, and in addition to wildfires, artificial burning is also carried out during the dry season. Because of this, the understory vegetation is burned, so the canopy trees are fire-resistant. On the other hand, summer-green forests develop widely in the northern temperate zone of the Northern Hemisphere. Although leaf loss is an adaptation to low temperatures, leaf loss can also occur when temperatures are maintained high. The northern limit of deciduous forests is 120 days or more with an average daily temperature of 10°C, below which they are replaced by coniferous trees.

[Masahiko Osawa]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

生育不適期に葉を落とす樹木のことで、乾期に落葉する雨緑樹と低温期に落葉する夏緑樹がある。雨緑林は、4～6か月の乾期をもち、1000～2000ミリメートルの年降水量を有する熱帯から亜熱帯地域に発達し、熱帯多雨林からサバンナへの移行部で、多雨林を取り巻くように分布する。チークTectona grandis L. F.、サラソウジュ（サル）Shorea robusta Gaertn.は南アジアの雨緑林の構成種で、造林樹種としてもよく知られている。林内は放牧に利用され、乾期には野火のほか人為的な火入れも行われる。このため、下層植生が焼かれるので、林冠木は耐火性をもっている。一方、夏緑林は北半球の温帯北部に広く発達する。落葉は低温に対する適応であるが、落葉は温度を高く維持してもおこる。落葉樹林の北限は日平均気温10℃が120日以上までで、それ以下になると針葉樹に置き換わる。

［大澤雅彦］

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