森林立地学会誌 森林立地45(2),2003,99〜102
Jpn.J.For.Environment
45(2), 99-102 2003
スギ樹冠における葉齢別窒素含量の季節変動
小林 元＊・田代直明＊＊
＊九州大学農学部附属北海道演習林・＊＊九州大学農学部附属演習林
スギの葉の窒素含量の季節変動を葉齢別に調べた。面積あたりの窒素含量は，当年葉においては6月から10月に増加し，その後，低下した。１年葉においては月を経るにつれて緩やかに低下したが，２，３年葉においては明瞭な季節変動は見られなかった。窒素含量は，葉齢別では齢の古い葉ほど低い値を示した。葉の寿命は３年で，落葉時における窒素の回収率は49%であった。窒素含量から既報の文献を用いて光合成速度を推定し，さらに葉の寿命と窒素の回収率を用いて窒素利用効率を計算した。得られた窒素利用効率は，これまでに報告されている常緑針葉樹のなかでも中程度の値であった。葉の寿命が短く，窒素の回収率もそれほど高くないスギは，窒素量あたりの光合成速度を高めることで窒素利用効率を高めているといえた。
Hajime Kobayashi and
Naoaki Tashiro : Temporal variation in leaf nitrogen content in a Cryptomeria
japonica canopy
The temporal variation in leaf nitrogen content was investigated in an
8m tall Cryptomeria japonica canopy. Leaf nitrogen content (expressed on
area basis) decreased with increasing leaf age. The nitrogen content of current
leaves increased from June to October and then decreased thereafter. Nitrogen
content of one-year-old leaves decreased gradually as the season progressed.
There was no clear variation in leaf nitrogen content in two- and
three-year-old leaves. The oldest leaf was three-year-old, and about half
amount of the nitrogen was resorbed prior to leaf abscission. Nitrogen use
efficiency was calculated from leaf life-span, resorption efficiency, and
photosynthetic capacity. Photosynthetic capacity was estimated from leaf
nitrogen content. Nitrogen use efficiency of Cryptomeria japonica was
intermediate to that of other conifer species previously described. We
concluded that Cryptomeria japonica, which has short-life leaves and
moderate resorption efficiency, increases nitrogen use efficiency with
increasing photosynthetic capacity per unit leaf nitrogen.