スギ若木のモジュールの形態, 構造及び動態 : 林冠ギャップ下と閉鎖林冠下の比較

Abstract

スギ・落葉広葉樹林の林冠ギャップ下と閉鎖林冠下におけるスギ若木の当年枝(モジュール)の長さ分布, 当年枝当たりの成長点数, 葉の形態, 当年枝の伸長様式, 葉齢を調べた.閉鎖林冠下における当年枝の長さ分布は正規分布であったが, 林冠ギャップ下では長い当年枝が見られ正に歪んだ分布となっていた.当年枝当たりの成長点は閉鎖林冠下(1.4個)よりも林冠ギャップ下(3.0個)で有意に多かった.当年枝の単位長さ当たりの針葉数と当年枝の単位長さ当たりの重さは閉鎖林冠下の方が有意に小さかったが, 軸に対する針葉の開度は林冠ギャップ下の方が有意に小さかった.当年枝の伸長期間は3ケ月前後と長期に及び, 林冠ギャップ下は閉鎖林冠下の1.6倍となっていた.これらの結果から, 光資源が乏しい閉鎖林冠下のスギ若木は針葉の重複回避によって効率的な同化生産のための調整を行う一方で, 比較的短い伸長期間にともなう成長点が少ない短くて軽い当年枝を伸ばすことによって樹冠の増加を抑えていることが明らかになった.また葉齢は閉鎖林冠下で有意に高かったが, これは被陰への対応に伴う若木の着葉量減少を葉の長寿命化によって補おうとした行動と推察された.

Morphology, structure and dynamics of current-year shoots (modules) of Cryptomeria japonica saplings were studied in canopy gaps and in closed canopy of a natural conifer-hardwood forest. Shoot length distribution was normal in closed canopy while the distribution skewed positively in canopy gaps. The number of growing points per shoot in closed canopy (1.4) was significantly lower than that in canopy gaps (3.0). The number of needles per shoot length and the shoot weight per shoot length were significantly lower in closed canopy although the open angle of needle along shoot was significantly lower in canopy gaps. The periods of shoot elongation (> 3 months) in canopy gaps was 1.6 times longer than that in closed canopy. These results clarified that C. japonica saplings adjust needle morphology to achieve efficient photosynthesis through avoiding mutual shading among needles and reduce the increment of crown by producing shorter and lighter shoots with lower number of growing points in the light limited condition of closed canopy. In addition, needle longevity was significantly higher in closed canopy, suggesting that C. japonica saplings compensate the reduced amount of needles by prolonging needle longevity in closed canopy.