せき悪地に造成したスギ林の施肥効果 : (II) 施肥後の成長促進過程

Abstract

極めて劣悪な場所に造成された28年生スギ林の施肥前の生産力調査に続き, 施肥後3年目の生産力調査とその間の成長経過を調べた。この一連の調査によって, 施肥後の高い成長速度がどのように達成されたかを検討し, 次ぎの諸点が明らかになった。1. 施肥3年後に, 葉量 (y_L) は約2倍に, 生産能率 (P_n_*/y_L) は約3倍になり, 成長促進にはy_LとP_n_*/y_Lの両者が影響していた。2. 個体でも林分でも, 施肥した年の成長促進は明確でなく, 2年後, 3年後と次第に速度が高まった。その間, 施肥によって新葉量が年々増大し, その結果として葉量, 生産能率が促進され, 生産力の増大が起こると推測された。3. スギ林の生産量 (地上部生産量; P_n_*) と葉量とは一次式の関係 ((2) 式) があてはめられ, この結果として葉量と能率の関係が定式化され ((3) 式), 両者の関係が明かとなった。施肥によって葉の生産能率が著しく高まるのは, 土地条件が劣悪で葉量が少ないスギ林であり, 葉量が多く閉鎖したスギ林では葉の能率はそれほど大きくならないものと予測された。4. 施肥によって新葉量 (y_<NL>) の増加が促進されるが, 葉量に対する新葉の割合は安定した値を示した ((4) 式)。この性質と (2) 式の性質から, 地上部生産量のうちの葉への分配率 (⊿:y_L/P_n_*) が決まり, 分配率と生産量の関係は双曲線関係 ((5) 式) で示された。

To examine the forest productivity corresponding to forest fertilizer, the production rate of the above ground parts (Pn*) in three years after fertilization and growth courses of stem, branch, leaf efficiency were investigated in a 31-year-old poor forest of Japanese ceder (Cryptomeria japonica D. Don), and were compared with the same stand data examined in 1985 before fertilization. The annual stem and branch production were estimated by usual allometric method, and the leaf production rate was estimated by the method of cutting the new part, as the former study. The biomass and growth rates of them were shown in Table 1. The leaf biomass (yL), net production and leaf efficiency (Pn*/yL) were raised about three times during these three years, though this stand were very low due to the sterile condition. Stem and leaf growth and leaf efficiency were accelerated gradually, reached maximum at three years after fertilization (Fig. 2). These initial slow growth were seemed to be a feature in species of long span of life of leaf such as Japanese ceder. By the temporizing improvement in soil condition by fertilizer, the high allotment ratio of leaf production to the Pn* (⊿yL/Pn*) declined and leaf efficiency (Pn*/yL) increased. The change of the allotment ratio was shown by the hypabola relation to leaf biomass (Eq. (3), Fig. (5)), and the increase of leaf efficiency was shown by the other hypabola relation to the Pn* (Eq. (5), Fig. 6, B). These relationns introduced from the empirical equations (2) and (4). The response of the fertolizer may be clear in the low leaf biomass, and become obscure in high leaf biomass stands, as shown from Eq. (3).