小径木間伐に関する研究 (I) : 第1回間伐前後の林況の変化について

Abstract

人工一斉林において小径木から順に間伐した場合, その林分の状態はどのように変わり, またどのような径過をへて林分が回復していくかを知るために試験地をもおけて実際に間伐を行った。本論文では, このうち主に間伐による林分の現存量ならびに林内照度の変化についてのべる。試験地は奈良県吉野郡東吉野村にある10年生のスギ林である。試験地の概要は次のとおりである。1) 各プロットとも緩斜地にあるが, プロット1, 2, 3, は比較的谷筋に, プロット4, 5はむしろ尾根筋近くに位置する。2) 林内植生はプロット1 - 4では見当らなかったが, プロット5にはササ, ススキなどの存在が認められた。3) 立木密度は4, 000本/ha前後であった。4) 林分平均樹高はプロット1 - 3が7. 5 - 8. 0mで, プロット4, 5は6. 1 - 6. 5mであった。5) 胸高断面積合計はプロット1 - 3が約30m_2/ha, プロット4, 5が約23m_2/haであった。間伐前後の樹体各部の林分現存量は, 試料木 (伐採木) の単木現存量と各プロットの胸高直径と樹高から相対生長法をもちいて推定した。間伐前の現存量は, 幹でプロット1 - 3は約43ton/ha, プロット4, 5で27ton/haであった。この差は主として林分平均樹高の差により生じている。従って立地条件の差によるものと思われる。枝, 葉の現存量は林分によってほとんど差はなく, 枝量は約4ton/ha, 葉量は17ton/ha前後の値と推定された。これらの値は立木本数4, 000本/ha前後の10年生のスギ林の蓄積としては, 異常な値ではない。樹幹解析による幹の生長量は, プロット1 - 3で8 - 9ton/ha, プロット4, 5で5ton/haと推定された。これらの値は, プロット4, 5は少し悪いが, 10年前後のスギ林の生長量として普通の値である。間伐によって取り去られる本数割合と現存量割合は, 大略本数で1/4間伐すると1/10程度の現存量が失われ, 1/2で約1/3が失われる。林内の相対照度は間伐前が4％前後 (プロット5は12％), 間伐後は葉の除去される割合でことなるが, その割合とほぼ比例して明かるくなり, 25％の葉が除去されると林内相対照度は約3・5倍, 50％で約6倍の明かるさを示した。

The investigation has been carried out mainly to test how the standing crop and relative light intensity of the plots were changed by the thinning. The authors attempted in this paper to describe the stand conditions in terms of standing crop and light intensity at the time of first treatment, i.e., thinning was conducted. Five experimental plots were chosen in pure Cryptomeria japonica stands of about 10 years old, located in Yoshino District, Nara Prefecture. The plots were treated with thinning ratios of different levels. The trees measured in the plots were arranged in order of diameter. The selection of trees to be thinned was to begin with the removal of smaller sized trees. In the experimental treatment, the highter the thinning ratio, the larger the critical diameter was raised. The topographical and floristic character of the plots were as follows; 1. The five plots are situated on a very gradual slope. Plots No. 1-No. 3 are comparatively near a ravine, but Plots No. 4 and No. 5 are near the ridge. 2. No undergrowth is found in Plots No. 1-No. 4, but Plot No. 5 has a few bamboo grasses. 3. The stand density is about 4, 000 trees/ha in each plot. 4. The mean height of Plots No. 1-No. 3 is 7.5-8.0m and that of Plots No. 4 and No. 5, 6.1-6.5m. 5. The total basal area at breast height of Plots No. 1-No. 3 is about 30 m2/ha and that of Plots No. 4 and No. 5 about 23 m2/ha. Utilising the regression equation of allometry, the standing crop of boles, branches and leaves as well as annual increment of the boles in terms of weight (ton/ha) in each plot were estimated. The amount thinned and the amount left, after thinning makes the standing crop before thinning. The standing crop of the boles before thinning averaged 43 tons/ha for Plots No. 1-No. 3 and 27 tons/ha for Plots No. 4 and No. 5. These differences noted were perhaps due to the mean height of the trees in the plots, which in turn is affected by the habitat. The standing crop of the branches and the leaves were estimated at about 4 tons/ha and 17 tons/ha respectively. No significant differences were apparent among the plots. It seemed that these standing crops were normal for Cryptomeria stands of 10 years old whose density is about 4, 000 trees/ha. The annual increment of the bole found by stem analysis was estimated at 8-9 tons/ha at Plots No. 1-No. 3 and at 5 tons/ha at Plots No. 4 and No. 5. The estimated values for Plots No. 1-No. 3 seemed to attain the general standard for 10 years old stands in the region concerned, but that for Plots No. 4 and No. 5 seemed lower than the general standard. When one-fouth of the trees in the stand were removed, the standing crop after thinning becomes nine-tenths of that before thinning. When half of the trees were removed, the standing crop becomes two-thirds. The relative light intensity at ground level before thinning avereged 5% (except Plot No.5 in which it averaged 12%). After thinning, the light intensities on the floor became higher according to the ratio of the removal of the leaves. The increasing ratio of the relative light intensity was in proportion to the thinning ratio of the leaves. When 25% of the leaves was removed, the relative light intensity of the floor after thinning was about three and a half times as intense as that before thinning and when 50% was removed the intensity became about six times higher than that before thinning.