テーダマツ壮令林の物質生産機構

Abstract

外国産マツ林に関する物質生産機構の一連の研究として, 熊本営林署管内西木原国有林の標高200m, 傾斜約18°のほぼ西向の丘陵地に造林されている34年生テーダマツ (Pinus taeda) 林の総合的な物質生産のしくみを明らかにしようとした。1970年10月初旬の調査時において, 2林分の平均で立木本数はhaあたり約700本, 平均樹高約21m, 平均胸高直径約26cm, 断面積合計約41m_2/haでほぼ閉鎖し最多密度に近い状態にあったが, 自然間引による枯死木は少なかった。本調査林分の資料木13本とこれまで調べられた幼令林の資料を合せた胸高直径 (D, cm) と樹高 (H, m) のD_2Hに対する幹材積 (V, cm_3) と幹乾重 (w_S, kg) の相対生長関係は比較的適合度がよく, その近似式はつぎのようであった。log V=0. 9439 log (D_2H)+1. 7692 log w_s=0. 9952 log (D_2H)-1. 7715 この両式の勾配値は, 胸高に比して樹高の低い幼令林からもとめた値より大きく, 上式は今後わが国で育成されるテーダマツのD, H_2変数の立木材積表として暫定的に適用できるであろう。調査した2林分の林分構成が類似していたので, 地上部各部分の現存量はD_2に対する相対生長関係を用いて推定した。調査時におけるテーダマツ林の幹材積現存量は2林分の平均で約320m_3/ha, 乾重現存量では幹約160ton/ha, 生枝約21ton/ha, 枯枝約7ton/ha, 葉約9ton/ha, 球果約0. 4ton/haと推定された。幹量は同林令の原産地の収穫表や付近のアカマツ, クロマツ林に比較して上位にあたる。林分葉量は密なテーダマツ幼令林より30％ほど少ないが, 夏の台風害による影響があったものと思われる。しかしマツ属中他樹種に比較してかなり多い。最近1年間の幹生産量は約14m_3 (7. 4ton/ha) で, 幼令林ほどではないが, 同様の立地環境にあるアカマツ, クロマツに比較してきわめて高い生産力を示した。林分の生産構造はアカマツに類似し, 林冠下の平均相対照度は24. 1％, 相対照度20％以上の "日もれ" は全体の1/3以上を占め, 比較的明るかった。したがって地床植生も多く乾重で約3. 9ton/haあった。A_o層の堆積量は17 - 19ton/haでかなり多かったが, その平均分解率は25％程度でマツ類の中で非常に高かった。調査地の土壌は両林分とも砂岩, 頁岩, 礫岩を母材としており, 土壌層の厚さはほぼ100cmであった。土壌型はA層が薄く, B層の厚いB_B型であった。土壌の通気透水性などの理学的性質はかなり良好であったが, 化学的性質は強酸性を呈し, 特に可給態のPの含有率は低かった。テーダマツの葉の養分要素含有率は白浜試験地で測定した幼令木とあまり差はなかった。しかし幹の含有率は幼令木に比べ, Nが減少し, Caが多い傾向がみられた。全生態系内の養分要素の現存量はhaあたりN約6, 200kg, P_2O_5 約630kg, K_2O 約890kg, CaO 約1, 410kg, MgO 約790kgで, これまで調査してきた幼令林分に比較して明らかに多く, 高nい生産力を示したことが理解できる。しかし生態系内の配分比率をみると, P, K, Caの3要素の土壌中の余裕は比較的少ないように思われた。各養分要素の平均分解率はアカマツに比べると非常に高く, このことがテーダマツの生長量の大きいことの原因の1つになっていると思われた。また毎年幹に吸収される各養分の吸収速度から, 土壌中の養分要素の消費の速さを推定した結果, NとMgはかなり余裕があると思われたが, 可給態のP, K, Caは将来不足をきたすのではないかと思われた。

In a series of studies on the productivity of man made exotic pine stands in Japan, some important mechanisms in matter production were investigated in a 34-year-old Loblolly pine (Pinus taeda) stand planted in the Nishikihara national forest (Kumamoto Pref.). Emphasis on the biomass of the upper ground parts, productivity of stem, production structure, soil conditions and the circulation of nutrients within the ecosystem are discussed in this paper. This stand is situated about 200 meter above the sea-level and a gentle slope facing the west. Measurements taken in October, 1970, of stand density, mean diameter breast height, mean height and basal area of sample plots are shown in Table 1. The biomass of the upper ground parts per stand was estimated by the method of allometric relation between each parts and D2H or D2 (D; diameter breast height, H; tree height). Sample trees of various sizes were cut down at the base, and the stem, branches and needles of each tree were separately weighed using the stratified clip technique. The fresh weight data were converted into oven-dry weights, and the volume of stem was measured by stem analysis. The allometric relations of the volume (V, cm3) and the dry weight (ws, kg) of stem to D2H (D, cm, H, m) closely fitted in a linear relation shown in the logarithm (Fig. 1). These regression formulas were found as follows: [Figure omitted] The above formula can provisionally apply to the standing stem volume-table of Loblolly pine in Japan. The stem volume per hectare was estimated at about 320 m3 on the average of sample stands, and the dry weight of stems at about 160 tons, living branches at about 21 tons, dead branches at about 7 tons, foliage at about 9 tons and cones at about 0.4 tons respectively (Table 2). The biomass of stems and foliage tended to be markedly larger in value than those of Japanese Red pine or Black pine forest having similar stand composition. However this foliage biomass was smaller by about 30% than the value of young Loblolly pine stand in Shirahama Experimental Station of Kyoto University Forest (Wakayama Pref.). As a result of seasonal changes of needle-fall amounts in young Loblolly pine stand as reported, the amount of foliage measured before the defoliation of needles of the previous year was presumed to be the largest of all seasons. The current annual increment of stem volume was estimated at about 14 m3 (7.4 tons in dry weight)/ha. And its production seemed to be remarkably larger than pine forest of other species in Japan, but its increment was smaller by about 50% than a young Loblolly pine stand in Shirahama. As regards the vertical distribution of foliage, it seemed that the Loblolly pine stand was the herb-type whose foliage appeared mainly in the upper strata of the crown as shown by a production structure diagram (Fig. 8). The average light intensity under the crown of sample plot showed 24.1% of full daylight, and the rate over 20% of relative light intensity accounted for about 30% of the whole. The ground flora in the stands was rich and was estimated at about 4 tons/ha. The oven dry weight of the A0 layer (consisting of litter, raw humus and humus) in the sample plots was estimated at about 18 ton/ha, the average decomposistion rate of the A0 layer was accounted at about 25%, so it seemed that the decomposition rate of A0 layer in the Loblolly pine stand was higher than that of the Japanese Red pine stand (Table 3). The characters of soil profile in the sample plots are shown in Table 4. The parent rock of the soil was composed of sand ston, shale and conglomerata, and the depth of soil layer was about 100 cm. It was presumed that the permeability of water and air in soil was suitable for growth of trees. The concentration of carbon and nitrogen in the soil were higher than those of other exotic pine stand in Japan, but the available phosphorous concentration was remarkably small (Table 5, 6). The concentration of nitrogen in the stem of the thrifty Loblolly pine stands was lower than the young stands in Shirahama, on the contray the calcium concentration was higher (Table 7). The amounts of total nitrogen in the forest ecosytem consisting of the upper ground parts (ground flora included) and the soil to 100 cm depth (A0 layer included) was estimated at about 6, 200 kg/ha, and as the amounts in soil was calculated by available forms, P2O5 at about 630 kg/ha, K2O at about 890 kg/ha, CaO at about 1, 410 kg/ha and MgO at about 790 kg/ha respectively (Table 8, 9). The amounts of each nutrient were found in abundance generally. The average decomposition rates of each nutritive element were higher in Loblolly pine stand than those of other species pine stand in Japan (Table 10). Although a part of those nutrients accumulate annually in the stem, the amounts of nitrogen and magnesium in the soil remain at over about 200 times the annual up take, and potassium at about 70 times, but phosphorous and calcium at only about 30 times (Table 12). Judging from the circulation of nutrients, it seems that phosphorous and calcium quantities in the ecosystem were not sufficient for the continuation of present growth.