スラッシュマツ幼令林の物質生産機構

Abstract

この研究は1961年3月, 京都大学農学部白浜試験地に密度と施肥量を組合せて植付けられ, 立木密度の高い一部の林分はすでに閉鎖状態に達した8年生のスラッシュマツ (Pinus elliottii) 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ものである。施肥した林分にはチッソ, リンサン, カリがそれぞれ15 : 13 : 12の組成の燐安尿素系肥料を, 植付け密度に反比例させて1本あたり100, 50, 25gの基準量とその倍量ずつ植付け時と翌年の2ヵ年与えた。1968年10月現在, 施肥した林分の立木密度はhaあたり2, 200 - 2, 000本 (疎), 2, 800 - 4, 000本 (中), 5, 400 - 5, 800本 (密) で, 平均胸高直径はそれぞれ12. 1 - 12. 2cm, 10. 4 - 9. 5cm, 7. 9 - 9. 6cmに達し, 平均樹高は密度に関係なくほぼ6 - 7mであった。しかし施肥しなかった林分はいずれの密度でも生長が悪く, 平均樹高はまだ2mに達していなかった。林分の地上部各部分の現存量は, 資料木の層別刈取りによる相対生長法から推定した。胸高直径 (Dcm) と樹高 (Hcm) のD_2Hに対する幹材租 (Vcm_3), 幹乾重 (w_s kg) ならびに密な林分の枝乾重 (w_B kg) と葉乾重 (w_L 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. 9. 38 log (D_2H) +0. 1332 log w_s=0. 9200 log (D_2H)－3. 3819 log w_B=1. 2610 log (D_2H)－59440 log w_L=10. 351 log (D_2H)－4. 5584 倍量施肥した密なスラッシュマツ林分の幹材積は196m_3/haで, 乾重現存量では幹72ton/ha, 枝9ton/ha, 葉18ton/haと推定された。葉量は落葉期直前の2年分の着葉量であったが, 隣接のテーダマツや他のマツ属よりかなり多かった。同じ林分の最近1年間の幹材積生産量は約28m_3 (10ton)/haで, テーダマツ林より約20％少ないが, 他のマツ属より著しく大きい。密な林分の生産構造は葉が上層にかたよった広葉型で, その生枝下の平均相対照度は約5％であった。そして林内の明るさに対する生枝下高や地床植生量の相関はかなり高かった。施肥した林分のA_0層 (落葉, 落枝, 粗腐植, 腐植) の堆積量は乾重で4. 5 - 11. 7ton/haとなり, 立木密度の高い林分ほど多かった。しかしスラッシュマツ林分の平均分解率は80 - 120％で著しく大きく, 落葉は1年前後でほとんど分解されるようであった。白浜試験地の土壌の構造ならびに理化学性はきわめて悪かった。しかし林分が成立することによって表層5cm位まではいくらか物理性の改善が認められる。肥料として施用したチッソ, リンサン, カリの3成分は, 葉中の含有率が無施肥のものより高くなり, また特に可給態のリンサン, カリの現存量は地上部 (地床植生も含む) とA_0層を含む地下35cmの深さまでの土壌を合せた生態系で, 根も考慮するとそれぞれの施肥量にほぼ等しい増加を示した。さらに物質循環上閉鎖系に属する可給態のリンサン, カリや置換性のカルシューム, マグネシュームは, 地上部に吸収保持された量だけ土壌中から減少していた。チッソ, リンサン, カリの推定平均分解率はきわめて高く, A_0層中の養分は1年前後でほぼ分解されるようであった。しかし毎年幹に蓄積される吸収速度から土壌中の可給態養分の残存年数を推定すると, 密な林分ではリンサン, カリが30 - 40年, カルシュームが15年分しかないことになり, 物質の循環からみてこれまでのような高い生産量を持続することは困難であると思われた。

The subject of the studies is to account for the important mechanisms in matter production in an 8-year-old Slash pine (Pinus elliottii) stand planted in the Shirahama Experimental Station of Kyoto University Forest (Wakayama Pref. ). This stand was planted in three densities and these three were fertilized in various amounts. 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. In the forest stands, after planting, ammonium phosphate-urea complex fertilizer containing nitrogen (15%), phosphorous (13%) and potassium (12%) were applied to plots of the three densities each year for two years in dosage of 100, 50 and 25 grams per tree. Also applications of 200, 100 and 50 grams were made in inverse density proportions. And three unfertilized plots were also observed. The amounts of elements applied per hectare for two years are shown in Table 13. Measurements taken in October, 1968, of stand density, mean diameter breast height (DBH), 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. 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 stem volume (V cm3), the stem dry weight (WS kg), branch dry weight (WB kg) and needle dry weight (WL kg) to D2H (D cm: DBH, H cm: tree height) in the high density plots closely correlated to with the linear relation shown in the logarithm (Fig. 1). These regression formulas were found to be: [Table omitted] In the high density plot with double quantity fertilizer applications, the stem volume per hectare was estimated at about 196m3, and the dry weight of stems at about 72 tons, branches at about 9 tons and foliage at about 18 tons respectively (Table 2). These biomasses except for the branches tended to be larger in value than those of Loblolly pines and pine forests of other genuses as reported by many workers. As a result of seasonal changes of the foliage biomass and leaffall amounts in a forest, 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 in the same plot was estimated at about 28m3/ha (Table 3) and its production seemed to be remarkably larger than pine forests of other genuses, but its increment was smaller by about 20% than a Loblolly pine forest planted contiguous to the Slash pine stand. The vertical distribution of oven-dry weight of stems, branches and foliage differed in various closed plots as shown by a production structure diagram (Fig. 7). As regards the vertical distribution of high density plots it seemed that the Slash pine forest was the herbtype whose foliage appeared mainly in the upper strata of the crown. The average light intensity under the crown in a closed plot showed about 5% of full daylight, with the average including sun-flecks (Table 4). And the correlation between the height of the lowest living branch and the relative light intensity seemed to be high. The oven-dry weight of the A0 layer (consisting of litter, raw humus and humus) in the fertilized plots was estimated at 4.5-11.7 ton/ha (Table 6), and these accumulated amounts increased in proportion to the density plot. The average decomposition rate of the A0 layer was estimated at 80-120%, it was presumed that the litter took about a year to decompose. The forest stand of the Experimental Station consisted of extremely sterile soil, compact (Table 8) and lacking in nitrogen, phosphorous and potassium (Tables 9 and 12). It was observed that as the forest grows and thus becomes denser the physical soil properties were improved in the surface layer (0-5cm). The concentrations of nitrogen, phosphorous and potassium in the needles in the fertilized plots were higher than in the non-fertilized (Table 10). The amounts of phosphorous and potassium in the forest ecosystem in the upper ground parts (ground flora included) and those available in the soil to 35cm depth (A0 layer included) increased as the amounts of fertilized applied increased (Table 12). But in the soil, available phosphorous and potassium or exchangeable calcium and magnesium which belong to a closed system, from the point of view of nutrient circulation, decreased in proportion to the amount taken up in the upper ground parts. The average decomposition rates of nitrogen, phosphorous and potassium in the Slash pine stand were higher than those in Loblolly pine stands and many others (Table 15). Because part of the nutrients accumulate annually in the stem, the amounts of phosphorous and potassium in the soil remain at 30-40 times the annual uptake in the high density plots, and calcium at only 15 times (Table 17). Judging from the circulation of nutrients, it seems that these quantities were not sufficient for the continuation of present growth.