テーダマツ幼令林の物質生産機構

Abstract

この研究は, 京都大学農学部白浜試験地に, 密度と施肥量をかえて植付けられ, 一部はすでに完全な閉鎖状態に達した7年生のテーダマツ林 (Pinus taeda) と, 上賀茂試験地の苗畑に床替後5年間放置され, すでに自然間引が起こり最多密度の状態に達した小林分とを対象に, それらの林分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N, P_2O_5, K_2Oがそれぞれ15 : 8 : 8の化成肥料と, 同比率の配合肥料の2種を1本あたり100gずつ植付け時から3ヵ年与えたが, 1967年10月現在, 立木密度はhaあたり約2, 100本 (疎), 約3, 800本 (中), 約6, 500本 (密) で, 平均胸高直径 (d) はそれぞれ約10. 2cm, 7. 8cm, 8. 0cmに達し, 平均樹高 (h) 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, 約30%の個体がまだ胸高に達していなかった。上賀茂の小林分は密度約93, 000本/ha, dは1. 7cm, hは2. 8mで, 約111, 000本/haの枯死木が認められた。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が, 上賀茂の小林分は総刈りを行なった。胸高直径 (Dcm) と樹高 (Hcm) のD_2H対する幹材積 (Vcm_3) ならびに密な林分の幹乾重 (w_s, kg), 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. 8638 log (D_2H)＋0. 2810 log w_s=0. 8404 log (D_2H)-3. 0073 log w_B=1. 2253 log (D_2H) -5.4815 log w_L=1. 0078 log (D_2H)-4. 4445 白浜における密な林分の幹乾重は55 - 59ton/ha, 枝乾重と葉乾重はそれぞれ13 - 14ton/haと推定されたが, 葉量は他のマツ属よりかなり多いようであった。この葉量は落葉期前の2年分の着葉量であるが, 春5月, 1年葉のみを着生していた上賀茂の小林分でも約10ton/haと計測された。密な林分の年間幹材積生産量は32 - 35m_3/haで, 他のマツ属より著しく大きい。林分の生産構造は葉が上層にかたよった広葉型で, 密な林分の生枝下の平均相対照度は1 - 2％であった。そして樹高に対する生枝下高の相対値と林内の明るさとは相関が高いようであった。L型の断面積分布や自然間引の状態からみて, 上賀茂の小林分は最多密度 (ρ, 本/ha) に達していると思われたので, これから最多密度曲線を暫定的にもとめてみた。 log ν=-1. 5 log ρ＋4. 3075 粗腐植 (A_o層) の堆積量は乾重でほぼ9 - 12ton/haとなり, 立木密度による差はほとんど認められなかった。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。肥料として施用したN, P, Kの3成分は, 施肥量が増加するとともに地上部 (テーダマツ以外の他植物も含む) と地下45cmの深さまでの土壌 (粗腐植を含む) を合せた生態系内の各部分で明らかに増加し, とくに可給態のPとKは施用量にほぼ比例して, 地上部, 地下部とも増加し, はじめの養分不足を補って順調な物質循環を繰返すようであった。これに反して, 施肥しなかった置換性のCaとMgは生態系内の増加はなく, 地上部に吸収保持された量だけは土壌中から減少していた。N, P, Kの推定平均分解率はきわめて高く, 粗腐植中の養分は1 - 3年でほぼ分解されるようであった。しかし幹に毎年蓄積される吸収速度から土壌中の可給態養分の残存量を推定すると, Pは約10年分, Kは約20年分しかない計算になり, 物質の循環からみて, 近い将来生長が減退する可能性が大きいものと思われた。

The subject of the studies is to account for the important mechanism of mater production in a 7-year-old Loblolly pine (Pinus taeda) forest planted with three kinds of density and manured variously in the Shirahama Experimental Station of Kyoto University Forest (Wakayama pref.), and also, investigation of productivity was made in a small stand transplanted in the nursery bed of the Kamigamo Experimental Station (Kyoto city) five years ago. Especially, the biomass of the upper ground part in each pine forest, productivity of stem, production structure, soil conditions and the circulation of nutrients within ecosystem were discussed in this paper. In the manured forest stand, complex fertilizer which contains nitrogen (15%), phosphorus (8%) and potassium (8%) have been manured 100 grams per tree for 3 years after planting annually, and also, mixed fertilizer was used similary. The amount of applied element per hectare was shown in Table 12. In October 1967, stand density, mean diameter breast high (DBH), mean height and basal area of sample stands were shown in Table 1. Biomass of upper ground parts per stand was estimated with the method of allometric relation. The sample trees of various dimensions were cut down at the base, and the stem, branches and needles of each tree were separately weighed with the stratified clip technque. The fresh weight data were converted into oven-dry weight basis, and the volume of stems were measured with stem analysis. The allometric relations of the stem volume (Vcm3), the stem dry weight (wS kg), branch dry weight (wB kg) and needle dry weight (wL kg) to D2H (Dcm; DBH, Hcm; tree height) in high denity stand well fitted a linear relation shown logarithm (Fig. 1). These regression formulas were found as follows: [Figure omitted] In the high density stand of Shirahama, the dry weight of stems per hectare was estimated at about 55~59 ton, and also branches and foliage at 13~14 ton respectively. These standing crops tended to be larger than those values of other genus pine forests which have been reported by many workers. As the result that the seasonal change of the foliage biomass and leaf-fall amount in a forest, the amount of foliage measured before defoliation of grown needle in the previous year was presumed to be the largest of all seasons. But in May, the dry weight of foliage only consisting of a year needle in the Kamigamo's forest stand was about 10 ton per hectare. The current annual increment of stem volume in a high density stand was estimated at 32~35m3/ha, and its production was seemed to be remarkably larger than other genus pine forest. The production structure diagram showing the vertical distributions of oven dry weight of stems, branches and foliage differed in various closed stand (Fig. 8). As regards the vertical distribution of foliage it seemed that the Loblolly pine forest was the herb-type whose foliage appeared mainly in upper strata of crown. The light intensity under crown in closed stand ranged from 1 to 2 percent of full daylight, with average including sun-fleck (Table 4). And the correlation between the height of the lowest living branch and relative light intensity seemed to be high. It was presumed that the Kamigamo's stand where the distribution of cross sectional area was L type and natural thinning had already occurred had become formation of full density. As the results, a tentative curve was expressed as follows: [Figure omitted] The oven dry weight of litter and raw humus (A0 layer) was estimated at 9~12 ton/ha, and these accumulated amount did not differ too mucn in each stand. The forest stand of the Shirahama Experimental Station consisted of extremely sterile soil where the physical properties was compact (Table 7) and the nutrients of nitrogen, phosphorous and potassium were lacking in surface layer (Table 8 and 10). The amount of nitrogen, phosphorous and potassium in forest ecosystem consisted of upper ground parts (ground flora inclused) and soil to 45cm depth (raw humus included) obviously increased with fertilizer. Especialy, avaible phosphorous and potassium increased in proportion to applied amount of fertilizer in both under ground and upper ground parts. Therefore, it was considered that the circulation of nutrients had smoothly been promoted in forest ecosystem. But exchangeable calcium and magnesium applied as nonfertilizer did not increase in forest ecosystem. And it was found that the elements in soil decreased as much as was absorbed in upper ground part. The average decomposition rate of nitrogen, phosphorous and potassium was showed Table 14, it was presumed that the nutrients in raw humus decomposed for 1~3 years mostly. Because of part of nutrients accumulated in stem annually, phosphorous and potassium in soil have remained of only 10 and 20 times of annual uptake. It seems that these quantity was not too much for the continuation of present growth.