スギ林の根の現存量について

Abstract

林木の根量調査方法としては単位面積あたりの根量を土ごとほりとるブロック法, 消防ポンプなどで根を洗い出す水洗法, 抜根器でひきぬく抜根法などがある。本稿では芦生24年生スギ林を中心にして行なった抜根器による根量調査の問題点などについて論じた。地下部の相対生長関係では, w_s, w_T, D_2, D_2Hなどとw_Rとの組合せがよく, とくにD_2だけで根量推定をおこなってもD_2Hによるものと大差はみられなかった。抜根法では抜根の際, 切断されて土中に残る根の回収がもっとも問題になる。本調査結果では切断された根は直径5mm以下のもので, 実際にこの5mm以下の根を林内よりほりとると3. 46ton/haとなり, これは抜根して得た根量, 29. 9ton/haの約11. 6％にあたる。両者を加えあわせたものが林分の全根量で33. 4ton/haとなり, T/R率は3. 7であった。抜根法ではブロック法にくらべて垂直的な根の分布の正確な把握が困難とされる。この欠点をおぎなうため, 林内にトレンチをほり, そのトレンチ断面の根の本数, 直径を測定した。その結果, 直径2mm以下の細根は本数, 断面積ともに地下30cmまでに約80％が分布し, それ以下では急激に減少するが, 直径2mm以上の根では深さごとの本数は変化しないが, 断面積合計は地下40cmのところがもっとも大きい値を示した。苗畑において, 3年生, 4年生スギ苗について抜根法, 水洗法, ブロック法の比較をおこなった結果, それぞれの根量比は抜根法1 : 水洗法2 : ブロック法3という関係になり, T/R率では3方法の間に2. 16から9. 16の差がみられた。

The estimation of forest root biomass was carried out by three methods; namely 1) Pulling out method, 2) Power washing method, and 3) the Block method in two stands of Cryptomeria japonica; i.e., a 24-year-old stand in the Ashiu School Forest and a young seedling stand in the nursery of Kyoto University. The trench method was also used for the investigation of the vertical distribution of roots. 1) Stumps were pulled out by a Tractel Tirfor as a puller and the diameters at the cut ends of the roots were measured in the field in Ashiu. The allometric relations between the roots dry weight thus pulled out and the square of the diameters at breast height of the trees were used for the estimation of the root biomass. Root biomass estimated by this method was 29.9ton/ha. Since the roots less than 5mm in diameter on an average were assumed to be cut when pulled out, these small roots were extracted from four soil cores of 25cm in width, 1m in length and 80cm in depth drawn randomly from the stand. The weight of the small roots thus estimated was 3.46ton/ha. So the-total root biomass was 33.4 ton/ha. T/R ratio was calculated to be 3.7. 2) The root system of an individual tree was washed out by a power pump and weight. Though there were few samples, the estimated value of the root biomass by this method was similar to that of the pulling out method. 3) Ten trenches, each 1m in length and 1m in width were excavated to the depth of 80cm where no roots were found. A 100cm×80cm frame consisting of 80 small quardrats each 10cm×10cm in size was set at the profile of the trenches. The diameter at the cut end of the roots in each small quadrat was measured. The vertical distribution patterns of these roots ascertained by this method were that fine roots under 2mm in diameter were distributed from the surface to a depth of 30cm and bigger roots over 2mm in diameter were distributed mainly at the layer 40cm from the surface. A comparison in root biomass ratio between the three methods in nursery resulted in 1:2:3 for the Pulling out method, Power washing method and the Block method, respectively. T/R ratios of the Block, Power washing and Pulling out methods varied from 2.16 to 3.77 to 9.16.