森林の土壊呼吸に関する研究 (II) : 測定点数と測定日数について

Abstract

森林の土壌呼吸量測定に際して問題となる測定点間のバラツキと測定期間内の日別変動について若干の考察をおこなった。測定は京大演習林本部苗畑 (京都市左京区) にあるカンレンボク模型林分とその隣接裸地, 京大上賀茂試験地内ヒノキ林, 滋賀県蒲生郡日野町のヒノキ林, ならびに奈良県吉野郡杉谷阪本財団林内のスギ林, ヒノキ林で1967年4月より同年12月まで行なった。同一林分内での測定点間の変動係数は上賀茂ヒノキ林が最大で0. 22 - 0. 30, 吉野スギ林の1つが最小で0. 07 - 0. 13であった。測定地の表層土壌中の礫や根の量の多少がこの変動係数の大小を生ずる原因の一つだと考えられる。信頼限界95％, 抽出誤差20％で必要測定点数を推定すれば, 上賀茂ヒノキのような相当バラツキの大きい林分でも最大10点をとればよいことがわかった。比較的短かい1回の測定期間の日別変動に強く影響するのは, 降雨の有無とその間隔であり, 降雨後土壌呼吸量は増加する。その増加のしかたは土壌によって異なるようである。各測定点の日別変化の変動係数は測定期間中降雨の多かった苗畑内カンレンボク模型林分の1つが最大で0. 38, 最小は降雨のなかった上賀茂ヒノキ林の1つで0. 04であった。これより測定点数と同様に1回の必要測定日数を推定すると, 通常の森林では最長6日間, 苗畑のような場所では最長15日間となった。これらの推定に基づいて今回の土壌呼吸量測定結果と測定期間の平均気温との関係をみてみると, バラツキは依然残るものの各測定値をプロットする場合に比べれば非常に少なくなくその精度はずっとよくなる。安定した森林の土壌呼吸値を得るためには今後はさらに温度などの測定に検討を加えていく必要があると思われる。

Two important subjects for measuring the soil respiration of forests, a variation in the amount of soil respiration among plots within a stand and daily fluctuations of amount of soil respiration for short periods of time were studied. The experiments were carried out, between April and December 1967, on 3 Hinoki (Chamaecyparis obtuia) stands, two Sugi (Cryptomeria japonica) stands, a Kanrenboku (Camptotheca acuminata) nursery stand, and an open stand adjacent to the Kanrenboku stand. The Hinoki stands were at Yoshino in Nara, at Hino in Shiga, at the Kamigamo Experimental Forest in Kyoto. The Sugi stands were at Yoshino in Nara. The Kanrenboku stand and the open stand were in the Experimental Nursery in Kyoto. The largest coefficient of variation between plots within a stand was 0.22-0.30 in the Hinoki stand in the Kamigamo Experimental Forest, and the smallest was 0.07-0.13 in one of Sugi stands in Yoshino. The gravel and root contents in the surface soils seemed to be one of the important factors affecting the value of the coefficient. According to our estimation 10 measuring plots even in the Hinoki stand in Kamigamo have been sufficient to estimate within 95% confidence limits and with 20% sampling error. Concerning with the daily fluctuation, a rain fall would bring about great fluctuations for a short period. The amount of soil respiration increased subsequently to the rain falls, though the rate of increase differed among the stands. In daily amount of soil respiration, the largest coefficient of variation was 0.38 in a plot in the Kanrenboku stand, and the smallest was 0.04 in a plot in a Hinoki stand in Kamigamo, where there was no rain fall during the measuring period. According to our estimation, measurment for soil respiration for 6 successive days in usual forests would suffice. In an unusual stand, like the kanrenboku stand, however 15 days would be necessary. In order to investigate soil respiration more thoroughly, the above results can be applied to further clarify the relationships between soil respiration and mean air temperature.