ヒノキ林内におけるスギ直挿苗とヒノキ天然生稚樹および下層植生の生長について

Abstract

前更作業としての二段林の施業体系を確立し, 林地保全に与える効果を明らかにする目的で, 63年生のヒノキ林内に直挿しされたスギ苗と, 天然更新したヒノキ稚樹および下層植生の生長状態と土壌環境を生態学的に解析した (林分I, II)。また比較対照として26年生スギ林内のスギ直挿苗についても調べた (林分III)。調査は1981年10月, 岐阜県石原林材山林で行なった。林分I (スギ直挿し) : 上木のヒノキは1970年以降5年おきに間伐が行なわれ, 現在本数230本/ha, 平均胸高直径35cmでかなり疎開し, 林内平均相対照度は約31％である。1971年にスギの直挿しが行なわれたが, 相対照度約5. 6％のスギ林内のもの (林分III) より生長がよく, 年平均樹高生長量は約25cmとなっている。1981年冬雪害を被り残存本数は約3, 600本/haである。林内には下層植生が多少繁茂するが, 間伐後天然更新したヒノキ稚樹も多い。これらを本数調整することによって直掃ししたスギとの混交仕立てを行なうことは有望である。林分II (ヒノキ天然更新) : 林分Iに隣接し上木の年齢も同じであるが, 1965年以降ほぼ5年おきに間伐が繰返されたため, 現在本数は林分Iより30％も少なく, 林冠は疎開し平均相対照度は50％と著しく明るい。したがって下層植生もよく繁茂しているが, 5 - 19年生, 高さ50 - 300cmの天然生のヒノキ稚樹が70, 000本/haも密生し, 枯死木もみられる。そのため相対的に葉量の少ないものが多いので, 下層植生とともに本数調整のための除伐を行なう必要がある。各調査林分のA_o層量はスギ林 (林分II) より少ないが, 表層土中に混入したリター量は反対に多く, 雨水による流亡はほとんどないようである。また土壌の理化学性もPの含有率が少ない以外は良好である。したがって土砂の流亡を引起こしやすい石英斑岩を母材とするヒノキ林の取扱いとして, このような二段林施業は極めて適切であると判断された。

With the purpose of establishing a system of two-storied forest as pre-regeneration, the growing structure of Sugi (Cryptomeria japonica) direct cuttings, Hinoki (Chamaecyparis obtusa) natural seedlings and undergrowths was investigated in a 63-year-old Hinoki stand existing at the Ishihara Rinzai (Forest management) Co. which locates in the middle of Gifu Pref., in October 1981 (Stand I, II). In addition, the growth of Sugi direct cuttings in a 26-year-old Sugi stand (Stand III) is measured in order to compare with those in Hinoki stand. Moreover, some effects on forest land conservation by the underlayer vegetation are discussed in this paper. Stand I (Sugi direct cutting in Hinoki stand): Hinoki trees of upper stratum have been thinned at five years intervals since 1970. At present, the tree number is about 230 per hectare and mean diameter at breast height is about 35cm, therefore the forest canopy is a sparse state and the light intensity under the crown shows about 31% of full daylight (Table 1, 2). In this stand, the cuttings of Sugi branch cut at about 60cm length have been directly planted in 1971. These cuttings grow more rapidly than those planted in closed Sugi stand (Stand III) where relative light intensity is only about 5.6%, and the mean height increment of cuttings was estimated at about 25cm so far (Fig.3, Table 1, 2, 5). In winter 1981, the cuttings of Stand I have suffered from snow damage, and the number of remained cuttings is only about 3, 600 per hectare now. On the other hand, the natural Hinoki seedlings are numerously regenerating after thinning, but the undergrowth is established more or less, too (Table 3, 4, 5). Therefore, it certainly is possible that there are cultivated the mixed forest of Sugi and Hinoki if the number of Hinoki seedlings and undergrowth are adequatly controlled by improvement cutting and weeding. Stand II (Hinoki natural seedlings in Hinoki stand): This stand neighbor with Stand I, and an age of upperlayer Hinoki tree is the same. The stand has been thinned in order to cultivate a naturally regenerated seedlings at intervals of about five years ever since 1965, and at a result, the existing tree number is fewer by about 30% than its Stand I. Therefore, the crown density of Stand I. is very low, and the relative light intensity in the stand indicates high value such as about 50% (Table 1, 2). At present, although there are overgrown various undergrowths, natural Hinoki seedlings at 50 to 300cm high and regenerated 5 to 19 years ago are densely established so much as 70, 000 seedlings per hectare. Furthermore, there are found many dead seedlings in under stratum less than 1m high (Table 3, 4, 5, Fig.4). In general, the bigger seedlings grow more rapidly than smaller ones, and it is recognized that the amount of leaf per individual is scanty (Fig. 6, 7). Judging from the above regenerating process, it is necessary to be adjusted the number of Hinoki seedlings and undergrowths by improvement cutting or weeding. The amount of litter and raw humus (Ao layer〕in two Hinoki stands are fewer than its in Sugi stands (Stand III), but on the contrary, the litter mingled with the surface soil is abundant in Hinoki stands. As a result, it was presumed that the soil and litter were scarcely drifted down by the runoff from a heavy rainfall (Table 6). On the other hand, the physical and chemical properties of the soil are very good as a whole, except of lacking phosphorus. Viewing from the results mentioned above, especially, on working of the Hinoki stand consisting of mother rock of quartz-porphyry, it was understood that the system of two-storied forest is the most proper method for preservation of soil productivity.