Successive mapping of secondary pine forests affected by pine wilt disease and subsequent forest management in Miyajima Island, SW Japan

摘要

Recently secondary pine forests in which Finns densiflora is the dominant species have drastically declined due to the spread of pine wilt disease and the predominance of understory species accompanied by the abandonment of human traditional management. In order to examine the longterm change of secondary pine forests affected by pine wilt disease and subsequent forest management; i. e.; cutting and removing dead pine trees; we compared three detailed vegetation maps of the diseased pine forest successively drawn in Miyajima Island; SW Japan. The detailed vegetation maps were made based completely on field works at a scale of 1: 2;500 just before the forest management (1974) and after the management (1980 and 2000). Map legends employed were the Dicranopteris linearisGleichenia japonica community and several variants subordinated to the subasso ciationSymplocoPinetum densiflorae myrsinetosum seguinii. Successive maps recognized both the progressive and retrogressive successional changes; the vegetation of the lower part of slopes and valley bottom sites mostly changed into a speciesrich vegetation unit regarded as a later successional stage; the Symplocos glauca variant; whereas the vegetation of the upper and middle parts of slopes was patchilysegmented by floristically retrogressive vegetation units; the Dicranopteris linearisGleichenia japonica community (typical group) and the typical variant. One possible cause responsible for the expansion of those retrogressive vegetation units was the proliferation of sundemanding ferns; Dicranopteris linearis and Gleichenia japonica (Gleicheniaceae); which was probably facilitated by forest management (cutting and removing dead pine trees) and deer grazing. Removing dead pine trees not only directly damages later successional species established below pine canopies but also decreases potential nutrient resources and provides excessive light resources for the growth of them. For the maintenance of ecosystem functions; leaving dead pine trees is considered to be more appropriate than removing them; especially in the diseased pine forests where the risk of soil nutrient loss with management is high.