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タイトル: 植生からみたオーストラリア半乾燥地域の「砂漠化」現象
著者: 武内, 和彦
大森, 博雄
著者(別言語): Takeuchi, Kazuhiko
Ohmori, Hiroo
キーワード: desertification
land and vegetation degradation
semi-arid region
vegetation change
Australia
発行日: 1988年
出版者: 日本地理学会
掲載誌情報: 地理学評論, 61, 1988, 124-142
抄録: In the Australian Continent, in those areas occupied by Eurpoeans during the past 200 years, the natural environment has been modified; the change are most severe in the semi-arid regions. Prior to Eurpoean settlement, for some tens of thouthands years of Aboriginal occupation, there were slow changes mainly due to occasional burning of vegetation so that the Australian flora was gradually moved towards fire-resisted species. With the introduction of agricuture and grazing drastic changes in the density, floristics, and structure of native vegetation occurred. Disturbance or removal of vegetation has also affected surface and sub-surface hydrology. These changes have resulted in water or wind erosion and salinization. Where vegetation has been regenerated the species established are often those which are unpalatable to grazing stock or can withstand frequent burning. This leads to "desertification". The concept of "desertification" is ecologically defined as follows: The natural vegetation in equilibrium with the potentiality of regional climate (climatic climax) is replaced by the vegetation which grows naturally and originally in more arid region due to environmental stress directly induced by man and the accompanying land degradation. The Murray-Mallee Region in the southern part of Austrralia, where the late Pleistocene and Holocene sand dunes are widely distributed, is a semi-arid region with annual rainfall of 250-400 mm; it is wholly covered with mallee (Eucalyptus spp.) scrub. The sand dunes which were stable covered totally with such vegetation during the pre-European days have become unstable at many sites due to vegetation clearing, farming and grazing associated with European settlement since the late 19th Century. The remobilization of sand dunes is one of the typical events of "desertification" and, in extreme cases, the surface remains unvegetated. Various stages in vegetation change can be observed showing "desertification" or regeneration process. The authors describe the processes involved in land and vegetation degradation, and discuss the measures to prevent "desertification" based on the examination of the relationships between activity of sand drifting, thickness of loose sand overlying the stable substrata, floristic composition, community height, and activlty of rabbits (Olyctolagus cuniculus). The natural mallee scrub in the conservation park is dominated by Eucalyptus spp. accompanled by rlch lower shrub layer species such as Casuarina stricta, Callitris verrucosa and Hakea leucoptera. The mallee scrub remaining on farmland, however, is very poor in understory resulting from grazing. Triodia spp. and Acacia spp. invade in the understory of the mallee scrub after bushfires. Acacia spp. regenerate easily and are dominant on the land where the surface is disturbed by vegetation clearance and/or wind erosion. Natural mallee scrub will regenerate through sprouting from so-called mallee roots (lignotuber) persisting after surface disturbance. When the lignotuber is exposed by wind erosion or covered by sand accumulation, the recovery to natural mallee scrub becomes very difficut. On such sites, Triodia colonies and/or Acacia scrub stand as disclimax communities with a life cycle of 30 to 50 years (Figs. 6 and 7). The thicker the loose sand is, the more active and continuous is the sand drifting (Fig. 8). In particular, on many dunes with loose sand thicker than 80 cm, serious drifting has continued for recent tens of years exacerbated by the large amount of mobile sands and the difficulty of vegetation invasion. At the sites with thick loose sands, the population of rabbit introduced from Europe is larger, suggesting that the reactivation and acceleration of sand drifting is induced by rabbit infestation (Fig. 9). On the other hand, where vegetation height is greater than the line shown in Fig. 8, suggesting a minimum height of vegetation for erosion control, sand dunes tend to be stabilized. The results of investigation as mentioned above show the various processes of vegetation changes according to the combinations of local difference in natural conditions and the importance of human impacts, and imply many stages of"desertification". The remobilization of sand dunes is regarded as an extreme stage of "desertification" caused by a combination of many factors. The soil conservation and the vegetation management based on detailed survey of physical and ecological characteristics of land will identify the factors contributing to landscape instability and "desertification" and so provide a framework for reversing the processes of land and vegetation degradation.
URI: http://hdl.handle.net/2261/7257
ISSN: 00167444
出現カテゴリ:014 自然科学
1162010 学術雑誌論文

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