Ogata Takayuki

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<Research Reports>Peaty hummocks as anenvironmental indicator : a case of Japaneseupland mire著者 Ogata Takayukijournal orpublication titleTsukuba geoenvironmental sciencesvolume 1page range 33-38year 2005-12-26URL http://hdl.handle.net/2241/10167033Peaty hummocks as an environmental indicator: a case of Japanese uplandmireTakayuki OGATA*Tsukuba Geoenvironmental Sciences, Vol. 1, pp. 33-38, Dec. 26, 2005.* Researcher, Graduate School of Life and Environmental Sciences,University of Tsukuba, JapanAbstract    Hummocky microforms are formed in Japanese highlatitude and altitude regions under seasonal frostenvironments. The Senjogahara mire, a representativeupland mire in Japan, provides shallow water table,adequate snow depth and frost penetration, which havepromoted the formation of peaty hummocks. Thesehummocks are classified into active and inactive typesin terms of the vegetation characteristics. The activeforms occur on the seasonally waterlogged mire surface,whereas the inactive forms are located on the surfacewith perennial underground water. This suggests theenvironmental significance of hummocks as a hydro-logical indicator.Key words: hummocks, mire, seasonal frost, snowdepth, water table1. Introduction    Hummocky microforms, commonly referred to as“earth hummocks”, “thúfur” and “pounus”, are oftendistributed in continuous and discontinuous permafrostand seasonal frost zones. Although the hummocks mustbe termed by internal and external morphology andthermal characteristics controlling the hummock formation,Grab (2005) described that diverse morphological andsedimentological features of the hummocks haveproduced various academic terms. Such a situation hasresulted in typological and terminological confusions.    Similar hummocks are also formed in Japanese highaltitude and latitude regions under cold environments,where they are locally called “Yachi-bozu” or “Tokachi-bozu”. These two types are distinguished by vegetationand soil characteristics. The former generally includesa peat core and is covered with tussock grass such assedge, whereas the latter mainly consists of volcanicash and non-tussock grass. Few scientific investigations,however, have been conducted to evaluate environmentalcontrols on the formation of these hummocks. This leadsto the typological and terminological problems of theJapanese hummocks, and prevents the study on palaeo-environmental reconstructions using the hummocks.    This paper classifies peaty hummocks called “Yachi-bozu” in terms of vegetation types and discusses theenvironmental significance of hummocks, based on dataon climatic and hydrological conditions influencing thehummock formation in a Japanese upland mire (Sen-jogahara mire, shown in Fig. 1) located in a seasonalfrost environment.2. Study area and methods    The Senjogahara mire (about 1400 m in elevation),which has a peat layer about 4 m in depth, stretchesover an area about 1.5 km in length and 1 km in widthsurrounded by volcanic mountains (highest peak, 2484m in elevation) in the Nikko National Park. The mirehas been drying and shrinking in recent decades.Yamakawa et al. (1999) reported that the extent of themire decreased by 34.6 % from 1918 to 1997, and thatthe decreasing rate is one of the highest values inJapanese mires.    Climatic conditions in the study area are estimatedfrom meteorological data at the Oku-Nikko station (1292m in elevation, 7 km southeast of the study area) sinceno climatic data are available in the mire. The meanannual air temperature was 6.7˚C and the total meanannual precipitation was 2102.9 mm at the stationbetween 1971 and 2000 (Table 1). In the study area,however, a cold air lake and higher altitude are likely toproduce slightly lower temperature and larger snowamount.    Peaty hummocks, 20-40 cm in both height anddiameter and composed of tussock grass (sedge: e.g.Carex thunbergii, Eriophorum vaginatum), are extensivelydistributed over the Senjogahara mire. Transect A-A’50 m in length and 5 m in width was established at themargin of the mire (Fig. 1), where pioneering trees (e.g.birch: Betula platyphylla; larch: Larix kaempferi) haveinvaded on sandy deposits above the peat layer (Ogata,2003). Along the transect, longitudinal profile wasmeasured with a transit and laser distance meter. Thetransect was divided into ten quadrates (Q1-Q10, 5 m ×5 m), in which the total number and height weremeasured for the hummocks over 20 cm in height. Snowdepth and water table were investigated at half-month34 Ogata, T.intervals between December 2001 and March 2003 attwo sites (Sites 1 and 2), where snow scales and wellswere installed with a hand auger.3. Classification    Figure 2 displays the number of hummocks in eachquadrate with the landform cross section. The longi-tudinal profile shows that the mean gradient is 1/93,and that relative height between points A and A’ is 53.5cm (Fig. 2a). The hummocks are classified into “tree-invasion type” (with tree species), “shrub-invasion type”(with shrub but without tree species), “herb-invasiontype” (with non-tussock herb but without tree nor shrubspecies) and “no-invasion type” (with only tussock herb)based on the invading plant species. The last typehummocks are formed only in the lower area, whereasthe upper area allows the formation of the otherhummocks (Fig. 2b).Fig. 1 Study area.Table 1   Mean monthly and annual temperature (T) and precipitation (P) at Oku-Nikko station in 1971-2000.35Peaty hummocks as an environmental indicatorFig. 2 Landform cross section and the number of peaty hummocks over 20 cm in heightalong Transect A-A’. (a) Landform cross section. (b) The number of hummocks.Fig. 3 Histograms for the height of peaty hummocks over 20 cm in height along Transect A-A’.36Ogata, T.    The no-invasion type is considered to be active andthe other types are thought to be inactive, because thehummock growth requires the tussock grass. Theinactive hummocks suggest the vegetation successionto drier meadow or pioneering forest.    Figure 3 presents histograms of the height of activeand inactive hummocks in the pioneering forest (Q1-Q5, 63 hummocks in total) and in the mire (Q6-Q10, 62hummocks in total). The inactive hummocks accountfor 90.5 % in the pioneering forest and 27.4 % in themire. The percentage of hummocks higher than 30 cmoccupies 14.3 % in the pioneering forest and 32.3 % inthe mire. In addition, the range of height increases withthe occurrence of active hummocks (Table 2). Thesefacts indicate that various ages of hummocks coexist inthe mire, and that the hummocks are growing in the mireand influenced by succession in the forest.Table 2   The number and height of peaty hummocks over 20 cm in height in each quadrate.4. Environmental conditions    Figure 4 shows changes in snow depth and water tableat Sites 1 and 2 between December 2001 and March2003. In the study area, continuous snow covered theground for about four months from December to March.The maximum annual snow depth reached 91 cm (2002)and 106 cm (2003) at Site 1, and 132 cm (2002) and110 cm (2003) at Site 2. Such a snow cover insulatesthe ground surface from diurnal variation in airtemperature, while the meteorological data (at Oku-Nikko station) on air temperature suggest that diurnalfreeze-thaw cycles occur at the study sites in autumn(September to November) and in spring (April to May)when snow cover is absent. In fact, in the observationperiod, the freeze-thaw cycles occurred 25 times inautumn, 2001, 9 times in spring, 2002, 27 times inautumn, 2002 and 15 times in spring, 2003. This suggeststhat freeze-thaw action affects the snow-free ground atFig. 4 Changes in snow depth and water table (depth below the surface) at Sites 1 and 2 between December 2001and March 2003.37Peaty hummocks as an environmental indicatorFig. 5 (a) Inactive peaty hummocks in the pioneering forest (3 September 2005). Watertable lies below the ground surface. Pioneering trees invaded on slightly higher groundresult in a drier condition. (b) Active peaty hummocks in the mire (3 September2005). Water table lies above the mire surface.38Ogata, T.the study sites.    Groundwater data indicate a significant difference inthe water table between the two sites throughout a year.The water table rises above the surface seasonally atSite 2 located at the boundary between the pioneeringforest and the mire, while it never rises above the surfaceat Site 1 located in the forest. The hydrological conditionseems to influence the hummock growth, because theinactive hummocks are distributed where the water tablealways lies below the ground surface. This suggests thatdrying in the pioneering forest is likely to have promotedthe vegetation succession, which in turn has inactivatedthe hummocks (Fig. 5a). In contrast, the active hum-mocks are considered to have grown under theenvironment where the water table fluctuates across theground surface (Fig. 5b).    Thus, the hummock formation requires both a highwater table and paucity of snow which favor frost action.The peaty hummocks in Japan are distributed mostly inthe mires under cold environments located in the PacificOcean side, such as eastern Hokkaido (e.g. Kushiro,Kiritappu and Nemuro) and central Honshu (e.g.Kirigamine and Senjogahara), lacking deep snow. Thisstrongly supports that peaty hummocks favor the mireswith winter frost action.    These climatic and hydrological conditions probablycontrol the morphology of hummocks. In FinnishLapland, pond ice excavates a notch at the foot ofhummocks which are located in a waterlogged condition(Van Vliet-Lanoë and Seppälä, 2002). Active peatyhummocks in the Senjogahara mire often have a notchwhich is considered to be formed by frequent freeze-thaw alternations of surface water.5. Environmental significance    Some recent studies have pointed out the significanceof earth hummocks as an environmental indicator. Forinstance, in Finnish Lapland, Van Vliet-Lanoë andSeppälä (2002) suggested that earth hummocks indicatea general cooling trend associated with progressivedepletion of summer insolation. Luoto and Seppälä(2002) also described that earth hummocks react quicklyto variations in temperature, snow depth and precipi-tation. In the high Drakensberg, southern Africa, relictearth hummocks are indicative of hydrological changeswhich are caused by a gradual lowering of the watertable due to desiccation along wetland peripheries (Grab,2005).    Field data in the Senjogahara mire also suggest theenvironmental significance, especially for hydrology.The close relation between the hummock growth andthe seasonal groundwater level change permits theestimation of past hydrological environments around thehummocks. Especially, the inactive hummocks withperennial underground water suggest the groundwaterdeepening, because the hummock formation is con-sidered to require the seasonally waterlogged ground.    Future studies on the Japanese hummocks have toundertake terminological redefinition based on themechanism of hummock formation, which will enhancethe environmental significance of hummocks called“Tokachi-bozu” and “Yachi-bozu”.Acknowledgements    Ministry of the Environment provided the permissionfor field investigations in the Nikko National Park. Iwish to thank Eiji Matsumoto, Norio Tase and NorikazuMatsuoka for their helpful comments.ReferencesGrab, S. (2005): Aspects of the geomorphology, genesisand environmental significance of earth hummocks(thúfur, pounus) : miniature cryogenic mounds.Progr. Phys. Geogr., 29, 139-155.Luoto, M. and Seppälä, M. (2002): Characteristics ofearth hummocks (pounus) with and withoutpermafrost in Finnish Lapland. Geogr. Ann., 84 A,127-136.Ogata, T. (2003): Physiographic factors controlling thedegradation of the moor at the foot of an alluvialfan in the Senjogahara, Nikko National Park, Japan.Geogr. Rev. Japan, 76, 1025-1039. (in Japanesewith English abstract)Van Vliet-Lanoë ,  B. and Seppälä ,  M. (2002) :Stratigraphy, age and formation of peaty earthhummocks (pounus), Finnish Lapland. TheHolocene, 12, 187-199.Yamakawa, O., Otoi, K., Nakata, S., Yaguchi, H.,Sekiguchi, T., Hatakeyama, Y. and Numata, Y.(1999): Areal changes of marshes in whole country.J. Geogr. Surv. Inst., 92, 57-67. (in Japanese)Received 6 September 2005Accepted 5 October 2005

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