Study on the Vegetation Zonation in Doi Inthanon National Park and Its Application to Environmental Educationドイインタノン国立公園における植生帯構造の研究と研究成果の環境教育への応用

Authors著者名

Pongsak Sahunaluポンサク サフナル1), Sakhan Teejuntuk1), Chettha Sungpalee1), Anusit Methavararuk2)

Affiliations著者所属

  1. 1) Dept. of Silviculture, Faculty of Forestry, Kasetsart University, Bangkok 10900, Thailand.
  2. 2) Doi Inthanon National Park, National Park Division, Royal Forest Department, Chiangmai 50160, Thailand.

Abstract要旨

Forest vegetations (trees with DBH ≧ 4.5 cm) in 46, 0.16 ha plots of six forest types; dry dipterocarp, mixed deciduous, dry evergreen, pine-dipterocarp, pine-oak and montane forests distributed along the altitudinal gradients ranging from 400 to 2,320 m asl were enumerated together with soils, air temperature and humidity analyzed and recorded in Doi Inthanon National Park, Chiangmai, northwest Thailand. Two broad altitudinal zones are recognized as lowland zone at 400-1,000 and montane zone at 1,000 m asl upward. On the lower and part of upper zone are partially overlapped by dry dipterocarp forest and montane forest at about 1,020 m asl where pine-dipterocarp and pine-oak forests are overlapped recognizing as a transition to montane zone whereas on the intermediate altitude, mixed deciduous forest is intermingled in a narrower range of altitude with a small portion of dry evergreen forest occurs along the stream bank. About 293 species, 155 genera and 70 families of trees with few unidentified species considering as minor elements in some forest types are identified. Their distributions and dominances in different forest type on the altitudinal ranges can be preliminary discriminated into groups of families, genera and species. Some physical and chemical properties of surface and subsoils are obviously different toward the altitudinal shifts in correspond to the sequence of forest type occurrences. Monthly mean air temperature during the 9-month period covering dry and wet seasons inside the forests on various altitudes exhibit the clear trends and significant decline upwardly along the altitudinal gradients. Monthly mean air humidity in dry season greatly decline in deciduous forest types on the lower altitude while maintain relatively high throughout the period in evergreen forest types on the upper altitude. Further quantitative analyses are being carried out and expected to elucidate the forest vegetation zonation in relation to altitude, soil and climatic variables.


タイ北西部チェンマイ県のドイインタノン国立公園において、標高400mから2,320mの範囲に46個の0.16haの調査区を設置し、胸高直径4.5cm以上の樹木種を対象に森林植生の調査を行った。また、同じ調査区を利用して土壌調査と、林内気温ならびに空中湿度のモニタリングを行った。調査した標高の範囲には、乾燥フタバガキ林、混合落葉林、乾燥常緑林、マツ―フタバガキ林、マツ―カシ林、山地林などの森林タイプが含まれていた。調査地域では、大きな2つの垂直分布帯が確認でき、標高400mから1,000mの低地帯と、標高1,000m以上の山地帯に区分できた。低地帯で優占する乾燥フタバガキ林と山地林は標高1,020m付近で重複しており、さらにこの標高帯では、マツ―フタバガキ林とマツ―カシ林が混在し、低地帯と山地帯の移行帯を特徴づけていた。低地帯では乾燥フタバガキ林が最も広く分布し、混合落葉林の分布する標高帯はこれに比べれば狭かった。また河川に沿って乾燥常緑林がわずかに混在していた。46個の調査区からは、70科、155属、293種の樹木が出現した。これ以外に科レベルで同定されていない優占度の低い数種が出現した。このデータをもとに、科、属、種のそれぞれのレベルで、各森林タイプへの出現状況を予備的に解析し整理した。また、表層土と下層土の物理的、科学的な特性も、標高の推移とそれに伴う森林タイプの変化とともに明瞭に変化していった。乾期と雨期を通じて9ヵ月間連続して測定した、林内気温と空中湿度の測定結果も、標高傾度に沿って変化した。気温は標高の上昇とともに明瞭に低下した。空中湿度の月平均値は、低標高の落葉性の森林タイプで乾期に明瞭に低下するのに対し、高標高の常緑樹林では乾期を通じて相対的に高い湿度を維持していた。今後、定量的な解析をすすめることによって、森林植生の帯状分布の様子を、標高・土壌・気候条件と関連づけて明らかにすることができると期待している。

推薦者:神崎 護 訳

Introduction

There are few studies of the altitudinal zonation of vegetation on mountains in Southeast Asia. The most detailed ones have been that on some tropical mountains such as on Mount Kerinci, Sumatra, Indonesia (Ohsawa et al., 1985), Gunung (Mt) Kobipoto and Gunung Binaia, Seram, Maluku, Indonesia (Edwards et al., 1990), Mount Kinabalu,Sabah, Malaysia (Kitayama, 1992), Gombak and Genting Highlands, Selangor, Peninsular Malaysia (Nakashizuka et al., 1992), Gunung Silam, Sabah, Malaysia (Bruijnzeel et al., 1993; Proctor et al., 1988) and Bukit Belalong, Brunei (Pendry and Proctor, 1997). All these studies are apparently focussed in tropical rain forest and in humid regions where most areas lie closely to the equator and evergreen forest species prevail. Apart from these equatorial forests, forest vegetation types on mainland Southeast Asia are more diverse where climate is mostly alternated between dry and wet seasons and their types vary from those on lowland plain zone to montane zone on mountainous areas as well as from deciduous to evergreen types recognized as various formations and zonation of forest vegetation on these tropical mountains is different from that on temperate mountains (Ohsawa et al., 1985). Moreover, zonation studies of vegetation in tropical forest are always concentrated on a single hill range and generally dealt with one or a particular forest type, less studies have been done in a complex hill range with different forest types and several studies did not include soils and climatic conditions in explaining the relationships between these factors with the zonation of vegetations in various ranges of altitude.

Doi (Mount) Inthanon is a representative name of a group of 15 hill ranges with different ridge height and some small plains on the valley along these mountain ridges included in an area cover of 482.4 km2 and designated as a national park in Thailand in 1978. It is located on the Thanonthongchai range stretches along the northwest highland of Thailand (18° 24'N, 98° 24' E). Its landscape is characterized by the hilly and complex mountains which supports various types of vegetation with different physiognomic, species composition and richness. This mountain complex vary in altitude ranging from 400 m asl at the foothill up to 2,565 m asl at the summit of Doi Inthanon, represents the highest peak in the country. The great difference in altitude makes up six forest types (Faculty of Forestry, 1992) distributing along the altitudinal gradients : dry dipterocarp forest (400-1,000 m asl), mixed deciduous forest (400-800 m asl), dry evergreen forest (400-800 m asl), pine-dry dipterocarp forest (800-1,200 m asl), pine-oak forest (1,000-1,400 m asl) and montane forest (1,000-2,565 m asl). These forest types are well preserved and represent a majority of the terrestrial plant community in Thailand. As the area lies in the northwest highland (Samapudhi, 1957), it is underlain by a granitic massive intruded into the range lying some 60 km to the southwest of Chiangmai and divides between the Ping and Maejaem rivers. Three major rock types are found in the highland according to Pendelton's reconnaissance geologic map (Pendelton, 1962). Parent materials are gneiss, connecting the northwest range with the east range of Doi Suthep, a narrow pocket of shale, siliceous sandstone, quartzite and slate around the middle part of the area and at the main hill of Doi Inthanon is a granitic massive. Climate of the area is recognized as monsoonal type with a strong alternation of wet and dry seasons. Heaviest rain falls during August and September coinciding with the wet-carrying northwest monsoon and driest month is December conforming to dry northeast monsoon. Mean annual rainfall of the northern region is 1,500 mm although the detailed record of climate in Doi Inthanon is not available, only local figures are from the lowland station. Minimum temperature is 3.7-7.5℃ during November to February and maximum temperature is 39.6-41.5℃ during March to May. Annual rainfall at the nearest station outside the park is 1,000-1,400 mm during July to October. However, annual rainfall on the upper part of Doi Inthanon may exceed 2,000 mm (Robbins and Smitinand, 1966).

Studies on forest formations of the northwest highland particularly in Doi Inthanon was initiated by Ogawa et al. (1961) followed by Robbins and Smitinand (1966) and Faculty of Forestry (1992). Santisuk (1988)'s account of forest vegetation of northern Thailand has also included the forest formation of Doi Inthanon and Pengklai (1996) pointed out the significance of plant species richness of this area. These studies are mainly focussed on the broad perspective of the forest in the area and provided fundamental understanding of the forest formation and plant species composition of each forest type. Ecological studies focussing on their vegetation zonation in relation to altitudinal gradient especially in association with soils and some climatic conditions have been scarcely carried out. Soils in Doi Inthanon have been studied by some investigators. This included some details of soil types (Pendelton, 1962), soil properties in some dry dipterocarp subcommunity types (Sukwong et al., 1976; 1977) and in hill evergreen forest (Robbins and Smitinand, 1966). Obivously, there are no record of soils in other forest types in Doi Inthanon and also no relationships between soil properties and vegetation distribution along the altitudinal gradient to assess their interrelationships. Furthermore, climatic conditions which are generally covaried with altitude may play an important role as a profound factor determining the distribution of various forest types and species abundance have never been investigated in this site. This study is therefore aimed to explore the floristic and structural variations accounted for by the altitudinal zonation, together with the investigation of species diversity patterns along the altitudinal gradients and to relate these vegetation parameters to some edaphic and climatic variables at a range of altitude, particularly in Doi Inthanon. This paper is a preliminary report on the results of this study. The detail of study analyses will be dealt with in another paper under preparation. This study was financially supported by PRO NATURA FUND 1999 of NACS-J together with the mutual support and encouragement of Dr. M. Kanzaki, Kyoto University to whom the authors gratefully acknowledged.

Materials and Methods

Six study sites were selected based on the vegetation map with the altitudinal classes together with a topographic map (Faculty of Forestry, 1992). Each selected site located in each forest type was further randomly selected for 3 locations on 3 altitudinal ranges at lower, middle and upper altitudes except for one forest type; dry evergreen forest where the area cover is relatively small (less than 1 % of the total forest area), only one location and 1 plot was selected. In each location, 3 plots were set at the lower, middle and upper slope. The study plots were then composed of 9 plots each in dry dipterocarp forest (450-990 m asl), mixed deciduous forest (490-730 m asl), pine-dipterocarp forest (980-1,100 m asl), pine-oak forest (1,020-1,180 m asl), montane forest (1,340-2,320 m asl) and a plot in dry evergreen forest (715 m asl). Forest type distrubution, study locations and altitudinal range are shown in Figs. 1, 2 and 3 respectively.

Figure 1. Vegetation map of Doi Inthanon National Park.

Figure 1. Vegetation map of Doi Inthanon National Park.

Figuer 2. Location of study plots;1-3: dry dipterocarp forest, 4-6: mixed deciduous forest, 7-9: pine-dipterocarp forest, 10-12:pine-oak forest, 13-15: mounane forest, 16: dry evergreen forest.

Figuer 2. Location of study plots;1-3: dry dipterocarp forest, 4-6: mixed deciduous forest, 7-9: pine-dipterocarp forest, 10-12:pine-oak forest, 13-15: mounane forest, 16: dry evergreen forest.

Figuer 3. Altitude class of Doi Inthanon National Park.

Figuer 3. Altitude class of Doi Inthanon National Park.

In each plot (40x40 m2), trees of DBH equal to and over 4.5 cm were tagged, individuals counted and DBH measuned. All individual trees were identified into species and specimens collected for unidentified ones to examine their species at Bangkok Forest Herbarium of the Royal Forest Department and herbarium of the Faculty of Forestry, Kasetsart University. Tree censuses were carried out since the middle of October, 1999, species checked and specimens collected periodically since then.

Seven sets of thermometer and hygrometer equipped with data loggers were installed for monitoring the air temperature and humidity in the sample plots representing some climatic parameters in each forest type at different altitudes. They were fixed to the inverted plastic boxes and attached to the selected trees at approximately 1 m aboveground. These seven sets were distributed one set each inside dry dipterocarp forest, pine-dry dipterocarp forest and pine-oak forest plots and two sets both inside mixed deciduous forest and montane forest plots. These loggers were set since the middle of November, 1999 and data recorded at 30-minute intervals in data loggers were further stored for every 3 months and reinstalled. This report covers only 9-month period as field data record is still continued while this report is prepared.

Soils were studied in each group of sample plots by examining soil profile at the middle part of the middle slope plot together with undisturbed soil samples collected with core samplers at the surface and subsoils (0-5 and 20-25 cm soil depth respectively) for three-phases analysis, hydraulic conductivity and moisture characteristics (pF) measurements of soils in each horizon. Disturbed soil samples were also collected from the subsoil horizons downwardly varying to depth of each profile in each forest type from the middle slope plot only for further analyses of soil chemical and physical properties. For surface soil horizon, disturbed soil samples were collected from 5 random points at the four corners and the center of the sample plot then composited into one sample for soil property analysis. For upper and lower slope plots, core sample soils were collected for surface soil from the center of plots for bulk density and hydraulic conductivity measurements and disturbed soil samples were from 5 random points at the corners and center of plots then composited into one sample, whereas surface soils were from one point at the center of plots for analyses of chemical and physical properties. Soil hardness was also investigated at the center of every plots by using soil penetrometer. Soil analyses were carried out at the Laboratory of Forest Soils, Faculty of Forestry, Kasetsart University, Bangkok.

Results and discussion

Forest type distribution along the altitude

Six forest community types of Doi Inthanon National Park distribute partially overlap along the altitudes especially those on the altitude lower than 1,200 m asl however, these forest community types can be broadly classified into 2 altitudinal zones. First one is apparently from 400 m asl to 1,000 m asl recognized as lower or lowland zone and those above 1,000 m asl as upper or montane zone reaching to the summit at 2,565 m asl.

On the lowland zone, dry dipterocarp forest appears to distribute on wider range of altitude than mixed deciduous forest although both types are deciduous forests. While dry dipterocarp forest is found from 400 m asl up to 1,000 m asl altitude, mixed deciduous forest is always found in between at the altitude of 480-750 m asl with narrow range of altitude. Within the range of altitude covering the lowland zone, dry evergreen forest is observed along the large stream bank either it flows through the flat lowland dipterocarp or mixed deciduous forests. The upper limit of dry dipterocarp forest extending up to 1,000 m asl is found to overlap with the pine-dry dipterocarp forest at the altitude around 980 m asl where the transition zone between the lower and montane zones is generally postulated by Santisuk (1988). Pine-dipterocarp forest extends its upper altitude to 1,100 m asl where it also overlaps with the pine-oak forest in which the lower limit of montane zone is fused together. In this lowland zone therefore comprises of 9 plots of dry dipterocarp forest at 450-990 m asl. 9 plots of mixed deciduous forest at 490-730 m asl and 1 plot of dry evergreen forest at 715 m asl (Table 1).

Table 1. Summary of the study plots in Doi Inthanon National Park, trees of DBH ≧ 4.5 cm are enumerated.

Table 1. Summary of the study plots in Doi Inthanon National Park, trees of DBH ≧ 4.5 cm are enumerated.

On the montane zone, pine-oak forest appears to distribute from below 1,100 m asl which overlaps with pine-dry dipterocarp forest on the lower limit and extends to the upper limit about 1,200 m asl. The 9 plots of pine-oak forest are therefore located on the altitudinal range from 1,020 to 1,180 m asl. On the altitudinal range between 1,200 to 1,300 m asl, there is no primary forest except for some secondary forests and the disturbed montane forest which is the result of the swidden (shifting cultivation) practice (Faculty of Forestry, 1992). Above 1,300 m asl to the summit of Doi Inthanon, montane forest is predominant type covering the widest range of altitude. The 9 plots of montane forest are therefore located on the altitudinal range between 1,340 to 2,320 m asl (Table 1).

In similar local habitat of the same northern region at Doi Chiangdao (2,175 m asl), dry dipterocarp forest was found from the foothill (400 m asl) up to 700 m asl where two-needle pine (Pinus merkusii) was found mix with dry dipterocarp forest species upwardly to 1,000 m asl where three-needle pine (Pinus kesiya) was prevailed upward but no altitudinal limit is recorded for this forest type (Khemnark et al., 1972). In a more extensive area, several subcommunity types of dry dipterocarp forest are recognized from the lower altitude as low as 280 m asl where low-scrub type composing of Shorea siamensis is dominant species and sometimes as low as 130 m asl up to 370 m asl where medium-scrub type dominating by S. obtusa is found and at 350-900 m asl where Dipterocarpus obtusifolius-S. obtusa subtype is common (Sukwong et al., 1976; 1977; Bunyavejchewin, 1983a). However, this forest type is recognized to be common confining to dry sites of northeast India, Myanmar, Thailand, Laos, Vietnam and Cambodia recorded by several authors included in a most recent publication of Santisuk (1988).

Mixed deciduous forest on Doi Inthanon is however, prevailing almost at the same altitude as for dry dipterocarp forest but in a narrower range of altitude. Doi Chiangdao mixed deciduous forest was not delimited to be at any altitudinal boundary but it might not be observed at the altitude higher than 750 m asl throughout Thailand (Khemnark et al., 1972) while at a wider perspective this forest is mostly developed below 800 m asl (Santisuk, 1988) and often occurs alternately or in mosaic or transitions with dry dipterocarp forest. On the other hand, dry evergreen forest in Doi Inthanon is confined to a small area as pocket-like along large bank of main stream only where in other localities it may be found on flat area at the altitude approximately 720 m asl in the northeast region (Sahunalu et al., 1979) and not higher than 900 m asl in the northern mountainous region (Santisuk, 1988). At Doi Chiangdao, this forest type was found below 600 m asl (Khemnark et al., 1972). As topography in Doi Inthanon is generally rugged and steep where large area along stream bank is not frequently found except some broad valley where are always occupied by local inhabitants and devoid of primary vegetation, a small pocket-like plain with an area of dry evergreen forest less than 1% of total forest area is classified (Faculty of Forestry, 1992) therefore, it may be considered as a minor forest type in this area.

Above 900 m asl altitude, three-needle pine (Pinus kesiya) is common in association with several dry dipterocarp tree species in Doi Inthanon and the area extends upward to c. 1,200 m asl but does not clearly separate from pine-oak forest where it always distributes overlapedly at c. 1,010 m asl as the lower limit. This pine-dipterocarp forest will gradually changes to pine-oak forest as some members of Fagaceae are found co-occurrently. It is postulated that dry dipterocarp forest is advanced into hill evergreen or montane forest (Stott, 1974; Santisuk, 1988). At Doi Chiangdao, two local pines are found to be associated with dry dipterocarp forest tree species at two different altitudes starting one from 700 m asl and another one from 1,000 m asl upward but pine-oak forest was not observed (Khemnark et al., 1972).

Area of montane forest in Doi Inthanon is reported to be largest being approximately 54% of total forest area (Faculty of Forestry, 1992). This forest type is generally classified into two types: lower montane and upper montane forests by Santisuk (1988). Pine-oak forest classified in this study is therefore considered to be correspondent to lower montane forest and montane forest reported here is upper montane forest under forest type nomenclature of Santisuk (1988). These two types are generally known as hill evergreen forest while it was recognized as moist lower montane forest by Smitinand (1965) in Doi Chiangdoa, a limestone massive at 1,300-1,800 m asl altitude as well as always found in several mountain ranges under the management of the national parks, wildlife sanctuaries and watershed management units in Thailand (Royal Forest Department, 1962; Robbins and Smitinand, 1966; Smitinand, 1977; Santisuk, 1988). It is known as oak forest in Myanmar (Stamp, 1925) and montane humid evergreen broad-leaved forest in central and west Yunnan, China (Xiwen and Walker, 1986) in a vicinity of Doi Inthanon area northward.

Floristic composition along the altitude

About 293 species in 155 genera and 70 families of trees with DBH ≧ 4.5 cm are recorded in 46 plots with few unidentified ones which are considered as minor component species of some forest types only. Species numbers in all plots vary from 14 to 135 among each forest type. Table 2 shows the taxonomic break down in each forest type.

Number of family of trees is most spectacular in montane forest (49) follow by pine-oak forest (37) and identical in dry dipterocarp and mixed deciduous forests (30). Although in pine-dry dipterocarp forest is almost identical in number of family to dry dipterocarp forest but some families are found in both forest types, some are in separate forest type and the least is in dry evergreen forest (11).

Five families are found widely distribute and the most important families as indicated by highest relative basal area cover in all forest types (Table 2) are Bignonaceae, Euphorbiaceae, Labiatae, Leguminosae and Rubiaceae while other families are found in at least four forest types and some families in only a single forest type although each forest type has different dominant families such as Dipterocarpaceae in dry dipterocarp and dry evergreen forests in which in the latter forest it may be considered as a monodominant type of Dipterocarpaceae family forest in this area. Labiatae and Leguminosae are dominant families in mixed deciduous forest, Pinaceae in pine-dry dipterocarp and pine-oak forests, Fagaceae and Lauraceae in montane forest (Table 2) suggesting their predominating characteristics in correspondence to the altitudinal range differentiation governing each forest type.

Table 2. Relative basal area cover (%) by families in each forest type.

Table 2. Relative basal area cover (%) by families in each forest type.

Table 2. Continued

Table 2. Continued

With 55 genera of trees, there are no tree genus frequently found in all forest types but 5 genera : Dalbergia, Phyllanthus, Schima, Stereospermum and Vitex are found in widest range of occurrence across 5 forest types. 10 genera: Albizzia, Anneslea, Aporusa, Callicarpa, Canarium, Dalbergia, Gardenia, Glochidion, Stryrax and Wendlandia are frequently found in 4 forest types. Other genera are found in 3 or 2 types and the rest are only in a specific forest type (Table 3).

Table 3. Species composition and relative basal area (%) of trees (DBH equal or over 4.5 cm) in six forest types.Each type is consisted of 9 study plots except for dry evergreen forest, only 1 plot is investigated.

Table 3. Species composition and relative basal area (%) of trees (DBH equal or over 4.5 cm) in six forest types.Each type is consisted of 9 study plots except for dry evergreen forest, only 1 plot is investigated.

Table 3. Continued

Table 3. Continued

Table 3. Continued

Table 3. Continued

Table 3. Continued

Table 3. Continued

Table 3. Continued

Table 3. Continued

Table 3. Continued

Table 3. Continued

Table 3. Continued

Table 3. Continued

Most widely distribute species represented by frequent occurrence across the 5 forest types are Dalbergia dongnaiensis, Phyllanthus emblica, Schima wallichii, Stereospermum neuranthum, Vitex peduncularis and Wendlandia tinctoria. The rest may be found only in some or even in one forest type. These species are the most dominant component species in separate forest type. The most dominant species in terms of % basal area cover in each forest type as shown in Table 3 are Dipterocarpus tuberculatus, Quercus ramsbottomii, Shorea obtusa and S. siamensis in dry dipterocarp forest. Dipterocarpus costatus, Homalium ceylanicum and Xylia xylocarpa var. kerrii are in dry evergreen forest. Lagerstroemia calyculata, Milletia leucantha, Pterocarpus macrocarpus and Tectona grandis are predominant species in mixed deciduous forest. Pine-dry dipterocarp forest which is on the highest altitude of dry dipterocarp forest is dominated by Aporusa villosa, D. tuberculatus, Gluta usitata, Pinus kesiya, Q. ramsbottomii with minor dominance of S. siamensis and W. tinctoria. In pine-oak forest where most of evergreen tree species are found are dominated by A. villosa, Lithocarpus elegans, P. kesiya and S. wallichii. Most evergreen broad-leaved tree species occupying the high altitude in montane forest are Actinodaphne sp., Castanopsis ferox, C. tribuloides, L. aggregatus, Litsea dubele, Mangleitia garettii, Mastixia euonymoides, Neolitsea pallens, Q. lenticellata, S. wallichii and Syzygium ankae. These floristic composition which only trees were enumerated are observed to be more or less diverse than ever previously reported by Pengklai (1996) in this area, especially for trees with D ≧ 4.5 cm.

Obviously these species distribute along the altitudinal gradients differently suggesting their amplitude responses and preferences in occupying the different breadth of habitat so as to correspond to their most favourable habitat conditions. Their quantitative structural characteristics, distribution, species diversity as well as other ecological parameters will be analyzed in relation to soils and some climatic variables in order to discriminate their zonations along the altitudinal range of Doi Inthanon national park.

Soil properties along the altitude

Soils of Doi Inthanon differ in their textural compositions in both horizons across forest types located on different altitudinal range, varying from sandy loam to sandy clay loam in surface soils and clay or clay loam in subsoils. Their average gravel contents as well as other physical properties also vary considerably among the 6 forest types (Table 4).

Table 4. Soil texture and some physical properties in Doi Inthanon forests.

Table 4. Soil texture and some physical properties in Doi Inthanon forests.

Soils of dry dipterocarp forest and montane forest are more gravelly and sandy in surface horizons than those in other forest types. As gravel contents are large downwardly to subsoils of dry dipterocap forest but on the contrary they decrease considerably in subsoils of montane forest while sand compositions are large in both forest types. Dry evergreen forest soils, on the other hand and although investigated from one plot only, are exceptionally low in gravel and sand particles in surface soils but greatest sand particles are found in subsoils. Silt compositions are most abundant in both horizons in mixed deciduous forest but least in surface soils of montane forest and in subsoils of pine-dipterocarp forest. Clay fractions are abundant in surface soils of dry evergreen and pine-dry dipterocarp forests and few in montane forest but most abundant in subsoils of pine-dipterocarp forest and fewest in dry evergreen forest.

Bulk density of soils in dry dipterocarp forest are greatest in both horizons and lowest in montane forest although the coarse particles such as gravel and sand are large in both forest soils. Their organic matter contents may be greatly different as indicated by their porosities which are most porous in both horizons suggesting their loose structure in soils of montane forest but more compact in both horizons of soils in dry dipterocarp forest. Bulk density of surface soils in dry dipterocarp, mixed deciduous and dry evergreen forests are found higher than in similar forest types in Doi Chiangdao (Khemnark et al., 1972) but fall within the ranges of bulk densities in soils of dry dipterocarp forest of several subcommunity types in Thailand (Sukwong et al., 1976; 1977; Bunyavejchewin, 1983a) but lower than in mixed deciduous and dry evergreen forests elsewhere (Bunyavejchewin, 1985; 1986). However, bulk densities in soils of pine-dipterocarp, pine-oak and montane forests are lower than in Doi Chiangdao soils investigated by the above workers.

Although particle density of soils in both horizons of mixed deciduous forest are observed to be greatest but all soils may not be greatly different in their values in the six forest types. However, particle densities of soils in dry dipterocarp and mixed deciduous forests are found to be greater than in similar forest types elsewhere recorded by Bunyavejchewin (1983a; 1985; 1986). These physical properties of soils although have been partly analyzed, they may exhibit some significant effects to the distribution of different tree species, diversity and community type on altitudinal gradients as postulated by Burnham (1974).

Some soil chemical properties have been investigated (Table 5) and found that soil pH range from neutral to strong acidity in surface soils and all strong acidity in subsoils of all 6 forest types. pH of both surface and subsoils are likely to follow the similar trend, being relatively high in the former and low in the latter horizons and the tendency of soil acidity obviously increase from those underneath the lower to the upper altitude forests except for dry evergreen forest which always distribute on soils along the stream bank at moderate low altitude where moderate acidity of soils in both horizons are observed as compared to other forest types.

Table 5. Soil reaction (pH) and some chemical properties in Doi Inthanon forests.

Table 5. Soil reaction (pH) and some chemical properties in Doi Inthanon forests.

Carbon and nitrogen contents of soil in both horizons are virtually different among the 6 forest types distribute along altitudinal gradients, being lowest in dry dipterocarp forest and largest in montane forest (Table 5). However, while carbon and nitrogen contents are found to be closely associated and decline downwardly in the similar magnitude in subsoils of all forest types, their average contents in surface soils are slightly different especially in soils of mixed deciduous and pine-dry dipterocarp forests where carbon contents of the former forest are low next to that in soils of dry dipterocarp forest but nitrogen contents of latter forest are lower than that in the former forest. This difference may be due to surface soils of mixed deciduous forest may have some influences from its species composition as it is found that members of Leguminosae tree species; Dalbergia nigrescens, Millettia leucantha, Pterocarpus macrocarpus and Xylia xylocarpa var. kerrii are found exceptionally abundant in this forest type (Table 3) which may provide better nitrogen contents to soils than those in dry dipterocarp forest as it is clearly observed that average nitrogen contents in soils of pine-dipterocarp forest are similarly low as in soils of dry dipterocarp forest even in the subsoils of this forest type also exhibit the same magnitude. C/N ratios of soils are found to be highest in pine-oak forest for both horizons (Table 5) but lowest in mixed deciduous forest for surface soils and in pine-dipterocarp forest for subsoils. However, tendency of C/N ratios in soils of forests distributing in xeric habitat on lower altitudes toward those in mesic habitat on upper altitudes are similar to that recorded by Ogawa et al. (1961). Other soil properties included soil moisture characteristics, moisture permeability, hardness, minerological properties and some chemical contents: P, K, Ca, Mg, Na, Si, Fe and Al are being analyzed. pH and some chemical properties of soils in various forest types in Thailand show similar trend with some variations in different localities as studied elsewhere in alternate dry and wet season sites (Khemnark et al., 1972; Sukwong et al., 1976; 1977; Bunyavejchewin, 1983a; 1985; 1986; Sahunalu et al., 1994; Sakurai et al., 1998). Doi Inthanon soil properties as variably differentiated due to altitude and vegetation zonation will be assessed as part of environmental factors determining their species composition and diversity distribution along the altitude.

Temperature and humidity along the altitude

Monthly mean air temperature inside the forests during the 9-month period from December, 1999 to August, 2000 vary clearly among the forest cover types of each measuring site along the altitudinal gradients, being highest in dry dipterocarp forest at lower altitude, lowest in montane forest at upper altitude and intermediate between the two lower and upper altitudes where they are mixed deciduous, pine-dipterocarp and pine-oak forests which distribute upwardly by altitudes suggesting temperature lapse rate pronounce clearly along the altitudinal difference. While mean air temperature in the lower altitude where two deciduous forest types are located rise up greatly in dry season (December through April), those in evergreen forest especially in pine-oak and montane forests do not greatly change (Fig. 4A). During wet season (May to August), while mean air temperature inside all forest types do not fluctuate much but inside those on lower altitude; dry dipterocarp and mixed deciduous forests maintain their mean air temperature as high as above 24℃ but those on upper altitude especially in pine-dipterocarp, pine oak and montane forests are always lower than 23℃. On the uppermost site of montane forest, it has never exceed 15℃ all over the period of 9 months and being almost constant at 14℃ throughout the wet season. These temperature measurements may not directly show the real air temperature regime over the sites as they are inside the forests and their fluctuations may not as great as in the area outside the forests but in the relatively open forests such as in dry dipterocarp and mixed deciduous forests, daily fluctuations are observed to be great as compared to the closed and evergreen forests; pine-oak and montane forests.

Figuer 4. Monthly mean air temperatuer (A) and relative humidity (B) during the 9-month period in 7 locations varying in different altitude (m asl) of Doi Inthanon from December, 1999 to August, 2000.

Figuer 4. Monthly mean air temperatuer (A) and relative humidity (B) during the 9-month period in 7 locations varying in different altitude (m asl) of Doi Inthanon from December, 1999 to August, 2000.

Available climatic data at a weather station situated at 1,300 m asl which may be comparable to those measured inside pine-oak forest at 1,110 m asl in this study, mean monthly air temperature in 1992-1997 during the 9 corresponding months are always greater than these observed values in the range of 0.5-3℃. On the basis of temperature lapse rates, tentative assessment during cold and dry seasons (December to April) and wet season (May to August) found that there are small variation during the two periods being about 0.7℃/100 m estimated by y = 27.509-0.00701 x, r2 = 0.976 and y = 29.735-0.00709 x, r2 = 0.977, where y = monthly mean air temperature, ℃; x = altitude, m asl; r2 = coefficient of determinations during the two periods respectively. It is noted that these estimations are from air temperature measurements inside the forests. This lapse rate is close to the value given by Whitmore (1984) for the tropical rain forests in the Far East of 0.67℃/100 m. However, several evidents from studies in Malaysia, lapse rates varied between 0.55 to 0.61℃/100 m (Burgess, 1969), 0.6℃/100 m from sea level to 2,000 m asl and 0.5℃/100 m above 2,000 m asl in Malaysian mountains (van Steenis, 1962), 0.7 and 0.51℃/100 m for maximum and minimum of mean weekly air temperature and 0.44 and 0.43℃/100 m for maximum and minimum of mean of 2-year data in Selangor, Malaysia (Nakashizuka et al., 1992) but it was 0.43℃/100 m in Equador (Grubb and Whitmore, 1967). In temperate region for instance; in Mount Emei, Sichuan, China, Tang and Ohsawa (1997) observed as 0.54℃/100 m in mean annual air temperature, 0.6℃/100 m in May, 0.45℃/100 m in December being low in winter and high in summer while in Wenchuan, Baoxing and Yaan, China, as 0.49℃/100 m in mean annual temperature, 0.58℃/100 m in May and 0.45℃/100 m in December.

Monthly mean air humidity inside the forests during the same period show large difference in dry (December to April) and wet (May to August) seasons especially in the open and on lower altitude forests (Fig. 4B). Forests on upper altitudinal zone especially pine-oak and montane forests maintain high air humidity almost all over the period as these zones are always wet and foggy days often prevail. These differences in air humidity suggest the atmospheric condition controlling forest type clearly. These climatic parameters will be further studied in relation to moisture condition both inside and outside the forest conditions together with soil parameters in association with the distributional patterns of tree species and forest community types along the ranges of altitude in Doi Inthanon.

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