An extreme feature of Himalayan vegetation is that there is a difference in plant communities between slopes facing south and those facing north (Haffner, 1981; Kikuchi and Ohba, 1988; Kojima, 1985; Ohsawa et al., 1986; Shresta, 1982). Alpine grasslands in the Himalaya are an important natural resource for the grazing of yak, but they are often lacking on slopes facing north, where a Rhododendron anthopogon scrub usually predominates (Kikuchi and Ohba, 1988). Such a phenomenon is probably caused by slope-to-slope differences in grazing intensities, geological or geomorphological properties, drought conditions related to exposure to the sun, etc. This paper deals with vegetation patterns on talus facing various directions in the alpine region of the Himalaya, with particular focus on the effects of different slope exposures on vegetation under the same sort of geological and geomorphological conditions.

Study Area

Investigations were carried out in the southern part of Rolwaling Himal, near the northeastern end of Central Nepal (Fig. 1). In that area, Abies spectabilis forests are distributed up to about 3,800 m above sea level where a Rhododendron campanulatum scrub replaces it and extends up to about 4,000 m above sea level. The talus slopes examined in the present study, at about 4,200-4,300 m above sea level, are located in the alpine region above the upper limit of the R. campanulatum scrub. Alpine plant communities in this area, including the present sites, have been described by Kikuchi and Ohba (1988). Strata in the study area usually dip to the north and slopes facing north approximately coincide with the stratification of such beds. On the other hand, slopes facing south often consist of unconsolidated sediment originating from destroyed beds. Thus, not only the vegetation, but also the general form of a slope differs with different exposure. To exclude influences of such general landforms and to insure common geomorphological habitat conditions, talus slopes were chosen as sites for the present analysis. Cross-sections along transects across the talus slopes investigated are shown in Fig. 2. Site 1 faces S60° E and Site 2 faces N30° E.

Vegetation Pattern on Talus Slopes Facing East and North

The surface of a talus slope can usually be divided into a number of sections more or less different in inclination and exposure because of relief induced by gully erosion as shown in Fig. 2. Vegetation sequences along transects of the slopes at Sites 1 and 2 are given in Tables 1 and 2, respectively. According to the classification of plant communities by Kikuchi and Ohba (1988), four plant communities are distinguishable along the transects: the Kobresia hookeri community, the Potentilla peduncularis community, the Rhododendron anthopogon community and the Bistorta vaccinifolia community. Each of the communities at Site 1 seems to occur on a different kind of section. In particular, grasslands of the K. hookeri and the P. peduncularis communities, and the R. anthopogon dwarf scrub community seem to be found on sections of opposite exposure. On the other hand, R. anthopogon communities are more or less common at Site 2. The B. vaccinifolia communities are found in stony gully bottoms at both sites.

Calculation of Section Exposures on a Talus

The exposure T of a section of a talus facing direction X is shown by the formula
T = X + ∠ABC (1)
where T and X are given as NT°E and NX°E, respectively, and ∠ABC is given in a tetrahedron consisting of a horizontal plane, a vertical plane parallel with the talus exposure (NX° E), a vertical plane across the talus exposure and the section surface itself as shown in Fig. 3.

∠ABC = tan^-1AC/BC.
AC = CD/tanY and BC = CD/tanZ
where Y is the inclination of the talus and Z is the inclination of the section along the cross-section across the talus. Thus, formula (1) can be given as
T = X + tan^-1(tan Z / tan Y). (2)
Z is given in minus degrees when sections incline left facing downward on the talus.

Based on formula (2), section exposures on a talus facing north (X=0) are calculated as shown in Fig. 4 against the talus inclinations (Y) of 15, 30, 45, 60 and 75°.

Micro-scale Vegetation Patterns on Talus in Relation to Exposure

Exposures of sections along a transect of Site 1 (Fig. 2) were calculated by formula (2) based on inclination along the cross-section(Z values). They are shown in Table 3 together with the community type existing on each section. Sections supporting a K. hookeri. community or a P. peduncularis community face south from a direction of N88°E, while those supporting a R. anthopogon community face north from the direction of N95°E. Thus, critical slope exposure differentiating the type of vegetation tends to fall on the east (T=90° ) and, probably, the west (T=270° ), although R. anthopogon communities were found to some extent on slopes facing N106°E and N137°E. Fig. 5 shows the distribution of T values, section exposure calculated against talus exposure (X) and section inclination (Z) when the talus inclination (Y) is 30° . According to the figure, talus slopes facing east or west should be expected to include sections inhabitable by both types of plant communities. However, those facing north or south, except for extraordinarily steep sections, provide habitats occupied by a certain type of community. The talus of Site 2, facing N30° E could be expected to consist of sections that support R. anthopogon communities (Fig. 5) and, in fact, these slopes were occupied by this community as shown in Tables 2 and 3. Nevertheless, the B. vaccinifolia community also occurs there along the gully bottoms as at Site 1, which faces south.

Conclusion

Different kinds of plant communities such as grassland communities of K. hookeri and P. peduncularis, and dwarf scrub communities of R. anthopogon are found on the same sort of landform as talus, but each of them occur on different exposures; the former two inhabited south-facing exposures, and the latter were found on northfacing exposures. Differences in vegetation between slopes facing north and south should not be caused by geological nor geomorphological conditions but rather by the differences in exposure, probably due to differences in drought conditions induced by solar radiation. In spite of such differences in vegetation correlated with differences in exposure, B. vaccinifolia communities commonly occur in rocky habitats along gully bottoms of talus slopes facing any directions. Vegetation of talus slopes comprises of two kinds of plant communities, one depending on exposure of the talus and the other depending on edaphic site conditions on the talus. The occurrence of such plant communities can be represented as in Fig. 6.

References

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