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15 - Temperature and humidity as determinants of the transition from dry pine forest to humid cloud forests in the Bhutan Himalaya

from Part II - Regional floristic and animal diversity

Published online by Cambridge University Press:  03 May 2011

By
P. Wangda  and
M. Ohsawa
Edited by
L. A. Bruijnzeel ,
F. N. Scatena  and
L. S. Hamilton
P. Wangda
Affiliation:
The University of Tokyo, Japan
M. Ohsawa
Affiliation:
The University of Tokyo, Japan
L. A. Bruijnzeel
Affiliation:
Vrije Universiteit, Amsterdam
F. N. Scatena
Affiliation:
University of Pennsylvania
L. S. Hamilton
Affiliation:
Cornell University, New York
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Summary

ABSTRACT

Air temperature and humidity, as well as soil moisture conditions were decisive in determining forest composition and structural changes along an altitudinal transect from a dry valley bottom at 1250 m.a.s.l. to a ridge top at 3550 m.a.s.l. in the Bhutan Himalaya. Mean annual temperature and soil moisture content were inversely related. Mean annual temperature decreased from 18.2 °C in the valley to 4.3 °C at the ridge top, with an average lapse rate of 0.62 °C 100 m−1. Conversely, relative humidity, soil moisture content, and mean annual precipitation all increased with elevation, from 71.0%, 14.7%, and 584 mm in the valley to 93.5%, 73%, and 1576 mm at 3550 m. Based on quantitative vegetation data, climatic conditions, and observations on the presence of mosses and epiphytic vascular plants, the cloud-affected zone was defined as lying above c. 2500 m.a.s.l. and coincided with the transition from moist broad-leaved forest to moist/wet broad-leaved forest. Two floristic/physiognomic transitions were identified along the altitudinal gradient between broad-leaved and coniferous forests; the lower one at 2000 m a.s.l. between dry Pinus roxburghii forest and moist evergreen broad-leaved Quercus lanata forest (intervening with deciduous broad-leaved Q. griffithii in between). A critical soil moisture content of c. 20% marked the limit of downward extension of broad-leaved trees into the shade-intolerant but drought-tolerant pine zone.

Type
Chapter
Information
Tropical Montane Cloud Forests
Science for Conservation and Management
 , pp. 156 - 163
Publisher: Cambridge University Press
Print publication year: 2011

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