Can Rapoport's rule explain tree species richness along the Himalayan elevation gradient, Nepal?

Authors

  • Khem Raj Bhattarai,

    Corresponding author
    1. Department of Biology, University of Bergen, Allégaten 41, N-5007 Bergen, Norway,
      *Correspondence: Khem Raj Bhattarai, Department of Biology, University of Bergen, Allégaten 41, N-5007 Bergen, Norway. E-mail: khem.bhattarai@bot.uib.no
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    • Current address: Department of Pant Resources, National Herbarium and Plant Laboratories, GPO Box 2270, Godawari, Nepal. E-mail: khem.bhattarai@bot.uib.no

  • Ole R. Vetaas

    1. Centre for Development Studies, University of Bergen, Nygaardsgaten 5, Norway
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*Correspondence: Khem Raj Bhattarai, Department of Biology, University of Bergen, Allégaten 41, N-5007 Bergen, Norway. E-mail: khem.bhattarai@bot.uib.no

ABSTRACT

Rapoport's rule applied to an elevation gradient predicts a positive correlation between elevation ranges and elevation. This is supposed to be caused by the increasing magnitude of the climatic extremes at higher elevations, and thus, it is deduced that species richness should decrease with increasing elevation. The distribution of 614 tree species was used to test Rapoport's elevational rule along a gradient from 100 to 4300 m a.s.l., in the Nepalese Himalaya. The relationship between species richness and elevation was analysed by using generalized linear models (GLM). Generalized additive models (GAM) were used to examine the relationship between elevational range and the elevational mid-point of a species along the gradient. The widest elevation ranges are observed at mid-elevations, and narrow elevation ranges are observed at both ends of the gradient. This does not support Rapoport's elevation rule, as proposed by Stevens. There is a peak in species richness between 900 and 1000 m, and not in the tropical lowland as projected by Rapoport's elevation rule.

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