Gymnosperm species richness patterns along the elevational gradient and its comparison with other plant taxonomic groups in the Himalayas

Suresh C Subedi,Jay P Sah,Khem Raj Bhattarai,Timothy M Perez

doi:10.21425/f5fbg44232

Abstract

Phylogenetic constraints on ecophysiological adaptations and specific resource requirements are likely to explain why some taxonomic/functional groups exhibit different richness patterns along climatic gradients. We used interpolated species elevational distribution data and climatic data to describe gymnosperm species richness variation along elevational and climatic gradients in the Himalayas. We compared the climatic and elevational distributions of gymnosperms to those previously found for bryophytes, ferns, and angiosperm tree lineages to understand the respective drivers of species richness. We divided our study location into three regions: Eastern; Central; and Western Himalayas, in each calculating gymnosperm species richness per 100-m band elevational interval by determining the sum of species with overlapping elevational distributions. Using linear regression, we analyzed the relationship between species’ elevational mid-point and species’ elevational range size to test the Rapoport’s rule for gymnosperms in the Himalayas. Generalized linear models were used to test if potential evapotranspiration, growing degree days, and the number of rainy days could predict the observed patterns of gymnosperm species richness. We used the non-linear least squares method to examine if species richness optima differed among the four taxa. We found supporting evidence for the elevational Rapoport’s rule in the distribution of gymnosperms, and we found a unimodal pattern in gymnosperm species richness with elevation, with the highest species richness observed at ca. 3000 m. We also found a unimodal pattern of gymnosperm species richness along both the potential evapotranspiration and growing degree day gradients, while the relationship between species richness and the number of rainy days per year was non-significant. Gymnosperm species richness peaked at higher elevations than for any other plant functional group. Our results are consistent with the view that differences in response of contrasting plant taxonomic groups with elevation can be explained by differences in energy requirements and competitive interactions.

Highlights

The latitudinal diversity gradient, i.e., the increase in species richness from polar to equatorial regions, a conspicuous feature of global biogeography, has long intrigued biogeographers (Davidowitz and Rosenzweig 1998, Hawkins et al 2003a, Willig et al 2003, Qian and Ricklefs 2007, Weiser et al 2018)
In the Himalayas, gymnosperm species richness had a unimodal hump-shaped relationship with elevation, the maximum gymnosperm species richness tended to be skewed toward higher elevations within the elevational range covered by gymnosperms
We found that gymnosperm species richness peaked at higher elevations, lower mean annual potential evapotranspiration (PET) values, and at shorter mean annual growing degree days than any other functional plant lineage

Summary

Introduction

The latitudinal diversity gradient, i.e., the increase in species richness from polar to equatorial regions, a conspicuous feature of global biogeography, has long intrigued biogeographers (Davidowitz and Rosenzweig 1998, Hawkins et al 2003a, Willig et al 2003, Qian and Ricklefs 2007, Weiser et al 2018). Several hypotheses that attempt to explain the latitudinal diversity gradient involve the direct or indirect role of climate in mediating biotic interactions (Pianka 1966, Janzen 1967, Hawkins et al 2003b, Usinowicz et al 2017). Climate has been suggested to be an important factor influencing interspecific competition across latitudes (Usinowicz et al 2017), with its influence mediated by species’ intrinsic climatic tolerances (Janzen 1967, Perez et al 2016). Fewer species can occur at higher latitudes, but they can occupy broader climatic niches due to the lower interspecific competition (Pianka 1989). One way that the interaction between climate and competition is thought to manifest is through species’ range size distributions, which depend on species’ intrinsic tolerance to climatic fluctuations

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