Ecological modelling of vascular plant diversity under different climate and land use change scenarios in Nepal Himalaya
Date
2021
Authors
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Department of Botany
Abstract
The effect of climate, topography and land-use on the plant species richness and
composition are widely studied topics in ecology. Recent scenarios of climate change
studies also show the effect on the species diversity and gain in new area and loss in
old habitats. The species richness is considerably high in Nepal with respect to its
surface area due to its topography and diversity in micro climates. Climate change and
land use change are affecting such richness and composition world widely. Current
study was focused on assessing the species richness of vascular plants, their
composition and finding the environmental variables which affect them in different
elevations and land use gradients in the Nepal Himalaya. We also assessed the effect
of climate change on the future distribution of Betula utilis and Larix potaninii var.
himalaica which are both tree-line vascular plant species in Nepal Himalaya. The
elevation of study sites ranged from 2200 m to 3800 m in six river valleys of Gorkha,
Solukhumbu and Taplejung districts of Nepal. Four land use types namely crop field,
meadow, exploited forest and natural forest were surveyed. Species were recorded in
25m x 2.5m plots in each land use type. We measured soil temperature as well as air
humidity and temperature with data loggers on all study sites. The recorded data show
a clear gradient of elevation and land use types designed in current study. Two visits
were done for all plots in two different seasons. Although there are some common
species recorded in both visits, the species record in individual visits are also unique.
The field survey revealed a total of 840 vascular plant species belonging to 492
genera of 120 families. Among them 631 were dicots, 159 monocots, 12
gymnosperms and 38 pteridophytes. There were 96 tree species, 110 shrub species,
586 herbs and 48 climber species. Asteraceae comprised the highest number of
species (85 spp.). Multivariate ordination techniques such as Detrended
Correspondence Analysis (DCA) and Canonical Correspondence Analysis (CCA)
were used to analyze the patterns of species composition and Generalized Linear
Model (GLM) was used to find out the significant environmental variables affecting
the species richness. In DCA, the first two axes were more than 4.0 standard units
inferring high beta diversity. Three sets of environmental variables namely
microclimate, bioclimate and topography were used for the ordination. In CCA,
isothermality, minimum temperature of the coldest month, precipitation seasonality,
precipitation of the coldest quarter were significant contributing bioclimatic variables.
Similarly, exposition, land use types, elevation, aspect, slope, region, and valley were
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significant constraining variables for the species composition. Same set of predictor
variables were used to develop three models using GLM. In addition, a synthetic
model was developed by combining significant predictor variables from each set of
the variables. Linear combination of mean soil temperature, mean humidity, minimum
humidity, maximum temperature of warmest month, precipitation of the driest month,
precipitation seasonality, valleys, land use types, elevation and latitude produce the
best model in forward selection procedure. Maxent modelling was done to predict the
future distribution of vascular plants under different climate change scenarios by 2050
and 2070. We chose two species namely Betula utilis and Larix potaninii var.
himalaica for this purpose. Both plants occur on high Himalaya up to tree-line. Larix
is a deciduous conifer which has limited distribution in central Nepal and adjoining
region of Tibet, China. Betula utilis is a deciduous broad-leaved plant with wide
distribution from Pakistan to China. It occurs in almost all high mountains of Nepal.
The bioclimatic variables were taken from CCSM4 climate database with RCPs 2.6,
4.5, 6.0, 8.5 for 2050 and 2070. Additionally, elevation, aspect and land use types
were taken as predictor variables which were taken from the land cover map of Nepal
2010. All models were validated with various statistical tests. All models showed
AUC > 0.9, accuracy between 88.4875 and 95.2858, error rate between 0.1151 and
0.0471, and TSS between 0.6359 and 0.7613. The validation parameters showed the
robustness of the models. The elevation and annual mean precipitation were main
contributing factors for the distribution of Betula utilis and Larix potaninii var.
himalaica respectively. The distribution area of Betula utilis will decrease by -18.72%
under RCP 2.6 but will increase in all other scenarios by 2050 and 2070. The new
gains in area will be distributed towards the western mountains at all RCPs in 2050
and 2070. The predicted area of Larix potaninii var. himalaica did not show specific
trend of increment or decrease at all RCPs by 2050 and 2070. The findings of the
current study will be applicable in the assessment of effect of environmental and land
use variables on the species composition and richness other group of plants too.
Current study will be also helpful in devising future policies of conservation of rare,
vulnerable, and threatened plant species whose distributions are more likely to be
affected by the climate change in the future.
Key words: Biodiversity, SDM, Species composition, Species distribution, Treeline
Description
Keywords
Biodiversity, Species composition, Species distribution, Tree line