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Modelling the potential distribution of Betula utilis in the Himalaya

Bobrowski, Maria, Gerlitz, Lars, Schickhoff, Udo
Global ecology and conservation 2017 v.11 pp. 69-83
Betula utilis, biogeography, climate, climate change, data collection, environmental factors, linear models, model validation, mountains, niches, prediction, statistics, temperature, treeline, Himalayan region
Developing sustainable adaptation pathways under climate change conditions in mountain regions requires accurate predictions of treeline shifts and future distribution ranges of treeline species. Here, we model for the first time the potential distribution of Betula utilis, a principal Himalayan treeline species, to provide a basis for the analysis of future range shifts. Our target species Betula utilis is widespread at alpine treelines in the Himalayan mountains, the distribution range extends across the Himalayan mountain range. Our objective is to model the potential distribution of B. utilis in relation to current climate conditions. We generated a dataset of 590 occurrence records and used 24 variables for ecological niche modelling. We calibrated Generalized Linear Models using the Akaike Information Criterion (AIC) and evaluated model performance using threshold-independent (AUC, Area Under the Curve) and threshold-dependent (TSS, True Skill Statistics) characteristics as well as visual assessments of projected distribution maps. We found two temperature-related (Mean Temperature of the Wettest Quarter, Temperature Annual Range) and three precipitation-related variables (Precipitation of the Coldest Quarter, Average Precipitation of March, April and May and Precipitation Seasonality) to be useful for predicting the potential distribution of B. utilis. All models had high predictive power (AUC ≥ 0.98 and TSS ≥ 0.89). The projected suitable area in the Himalayan mountains varies considerably, with most extensive distribution in the western and central Himalayan region. A substantial difference between potential and real distribution in the eastern Himalaya points to decreasing competitiveness of B. utilis under more oceanic conditions in the eastern part of the mountain system. A comparison between the vegetation map of Schweinfurth (1957) and our current predictions suggests that B. utilis does not reach the upper elevational limit in vast areas of its potential distribution range due to anthropogenically caused treeline depressions. This study underlines the significance of accuracies of current environmental niche models for species distribution modelling under climate change scenarios. Analysing and understanding the environmental factors driving the current distribution of B. utilis is crucial for the prediction of future range shifts of B. utilis and other treeline species, and for deriving appropriate climate change adaptation strategies.