The Himalayan region, with the largest snow and ice cover mountainous region in the world, regulates climate of a large landscape in the South Asia. It is home to major river systems, largest cryosphere area outside the polar region, one of the global biodiversity hot spots, and support life of nearly 300 million people (Xu et al., 2009). This region also holds significant importance in terms of biological and socio-cultural diversity, and encompasses wealth of a large number of endangered endemic species. This region is experiencing swift changes driven by climate change and other anthropogenic factors such as urbanization, infrastructure development, migration, tourism and globalization, which may lead to enormous consequences; both at regional and global level (Sharma et al., 2019). The rate of warming in Himalayas during past 102 years (1901–2003) in the last century has been higher (0.9-1.60C) than average global warming rate with relatively higher warming during the recent decades (Bhutiyani et al., 2007; Joshi and Kumar, 2013). Rise in both minimum and maximum temperatures has resulted into relatively warmer winters in the region (Dimri and Dash, 2012). Earlier studies have also reported enhanced warming in mountains with higher rates than land surfaces and greater increases in minimum temperatures than maximum temperatures (Diaz and Bradley, 1997; Beniston et al., 1997; Rangwala et al., 2009; Liu et al., 2009; Qin et al.; 2009).
There are growing evidences that the rate of warming is amplified with elevation; high elevation areas including alpine region experience higher increase in temperature than lower elevation areas (Pepin et al., 2015). This elevation-dependent warming (EDW) can modify various ecosystems in the mountain, including cryosphere, hydrological regimes and biodiversity. EDW is governed by important feedback mechanisms of snow, albedo, land surface, water vapor, latent heat release, and radiative flux changes, temperature change, and aerosols. Combinations of these factors may account for contrasting regional patterns of EDW (Rangwala and Miller, 2010; Rangwala and Miller, 2012; Pepin et al., 2015). The study from Tibetan Plateau by Yan and Liu, (2014) for 1961–2012 shows increase in warming rate with elevation for annual mean temperature and minimum temperatures, both at annually and during autumn and winter season. Similarly, EDW has been observed across other mountain regions (e.g. the Swiss Alps, the Colorado Rocky Mountains, the Tibetan Plateau, the Himalayas, and the Tropical Andes) in response to global or regional climate change (Pepin and Losleben, 2002; Rolland, 2003; Pepin and Seidel, 2005; Blandford et al., 2008; Liu et al., 2009; Qin et al., 2009; Rangwala and Miller, 2010; Gilbert and Vincent, 2013, Joshi et al., 2018). These studies confirm that high elevation regions including alpines are warming more rapidly than lowland areas across the world.
The high-altitude environments are influenced by the free atmosphere temperature gradients or temperature variation with altitude known as "temperature lapse rate (TLR)". TLR varies on daily and seasonal scales in mountains (Müller and Whiteman, 1988; Blandford et al., 2008; Joshi et al., 2018) due to local surface energy balance (Marshall et al., 2007) and different weather types or synoptic conditions (Pepin, 2001; Kirchner et al., 2013). It is useful for determining the elevational distribution of temperature along a transect in absence of the in-situ temperature measurements. In such cases, average temperature gradients of -0.60 0C (Dodson and Marks, 1997) or -0.65 0C/100 m (Barry and Chorley, 1987) are often used to simulate and model various ecological processes when high precision is not required. However, temperature and precipitation gradients vary considerably with space and time and controlled by topographical features in mountains. Hence, assuming a constant value of TLR may lead to inaccurate results while examining elevation dependent warming and its impacts on different high-altitude ecosystems.
Studies on temperature gradients for treeline environments and other high-altitude ecosystems in Himalayan region are scarce (Shrestha et al., 1999). However, pioneering study by Joshi et al. (2018) gave initial estimates of TLR and its variation for treeline environment in Western Himalaya. The study showed that TLR for treeline in Himalayas, which varies seasonally and along aspect, is distinctly lower (-0.53 0C/100 m) than commonly used value. In recent decades, air temperature at higher elevations has increased more rapidly thereby decreasing near-surface air temperature lapse rates in warmer climate (Liu and Chen, 2000; Qin et al., 2009; Wang et al., 2005; Rangwala et al., 2010; Pepin, 2001). Hence, the lower value of mean TLR for treeline region in Western Himalaya may be due to the enhanced EDW in high altitude areas and a consequence of global warming. The existing knowledge gap in this domain calls for further studies to analyse seasonal and synoptic variations in TLRs for climatically sensitive environments such as treeline, alpine meadows, and cryosphere regions in Himalaya.
The major objective of the present study is to quantify the temperature lapse rate and examine spatial variations of mean annual, seasonal and monthly TLRs for treeline transects representing different climate regimes along Himalayan arc based on two year's in-situ instrumental records from 20 stations. Our study gives statistics of monthly and seasonal TLR for tree line environments within Indian part of Himalaya and explains the influence of important factors (air temperature, rainfall and relative humidity) on lapse rate. We further examined whether treelines in Himalayas are warmer than the other mountain ranges across the world and TLR for these environments in Himalaya are shallow than the commonly used values. We made efforts to explain that the EDW, signified by shallow TLRs, is amplified with elevation in Himalayan region under the influence of climate change. Considering that treeline environments are extremely temperature-sensitive transition zone for many plant species endemic to the region (Körner, 1998; Walther et al., 2005), we discuss possible implications of reduced or shallow TLR on treeline vegetation and future needs to increase understanding of Himalayan climate, their controlling mechanisms and impacts on critical ecosystems. Considering the existing knowledge gap in the region, this study is a major effort to analyze temperature lapse rate for treeline areas of Himalayas as the study used observed daily temperature data for two years and the sites encompass much of the Himalayan Arc and major variability in precipitation in the region. However, to achieve more reliable estimates of the lapse rate and explaining EDW as a consequence of climate change, denser station networks and longer observations are required across the Himalaya.