Elsevier

Dendrochronologia

Volume 41, January 2017, Pages 57-64
Dendrochronologia

Tree-climate relations along an elevational transect in Manang Valley, central Nepal

https://doi.org/10.1016/j.dendro.2016.04.004Get rights and content

Abstract

Elevation is a strong determinant of local climate and may therefore be an important factor to consider when examining the association between climate and tree growth. In this study, we developed a set of tree-ring width records for Abies spectablis (D.Don Spach) in the Manang Valley of central Nepal Himalaya and tested how tree growth and the relationship between tree growth and climate varied across a 450-m elevation transect. The sampled trees had a median age of 115 years, and the oldest individual specimen, which was located at 3775 m, had more than 212 rings. The common signal shared across the tree-ring series was relatively weak, which is typical for ring-width chronologies from the Himalayas. Even though these forests are located within a semi-arid climate, temperature had a stronger and more consistent influence on Abies growth than precipitation. All three chronologies across the transect exhibited a negative relationship with mean March–June temperatures, which could reflect the impact of warm weather during the early part of the growing season, possibly mediated through its influence on evapotranspiration and soil moisture. While interannual fluctuations in tree growth were synchronous across sites, longer-term trends in growth varied across the transect, with high-elevation trees showing elevated growth during the last two or three decades and lower-elevation trees behaving just the opposite. These disparate trends suggest the factors that control longer-term trends in forest productivity vary substantially with elevation. For studies intending to use tree-ring width records in the Trans Himalaya as climate proxies, it may be preferable to collect specimens at lower forest sites, where the agreement across the population of trees is stronger. Because longer-term trends in ring width can differ substantially from one elevation to another in this region, it may also be necessary to collect a greater number of samples from several positions along an elevation gradient.

Introduction

Alpine forests in Nepal are important resources for an array of ecological, social, and economic uses. At lower elevations, trees are harvested for firewood and infrastructure development, while high-elevation forests are crucial for pastoral animal grazing and wildlife habitat (Stevans, 2003, Adhikari et al., 2004). In addition, tourist traffic, which is one of the country’s leading sources of revenue, is concentrated at locations within and adjacent to alpine forests and national parks (Nepal, 2000). Because atmospheric temperature in Nepal has warmed rapidly over the last four decades, even more so at locations above 3000 m, (Shrestha et al., 1999, Shrestha et al., 2000, Shrestha and Aryal, 2011), alpine forests may be particularly susceptible to climate-induced changes in productivity or survival (Beniston, 2003). The alpine treeline in central Nepal has been shifting upwards by 24–35 m per decade (Suwal, 2010, Gaire et al., 2014), which in the long term may threaten the viability of cold-adapted trees and narrow habitats for rare wildlife species (Forrest et al., 2012).

Despite the obvious changes occurring at treeline, to date it is not clear how tree growth across the Nepal Himalaya is connected to specific aspects of the local climate. Analysis of a network of tree-ring width records across Nepal indicated that, for sites close to 3000 m elevation, tree growth is primarily controlled by temperature (Cook et al., 2003). A more recent synthesis of tree-ring records from Betula utilis growing at treeline (4000 m) concluded moisture is the primary factor limiting the growth of high-elevation forests (Liang et al., 2014). Similar studies conducted at intermediate elevations suggest that some combination of temperature and precipitation, perhaps mediated through their common influence on soil moisture, is the dominant control of tree growth (Sano et al., 2005, Gaire et al., 2011, Sano et al., 2011, Dawadi et al., 2013, Thapa et al., 2013, Thapa et al., 2015, Kharal et al., 2014). However, all of these studies have evaluated tree-climate relations at either a single location (Sano et al., 2005, Dawadi et al., 2013, Thapa et al., 2013, Thapa et al., 2015) or at multiple sites within roughly the same elevational range (Cook et al., 2003, Liang et al., 2014).

Prior research has demonstrated the association between climate and tree growth often varies as a function of elevation (Fritts et al., 1965). In most cases, trees growing at higher elevations experience colder, wetter climates and, as a result, their growth is primarily limited by temperature (LaMarche, 1974). By contrast, trees growing at lower elevations, which are often warmer and drier, respond more directly to year-to-year changes in moisture availability (Fritts et al., 1965, Peng et al., 2008, Yang et al., 2013). However, it is also clear forests do not always follow these tendencies. For example, tree growth at high elevation in Argentina (Morales et al., 2004) and the Tibetan Plateau (Fang et al., 2012, Yang et al., 2013) is primarily governed by moisture, not temperature. Several studies in mountains including the Tatras (Büntgen et al., 2007), the Alps (Frank and Esper, 2005, Di Filippo et al., 2007), and the Andes (Villalba et al., 1997) have assessed tree-climate relations as a function of elevation, but so far no such studies have been carried out in the Trans Himalaya.

Because the Nepal Himalaya offers the greatest vertical relief anywhere on the planet, elevation may be a particularly important factor to consider when attempting to evaluate the association between climate and tree growth. We carried out this study with the objective to test how tree growth Abies spectabilis (D. Don) Spach and its relationship with climate varies across an elevation transect spanning 450 m in the Manang Valley of central Nepal. We developed a set of tree-ring width records for A. spectabilis from three sites at different elevation in the study area. We compared mean tree-ring width at each elevation against local weather observations to identify the primary climate factor that limits tree growth, and identified trends in annual growth across the transect during the last two centuries. We conclude by suggesting potential strategies to improve the quality of environmental information recovered from tree-ring width data in Nepal’s Trans Himalayan region.

Section snippets

Study area and sample collection

We carried out this study along an elevation gradient in the Manang Valley of central Nepal (Fig. 1). Characterized by extremely rugged topography, the Manang Valley has significant natural areas important for biodiversity and tourism (National Trust for Nature Conservation, 2008). Because the valley is located in the Trans Himalayan zone, the rainshadow influence of the surrounding mountains makes it one of the driest locations within the Nepal Himalaya (Fig. 3; Karki et al., 2015). Under the

Tree-ring width characteristics

A. spectabilis growing along our study transect in the Manang Valley had a median age of 115 years, and the oldest individual specimen, which was located at the high-elevation site, had more than 212 rings. The total span of the combined ring-width chronologies extend back to the early 19th century (Table 3). Locally-absent rings, which occur when the vascular cambium along some portion of the stem remains dormant throughout the growing season (Schulman, 1941) were relatively rare at the mid-

Conclusions

We established a transect of tree-ring width records within Nepal’s Manang Valley to evaluate the potential influence of elevation on the strength and nature of climate information recorded by A. spectabilis growth rings. These new ring-width records extend back to the early 19th century, and the quality of common signal is somewhat weak but comparable to similar records from the broader Himalaya region. Across the elevational transect, temperature is the primary climatic factor affecting tree

Acknowledgements

This research was funded by the Community-based Forest Management in the Himalaya Project, a joint Danida-funded initiative of the University of Copenhagen, the Department of Forest Research and Survey (DFRS), Nepal and the Institute of Forestry, Tribhuvan University. We thank Mr. Rabindra Maharjan for his support in fieldwork and sample preparation, and Mr. Narayan Gaire for helpful advice on sample collection and analysis. Dr. Kurt Kipfmueller provided constructive advice at an early stage in

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