Topographic influences on ring widths of trees and shrubs across alpine treelines in southwest Yukon

Katherine D Dearborn,Ryan K Danby

doi:10.1080/15230430.2018.1495445

Abstract

ABSTRACT Growth responses of trees and shrubs to climate often exhibit unexplained variation in alpine regions, making it difficult to predict how they will respond to future changes in climate. We sought to characterize and explain this variability in southwest Yukon, a topographically complex region of subarctic Canada. We collected cores and sections from 360 spruce trees and 480 willow shrubs across treelines on north and south aspects in six valleys spanning two mountain ranges. We compared growth rates, growth patterns, and climate-growth responses between species and topographic factors. South aspects had wider tree rings and higher tree and shrub interseries correlations than north aspects, likely because of shallow active layers on the latter. Growth patterns and responses to climate did not vary between aspects or elevations but differed slightly between mountain ranges, likely because of differences in spring soil moisture content between ranges. Growth responses of both species to summer temperature were positive, but tree growth was negatively correlated to spring temperature and shrub growth was negatively correlated to summer precipitation, both of which are projected to increase along with summer temperature. Future changes in climate could therefore reduce the growth of one or both species in southwest Yukon.

Highlights

Satellite imagery has revealed that many regions in the circumpolar north have experienced an increase in plant productivity in recent decades, much of which has been attributed to changes in the abundance and growth of woody vegetation (Frost, Epstein, and Walker 2014; Goetz et al 2005; Jia, Epstein, and Walker 2009)
A global metaanalysis revealed that only half of all treelines worldwide have advanced within the past century, despite nearly ubiquitous warming (Harsch et al 2009), and several studies have found evidence that the ring widths of trees growing at treeline are becoming decoupled from growing-season air temperatures
We briefly investigated the viability of S. glauca for dendroclimatological analysis in southwest Yukon, because only two previous studies have examined climate-growth relationships in S. glauca populations, both of which were conducted in western Greenland (Jørgensen et al 2015; Young et al 2016)

Summary

Introduction

Satellite imagery has revealed that many regions in the circumpolar north have experienced an increase in plant productivity in recent decades, much of which has been attributed to changes in the abundance and growth of woody vegetation (Frost, Epstein, and Walker 2014; Goetz et al 2005; Jia, Epstein, and Walker 2009). Dendrochronological studies on both trees and shrubs have revealed predominantly positive relationships between annual ring widths and rising air temperatures, especially near the elevational or latitudinal range limits of the study species (Salzer et al 2009; Forbes, Fauria, and Zetterberg 2010; Blok et al 2011; Buras, Hallinger, and Wilmking 2012; Macias-Fauria and Johnson 2013; Buchwal et al 2013; Jørgensen et al 2015) These changes have the potential to alter the composition of tundra plant communities and initiate a positive feedback loop between rising air temperatures and increasing woody plant abundance by reducing tundra surface albedo (Chapin, Sturm, and Serreze 2005; Greenwood and Jump 2014). A global metaanalysis revealed that only half of all treelines worldwide have advanced within the past century, despite nearly ubiquitous warming (Harsch et al 2009), and several studies have found evidence that the ring widths of trees growing at treeline are becoming decoupled from growing-season air temperatures, e1495445-2

Methods

Results

Conclusion