Multiscale Influences of Climate on Upper Treeline Dynamics in the Southern Rocky Mountains, USA: Evidence of Intraregional Variability and Bioclimatic Thresholds in Response to Twentieth-Century Warming

Abstract

We examined the influence of twentieth-century climate on upper treeline dynamics in the southern Rocky Mountains to better understand the role of temperature and precipitation on tree establishment and to determine whether bioclimatic thresholds have been exceeded as a result of warming during the twentieth century. By using dendrochronological techniques, we reconstructed tree establishment at upper treeline on six mountain peaks within the Front Range and Sangre de Cristo Mountains. We compared age–structure data with climate using Spearman's rank correlation coefficients between annual and seasonal climate indexes and tree establishment dates at both regional (southern Rockies) and landscape scales (mountain range). Regime-shift analysis detected thresholds in temperature, precipitation, and tree establishment data. Tree establishment has increased substantially at upper treeline throughout the southern Rockies, leading to varying degrees of treeline advance upslope. Tree establishment in the Front Range significantly correlates with temperature, but no significant correlations with temperature variables exist in the Sangre de Cristos. Significant inverse correlations exist with precipitation but remain confined to north-facing slopes in both mountain ranges. Synchronous regime shifts (within five years) occurred in the Front Range between temperature and tree establishment during the early 1950s (1950–1954), suggesting that increasing temperatures provided a possible mechanism for abrupt increases in establishment. This research highlights the intraregional variability in treeline sensitivity to climate in the southern Rocky Mountains and the usefulness of using a multiscale approach coupled with regime-shift analysis to examine the influence of twentieth-century climate on treeline.