Abstract
ABSTRACT In this study we explore radial growth rates and climatic responses of alpine larch trees (Larix lyallii Parl.) growing in high elevations of the northern Rocky Mountains of Montana, USA. We examine responses between two stands of alpine larch that are separated by less than one kilometer and are growing at similar elevations, but with different aspects. Radial growth rates from trees sampled on the southern aspect of Trapper Peak (TPS) were largely controlled by January snow-water equivalent, while summer maximum temperature was the principal radial-growth driver for trees sampled on the northern aspect of Trapper Peak (TPN). Following the coldest summer (1993) in the century-long instrumental climate record, the radial growth at TPN became greater than at TPS and was the reverse of what occurred pre-1993. We posit that an upward trend in maximum summer temperature is preferentially benefitting the trees growing on the north-facing TPN site by extending the growing season and causing earlier snowmelt, and this has caused the growth rate divergence during the past two decades. As such, our study illustrates that the growth-divergence phenomenon noted in other high-elevation species, whereby macroenvironmental changes are eliciting responses at the microenvironmental level, occurs within stands of alpine larch growing in western Montana.
Highlights
Microenvironmental variations have emerged in recent decades concurrent with increasing temperature, resulting in differential stressors that are driving forces for tree growth (Wilmking et al 2005)
On examination of the climate-growth relationships from a chronology developed using all trees, we found that they were weaker than what we had found from nearby sites (Knapp and Soulé 2011) and decided to divide the chronology based on aspect
For this study we developed two tree-ring chronologies from a contiguous grouping of alpine larch trees but separated by aspect and: (1) searched for the dominant climatic drivers of radial growth, (2) examined the climate/growth responses for long-term temporal continuity, (3) examined the long-term temporal patterns of standardized and raw radial growth rates to determine if and when growth divergence occurred between sites, and (4) hypothesized on environmental and climatic controls for radial-growth and climate/growth differences between sites
Summary
Microenvironmental variations have emerged in recent decades concurrent with increasing temperature, resulting in differential stressors that are driving forces for tree growth (Wilmking et al 2005). The causes of divergence are poorly understood because of the potentially large suite of interrelated variables affecting tree growth (D’Arrigo et al 2008) and Büntgen et al (2009) suggest that the divergence problem is not a response to a single mechanism but, rather, should be examined at more localized scales before making broad conclusions. The divergence problem arises when calibrating the instrumental climate record with tree growth. When tree-ring indices that contain recent decades are used in climate calibrations, the divergent tree growth does not allow for a precise fit, resulting in an underestimation of temperatures (D’Arrigo et al 2008). Growth divergence and associated problems need to be critically considered as a limiting factor in the effectiveness of a given tree-ring chronology to accurately model and predict future temperatures, especially with the observed warming in the twenty-first century
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