Co-Occurring Wyoming Pinus Species Exhibit Differing Climate–Growth Relationships

Abstract

The North American Dendroecological Field week (NADEF) is an intensive dendrochronology workshop, funded in part by the National Science Foundation. The 2019 Introductory Group at NADEF developed two precisely dated tree-ring width chronologies for Pinus contorta (lodgepole pine) and Pinus flexilis (limber pine) at the Wolf Knob site ca. 5 km west of Beartooth Lake, WY, within the bounds of the Shoshone National Forest (SNF), in the Greater Yellowstone Ecosystem (GYE). Wolf Knob is a semi-arid, S- to SW-facing, mid- to high-elevation site, making it an ideal location to examine the climate sensitivity of annual tree-ring width increments. Here, we show that two co-located Pinus species exhibit differing climate–growth relationships, with P. contorta exhibiting relatively weak correlations with precipitation (r = 0.37; p < 0.01) and temperature (r = –0.23; p < 0.05) during the late summer, and P. flexilis exhibiting stronger overall correlations with both cool-season (r = 0.48; p < 0.01) and warm-season precipitation (r = 0.51; p < 0.01) as well as with snowpack records (r = 0.45; p < 0.05). Our results suggest these two Pinus species may face disparate threats in the face of regional climate change, with P. flexilis being particularly vulnerable to drought conditions and declining snowpacks in the GYE. The differing seasonal climate sensitivities of the two species is likely caused by microsite conditions (e.g. soil moisture capacity, incoming solar radiation) and distinct species-climate responses, underscoring the importance of not only site selection, but also microsite and individual selection in dendroclimatological sampling. Finally, this work contributes to identifying Snow Water Equivalent (SWE)-sensitive tree-ring proxies in the GYE, critical for understanding ongoing warming-induced snowpack declines across western North America, particularly given the projections of a largely snow-free (April 1) GYE by 2075.