From treeline to species line: Thermal patterns and growth relationships across the krummholz zone of whitebark pine, Sierra Nevada, California, USA

A multiyear study of forest-to-alpine ecotones across extensive krummholz zones in whitebark pine (WBP; Pinus albicaulis), Sierra Nevada, California, resolved mean treeline growing season temperature (GST) of 9.3°C, 2.6°C warmer than global thresholds previously described, and mean growing season length of 143 days. Temperatures declined with increasing elevation; GST at the upper krummholz line (8.9°C), however, was 2.2°C warmer than the mean global treeline threshold, suggesting that by thermal criteria these environments should support tree growth. Possible explanations for the warm conditions and persistence of krummholz rather than treeline advance include a role for moisture limitations, disequilibrium with Little Ice Age temperatures, and the influence of krummholz as a buffer to treeline dynamics. Radial growth in treeline WBP trees was negatively correlated with maximum annual temperature and positively correlated to water year precipitation. Krummholz stems had low correlations to one another, to treeline trees, and to climate, suggesting nonclimatic controls on growth. These findings underscore the variable nature of treeline response to climate change, suggest that krummholz ecotones behave differently from diffuse treelines, and add to examples of mountain conifers that may be exhibiting lag effects and have not shifted with contemporary warming.