Annual evidence of moisture limitations at treeline in the Sangre de Cristo Mountains

Abstract

This study is focused on capturing a recent regeneration patterns on an annual level over the landscape of the Sangre de Cristo (SDC) mountain range. Modern temperature trends (post-1945) of this area are dominated by sharp rises in minimum temperature during the warm and cool seasons. Coincidentally, the onset of heat-induced drought stress is impacting trees throughout the mountain forest belt, though research is lacking across broad spatial scales at treeline. I used dendroecological techniques to destructively sample seedlings on contrasting north and south facing slopes at upper treeline in the SDC to investigate moisture interactions at treeline. I hypothesized that if patterns of tree regeneration are primarily driven by temperature, then I would expect seedling establishment to be more abundant on south-facing slopes. Alternatively, if heat induced drought stress is an important driver, I would expect seedling establishment to be more confined to north-facing slopes. Results show that seedling establishment is significantly (p < 0.01) favored on north-facing slopes (n=169) verses south-facing slopes (n = 66). These seedlings are also significantly younger (p < 0.05) and smaller (p < 0.01) than their counterparts on south-facing slopes. The relationship between climate and annual establishment patters were investigated to identify any important drivers. Results show that there is a negative relationship between drought conditions and establishment events. Across all sites (n=6) and slope aspects, there was no establishment found after 2009, possibly indicating a minimum threshold has been surpassed and conditions are no longer suitable to regeneration above treeline. This indicates the possibility of moisture limitations at treeline and brings into question the future structure and extent of the upper forest border under a warmer and drier climate.