Seasonal temperature-moisture interactions limit seedling establishment at upper treeline in the Southern Rockies

Over recent decades, sharply rising temperatures without an accompanying increase in precipitation have created widespread heat-induced drought stress, or hotter drought. Tree-ring reconstructions have discovered significant declines in seedling establishment from hotter drought across montane and subalpine forest belts during this time, yet comparable studies at upper treeline are non-existent. In this study, we reconstruct annual patterns of seedling establishment at upper treeline in the Southern Rocky Mountains to test the hypotheses that establishment is governed by temperature-moisture interactions and that slope aspect mediates the influence of hotter drought. To test these hypotheses, we destructively sampled seedlings along a network of six study sites to reconstruct annual patterns of establishment on opposite north-facing and south-facing slopes. Results from this research can be summarized into two main points with respect to the influence of climate on seedling establishment at upper treeline over approximately the last three decades (1991-2019). First, temperature-moisture interactions throughout the year play a crucial role in facilitating successful seedling establishment. Second, and perhaps most striking, is the complete lack of establishment at all sites over the past decade. This could signify that a threshold has been surpassed and conditions are now beyond the climatic optimum for successful seedling establishment above treeline moving forward. These results expand upon similar findings from forests at lower elevations, introducing the likelihood that seedling establishment along the entire mountain forest belt of the Southern Rocky Mountains is being impacted by hotter drought. This means that any declines in the rate of seedling establishment across montane and subalpine forests will not be offset by increased recruitment at treeline.

Automated summary

Based on the study of seedling establishment at upper treeline in the Southern Rocky Mountains, it was found that temperature-moisture interactions are crucial for successful seedling establishment, and there has been a complete lack of establishment in the entire region over the past decade. This could indicate that climatic conditions above treeline have surpassed the optimum for successful seedling establishment.