Overstory density, short growing seasons, and moisture limit Engelmann spruce establishment over time in high-elevation managed stands

For tree species with narrow ranges of climate suitability for regeneration like Engelmann spruce, understanding biophysical influences on juvenile establishment over time is crucial for supporting silvicultural practices responsive to climate change. We developed count-based models to identify abiotic and biotic variables explaining annual spruce establishment over 20-30 years following silvicultural regeneration treatments across 24 spruce stands in Colorado. Covariates retained in our final model encompassed information on climate; physiography; stand structure; and time. Establishment was positively related to germination- and second-year frost-free period beginning dates, indicating that moisture limitations at early life-stages may be alleviated by longer retention of snowpack. Yet, recruits were positively influenced by both lengthier frost-free periods and lower heat-moisture indices, suggesting a tension between growing season length and moisture limitations for physiological maturation, lasting five years after germination. Establishment was also limited by higher elevations and lower solar radiation, further suggesting cold-limitations in these stands. Moreover, overstory basal area negatively influenced recruitment, suggesting an inhibitive effect on light and precipitation availability for seedlings. Our findings extend similar conclusions from harsher treeline environments to high-elevation managed stands, but illustrate stronger relative effects of overstory density and cold-related limitations, and suggest transitions in climate effects with life-stages.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author