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

Abstract

For tree species with narrow ranges of climate suitability for regeneration such as Engelmann spruce (Picea engelmannii Parry ex Engelm.), 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 5 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.