Tree species effects on ecosystem water-use efficiency in a high-elevation, subalpine forest

Area of Science:

• Forest ecology
• Ecosystem science
• Plant physiology

Background:

• Ecosystem water-use efficiency (eWUE) integrates physical and biological processes.
• Leaf water-use efficiency (lWUE) is primarily driven by stomatal dynamics and varies by species.
• The relationship between lWUE and landscape-scale eWUE remains poorly understood.

Purpose of the Study:

• To assess how leaf-scale variation in lWUE influences landscape-scale eWUE in two neighboring forest communities.
• To investigate the contribution of species-specific traits and canopy structure to observed eWUE differences.

Main Methods:

• Analyzed 9 years of eddy covariance data to measure eWUE.
• Utilized C isotope ratios in needles to assess lWUE patterns.
• Scaled lWUE differences to account for leaf area distribution and canopy structure.

Main Results:

• Mean eWUE was 19% lower in spruce-fir dominated communities compared to lodgepole pine.
• lWUE differences among species and canopy positions were detected but explained little of the eWUE variation.
• Canopy structure, independent of needle physiology, showed potential to explain eWUE differences between communities.

Conclusions:

• Leaf-level physiological differences (lWUE) alone do not adequately explain landscape-scale eWUE variations between forest types.
• Soil, understory, and non-needle tissues likely contribute significantly to observed eWUE.
• Canopy structure plays a more critical role than species-specific needle physiology in driving eWUE differences when species effects are significant.