Altitude trends in conifer leaf morphology and stable carbon isotope composition

Area of Science:

• Ecology
• Plant Physiology
• Stable Isotope Biogeochemistry

Background:

• Stable carbon isotope ratios (δ¹³C) in plant tissues reflect the balance between atmospheric CO₂ supply and leaf internal demand.
• Altitudinal gradients present varying environmental conditions that can influence leaf structure, chemistry, and gas exchange.
• Understanding these relationships is crucial for predicting plant responses to environmental change.

Purpose of the Study:

• To develop a general model explaining variation in δ¹³C across altitudinal gradients in North American evergreen conifers.
• To investigate the influence of leaf structural (stomatal density, leaf mass per area) and chemical (leaf nitrogen content) traits on δ¹³C.
• To assess species-specific functional plasticity in response to environmental gradients.

Main Methods:

• Field sampling of four conifer species (Pseudotsuga menziesii, Abies lasiocarpa, Picea engelmannii, Pinus contorta) across an 1800 m altitudinal range.
• Measurement of leaf stable carbon isotope ratios (δ¹³C), stomatal density, leaf nitrogen content, and leaf mass per area (LMA).
• Statistical modeling to correlate δ¹³C with leaf traits and altitude, including derived variables representing CO₂ supply-demand balance.

Main Results:

• Significant species-specific variation in the rate of δ¹³C increase with altitude was observed.
• Leaf structure varied with altitude: stomatal density decreased, and LMA increased, while leaf nitrogen content remained constant.
• Derived variables (stomata per gram of nitrogen, stomata per gram of leaf mass) were negatively correlated with δ¹³C, with stomatal density per gram showing the best fit (r²=0.72).
• A general relationship was found between δ¹³C and LMA (r²=0.45), suggesting internal resistance as a key factor in inter-specific isotopic variation.

Conclusions:

• Evergreen conifers exhibit species-specific functional plasticity in response to altitudinal environmental gradients, as indicated by varying isotopic shifts.
• Leaf mass per area (LMA) emerged as a general predictor of δ¹³C across species, highlighting the role of internal leaf resistance.
• The findings underscore the importance of considering both species-specific traits and general physiological constraints when interpreting isotopic variation in plants.