Stomata Are Driving the Direction of CO2-Induced Water-Use Efficiency Gain in Selected Tropical Trees in Fiji

TL;DR

This study shows that changes in stomata, not just photosynthesis, drive how tropical trees in Fiji use water more efficiently as CO2 levels rise.

Contribution

The study reveals that stomatal density, not size, is the key driver of water-use efficiency in tropical trees under rising CO2.

Findings

The response to rising CO2 varied significantly among the five tropical tree species studied.

Stomatal density had a greater influence than pore size on maximum stomatal conductance.

Stronger increases in water-use efficiency were linked to stronger declines in stomatal conductance.

Abstract

Understanding how plants respond to increasing atmospheric CO2 is crucial for predicting future climate interactions. However, the long-term effects of rising CO2 on plant physiology, especially in tropical regions, are not well known. To investigate this, we studied how a CO2 increase of about 95 ppm from 1927 to 2015 affected five tropical tree species in Fiji. We analysed historical leaf samples to measure the following two key traits: how efficiently the trees use water (intrinsic water-use efficiency) and the maximum rate of conductance through leaf pores (maximum stomatal conductance). Our results showed that the responses to rising CO2 varied significantly by species. Generally, the number of stomata on the leaves was more important than their size in determining the trees’ response to higher CO2 levels. While photosynthesis is a major factor in improving the water-use efficiency, changes in stomatal conductance primarily drive this trend across different species. Trees that showed greater increases in the water-use efficiency also displayed a greater reduction in stomatal conductance. Overall, our study shows the importance of considering differences in the maximum stomatal conductance when predicting how different tree species will react to increasing CO2 levels.

Understanding plant physiological response to a rising atmospheric CO2 concentration (ca) is key in predicting Earth system plant–climate feedbacks; however, the effects of long-term rising ca on plant gas-exchange characteristics in the tropics are largely unknown. Studying this long-term trend using herbarium records is challenging due to specimen trait variation. We assessed the impact of a ca rise of ~95 ppm (1927–2015) on the intrinsic water-use efficiency (iWUE) and maximum stomatal conductance (gsmax) of five tropical tree species in Fiji using the isotopic composition and stomatal traits of herbarium leaves. Empirical results were compared with simulated values using models that uniquely incorporated the variation in the empirical gsmax responses and species-specific parameterisation. The magnitude of the empirical iWUE and gsmax response was species-specific, ranging from strong to negligible. Stomatal density was more influential than the pore size in determining the gsmax response to ca. While our simulation results indicated that photosynthesis is the main factor contributing to the iWUE gain, stomata were driving the iWUE trend across the tree species. Generally, a stronger increase in the iWUE was accompanied by a stronger decline in stomatal response. This study demonstrates that the incorporation of variation in the gsmax in simulations is necessary for assessing an individual species’ iWUE response to changing ca.

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