Century-long apparent decrease in intrinsic water-use efficiency with no evidence of progressive nutrient limitation...

Area of Science:

• Forest ecology
• Plant physiology

Background:

• Rising atmospheric carbon dioxide (CO2) is predicted to increase tree water-use efficiency and induce nutrient limitation in tropical forests.
• Long-term observational data to validate these predictions are scarce, especially for large ecosystems like the Congo Basin.

Purpose of the Study:

• To investigate changes in intrinsic water-use efficiency (iWUE) and nutrient limitation in central African tropical trees over the past century.
• To assess the impact of environmental changes, including CO2 rise and temperature, on forest productivity.

Main Methods:

• Analysis of herbarium leaf samples from the Congo Basin dating back to 1938.
• Measurement of leaf stable isotopes (δ13C) to infer changes in iWUE.
• Examination of leaf stoichiometry (C:N and N:P ratios) to assess nutrient limitation.

Main Results:

• Despite increasing CO2 concentrations, intrinsic water-use efficiency (iWUE) has decreased in Congo Basin tropical trees.
• Leaf-level adaptations, such as increased photosynthesis-related nutrients and decreased stomatal conductance, were observed.
• Leaf δ13C values indicate a significant decline in iWUE over time.
• Stoichiometric ratios show no evidence of progressive nutrient limitation, with C:N and N:P ratios remaining constant since 1938.

Conclusions:

• Central African tropical trees have responded to recent environmental changes in unexpected ways, with decreasing iWUE.
• Increasing temperatures, rather than nutrient limitation, are suggested as a potential factor negatively affecting net photosynthesis and iWUE.
• The findings underscore the need for continued on-ground monitoring in the Congo Basin to understand forest responses to global change.