Elevated CO2 reduces sap flux in mature deciduous forest trees

Area of Science:

• Forest Ecology
• Plant Physiology
• Climate Change Research

Background:

• Mature forest ecosystems play a crucial role in carbon cycling and water regulation.
• Understanding how elevated atmospheric carbon dioxide (CO2) affects tree water use is vital for predicting forest responses to climate change.
• Previous studies have yielded mixed results on the impact of elevated CO2 on forest transpiration.

Purpose of the Study:

• To investigate the effect of elevated CO2 on the water use of mature, broad-leaved forest trees.
• To quantify changes in sap flux density (JS) under elevated CO2 conditions.
• To determine how species-specific responses and environmental factors influence CO2-induced water savings.

Main Methods:

• Canopy CO2 enrichment of 14 broad-leaved trees in a Swiss forest.
• Measurement of sap flux density (JS) using the constant heat-flow technique before and during CO2 exposure.
• Analysis of short-term responses to transient CO2 level changes and varying evaporative demand (VPD).

Main Results:

• CO2 enrichment led to a 10.7% average reduction in daily sap flux density across species.
• Species like Carpinus, Acer, Prunus, and Tilia showed the most significant reductions, while Fagus showed negligible changes.
• Water savings were higher under low vapor pressure deficit (VPD) and diminished under high VPD or dry conditions.

Conclusions:

• Elevated CO2 can significantly reduce water use in mature forest trees, potentially improving their water status during dry periods.
• The magnitude of CO2's effect on stand transpiration is influenced by rainfall patterns and species composition.
• Forests dominated by species like Carpinus may experience greater water savings under elevated CO2 compared to monocultures like Fagus.