Foliar temperature as a tool for quantifying whole-plant transpiration in tree seedlings under laboratory and greenhouse conditions

A proper mechanistic understanding of how whole-tree transpiration varies as a function of environmental conditions is essential for predicting how vegetation will respond to climate change, particularly to changes in soil moisture dynamics expected under novel rainfall regimes. Whole-plant transpiration studies in trees rely on Granier-type sap-flow gauges that can be difficult to deploy. A potential alternative is provided by thermal indices of transpiration, used quite extensively in agricultural research and applications but so far largely ignored in ecological applications. We conducted a study under controlled conditions to test the ability of a simple foliar thermal index (based on the difference between leaf temperature and a dry reference temperature) to track whole-plant transpiration in savanna tree seedlings. We found that this index (Δ T ) provided a good fit to actual hourly transpiration data in five of seven tree species. We applied the index to four trees and one grass tussock growing together in a large container under greenhouse conditions over a two-month period, and found that the index tracked fluctuations in soil moisture availability reasonably well, suggesting that it has power to detect reductions in transpiration linked to soil moisture limitation, at least under controlled conditions. We suggest that this technique provides a viable method for collecting transpiration time series for whole-plant functional trait studies.