Print PageSap flow measurements have become integral in many physiological and ecological investigations. There are a number of methods described to estimate sap flow rates in trees, but probably the most popular is the Granier thermal dissipation method because of its affordability and ease of use. However, there have been questions about the use of the Granier system in ring porous species and whether or not individual species calibrations need to be performed. We made concurrent measurements of sap flow rates using Granier sensors and the Cermák heat balance method in two oak species (Quercus prinus and Quercus velutina) and one pine species (Pinus echinata). We also made concurrent measurements of sap flow rates using both 1cm long and 2cm long Granier style sensors in both oak species.
Results/Conclusions
We found that both systems matched well in the pine species, but sap flow rates were underestimated by the Granier system in the two ring-porous oak species. Underestimation of sap flow rates by the Granier system was nonlinear and increased at higher flow rates. We also found that 2cm long Granier sensors tended to underestimate flow rates compared to 1cm long sensors and these underestimations also increased at greater flow rates. When 2cm long sensor data were scaled using the regression with 1cm long data, daily flow rates matched well with the rates measured by the Cermák system. These results suggest that 1cm long sensors are appropriate for use in ring porous species and 2cm long sensors are appropriate in species with deeper sapwood like pines. This study shows that as long as the appropriate methodologies are taken, the Granier method can provide reliable sap flow data that can be useful in better understanding water use in both individual species as well as whole tree stands.
