Measuring water flow through African forest trees

We have just completed the installation of tree water use measuring equipment at the Bobiri field site in Ghana. This is one of our key intensively monitored sites which run in a wet-dry gradient in Ghana, from wet rainforest to woody savanna. Bobiri sits in the middle of the transect, a patch of forest reserve just east of Kumasi and conveniently close to the Forest Research Institute of Ghana (FORIG). With our FORIG colleagues we have been monitoring the carbon cycle of this forest in detail for the last five years, and have shown it to be the most productive tropical forest we have ever monitored. We are still trying to puzzle out why this site is so productive.

I am with Lucy Rowland from Exeter University (who has worked on studying water flow in a drought experiment in Brazil for many years) and our Ghanaian colleagues Stephen Adu-Bredu, Akwasi Duah Gyamfi, Mickey Boakye and Patrick.

Now, with the installation of water flux sensors, we are adding an understanding of the use of water by different trees. Our main measurements are done using sap flow sensors, which measure water flow (transpiration) through the stem by looking at the rate of dissipation of heat injected into the tree via electrodes. The more sap flow, the faster the dissipation. Installation involves hammering electrodes into the tree (often perched precariously on a ladder in the case of large buttressed trees).

Initial data suggest phenomenal levels of water flow through these trees, much higher than we have seen at previous work in eastern Amazonia. This may be related to the high productivity and fertility that we see at the Ghana site, with the fast growth rates requiring high water use, at least in the wet season. These are amongst the first (and maybe the very first) sap flow measurements in the wet tropical forests of Africa, and there is much to learn in the coming years.