Periodic Response of a Sweetgum Tree to Transpiration and Environmental Interferences

During the drought of 2001, researchers at Clark Atlanta University observed large diurnal fluctuations in the circumference of old deciduous trees at a suburban forested site. Additional observations were made in 2002 and 2003. A more detailed study of this phenomenon was planned in 2004 for a single tree species. Continuous measurements of the periodic change in sweetgum tree circumference due to transpiration and environmental interferences were made during a 5-week period in the summer of 2004. The study investigated links between environmental conditions and the biological responses, both on diurnal and longer-term scales, by observing changes of the tree trunk circumference of Sweetgum (Liquidambar styraciflua), a species native to the mid-Atlantic region. Monitored environmental parameters include insolation, volumetric soil moisture and temperature profiles, atmospheric temperature, relative humidity and related meteorological variables. Six Sweetgum trees were studied for evidence of periodic patterns in their respective trunk-circumferences during significant precipitation events. Three of the trees were located in the riparian zone of a small stream, while the remaining three were situated above the riparian zone. The soil at the site was a red clay loam typical of the Georgia Piedmont region, an area extending from the foothills of the Appalachian Mountains to the coastal plain. During high precipitation events, the soil under the lowland canopy would persist as a clay slurry mud retaining the moisture days after rain had fallen. For this reason, care was taken to select trees with clear canopy exposure. Harmonic analysis and nonlinear curve fitting of the circumference changes for each tree during precipitation events of half an inch or greater per hour showed distortions in the expected diurnal response to evapotranspirative pumping.