Sap flux and stem radius variations in mature cedrus libani trees during the growing season

Abstract

Analysis of continuous sap flux and stem growth measurements can help to elucidate how environmental conditions influence water status and stem growth in trees. Stem radius variations and sap flux densities of the drought-tolerant conifer Cedrus libani were measured using automatic point dendrometers and heat dissipation sensors, respectively. The study was conducted at the Elmalı Cedar Research Forest at 1665 m a.s.l. From April to September 2009, stem radius variation was measured hourly for nine C. libani trees. Concurrently, environmental variables (e.g., air temperature, precipitation, relative humidity, soil water content) were monitored within the study site. From the dendrometer records, maximum daily shrinkage (MDS) of the stem radius was calculated. From June to August, a time period of 2 months, four of the nine study trees were selected for hourly measurements of sap flux density (J s ). Daily courses of stem radius variation and J s were very uniform and similar for all studied trees. A daily hysteresis effect was observed, reflecting the pattern of water storage and retrieval. Means of MDS and J s varied between 0.09 and 0.15 mm and between 4.3 and 10.6 g m –2 s –1 , respectively. Daily maximum J s varied between 8.0 and 33 g m –2 s –1 . Correlation and regression analyses between MDS and J s and environmental variables revealed that MDS and J s were most closely related to vapor pressure deficit and photosynthetic active radiation, respectively. We conclude that C. libani is well adapted to the current environmental conditions at the study site (natural habitat) and that atmospheric conditions play a primary role in transpiration and tree growth.