The effects of temperature and precipitation gradient on radial growth and vascular characteristics of Fraxinus excelsior L. in Hyrcanian forests

Abstract

Vascular characteristics of the xylem determine the efficiency of the hydraulic system in broadleaved trees. To investigate the effects of elevation and longitude (temperature and precipitation gradient) on radial growth and vascular properties of common ash (Fraxinus excelsior) xylem, samples were taken from trees located in two wet sites in Ramsar (300 and 1300 m.a.s.l) and a semi-dry site in Sari (1260 m.a.s.l) of Hyrcanian forests. Tree-ring width (TRW), as well as early- and latewood width, average size (AVLA) and the number of vessels per area (VF), tree-ring porosity, relative specific hydraulic conductivity (RSC), and fiber length were measured and compared in the last ten growth rings of selected trees. According to the results, variations in vessel size were independent of other features and were affected by a different component. Compared with the highland of Ramsar, trees at low altitudes had wider annual rings while having lower RSC, VF, and porosity. Surprisingly, with a decrease in altitude (increasing temperature), the size of vessels shrank, which can be attributed to the inverse effect of over-threshold temperature on tree physiology. Despite the differences in AVLA and VF in the two highland sites of Ramsar and Sari, the combination of two features was in a way that the porosity, RSC, and TRW of trees in the two sites became comparable. In general, it can be concluded that in the western and central Hyrcanian forests, the effect of altitude – manifested in a decrease in temperature - on the radial growth and vascular system of ash trees is more pronounced than the effect of precipitation gradient (longitude).

Keywords

• elevational gradient
• wood anatomy
• tree physiology
• xylem conducting tissue
• ash