Annual growth of Fagus orientalis is limited by spring drought conditions in Iran’s Golestan Province

Abstract

Due to the lack of a uniform and accurate definition of ‘drought’, several indicators have been introduced based on different variables and methods, and the efficiency of each of these is determined according to their relationship with drought. The relationship between two drought indices, SPI (standardized precipitation index) and SPEI (standardized precipitation-evapotranspiration index) in different seasons was investigated using annual rings of 15 tree samples to determine the effect of drought on the growth of oriental beech (Fagus orientalis Lipsky) in the Hyrcanian forests of northern Iran. The different evapotranspiration calculation methods were evaluated on SPEI efficiency based on Hargreaves-Samani, Thornthwaite, and Penman–Monteith methods using the step-by-step M5 decision tree regression method. The results show that SPEI based on the Penman–Monteith in a three-month time scale (spring) had similar temporal changes and a better relationship with annual tree rings (R ² = 0.81) at a 0.05 significant level. Abrupt change and a decreasing trend in the time series of annual tree rings are similar to the variation in the SPEI based on the Penman–Monteith method. Factors affecting evapotranspiration, temperature, wind speed, and sunshine hours (used in the Penman–Monteith method), increased but precipitation decreased. Using non-linear modeling methods, SPEI based on Penman–Monteith best illustrated climate changes affecting tree growth.

Keywords

Climate change / Drought index / Hyrcanian forests / SPEI / Annual growth rings / Fagus orientalis