WOOD FORMATION IN JUNIPERUS EXCELSA SSP. POLYCARPOS IN THE HIGH MOUNTAINS OF NORTH-EAST IRAN

Cambial activity and of xylem formation in Persian juniper (Juniperus polycarpos) were studied during the 2010 growth season in the high mountains of Chaharbagh, Gorgan, north-east Iran. Wood microcores were collected weekly from March till May and biweekly from June till September from four trees. Thin sections were cut and anatomical features of the developing and mature tracheids were observed, counted and measured along three radial files of the growing xylem. Monthly tree-ring increments were correlated with monthly mean temperature and total rainfall. The dormant cambium contained 2–3 cell layers adjacent to the last formed phloem fibres in the previous year. Cambial cell division began on the second week of April and terminated at the end of August. The maximum number of cambium cells (8–9) and the maximum daily growth rates of all trees were achieved around mid-June. Rainfall and temperature showed a strong positive and negative relation respectively with monthly tree-ring increment. As the study area is cool and semi-arid, cambial reactivation is controlled by temperature. However during the growing period, water availability is the main limiting factor for growth.