This study was undertaken to elucidate the pattern of tree phenology, cambial growth and wood formation in Juniperus procera Hochst. ex Endl. growing in a low-altitude, semi-arid region of Al-Yazid, Saudi Arabia. Primary growth was found to continue for 5.5 months (mid-March to end of August) at a rate of 10.21 cm year-1. Following the new leaf formation, vascular cambium was active for five months (mid-March to mid-August), producing phloem for three months (April-June) and also xylem for three months (June-August). The maximum number of cell layers in the cambial zone (5-6) was observed during June at a high temperature (30 degrees C), moderate rainfall, low relative humidity (35.3%) and a high water-saturation deficit (44%). A close relationship was found between the emergence of new leaves and the initiation/continuation of cambial activity and tissue differentiation. Variations in the maximum and the mean monthly temperatures during the period of cambial activity showed a high degree of similarity with the variation pattern of width of the cambial zone. The same applies to the relationship between the cambial-zone width and the water-saturation deficit during the active period of cambium. The wood produced by the cambial activity was dense (0.69 g cm-3) and composed of relatively narrow and short component cells; average width of the annual growth ring was 284 mu m.