Abstract
This study, aimed at elucidating cambial activity and wood formation in Acacia tortilis subsp. tortilis growing in the dry region of Al Baha in Saudi Arabia, reveals that the vascular cambium was active for almost the whole year, producing phloem in two flushes (February to March and then June to August), and xylem for about 11 months. The maximum number of cell layers in the cambial zone (10-13) was observed during June at high temperature, high leaf water deficit and lowest relative humidity (52%) and rainfall. A close relationship existed between the emergence of new leaves and the initiation/acceleration of cambial activity and tissue differentiation. The mean and maximum monthly temperature of the site showed positive and highly significant correlations (p < 0.01) with the width of the cambial zone. The mean monthly relative humidity and mean monthly rainfall were negatively but significantly correlated with cambial activity (p < 0.05). Leaf water deficit had a favorable effect on the cambial activity (r = 0.894, p < 0.01). Acacia tortilis subsp. tortilis has thick-walled vessels of medium diameter and frequency, thick-walled fibers occupying a high tissue proportion, high wood density (0.905 g cm(-3)) and tannin-like deposits in the vestured vessel pits. The apparent drought tolerance of this species is hypothesized to be due to its deep root system, allowing optimal cambial activity during extreme drought and extremely high day temperatures.