Age structure, regeneration-gap of Ziziphus spina-christi populations and implications for its conservation

The study was conducted to determine the age structure, regeneration status and possible factors for regeneration-gap of Ziziphus population of the southwestern valleys of Saudi Arabia. The population age structure was determined through stem disc ring analysis of fallen trees and girth value measurements of live trees. The average number of rings and annual ring width of trees were determined based on analysis of 95 sampled tree discs and the number of rings correlated with their circumference values. The girth values of 1662 individual live trees from 75 plots were measured and their age class frequencies were used to determine the age structure and regeneration status of the populations. Possible factors contributing to regenerationgaps of the populations were assessed in relation to human interferences and trends of three decade climate data from the nearest metereolgical station. Of the total individual trees measured, only <0.96% were seedlings and 2.3% were saplings and the remaining 96.8% were adults. Based on age frequency distributions, only 0.42% were less than five years old and about 0.9% were between 5-10 years old, while 98.7% were between 10–400 years old indicating that the populations exhibited an even age conditions with serious regeneration gaps. Of the tatal adults about 12% of them were in a die-back condition. The few seedlings and saplings recorded in the study were observed to grow only in areas where associated with other thorny shrubs that might have protected them from animal grazing. The age structure distribution of the populations indicates that the regeneration gaps may have started many years back. The major contributing factors for the regeneration-gaps of the populations were associated with overgrazing, soil erosion and climate changes. It is concluded that if the trends continue in the same manner, the populations will disappear sooner or later. Intervention measures like area closure, planting of seedlings and restoring cultural practices with the full participation of communities would facilitate rehablitating and conserving the remaining relict forest patches and the very unstable population of Ziziphus species in particular.