Vegetation, Floristic Diversity, and Size-Classes of Acacia gerrardii in an Arid Wadi Ecosystem

Abstract

This study focuses on the vegetation ecology, floristic diversity, and dynamics of Acacia gerrardii populations in wadi Huraimla, Central Saudi Arabia. The floristic richness, plant cover, human impact, and demography of A. gerrardii population were monitored in open grazing and managed sites. The applications of TWINSPAN, DCA, and CCA multivariate analysis have produced nine vegetation groups; eight of them were associated with the distribution of A. gerrardii and one group with the surrounding agricultural lands. The plant communities were separated into three major vegetation groups, viz., Acacia gerrardii-Rhazya stricta representing the open grazing sites, Acacia gerrardii-Lycium shawii-Zilla spinosa representing the managed sites, and Cynodon dactylon-Heliotropium bacciferum-Cenchrus ciliaris dominating the agricultural lands. The CCA ordination indicated that the separation of plant species along the first axis was positively affected by EC, K, Na, species richness and human impact, and negatively affected by pH, Mg, and density and age of A. gerrardii populations. Alternatively, the second axis is positively correlated with altitude and Ca content of the soil. Human impact is positively correlated with soil salinity, K and Na contents and negatively correlated with altitude, soil texture, and Mg content. The density of A. gerrardii is positively correlated with pH, Fe, Mn, and Mg contents of the soil, whereas negatively correlated with salinity and K and Na contents, species richness, relative evenness, and human impact. Populations of A. gerrardii demonstrated different size-class distribution patterns with either continuous regeneration inputs or lack of regeneration.