Abstract
Thumamah Nature Park is located at about 100 km north of Arriyadh (Saudi Arabia), having an area of 170 km(2). The Park was established since 30 years ago. The aim of this study is to analyze the vegetation structure in relation to the environmental factors in different habitat types. The phenological activities around the year of the 20 dominant species were monitored. 119 species were identified, of which 51 (43 %) annuals and 68 (57 %) perennials after 30 years of exclusive human impact. The Saharo-Arabian component species were the highest among the monoregional species (64 %) in most life forms, while the Sahelien-Somali Masai attained the highest among the biregionals (46 %). The TWINSPAN, DCA and CCA analyses separated seven vegetation groups. The first two groups were dominated by psammophytic species, which occupy the lower sandy plain as shown in group I with Rhanterium epapposum-Rhazya stricta and group II with Pennisetum divisum-Haloxylon salicornicum. The escarpment habitat was characterized by three groups, viz., group III with Acacia gerrardii-Panicum turgidum, group IV with Panicum turgidum and group V with Acacia ehrenbergiana-Lasiurus scindicus. Vegetation in the upland plateau was represented by the remaining two groups; group VI with Helianthemum lippii and group VII with Helianthemum kahiricum. The environmental variables that affect the species distribution and diversity in the park include the altitude, soil texture, pH, EC, Ca, Mg and Mn. The increased plant species richness, turnover, evenness and cover were mostly due to the increase of the herbaceous species. Plant populations showed interspecific variations in their relative timing of phenological phases with reproductive activity period ranged between 3 and 6 months with unimodal flowering peak. Three flowering-fruiting activities were recorded during late winter-spring, summer and late autumn-early winter. In an attempt to explain the vegetation dynamics after 30 years conservation, the progressive succession varied among the different habitat types, including the lower sandy sites, the escarpment and the upland rocky habitats, which reflect the relationship between altitude, edaphic factors and the type of vegetation units in each habitat type after exclusion of the human impact.