WOODLANDS, OASES, AND FIRE IN SOUTHERN YEMEN DURING THE HOLOCENE HUMID PERIOD: PALEOECOLOGICAL EVIDENCE FROM ROCK HYRAX MIDDENS

Dryland ecosystems are particularly vulnerable to climate change. In particular, social changes coupled with environmental instability have the potential to rapidly accelerate ecosystem degradation. The Holocene Humid Period and its termination has been used as an iconic example for evaluating and testing the climatic and anthropogenic impacts to arid systems. Although long records of climate and vegetation can provide critical insight into the biotic and abiotic mechanisms that alter ecosystems, traditional archives for these records, such as lakes, are not available despite a rich archeological record. In order to better understand the processes that govern ecosystem structure and function in the region, we use pollen and stable isotopic analysis of rock hyrax (Procavia capensis) middens from Wadi Sana, Yemen, to look at vegetation change at the end of the Holocene Humid Period. Before 4.5ka, the pollen flora suggests that woodland trees were abundant in the now sparse landscape. These included multiple keystone species, such as Anogeissus and frankincense (Boswellia sacra), which have had a strong influence on local hydrology and the local economy, respectively. In fact, our analysis suggests that the presence of Anogeissus during this interval may have mediated and promoted groundwater storage as well as ponding in now dry wadi channels over 1° further north (480km) than today. Fire is also common during this period, suggesting continuous vegetation cover in an area which now is classified as bare ground. After 4.5ka, there is a shift in tree taxa to an increase in abundance of the now-dominant tree, Acacia tortilis-type. Finally, we observe that after 1.5ka increased sedge abundances at the expense of grasses and all trees which suggests the development of high desert conditions. This transition, which occurred after the maximum onset of aridity, is coeval with major changes in the archeological record. Thus this may have involved a feedback between continued dry conditions coupled with intensified pressure from domestic livestock (cows and camels) past the point of ecosystem resilience.