Environmental determinants of the ecology and distribution of Acacia tortilis under arid conditions in Qatar

Abstract

Scrub or woodland communities dominated by Acacia tortilis form one of the few tree-dominated natural ecosystems in the hyper-arid climate of Qatar, making it a very important tree species that provides an essential habitat both for native animals and domestic livestock. However, the conservation and sustainable management of this tree has so far been neglected and it is now severely impacted by overgrazing and wood fuel collection. This research investigates the main environmental, ecological and management factors affecting the growth and distribution of Acacia tortilis in Qatar, including the factors affecting its regeneration. It also aims to guide the implementation of conservation programmes and development of a strategy to forestall deforestation and prevent the extinction of Acacia tortilis in Qatar. Initially, field survey, remote sensing and GIS techniques, together with univariate and multivariate statistical modelling techniques, were used to explore environmental influences on distribution of A. tortilis in Qatar at a national scale. Different vegetation indices (VIs), normalized difference vegetation index (NDVI), soil adjusted vegetation index (SAVI), were derived for a time series of Landsat TM/ETM+ images for 1998 and 2010 and tested using ground-truth data to explore the temporal dynamics of Acacia-dominated ecosystems which indicated substantial reduction in vegetation greenness in 2010 than 1998. The initial approach had limited success due to difficulties of identifying Acacia tortilis communities accurately on satellite images due to the sparsity of tree cover and indicates the limitations of using remote sensing methods for tracing vegetation dynamics in Qatar and similar arid and hyper arid environments. The multinomial logistic regression model has a superior ability to predict Acacia distribution and is a suitable method in the prediction of the occurrence of different vegetation types. Phytogeographical investigations of the environmental and biotic factors that control the distribution of the Acacia tortilis at a local scale, in both areas protected and unprotected from human land use impacts, demonstrate that topographic factors and their control on soil and water conditions are fundamental determinants. The distinctive topography of Qatar has resulted in a heterogeneous soil landscape with extreme contrasts of chemical and physical soil conditions within and between depressions and more elevated positions in soil toposequences. Depressional land forms are more suitable for the Acacia tree growth than the surrounding higher ground because ENVIRONMENTAL DETERMINANTS OF ACACIA TORTILIS IN QATAR II depression soils have greater soil water content, soil depth, organic carbon and available phosphorus contents. Conversely, the absence of Acacia trees in summit areas is related to severe limitations for tree growth, including negligible soil water content and shallow soil depth caused by impeding bedrock or cemented horizons resulting in drought stress, as well as large contents of gypsum and/or CaCO3 in soils. The slope-controlled movement of eroded soil material, water and plant debris, and the localised leaching of soluble salts, are suggested to be important processes that lead to improved soil quality and better tree growth in depressions. The regeneration of Acacia tortilis through seedling establishment is perhaps surprisingly shown to be greater in the unprotected than in protected areas. This is attributed to the importance of ingestion by large mammals (mainly domestic herbivores) on the germination and recruitment of Acacia seedlings. The greater frequency of Acacia saplings in depressions within the unprotected areas is, however, also attributed to the presence of greater amounts of soil water, soil depth, available phosphorus, and organic carbon. Although the action of browsing may be regarded as positive, most anthropogenic impacts were shown to have negative effects on the condition and distribution of Acacia tortilis. The results proved that the impacts of cutting and browsing were severe in the unprotected sites, despite the evidence of more active regeneration. It is concluded that there is an urgent need to review the provision and management of protected habitats for Acacia tortilis in Qatar. It is suggested that cutting for domestic use should be restricted; that conservation efforts should be concentrated in depressions that favour tree growth; and that the livestock numbers should be limited to enable seedling establishment without excessive browsing.