Latitude, basin size, and microhabitat effects on the viability of Acacia trees in the Negev and Arava, Israel

Abstract

In the hyper-arid Arava Valley and eastern Negev desert of Israel, the acacia species of raddiana, tortilis, and pachyceras play a central role as keystone species. Many studies implemented during the last few decades have revealed considerable changes in acacia populations, including high mortality rates and low recruitment. The objective of this study aimed to examine the relationships between presumed water availability for acacia populations and their mortality by examining the effect on tree mortality of latitude, basin size, and microhabitat in the valley floor. A total of 1980 acacia trees in 11 wadis along the Arava and eastern Negev region were studied. The obtained results revealed a highly varied inter-wadi mortality rate, ranging between 6.7% and 72.2%. Overall, mortality within species occurred in 14.7% of Acacia pachyceras, 16.8% of Acacia tortilis, and 29.4% of Acacia raddiana. A highly negative correlation coefficient (r=−0.55; P<0.0001) was found between latitude and acacia mortality. However, a low and not statistically significant correlation coefficient was found between basin size and acacia mortality (r=−0.24; P=0.2103). In terms of overall occurrence in microhabitat across the valley floor, 6.4% of the trees were located in main channels, 9.8% in high banks, 10.4% in bars, 19.1% in secondary channels, and 54.3% in low banks. However, the effect of microhabitat on acacia mortality rates was not statistically significant (P=0.2240). In addition to these, we monitored the occurrence of the hemiparasite, mistletoe (Plicosepalus acaciae), which was observed in 8.2% of the trees. Also, the tree canopies' height and diameter were measured in order to calculate a size index for them. Values of this index were divided into four groups in order to represent the size distribution of trees. The overall occurrence of 3.6% of the smallest tree-size group is worrisome, as it presumably indicates very low recruitment rates.