Tree growth and water-use in hyper-arid Acacia occurs during the hottest and driest season
Drought-induced tree mortality has been recently increasing and is expected to increase further under warming climate. Conversely, tree species that survive under arid conditions might provide vital information on successful drought resistance strategies. Although Acacia (Vachellia) species dominate many of the globe’s deserts, little is known about their growth dynamics and water-use in situ. Stem diameter dynamics, leaf phenology, and sap flow were monitored during 3 consecutive years in five Acacia raddiana trees and five Acacia tortilis trees in the Arid Arava Valley, southern Israel (annual precipitation 20–70 mm, restricted to October–May). We hypothesized that stem growth and other tree activities are synchronized with, and limited to single rainfall or flashflood events. Unexpectedly, cambial growth of both Acacia species was arrested during the wet season, and occurred during most of the dry season, coinciding with maximum daily temperatures as high as 45 °C and vapor pressure deficit of up to 9 kPa. Summer growth was correlated with peak sap flow in June, with almost year-round activity and foliage cover. To the best of our knowledge, these are the harshest drought conditions ever documented permitting cambial growth. These findings point to the possibility that summer cambial growth in Acacia under hyper-arid conditions relies on concurrent leaf gas exchange, which is in turn permitted by access to deep soil water. Soil water can support low-density tree populations despite heat and drought, as long as recharge is kept above a minimum threshold.
KeywordsAcacia raddiana Acacia tortilis Leaf phenology Sap flow Desert Global warming Tree drought resistance Arava
We thank Victor Lukyanov (ARO Volcani Center, Israel) for technical assistance in the field. TK wishes to thank the Benoziyo Fund for the Advancement of Science; Mr. and Mrs. Norman Reiser, together with the Weizmann Center for New Scientists; the Edith and Nathan Goldberg Career Development Chair. CB and DO thank the Bayrische Forschungs (BayFor) Alianz, Germany, for supporting the visits to the Arava. GW thanks the Arava Drainage Authority and the Israeli Ministry of Science and Technology (MOST) for their continued support. Thanks to the anonymous reviewers and to the editor, Russel K. Monson, for their helpful comments and suggestions which significantly improved the MS.
Author contribution statement
GW initiated the study in 2013, with monitoring operated together with GR. DO, SC, and TK joined the study in 2015, with YW, IR, IP, and CB joining in 2016. TK started the analysis and manuscript drafting, with contributions from all authors.
Compliance with ethical standards
Conflict of interest
The authors declare no conflict of interest.
Video S1. “A year in the life of Acacia tortilis”, A lapse-rate video (MP4 66073 kb)
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