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Accueil du site → Doctorat → Israel → Monitoring drought stress In Acacia Trees in hyper arid zones : geo-ecophysiology aspects

Ben-Gurion University of the Negev (2017)

Monitoring drought stress In Acacia Trees in hyper arid zones : geo-ecophysiology aspects

Isaacson, Sivan

Titre : Monitoring drought stress In Acacia Trees in hyper arid zones : geo-ecophysiology aspects

ניטור עקת יובש בעצי שיטה באזור צחיח קיצון.

Auteur : Isaacson, Sivan,

Etablissement de soutenance : Ben-Gurion University of the Negev

Grade : Doctor of Philosophy (PhD) 2017

During the last decades, attention was drawn to reports about high mortality rates and lack of recruitment in the Acacia populations in arid and hyper-arid zones of the Middle East and North Africa. At the same time, the ecological importance of scattered trees such as Acacia in arid regions is being acknowledged. Arid environments are characterized by rare and highly variable rain events, both in time and space. As a result, plant populations in arid environments are known for their episodic recruitment and mortality dynamics. However, because of the slow developmental rate of woody species in these environments, direct records and observations of the population dynamics and its response to rain events are rare. Acacia trees in arid zones are considered to be “keystone species”, having strong and direct influence over the entire ecosystem function and over the habitat biodiversity. Explaining the spatial and temporal heterogeneity in those populations and between different habitats, and understanding the Acacia response to changes in water supply, is of high importance and essential in order to evaluate the entire ecosystem’s vulnerability.

A continuous, yearly-based, field monitoring of individual Acacia trees in the hyper arid southern Arava Valley, was conducted between the years 2000 and 2016 at seven sites, representing different hydrologic regimes ; wadis of small catchment (Shlomo, Ktora, Roded), wadis of large catchment (Zhihor, Zhnifim), margins of salt flat (Evrona, Yotveta). This region experienced a clear decrease in the accumulated annual precipitation during 1995-2009. The sequence of those dry years ended in 2010 with vast flood events.

Mortality in the study area, was initially observed in 2003 and reached values of seven to forty-four percent during the monitoring years (2000-2016). Moreover, the overall percentage of dead trees observed in those sites, including dead trees before 2000, reached seven to fifty-five percent. It was found that dead Acacia trees remain standing for at least 15 years in hyper-arid regions ; therefore mortality percent observed in the field should be related as cumulative mortality for the past fifteen years at least. We found high negative correlation between running average rainfall and mortality in sites of wadis of small catchment. In contrast, no correlation was found between rainfall and mortality or vitality estimation of trees in sites of margins of salt flats. In small wadis, the effective rain in relation to Acacia trees’ survival was determined as rain events greater than nine mm, as Acacia trees’ mortality was indifferent to rainfall events less than this threshold. Three recruitment events were identified during the monitoring years in : 2010, 2012 and 2014 ; however, only six percent of the new seedlings survived for more than three years. In order to expand the time scope of the Acacia population field survey, we implemented two different approaches : (1) Trees – individual-based change detection using Corona satellite images and (2) Spatial analysis of tree populations using high resolution CIR (color infra-red) aerial photograph, converting spatial data into temporal data. We used the spatial distribution of different parameters of Acacia trees as an indicator of past and present hydrological regimes within different segments of the wadi. Tree size distribution was used as an indicator of long-term (decades) geo hydrologic spatial processes affecting the Acacia population. The tree vitality (NDVI) distribution was used as an indicator of short-term (months to a few years) geo hydrologic spatial processes, such as the paths of recent flashfloods events.

The spatial analysis revealed that both tree size and NDVI distribution patterns were significantly clustered, suggesting that the processes responsible for tree size and tree vitality status (i.e., flash-floods spatial spreading) have a spatial expression. The distribution of the trees in the wadi (ephemeral river) was divided into three distinct parts : large trees with high NDVI values, large trees with low NDVI values, and small trees with medium NDVI values. Using these results, we divided the wadi into three sections, each representing a unique combination of long- and short-term geo hydrologic processes affecting the Acacia trees. These results suggest that the lack of spatial correlation between tree size and vitality status is a result of spatio-temporal changes in the water supply. The next phase of the temporal data extraction procedure was to implement change detection regarding each of the wadi sections defined by the spatial analysis result. The results of the change detection supported our findings of changes in the geo-hydrology regime from long- to short-term scale. The integration of field monitoring, spatial analysis and change detection provides a unique database that reveals the Acacia population response to changes in water supply caused either by temporal precipitation fluctuations or by spatial flow changes.

Mots clés : Acacia trees, Hyper-arid, Long-term monitoring, Remote sensing, Scattered trees, Spatial analysi

Présentation (PRIMO)

Page publiée le 7 janvier 2022