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Morpho-anatomical Changes in Water-conducting Tissue during Branch Dieback in Juniperus procera Trees in the Asir Forest of Saudi Arabia
Pervaiz R. Khan, Rahmah N. Al-Qthanin, Ibrahim M. Aref1, Muhammad Iqbal
Abstract
Juniper forest of Asir region in the southwest of Saudi Arabia is declining through branch dieback, leading to die-off. The all-time high greenhouse-gas concentration, land-surface temperature, ocean-heat content (OHC), and declining precipitation pattern have long been a persistent source of threat to the endangered Juniperus procera. The present study reveals the positive correlation of porosity of pit membrane on tracheid walls with the xylem vulnerability to embolism induced by water stress. Air-seeding and implosion in tracheids trigger cavitation/embolism that decreases hydraulic transport. In consequence, stomatal conductance and photosynthesis are reduced, which causes die-back of branches, leading to death of the whole tree. Huge amounts of chemicals given off by the axial as well as radial living parenchyma are deposited in tracheids, thus blocking the entire water-conducting media and giving dark appearance to the dead portion of a branch. Symptoms of carbon-starvation severity in dying or dead branches were identified by the presence of dead ray cells (66%) that lack the normal cell content and starch granules, unlike the healthy ray cells. The severity and frequency of internal symptoms of branch dieback seem to be inversely proportional to the level of altitude and water potential, and directly correlated to temperature of the collection site. It appears that J. procera has crossed the threshold point of temperature, water-saturation deficit (WSD), hydraulic-conductivity failure (HCF), and carbon-starvation severity (CSS), leading to branch dieback and tree die-off, under this era of climate change/global warming. Consequently, altitudinal-range shift has taken place in the marginal areas. Regeneration process has stopped at low altitudes.
Keywords
Altitudinal-range shift; Asir juniper forest; Embolism; Greenhouse-gas emission; Ocean-heat content
Author for correspondence
Muhammad Iqbal (iqbalg5@yahoo.co.in)
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