Xylem embolism impairs hydraulic conductivity in trees and drives drought-induced mortality, yet evidence of its occurrence in mature field-grown trees is not robust. Seasonal patterns of embolism were monitored in Aleppo pine (Pinus halepensis) trees growing in a dry Mediterranean forest using Optical visualization sensors (OV) and μCT. In addition, potted Aleppo pine trees were dehydrated in the greenhouse in order to examine the effect of embolism on their survival and recovery from drought.
In forest-grown trees, embolism increased from zero to ~12% along the dry season, with 77% of cavitation events occurring between 10:00 and 16:00. The probability for cavitation increased as vapor pressure deficit (VPD) increased, up to 42% chances for cavitation when VPD > 5 kPa. In potted pines, no trees with embolism levels lower than 60% had died, but stomatal conductance (gs) following one month of rehydration was lower in dehydrated trees, proportionally to the level of hydraulic damage.
The large difference between native embolism in forest-grown pines and mortality threshold in potted ones, suggests the mature trees are not approaching hydraulic failure. However, the impaired recovery of gs implies towards potential long-term effect of embolism on productivity, and the increased probability for cavitation with increase in VPD may encompass fatal consequences for these trees under future climate change.
Key message: Xylem embolism was frequent in forest-grown Aleppo pine under seasonal drought, but still low (12%) compared to the mortality threshold of >60%, measured in potted trees.