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by Alain Valette, phytosanitary expert

Every year, we are told stories of branches splitting and breaking during summer. “It was a hot and windy day. We heard a deafening sound and saw an enormous branch fall to the ground floor.” Daily regional newspapers all over France report every day on “narrowly avoided potential catastrophes.”

The increasing frequency of droughts foreshadows an increase in this type of terrifying experience for park managers. These branch breakages have a complex explanation. We do not offer a miracle cure, but an introductory explanation into this event, which seems inexplicable for most of those who have never been confronted by it.

The hydraulic system of a tree is unique and complicated. Water circulates from the bottom of the tree (at the roots) to the top (at the leaves) through canals for the entire length of the trunk. This is made possible by evaporation, as the column of water is pulled towards the pores of leaves. Water is transpired at the pores in order to cool the tree and absorb the carbon dioxide necessary for photosynthesis.

The water in the tree is under constant tension, like a cord pulled tight. During a drought, this sensitive mechanism can easily become imbalanced, and the “cord” can break following the absorption of air bubbles into the central canals. The tension level ranges from -1 Bar per 10m long branch. The effected duct empties almost instantly and becomes hydraulically functionless. The circulation of water is interrupted due to embolism.

As the intensity of the drought increases, the number of embolisms in the tree’s conduit system increase. If the water tension increases, this propagates step-by-step; the passage of an air bubble from a neighbouring canal occurs across the membranes, via microscopic pores named ‘punctuations.’ The propagation of an embolism can seriously impact water conduction and threaten the survival of the tree. The tension increases, whilst the leaves consistently transpire water, and the embolism migrates to a single branch. This causes an implosion of the system, during which the sap canals brutally collapse. A branch under this level of tension can therefore snap without warning.

… and could affect numerous tree species. There are no detectable warning signs, even for experienced people.