Will global warming deprive trees of sap?

• Gas embolism – a consequence of global warming – is the main driver of tree dieback, according to our partner The Conversation.

• The rate of lethal embolism is about 50% for conifers with evergreen leaves and 90% for angiosperms, in which the fall of the leaves makes it possible to limit dehydration in extreme situations.

• This analysis was carried out by Thierry Ameglio, research director in tree ecophysiology and Guillaume Charrier, researcher in ecophysiology.

Ongoing climate change is causing spectacular forest decline throughout the world.

The best known are associated with the lack of water and the high temperatures of summer heat waves.

However, they are not the only ones: other declines, such as those of yellow birch in Canada, are linked to winter warming.

Gas embolism in trees

In both cases, the person responsible is a stoppage of the flow of liquid water coming from the ground (the raw sap) due to the intrusion of air bubbles into the hydraulic circuit: this is called a gas embolism.

This gas embolism is the main factor in tree dieback.

By interrupting the hydraulic continuity between the roots and the leaves, by sucking air into the raw sap (summer embolism) or by freezing this same sap and forming a gas bubble (winter embolism), it interrupts irrigation. aerial tissues and can thus lead to their death by dehydration.

For forty years, our ability to quantify embolism in trees has greatly progressed, thanks to various direct methods (coloration, measurement of hydraulic flows, etc.) or indirect (visualization by X-ray microtomography, detection of bubble formation by acoustic method, etc.). .

These new methodologies have made it possible to follow the progression of the embolism in natural conditions according to water and climatic constraints.

It has thus been discovered that vulnerability to embolism varies greatly between species, making it possible to explain the broad distribution profiles of tree species according to the aridity of their habitats.

Thus, the lethal embolism rate (i.e. the percentage reduction in water flow in conductive tissues) is about 50% for evergreen conifers.

It is 90% for angiosperms, in which leaf fall limits dehydration in extreme situations.

Until very recently, these lethal rates were only reached for 100-year droughts… but are now unfortunately more frequent.

​The case of winter embolism

These studies also revealed that another type of embolism, occurring every year, is caused by freeze/thaw cycles: winter embolism.

Vulnerability to winter embolism depends on the anatomy of the wood and in particular the size of the xylem ducts.

Thus, a single freeze/thaw cycle is sufficient to embolize trees with very large vessels.

This is the case, for example, with oaks, all of whose large-diameter vessels are full of air in the spring.

Fortunately, most hardwoods have ways to combat this winter embolism.

Thus, the absence of leaves during this period slows down the drying of the tissues.

They also develop bypass and/or repair strategies in the spring to rebuild hydraulic continuity between the roots and the young leaves produced.

The oak, for example, makes new vessels by starting its secondary growth (production of new functional vessels within a new growth ring) before the development of new leaves.

Other species (birch, maple, beech, walnut, vine, etc.) repair their hydraulic system by increasing the pressure in the sap of the wood.

Concretely, this pressurization drives out the air bubbles and reconnects all the water columns between the ground and the end of the branches.

This pressure is observed empirically when a branch is cut, by a flow of sap at the cut end.

It is said that these ligneous plants “weep at the waist” and the gardener will speak of a rise in sap, whereas it is only a change in hydric state.

Because if the sap rises and can refill embolized wood vessels, it has never really descended!

Conversely, conifers are very resistant to winter embolism thanks to the small size of their ducts, which limits the propagation of air in the conductive system.

This allows them to supply water to their evergreen needles throughout the year.

This is why conifers are found in the mountains, where frosts are severe and frequent.

​The impact of climate change

Changes in environmental conditions linked to climate change (increased droughts and heat waves) are likely to lead to a greater frequency of situations bringing plants closer to their lethal summer embolism thresholds.

In fact, more than 70% of the planet's trees have a safety margin against gas embolism that is too small to allow them to withstand more intense droughts.

Winter embolism seemed less problematic until now, in particular with a decrease in the prediction of the number of frost days.

However, here again, climate change risks amplifying the damage linked to winter embolism.

This is the case in the mountains, where the increase in freeze/thaw alternations linked to greater thermal amplitudes in winter will accentuate the embolism rates at the end of winter.

In addition, the snow cover has a protective role vis-à-vis the integrity of the roots.

The decrease in its thickness and its duration of presence linked to warming will therefore limit the ability to repair the embolism by root pressure.

Finally, the summer water constraints and the decrease in photosynthesis and the storage capacity of carbon reserves also raise fears of a decrease in the capacity to repair the winter embolism before the development of new leaves.

Indeed, these carbon reserves are essential to the production of new vessels or to produce a pressure of the sap of the wood restorative of the winter embolism.

Our "TREES" file

Whether it is a summer or winter embolism, trees that are poorly acclimatized to the modifications induced by climate change risk "lacking sap" in the future, which will accelerate their decline...

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This analysis was written by Thierry Ameglio, research director in tree ecophysiology and Guillaume Charrier, researcher in ecophysiology (both at the National Research Institute for Agriculture, Food and the Environment – ​​INRAE) .

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