Dear Richard:

Traditional indrect method overestimates the xylem vulnerability to cavitation. The present understanding is that embolism does occur but not routine. Embolized vessels and tracheids can be refilled with water for a few times and then no longer be refilled becuase of cavitation fatigue. Besides xylem embolism, phloem disfunction and carbon starvation during drought could also cause tree mortality, someitmes these mechanisms may work together to cause tree mortality. 

Thank you Kunfang. This is exactly what I need to know.

Branching pattern and allometric scaling of leaf area to vessel area may change vulnerabilty,according to specific variations. There is a limited evidence for actual embolic mortality and age decline is linked to wood failure under stress like wind and diseases. Delzon and Cochard reviewed this topic recently in New Phytologist 2014.  Leaf embolism may however play a role in heteromorphic or patchy photosynthesis. Climate variations towards drought modify tree size population distribution and with lesser small trees one there will be a dominace change to shrub and grass.

What about tree deaths during droughts? Are these not linked to embolism?

It is rather obvious that you know the point . One (not 2) paper was about who came first water stress or Carbon (in Spruce seedling) death? http://onlinelibrary.wiley.com/doi/10.1111/nph.12331/pdf, but on these seedlings embolism was inferred from a very low water content. In the wood it is probably difficult that a big tree cohort does not reach water and that lateral water movements are absent. Leaf shedding can be a tthermal response as well.

Check these papers. Maybe they will help you.

Regards

Article Is xylem cavitation resistance a relevant criterion for scre...

Article Interspecific variation in xylem vulnerability to cavitation among

Article Vulnerability Of Xylem To Cavitation And Embolism

Thank you Manuel. These are interesting but they rely on indirect indications of cavitation. Papers published in the last few months in New Phytologist, Tree Physiology etc. suggest these indications are unreliable. But I suspect that by reading only what is published I am a year or so behind the current understanding in this field!

The tallest trees are living so far in north California (Koch Nature 2004). In tropics there are less dry spells than in giant redwood areas and smaller trees. But leaf mean area, mesophyll wall conductance and photosynthesis are down regulated with height as and more than water potential. To draw a conclusion about the possible incidence of embolic limit, like ina recent Nature vulnerability inference model, we should produce legal evidence of it  Failure characters are typical, involve a number of trees at one time and require monitoring near dry season ends

Yes, but the redwood genus, Sequoia, was eliminated from SW China, where I live, as a result of drying since the Miocene and is supported by fog in N California. A very recent paper (2015?) shows it cannot refill embolized tracheids and has to replace them with new xylem. A lot of tropical forests get 5-7 months with no rain at all and high temperatures. But I agree, we have no strong evidence that embolisms set the limit.

Brazil is facing a  severe drought spell, perhaps In Rondonia they will find an evidence. Fog has a limited effect but condensation can restore some soil humidity, but a  lesser leaf mean area (osmoregulation) and wall resistance (--C13 due to low humidity feedback)  are negative adaptations too and indicate the redwood strategy that reduces frictional drag.  If young trees are damaged for their shallower roots, seedling embolism occurs togheter with weed competition and then an entire genus can disappear after maximal life span attainement, like probably was the case for savannas establishement

Hi,

we did quite a number of experiments in the recent years to address specifically these questions. We have not worked with tropical trees, but I think the processes are quite generic (see or global review paper in Nature Choat et al 2012). 

There is actually two questions:

1- Are trees able to refill embolised vessels? There is a lot of confusion in the literature on this point because many previous observations are biased by faulty methodologies. X-ray micro-CT is probably the most reliable technique available today to study these processes and we have used it intensively recently (through Synchrotron facilities, or with our lab based systems). So far, our conclusion is that trees can refill embolised vessels only if they can generate positive root pressures. In other words, we have not been able to confirm the so called 'novel' refilling with these robust techniques. This means that embolism can be reversed only when transpiration is null (at night) and when soil is fully hydrated. I've seen this phenomenon on temperate trees only during spring on few species. A point that should be made clear is that cavitation is not routine in trees. Cavitation forms only under extreme drought conditions, typically after stomata have closed and leaves loos their turgor. We have never been able to confirm daily cycles of embolism/refiling with our new technologies (micro-CT). 

2- Are trees able to recover once embolism has formed? How resilient are they ? We have addressed this question experimentally by exposing trees so increasing levels of drought, measuring embolism and quantifying mortality. We have data for about 10 species and if we can generalize from this small data set we can say that the lethal level of embolism is around 90% for angiosperms and 50% for gymnosperms. Above these levels tree never recovered (=death). Below, they were able to regrow, but their growth was impaired and they would have eventually die in the field (less competitive).

There is of course a lot of experiments that need to be done to confirm these preliminary conclusions. The real world is probably more nuanced and several species probably behave different (especially in the tropics!). However, the key point is that you should be extremely critical about the methods you use and it is essential to confirm your findings with robust techniques such as micro-CT. Very sadly there is a lot of confusion in the literature, so please do not bring more confusion ! 

Here is a list of recent papers from our group addressing these questions. You can find them here on Resarchgate or on my website: http://herve.cochard.free.fr/publi.htm

Torres-Ruiz JM, Jansen S, Choat B, McElrone A, Cochard H, Brodribb TJ, Badel E, Burlett R, Bouche PS, Brodersen C, Li S, Morris H, Delzon S. 2015 Direct X-ray microtomography observation confirms the induction of embolism upon xylem cutting under tension. Plant Physiology 167: 40-43.

Cochard H, Delzon S, Badel E. 2015 X-ray microtomography (micro-CT): a reference technology for high-resolution quantification of xylem embolism in trees. Plant Cell and Environment. 38: 201-206

Martin-StPaul N K, Longepierre D, Huc R, Delzon S, Burlett R, Joffre R, Rambal S, Cochard H. 2014 How reliable are methods to assess xylem vulnerability to cavitation? The issue of “open vessel” artifact in oaks. Tree Physiology 34: 894-805.

Torres-Ruiz JM, Cochard H, Mayr S, Beikircher B, Diaz-Espejo A, Rodriguez-Dominguez CM, Badel E, Fernández, JE. 2014 Vulnerability to cavitation in Olea europaea current-year shoots: more support to the open-vessel artefact with centrifuge and air-injection techniques. Physiologia Plantarum. 152: 465-474 doi: 10.1111/ppl.12185

Delzon S, Cochard H. 2014 Recent advances in tree hydraulics highlight the ecological significance of the hydraulic safety margin. New Phytologist. 203: 355–358 doi: 10.1111/nph.12798

Cochard H, Delzon S 2013. Hydraulic failure and repair are not routine in trees. Annals of Forest Science 70: 659-661

Urli M, Porté A, Cochard H, Guengant Y, Burlett R, Delzon S 2013. Xylem embolism threshold for catastrophic hydraulic failure in angiosperm trees. Tree physiology 33: 672–683

Cochard H, Badel E, Herbette S, Delzon S, Choat B, Jansen S 2013. Methods for measuring plant vulnerability to cavitation: a critical review. Journal of Experimental Botany 64: 4779-4791

Barigah ST, Charrier O, Douris M, Bonhomme M, Herbette S, Ameglio T, Fichot R, Brignolas F, Cochard H 2013. Water stress-induced xylem hydraulic failure is a causal factor of tree mortality in beech and poplar. Annals of Botany 112:1431-1437

Choat B, Jansen S, Brodribb TJ, Cochard H, Delzon S et al 2012. Global convergence in the vulnerability of forests to drought. Nature 491: 752–755

Brodribb TJ, Cochard H 2009. Hydraulic failure defines the recovery and point of death in water stressed conifers. Plant Physiology 149: 575-584

Thank you Hervé. I have read several papers from your group and this is a very clear summary.