Thirsty forests

1 02 2019

Climate change is one ingredient of a cocktail of factors driving the ongoing destruction of pristine forests on Earth. We here highlight the main physiological challenges trees must face to deal with increasing drought and heat.

Forests experiencing embolism after a hot drought. The upper-left pic shows Scots (Pinus sylvestris) and black (P. nigra) pines in Montaña de Salvador (Espuñola, Barcelona, Spain) during a hot Autumn in 2015 favouring a massive infestation by pine processionary caterpillars (Thaumetopoea pityocampa) and tree mortality the following year (Lluís Brotons/CSIC in InForest-CREAF-CTFC). To the right, an individual holm oak (Quercus ilex) bearing necrotic branches in Plasencia (Extremadura, Spain) during extreme climates from 2016 to 2017, impacting more than a third of the local oak forests (Alicia Forner/CSIC). The lower-left pic shows widespread die-off of trembling aspen (Populus tremuloides) from ‘Aspen Parkland’ (Saskatchewan, Canada) in 2004 following extreme climates in western North America from 2001 to 2002 (Mike Michaelian/Canadian Forest Service). To the right, several dead aspens near Mancos (Colorado, USA) where the same events hit forests up to one-century old (William Anderegg).

A common scene when we return from a long trip overseas is to find our indoor plants wilting if no one has watered them in our absence. But … what does a thirsty plant experience internally?

Like animals, plants have their own circulatory system and a kind of plant blood known as sap. Unlike the phloem (peripheral tissue underneath the bark of trunks and branches, and made up of arteries layered by live cells that transport sap laden with the products of photosynthesis, along with hormones and minerals — see videos here and here), the xylem is a network of conduits flanked by dead cells that transport water from the roots to the leaves through the core of the trunk of a tree (see animation here). They are like the pipes of a building within which small pressure differences make water move from a collective reservoir to every neighbours’ kitchen tap.

Water relations in tree physiology have been subject to a wealth of research in the last half a decade due to the ongoing die-off of trees in all continents in response to episodes of drought associated with temperature extremes, which are gradually becoming more frequent and lasting longer at a planetary scale (1). 

Embolised trees

During a hot drought, trees must cope with a sequence of two major physiological challenges (2, 3, 4). More heat and less internal water increase sap tension within the xylem and force trees to close their stomata (5). Stomata are small holes scattered over the green parts of a plant through which gas and water exchanges take place. Closing stomata means that a tree is able to reduce water losses by transpiration by two to three orders of magnitude. However, this happens at the expense of halting photosynthesis, because the main photosynthetic substrate, carbon dioxide (CO2), uses the same path as water vapour to enter and leave the tissues of a tree.

If drought and heat persist, sap tension reaches a threshold leading to cavitation or formation of air bubbles (6). Those bubbles block the conduits of the xylem such that a severe cavitation will ultimately cause overall hydraulic failure. Under those conditions, the sap does not flow, many parts of the tree dry out gradually, structural tissues loose turgor and functionality, and their cells end up dying. Thus, the aerial photographs showing a leafy blanket of forest canopies profusely coloured with greys and yellows are in fact capturing a Dantesque situation: trees in photosynthetic arrest suffering from embolism (the plant counterpart of a blood clot leading to brain, heart or pulmonary infarction), which affects the peripheral parts of the trees in the first place (forest dieback).

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Cartoon guide to biodiversity loss LII

2 01 2019

The first set of six biodiversity cartoons for 2019 to usher in the New Year. See full stock of previous ‘Cartoon guide to biodiversity loss’ compendia here.

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With a Rebel Yell, Scientists Cry ‘No, no, more!’

29 11 2018

Adrenaline makes experiences hyper-real. Everything seems to move in slow motion, apart from my heart, which is so loud that I am sure people can hear it even over the traffic.

It’s 11:03 on a sunny November morning in central London. As the green man starts to shine, I walk into the middle of the road and sit down. On either side of me, people do the same. There can only be about 50 of us sitting on this pedestrian crossing, and I murmur ‘are we enough?’

‘Look behind you,’ says a new friend.

I turn. Blackfriar’s Bridge, usually covered in cars and buses, is filling with people. Citizens walking into the road and staying there, unfurling colourful flags with hourglass symbols on them. The police film us, standing close, but make no move to arrest anyone. Later, we discover that at least some of them encourage our disobedience.

Messages start coming in — 6,000 people are here, and we’ve blocked five bridges in central London with Extinction Rebellion, protesting for action to stop climate change and species extinctions. I’m a scientist participating in my first ever civil disobedience, and for me, this changes everything.


Left to right: protestors include kids, company directors, and extinct species.

What makes a Cambridge academic — and thousands of other people — decide that sitting in a road is their best chance of being heard? In short, nothing else has got us the emissions cuts we need. The declaration that global warming is real and that greenhouse-gas emissions need to be cut came in 1988, when I was a year old. Since then, scientists have continued to be honest brokers, monitoring greenhouse gases, running models, presenting the facts to governments and to the people. And emissions have continued to climb. The 2018 IPCC report that shocked many of us into action told us we have 12 years to almost halve emissions, or face conditions incompatible with civilisation. How did we end up here? Read the rest of this entry »

Global warming causes the worst kind of extinction domino effect

25 11 2018

Dominos_Rough1-500x303Just under two weeks ago, Giovanni Strona and I published a paper in Scientific Reports on measuring the co-extinction effect from climate change. What we found even made me — an acknowledged pessimist — stumble in shock and incredulity.

But a bit of back story is necessary before I launch into describing what we discovered.

Last year, some Oxbridge astrophysicists (David Sloan and colleagues) published a rather sensational paper in Scientific Reports claiming that life on Earth would likely survive in the face of cataclysmic astrophysical events, such as asteroid impacts, supernovae, or gamma-ray bursts. This rather extraordinary conclusion was based primarily on the remarkable physiological adaptations and tolerances to extreme conditions displayed by tardigrades— those gloriously cute, but tiny (most are around 0.5 mm long as adults) ‘water bears’ or ‘moss piglets’ — could you get any cuter names?


Found almost everywhere and always (the first fossils of them date back to the early Cambrian over half a billion years ago), these wonderful little creatures are some of the toughest metazoans (multicellular animals) on the planet. Only a few types of extremophile bacteria are tougher.

So, boil, fry or freeze the Earth, and you’ll still have tardigrades around, concluded Sloan and colleagues.

When Giovanni first read this, and then passed the paper along to me for comment, our knee-jerk reaction as ecologists was a resounding ‘bullshit!’. Even neophyte ecologists know intuitively that because species are all interconnected in vast networks linked by trophic (who eats whom), competitive, and other ecological functions (known collectively as ‘multiplex networks’), they cannot be singled out using mere thermal tolerances to predict the probability of annihilation. Read the rest of this entry »

Cartoon guide to biodiversity loss LI

23 10 2018

The six set of six biodiversity cartoons for 2018. See full stock of previous ‘Cartoon guide to biodiversity loss’ compendia here.

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Cartoon guide to biodiversity loss L

3 08 2018

The fifth set of six biodiversity cartoons for 2018. See full stock of previous ‘Cartoon guide to biodiversity loss’ compendia here.

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Cartoon guide to biodiversity loss XLIX

2 07 2018

The fourth set of six biodiversity cartoons for 2018. See full stock of previous ‘Cartoon guide to biodiversity loss’ compendia here.

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