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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Influential conservation ecology papers of 2018

17 12 2018

For the last five years I’ve published a retrospective list of the ‘top’ 20 influential papers of the year as assessed by experts in F1000 Prime — so, I’m doing so again for 2018 (interesting side note: six of the twenty papers highlighted here for 2018 appear in Science magazine). See previous years’ posts here: 2017, 20162015, 2014, and 2013.

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Perseverance eventually gets the policy makers’ attention

10 12 2018


My entry badge today to the South Australian Parliament (sorry for the shitty reproduction, but it’s a shitty photo of a shitty photo)

I’ve often commented on it over the years, as well as written about it both in my latest book, as well as featured it here on, that little of the conservation science we do appears to reach the people making all the decisions. This is, of course, a massive problem because so much policy that affects biodiversity is not evidence-based, nor do we seem to be getting any better at telling them how buggered our natural world is.

Even the Extinction Rebellion, or school kids screaming in the streets about lack of climate-change policies appears unable to budge the entrenched, so what hope do we lonely little scientists have of getting in a Minister’s ear? It’s enough to make one depressed.

look-at-me-girlSo, we go through the motions; we design ideal reserves with the aid of our computers, we tell people how much to fish, we tell them why feral species are bad, etc., etc., and then we publish our findings and walk away. We might do a little more and shout our messages loudly from the media rooftops, or submit comments to proposed policies, or even draft open letters or petitions. Yet no matter how hard we seem to try, our messages of urgency and despair largely fall on deaf ears.

It’s enough to make you reconsider and not bothering at all.

But! Despite my obviously jaded perspective, two things have happened to me recently that attest to how a little perseverance, sticking to your guns, and staying on message can reach the ears of the powerful. My examples are minuscule in the grand scheme of things, nor will they necessarily translate into anything really positive on the ground; yet, they give me a modicum of hope that we can make a positive difference.

The first event happened a few weeks ago after we did a press release about our paper on co-extinction cascades published in Scientific Reports. Yes, it got into a few big newspapers and radio, but I thought it wouldn’t do much more than peak the punters’ interest for the typical 24-hour news cycle. However, after the initial media interest died down, I received an e-mail from one of my university’s media officers saying that the we had been cited in The Senate (one of the two houses in the Australian Parliament)! An excerpt of the transcript is shown below (you can read the whole thing — if you could be bothered — 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 »

Why a (young) scientist should blog

12 11 2018

I started to blog in the middle of my PhD, exactly on 17 February 2011 — as a scientist I remember my first blog like a soccer-loving kid might remember his/her first soccer ball. Postgraduates from ACAD have recently asked me to give a talk about my blogging experience, and I couldn’t resist turning my talk into a blog.

Salvador Herrando-Pérez



The cover of the February (polar bears) and December (water flea) 2017 issues of the Spanish magazine Quercus featured two of my popular-science articles. Founded in 1981, and with a current print run of some 15,000 copies monthly, Quercus has pioneered the dissemination of ecological and environmental science with a conservation edge in Spain and survived the digitalisation age, which has recently deserved the prestigious 2018 BBVA prize for Biodiversity Conservation. My liaison with the magazine already spans seven years with 49 articles published in three theme series (conservation biology: 2011-2012; animal behaviour: 2013; and climate change: active since January 2017 in collaboration with my colleague David Vieites).

I write in blogs, but I am not a blogger in the sense of owning and managing a blog. More exactly, I write about science using a language that should be understandable by an audience of scientists and, primarily, non-scientists. The best English expression I have found to qualify such activity is ‘popular science’ (I use it interchangeably with ‘blog’ hereafter). And blogs are just one platform (internet) to publish popular science.

In fact, I publish popular science on a regular basis here in ConservationBytes, and in Quercus: a printed Spanish-language magazine about ecology and biodiversity. My articles in those outlets typically synthesise the findings, and expand the background and implications, of high-profile research papers from the primary literature. Sometimes, I also write blogs to maximise the audience of my own publications (e.g., here and here), or to discuss a topic of general interest (e.g., numerical literacy). I have listed all my blogs on ConservationBytes at the end of the text.

Frankly, I had never stopped to think why I started and why I keep writing popular science. So after a bit of brainstorming, I have come up with five personal motivations which will probably resonate with those of other scientists entering the Blogosphere (1) — see here Corey’s take on the virtues of blogging.


When you are in the early stage of your research career, letting your peers know that you exist is essential, unless one already publishes hot papers that everybody reads and cites, and/or you have already amassed quite a reputation in the scientific community (not my case). Let’s be clear: my blogs are bound to be read by more people than my research papers, because blogs magnify the chances of being detected by search engines (2), and because the size of the scientific community is dwarfed by the size of the internet community. Doubtless, self-promotion drew me into popular science in the first place, when I was just a PhD student — ahead of me lay some five to ten years over which I would have to compete hard for funding and publication space with a respectable crowd of other researchers, let alone to create new partnerships with colleagues in and out of my area of expertise. So, blogging initially meant like saying ‘hey! I am here, I am doing science’.


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Ecophysiological feedbacks under climate change

29 10 2018

Variability in heat tolerance among populations modifies the climate-driven periods of diurnal activity expected for ectotherm species. We illustrate this phenomenon for Iberian lizards in a paper we have just published in the Journal of Animal Ecology (blog post reproduced with permission by the Journal; see related blog).

Common wall lizard (Podarcis muralis, male) and three localities where the species is abundant in Spain, left to right including Valdesquí/Madrid (Central System), Peñagolosa/Castellón (Iberian System) and El Portalet/Huesca (The Pyrenees).

Iberia is a wonderful natural laboratory, with a complex blend of flat/hilly, open/woody and coastal/continental terrain, swept by climatic gradients of temperature and moisture. In 2013, I launched a BES-supported project about the thermal ecology of Iberian lizards and managed to drive over much of the Iberian Peninsula in fairly little time. Not being a reptile specialist myself, I was confronted by the consistent observation that lizard populations occupied very different habitats across the known distribution of each of the ~ 25 known Iberian species belonging to the family Lacertidae.

For instance, the common wall lizard (Podarcis muralis) likes water, rocks and mountains, but you can find this pencil-long reptile at the top of a summit, along the slopes or riversides of shallow and deep ravines, on little stones barely surfacing above peatland grasslands, or among the bricks of buildings. These animals must experience different local climates conditional on where they live, and adapt their thermal physiology accordingly.

Having then started a postdoc in Miguel Araújo’s lab — a world-class site for global change ecology and ‘big’ biodiversity patterns — I reviewed a sizeable body of literature looking into large-scale gradients of thermal tolerance. Most of those papers had collated (mostly) one estimate of tolerance from each of tens to thousands of species, then mapped them against regional and global metrics of climate change through sophisticated mathematical frameworks. But these studies rarely accounted for population-level thermal tolerance.

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Sex on the beach

2 10 2018
Female green turtles (Chelonia mydas) spawning (top) and diving (bottom) on Raine Island (Great Barrier Reef, Queensland, Australia) — photos courtesy of Ian Bell. This species is ‘Endangered’ globally since 1982, mainly from egg harvesting (poaching conflict in Mexico for olive ridley Lepidochelys olivacea featured by National Geographic’s video here), despite the success of conservation projects (39). Green turtles inhabit tropical and subtropical seas in all oceans. Adults can grow > 150 kg and live for up to ~ 75 years. Right after birth, juveniles venture into the open sea to recruit ultimately in coastal areas until sexual maturity. They then make their first reproductive migration, often over 1000s of km (see footage of a real dive of a camera-equipped green turtle), to reach their native sandy beaches where pregnant females will lay their eggs. Each female can deposit more than one hundred eggs in her nest, and in several clutches in the same season because they can store the sperm from multiple mating events.

When sex is determined by the thermal environment, males or females might predominate under sustained climatic conditions. A study about marine turtles from the Great Barrier Reef illustrates how feminisation of a population can be partitioned geographically when different reproductive colonies are exposed to contrasting temperatures.

Fortunately, most people in Western societies already perceive that we live in a complex blend of sexual identities, far beyond the kind of genitals we are born with. Those identities start to establish themselves in the embryo before the sixth week of pregnancy. In the commonest scenario, for a human foetus XY with one maternal chromosome (X) and one paternal (Y) chromosome, the activation of the Sry gen (unique to Y) will trigger the differentiation of testicles and, via hormonal pathways, the full set of male characteristics (1).

Absence of that gene in an XX embryo will normally lead to a woman. However, in just one of many exceptions to the rule, Sry-expression failure in XY individuals can result in sterile men or ambiguous genitals — along a full gradient of intermediate sexes and, potentially, gender identities. A 2015 Nature ‘News’ feature echoes two extraordinary cases: (i) a father of four children found to bear a womb during an hernia operation, and (ii) a pregnant mother found to host both XX and XY cells during a genetic test – with her clinical geneticist stating “… that’s the kind of science-fiction material for someone who just came in for an amniocentesis” (2). These real-life stories simply reflect that sex determination is a complex phenomenon.

Three ways of doing it

In nature, there are three main strategies of sex determination (3) — see scheme here: Read the rest of this entry »