Water droplets and burned leaves, continued

A few weeks ago (January 20 – “Help, help, the sky is falling”) I started a discussion about an article appearing in the peer-reviewed journal New Phytologist.  That posting focused on the methodology and results in the paper.  Today let’s take a look at the authors’ underlying arguments (their introduction to the study) and their conclusions.

1)  The authors’ premise is that “laymen and professionals alike commonly believe water drops on plants after rain or watering can cause leaf burn in sunshine.”  To support this statement, the authors surveyed “relevant topical websites.”  They found 29 sites (primarily .org and .com, but no .edu sites) that agreed with this statement and 9 sites (including 4 .edu sites) that disagreed.  How this translates to “professionals” believing that water drops cause sunburned leaves is unclear, especially when all the .edu sites surveyed disagreed.  In my opinion, the authors should have surveyed ONLY .edu sites to test their hypothesis about what professionals believe.  And why only 38 sites?  We’re not told how or why these sites were selected.

2)  Building on this shaky premise, the authors then address the apparently popular concern that water drops can cause forest fires.  They survey “the forestry literature” to find “the prevailing opinion is that forest fires can be sparked by intense sunlight focused by water drops on dried-out vegetation (Table S3).”  Table S3 is not included in the online article but is in a supplementary file.  Happily, it is short enough that I can paste it in here (so you can find the sites yourself):

Table S3 Survey of websites discussing the possibility of forest fires due to sunlight focused by water drops. We posed the question: “Can sunlit water drops spark forest fires?” The rate of the ’yes’ answer was 3 / 3 = 100%.


Title of article





Forest fire and water drops




Radó (2001) Role of vegetation in protection of the environment




Whether presence of water cause forest fire?



I must say this took my breath away.  This is not a survey of the “forestry literature.”  It is 3 websites, two in Hungarian and one in English, chosen for unknown reasons.  The first site is actually a stock photo website with comments about pictures of water drops on leaves.  The second is entirely in Hungarian and is not in the scientific databases.  The third is in English, and here’s what “wiki answers” has to say:

“When I was a youngster and could not afford a magnifying glass, I would twist a piece of wire around a pencil so that it formed a round piece at the end of the wire. I would then dip the rounded end into water so that a blob of water made a very small magnifying glass. I suspect that when it has rained this same effect is left on leaves, millions of tiny magnifying glasses all concentrating the suns rays onto what they happen to land on. Just one tiny focal point of a rain drop could possibly generate enough heat to start a fire.  Robert”

[Note to the editors at New Phytologist:  What I really want to know is how this kind of junk science can slip through peer review.  It is embarrassing.]

3)  The authors (none of them plant scientists) nevertheless address plant ecophysiology in the discussion:  “If, after rain, leaf blades were covered by a water film, they could not breathe, because gas exchange through the stomata would be blocked…To avoid this, plants evolved efficient water-repelling and water-channeling structures which build up and roll off rain drops. For example, water drops easily roll off the highly hydrophobic leaves of lotus, Ginkgo (Fig. 2b), and floating fern (Fig. 3b,c) if leaves are tilted or shaken.”

Two comments here:  the stomata through which terrestrial plants “breathe” are primarily on the underside of the leaves.  It is true that floating aquatic plants have most of the stomata on the upper leaf surface.  Which leads me to ask…if water drops easily roll off of floating fern leaves, then how did the researchers do the following?  “…the experiment was concluded by cutting and scanning several Salvinia leaves – still holding water drops – in the laboratory in order to document their sunburn.”

4)  The conclusion of a research article, as any Garden Professor knows, is meant to summarize the results of the experiment.  Yet the last paragraph of the conclusion reads as follows:  “Lastly, a similar phenomenon might occur when water droplets accumulate on dry vegetation (e.g. straw, hay, fallen leaves, parched grass, brush-wood) after rain. If the focal region of drops falls exactly on the dry plant surface, the intensely focused sunlight could theoretically spark a fire. However, the likelihood of this is considerably reduced by the fact that after rain the originally dry vegetation becomes wet, and as it dries water drops also evaporate. Thus, claims of fires induced by sunlit water drops on vegetation should also be treated with a grain of salt.”

Even though the authors seem to discount the possibility of these scenarios, they did NOT test the ability of water drops to cause combustion.  This speculation really belongs in the discussion, if anywhere at all.  So why is does it make up 50% of the conclusion?  The cynic in me says it’s because 90% of the people looking at this article will read only the abstract and the conclusion – and this is especially true of nonscientists.  It’s a great way to get immediate attention, even with a complete lack of supporting evidence.

Don’t believe me?  Just type in “water drops cause forest fires” without the quotes into Google.  146,000 hits, and all the top ones reference this article.

Help! Help! The Sky is Falling!!!

You all remember the story of Chicken Little, right? Chicken Little thought she’d been hit on the head by a piece of the sky and ran around alarming the rest of the barnyard animals, who assumed Chicken Little knew what she was talking about. Had they not all been eaten by Foxy Loxy, I’m sure they would have felt foolish discovering that an acorn, not a piece of the sky, had bopped Chicken Little on the noggin.

On to today’s science rant.

Ann McCormick, one of my GWA (Garden Writers Association) colleagues, alerted me to an online story from Live Science entitled “Water Drops Magnify Sunlight and Burn Leaves.” A quick look at the internet shows that this report has gone viral, with similar headlines from other websites including the venerable Scientific American (“A study in the journal New Phytologist confirms the gardener’s belief that droplets of water resting on some types of leaves can focus sunlight until the plant’s surface actually burns”). It gets more and more ridiculous by the day (“Sun shining?  Then don’t water your plants” courtesy of the Daily Mail in London; “Water droplets can form forest fires” from Calcutta). And so on.

I tracked down the original article in New Phytologist, entitled “Optics of sunlit water drops on leaves: conditions under which sunburn is possible.”  You should take a look at it, if only to become completely intimidated by the physics and computer modeling it contains. I’ll be honest – I didn’t even try to understand this portion but focused on the plant science.

The authors had three actual experiments in addition to the optical modeling.  The first experiment involved placing glass spheres on detached leaves (Acer platanoides – Norway maple) and exposing them to sunlight. Yup, glass spheres caused leaf burn on sunny days – no big surprise there. The second experiment substituted water droplets for the glass spheres and tested Ginkgo biloba as well as maple leaves. Not surprisingly (to me anyway) there was no damage to leaves of either species. The third experiment repeated the second, but tested the leaves of the aquatic fern Salvinia natans and voila! Leaf damage!

I have a lot of issues with this paper and maybe we’ll have to extend blog coverage for a day or two to keep today’s discussion as short as possible. Let me point out just two of the experimental problems

The leaves for experiments 1 and 2 were detached from the plant prior to treatment. How a detached leaf resembles anything in a natural situation is beyond me. Furthermore, these leaves were laid out, covered with glass spheres or water droplets, and left in the full sun for as long as nine hours. (Even so, the leaves covered with water droplets didn’t burn! You go, detached leaves!)

The Salvinia experiments were conducted on leaves scooped out of a pond, placed in two containers, sprinkled with water, and left in the sun for two hours. Then, in the authors’ words, “the experiment was concluded by cutting and scanning several Salvinia leaves – still holding water drops – in the laboratory in order to document their sunburn.” We are not told (a) how many leaves were scanned, (b) how the leaves were chosen, since they didn’t scan them all, and (c) how the reported damage was proven to be from water droplets. >Worse, there are no statistical data. We are simply asked to believe their report in the absence of any evidence except a handful of photographs. (As an aside, I really would like to know how they were able to cut and transport leaves without the water droplets moving!)

The lack of scientific rigor in this article is disappointing, especially in a peer-reviewed journal. Is there any plant scientist would consider detached leaves to be a model for those on an intact plant? Is there any gardener who would consider an aquatic fern comparable to trees and shrubs? Would any species – including aquatic ferns – easily burned by the combination of water and sunlight survive in the real world for long? I don’t think so – hence my myth posting on this very topic several years ago.

Let’s review: leaves of one species of aquatic fern was damaged by something – possibly sunlight – but without enough data presented to really evaluate the claim. None of the tree leaves tested were affected, even though they were detached from the plant and could not benefit from transpirational cooling. Yet the alarm has gone out! Don’t water your garden plants when it’s sunny, or the leaves will burn!!!

Be sure to take everything you read with a grain of salt – or an acorn.

(I will continue discussion of this article further if there is enough interest – that means you need to post a comment!)