Surprise: Leaves Maintain Temperature, new findings may put dendroclimatology as metric of past temperature into question

Dendroclimatology: thermometer or hygrometer?

Hot climate or cold, tree leaves stay in comfort zone

From the Google Climate Discussion Group, see an article also in Science News

Paris, June 11; Agence France-Presse

A new study that shows their internal temperature remains constant at 21.4deg could challenge the way trees are used to determine historical climate data

The internal temperature of leaves, whether in the tropics or a cold-clime forest, tends toward a nearly constant 21.4 degrees Celsius, reports a study released today.

It had long been assumed that actively photosynthesising leaves – using energy from sunlight to convert carbon dioxide and water into sugar – are nearly as cold or hot as the air around them.

The new findings not only challenge long-held precepts in plant biology, but could upend climate models that use tree rings to infer or predict past and present temperature changes.

For decades, scientists studying the impact of global warming have measured the oxygen isotope ratio in tree-rings to determine the air temperature and relative humidity of historical climates.

Oxygen atoms within water molecules evaporate more or less quickly depending on the number of neutrons they carry, and the ratio between these differently weighted atoms in tree trunk rings has been used as a measure of year-to-year fluctuations in temperatures and rainfall.

“The assumption in all of these studies was that tree leaf temperatures were equal to ambient temperatures,” lead researcher Brent Helliker told AFP. “It turns out that they are not.”

Helliker and University of Pennsylvania colleague Suzanna Richter turned those assumptions upside down in examining 39 tree species, across 50 degrees of latitude ranging from sub-tropical Columbia to boreal Canada.

They compared current observed records of humidity and temperature against the isotope ratios in the trees, and found that tree leaves were internally cooler than surrounding air temperatures in warm climes, and warmer in cool climes.

Even more startling was that in all cases the average temperature – over the course of a growing season – was about 21degC.

“It is not surprising to think that a polar bear in northern Canada and a black bear in Florida have the same internal body temperature,” because both animals have internal thermostats to prevent overheating or freezing to death, he said.

“But to think that a Canadian black spruce and a Caribbean Pine have the same average leaf temperature is quite astonishing,” he added.

Tree leaves keep cool through constant evaporation and reducing sun exposure through leaf angles or reflective qualities. Warmth is gained by decreasing evaporation and increasing the number of leaves per branch.

All these tricks should be seen as evolutionary adaptations that help the trees attain a maximum of nutrients through optimal photosynthesis, Helliker said.

The fact that part of this adaptation occurs at the level of entire forest canopies, and not just within individual leaves, is one reason direct measurements of tree temperatures have been so hard.

The new findings, published in the British journal Nature, are bolstered by a recent study of a mixed species forest in Switzerland based on infrared thermal imaging.

Measured across an entire growing season, the forest canopy temperatures were found to be 4degC to 5degC higher than the cool, ambient air in the Swiss Alps.

AFPar 12/06/08 08-05NZ

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June 13, 2008 7:50 am

I never thought about it before, but I suppose it isn’t that surprising given that humans have a relatively constant internal temperature despite geographical location.

June 13, 2008 7:53 am

How annoying! Is there no end to what we don’t know?

June 13, 2008 8:02 am

Trees are “warm-blooded”? Who’da thought?

June 13, 2008 8:10 am

Just WOW!!!
I wonder how Gavin and Hansen will respond to this? If this avg temp efficiency turns out to be case for the trunk as well as the leaves (would also provide another mechanism for trees to resist freezing in the winter), it would certainly explain the “Hokey” Stick’s stick.

June 13, 2008 8:25 am

Ouch! There is just bad news every day for the alarmists. I have always thought tree rings were a better indication of moisture than temperature anyway.

June 13, 2008 8:25 am

Has ring width as a proxy for temperature never been benchmarked?

Steve Keohane
June 13, 2008 8:32 am

Good to see someone still kicking the tires on this used model to see if they still hold good pressure. By all appearances they may be going flat.

Duane Johnson
June 13, 2008 8:50 am

Not to be a defender of tree ring climatology, but isn’t analysis of tree ring widths, not oxygen isotope ratios, the primary methodology relied on to produce the now discredited hockey sticks?

June 13, 2008 9:00 am

This is an astonishing discovery and illustrates how things that appear to be self evident quite often aren’t.
Also this discovery could have been made by anyone with an infrared camera. How many infrared pictures of trees have been taken and no one noticed that leaves were always the same color.

kum dollison
June 13, 2008 9:24 am

Or, they could have just asked any kid why he climbed up in his favorite tree in the summer.

June 13, 2008 9:37 am

That’s a great piece of information — truly non-intuitive, but it does makes sense when you think that work is being performed. Plants having a thermostatic mechanism; what concept. This just shows you how robust Nature is. Thanks for pointing it out. By the way, I am an avid gardener and I now see C02 as a fertilizer for plants; I don’t want C02 cuts: the biosphere seems to thrive — as you pointed in a previous post. Feed the plants! Keep the C02 coming.

Jack Simmons
June 13, 2008 9:39 am

I just assumed someone had already audited this basic feature of the proxy for temperatures.
Has someone bothered checking today’s tree rings versus temperature?
It’s amazing what gets overlooked in today’s world of science and politics.

June 13, 2008 10:06 am

The ring width to temperature relationship has always been based on an assumption. Its the manniacs of AGW that forgot about the problem with assumptions.
They found a curious statistical signal in the tree ring data that they thought allowed them to crow to the world that late 20th century warming was unprecedented.
It turns out that only thing that was unprecedented was their procedure, which either through naivety (ignorance) or malfeasance (possible but not likely) gave them the “smoking gun” the activists needed to raise the climate alarm.
I find it amazing that the hockey stick is still in use. I attended a public meeting just last March where a county official used it as part of a presentation on climate change in the PNW.
After the meeting I privately explained the facts. He hasn’t used it since, but the county remains stuck on stupid regarding AGW (this is the Seattle area).

June 13, 2008 10:14 am

Another stab at the Hockey Stick.
Somewhere in the wooly wilds of Ontario, Steve McIntyre is smiling. I hope. It’s a gorgeous day today.

Leon Brozyna
June 13, 2008 10:49 am

Oh heck.
Another inconvenient truth.
Didn’t these people get the message about what kind of results they were to come up with?
Just goes to show you what happens when real scientists actually study something and an assumption turns and bites the fashionably chic True Believers on their backsides.
Okay, enough venting. This is the sort of inquiry I always expect from science but too often am disappointed, most especially from the area of climatology.

Pamela Gray
June 13, 2008 10:52 am

The following is my own understanding, always open to new and improved insite:
Nutrients, light, and moisture. The big three.
Moisture: Tree rings demonstrate varying amounts of these essential ingredients. Note that a hot, dry, desert blooms infrequently, not because of temperature changes, but because it rains.
Temperature: Some plants withstand a great deal of temperature ranges, others are temp specific. Wine grapes in the PNW are an example of temp specific plant. GW encouraged growers to switch from temp insensitive orchards (like many kinds of plum, apple and cherry trees) to the glam of wine grapes. Hot dry summers in the inland NW prompted farmers to stop planting peas. The wine grower will have a much bigger downfall than the pea grower will. Interesting side note: Initial grape growers came from California. Pea farmers have ALWAYS been here. Pea fields have not been turned into grape fields. Orchards have. Not smart.
Nutrients: Bat poop makes the best fertilizer! Plus you can use it for gun powder when gas goes to $6.00 a gallon.

June 13, 2008 11:00 am

Does this represent another negative feedback mechanism to increasing temperatures from CO2? More atmospheric CO2 causes more plant growth – shouldn’t a global increase of plant matter at this constant temperature tend to pull the atmospheric temperature towards 21.4C?

Pamela Gray
June 13, 2008 11:07 am

Light: Some plants bloom at night, some during the day, some in shade, and some only in full sun. Some even bloom in spite of smog. Most plants show a preference for the sun. They bend towards the light in order to maximum benefit. Plants seem to have figured it all out. Without Gore. For me, I would like to say that I am at least as smart as a plant. Gore seems to have not risen to that level yet.

June 13, 2008 11:24 am

“For me, I would like to say that I am at least as smart as a plant.” Pam
“Pride goeth before a fall…” Proverbs
Why do I pick on Pam?
She makes it easy
and thus I can.
Plus her head, though not too green,
has quite a bit of red instead.

June 13, 2008 11:36 am

“shouldn’t a global increase of plant matter at this constant temperature tend to pull the atmospheric temperature towards 21.4C?”
Probably not. If the tree removes heat from it’s leaves it has to put that heat somewhere else, perhaps it dissipates the heat into the soil via the roots.

June 13, 2008 12:20 pm

I’ve been to the woolly wilds of Ontario. Steve lives about 300 km (200 mi) south of there in Toronto, a major urban centre (notice Canadian spelling) with over 3 million souls.
I’ll be up there again this summer (late July). Hoping that warming occurs before I get there. Black flies are nasty in cold summers. The summer of 1982 was one such year. I was working in central Labrador and nearly went nuts from the black flies.

June 13, 2008 12:41 pm

I think that there was an article on rings vs temp on here a while back. The conclusion seemed to be that tree ring size was more related to moisture, as long as the temperature was withing the plants ‘acceptable’ range.

June 13, 2008 1:22 pm

Philip_B (09:00:46) :

Also this discovery could have been made by anyone with an infrared camera. How many infrared pictures of trees have been taken and no one noticed that leaves were always the same color.

Maybe it’s because a typical IR photo looks like B&W. The ones that map IR to visible color are relatively recent and usually expensive. Here are more. The colors appearing in some of these were likely added using post-processing

June 13, 2008 3:06 pm

Actually, it is more likely because the kind of IR photograph that is commonplace is using “near-IR”, that is wavelengths just barely longer than visible red light. Temperature measurement with IR requires sensitivity to “far-IR” or much longer wavelengths. It is much more difficult to take meaningful photographs with far IR light. Until fairly recently, commercial cameras that could do this were tied up in military applications. FLIR is one of the primary patent holders, and their name reveals their military/aerospace origins.
At least part of the problem is that the sensor has to be carefully temperature controlled itself, or self heating distorts the image.
Also, one has to find materials that are transparent in the far IR range with which to construct the optics. I know that one material that works well is a KCl crystal, but it has problems with being both fragile and soluble in water. I am sure that the optical path in Anthony’s FLIR camera is not made of KCl…

June 13, 2008 3:19 pm

Other than the absolutely astonishing and fascinating finding, I’m not sure what this will actually affect in the record, nor which way it will go. But it does bring into question yet again our understanding of the historical temp record.
Within the last few thousand years, were temps more or less variable than we thought? The warmists tend to say they were less variable, the skeptics more. And with the discovery (not quite the right word) that the sun’s irradiance varies less than thought, our entire understanding of this history is clouded. With all this new research going on, we know less than we ‘knew’ before.

retired engineer
June 13, 2008 5:41 pm

Memory fails me, by someone’s Law of Minimums says plant groth is limited by what it has the least of. Moisture is one of the obvious limits. ’33/’34 were very hot and very dry. Trees didn’t grow much then.
As for IR camera lenses: Zinc selenide works fairly well at long IR, rather soft. Germanium works, but with a very high index of refraction, so lenses are very thin, and fragile. Good old salt works as well, but passing cattle tend to lick your optics.

Bill in Vigo
June 13, 2008 8:29 pm

I guess this explains why it is cooler under the old walnut tree in the dog days of summer when it is 95f/33c outside. I would think that the leaves would cool the air and the cooler air would “fall” to the ground and cool us. I always wondered why folks would plant walnut trees around the house in the old days, they make a terrible din when the nuts fall in the fall.
Now I know.
Bill Derryberry

June 13, 2008 9:27 pm

Helliker and Richter debunked the theory that O isotopes in trees rings are related to the temperature at which the ring formed. That theory has been widely used to do proxy estimates of past climates. Now we know the correlation was false, and those studies are largely bunkum.
Tree ring width also has little correlation to climate. First, trees are not machines operating in isolation. The proximity of other trees and competing vegetation has huge effects on diameter growth, regardless of climatic factors. The amount of competition can change over time, especially over longish periods like decades.
Second, each tree ring is cambial growth, right under the bark. As the diameter expands, so does the surface area of the tree. Each ring is actually a sheathe of new growth, like an elongated cone. If volume growth was a linear constant, tree rings would shrink in size every year, as they are formed farther and farther from the pith.
Third, volume growth is not linear, nor is diameter or height growth. Instead all three follow (generally) sigmoid growth curves. Maximum growth rate in height, diameter, and volume occur fairly early in a tree’s life, within the first few decades. After that growth diminishes in all aspects, even though the tree might live for centuries.
Fourth, external events occur, like fires, neighboring tree mortality, insect attacks, root damage from soil creep or burrowing animals, etc. that have nothing to do with climate.
Taking all these conditionals into account is very difficult, and hence climate conclusions from tree rings have large uncertainties. Unfortunately, those uncertainties are rarely considered or expressed in tree ring – climate studies.

Roger Carr
June 13, 2008 9:31 pm

An article in the New York Times within the last seven days had a fascinating story on the “electrical signals” in plants, and the capacity of some plants to move tendrils around in a specific search… Whatever the discipline of the study is called, it seems to genuinely infuriate those who study man and animal to the point where these ask in fury: “Are you looking for a brain in plants%”
This discovery relative to leaves and their capacity to control their “body” temperature would seem another step forward (almost a “brain” ) in the direction of both understanding and enlightenment as to the wonder of all things (bright and beautiful, all creatures great and small).
p.s. The “%” used above was because I could not find an interabang (?! combination) on the keyboard; and one was required.

Steve Keohane
June 13, 2008 10:04 pm

DAV, I had some color video made of Si wafers heating to about 100C to analyse any temperature gradient. I don’t know what equipment was used, but it was on hand, and highly portable, in our manufacturing plant circa 1988.

Roger Carr
June 13, 2008 10:07 pm

I found the plant story I mentioned above. Fits well with this thread’s “The internal temperature of leaves…
“Plants,” Dr. Dudley said, “have a secret social life.”
Loyal to its roots
CAROL KAESUK YOON – N Y Times, June 10, 2008

June 13, 2008 11:17 pm

Has ring width as a proxy for temperature never been benchmarked?

Kim, as close as I’ve seen it get is the guys exclaiming that “Tree rings aren’t working now.” When asked that question. Their theory is, of course, that man made influences have thrown off the tree-mometors. My theory, of course, is that their work has been bunk the whole time! ;0) This was from a realclimate thread, btw.

June 14, 2008 1:06 am

Who wudda thunk it………. Trees are WARM BLOODED!!!!
Cool!.. er… Warm…um….. aww now I’m confused…
Jokes aside…. I love it when real science wins out… and nifty bits of knowledge spring forth from the darkness…. Great stuff guys.

June 14, 2008 1:25 am

To Poetsam…. In defence for Pam, in posts above…. ’tis yor knight in shining armour, that plucks the broken legged lass safely ‘pon a mighty stead…
…. and take this foul sam!
He’s a poet and thinks he knows it,
and though ’tis a groove he’s in,
all that rhymes …. is not all reasoning.

June 14, 2008 4:58 am

It’s these moments, these special incrimental steps in knowledge, which keep me coming back to the global warming topic.
Everyday is an epiphany. I feel like Taylor, shaking the doll in Doctor Zaius’ face.
“Would an ape make a doll… that talks?”
Here’s a question. Would a tree… nay, a whole canopy of trees… no, a whole forest of trees… a whole continent of trees… would a whole planet of trees, have developed a temperature control mechanism… to cope with a warming which is unprecedented?
Anthony, could you make a special category to keep this post safe and available. I have a feeling that the SacBee is going to run some “climate change is killing the forest” reports in the near future, and it would be a shame if I couldn’t reprize this epiphany, for all the people of Sacramento to see.

June 14, 2008 6:15 am

J. Hansford
My poem though it rhyme
is not without reason.
If Pam I not like
I would not be teasing.
Oh, well. They can’t all be good.

June 14, 2008 6:43 am

Believe it or not, I’ve wondered about this recently while thinking about the effect of solar radiation on solid surfaces. Surfaces generally get “hot” while exposed to the sun, unless they’re white or reflective. Leaves (including grass-blades) aren’t either one, so I wondered why they didn’t get hot. As an engineer, I figured offhand that it had to be water evaporation — a wet sponge in the sun (even if colored black) won’t get hot either, until it dries out.
This helps explain why a forested area, even w/its low albedo (dark) surface, still remained much cooler than surrounding open/urban areas which usually have higher albedo. When I lived in a dense forest during the worst heat-waves the air temp could be 10F deg cooler than nearby urban areas — say 90F compared to 100F.
Ben Flurie

June 14, 2008 8:05 am

It’s as though they had never heard of evapo-transpirative cooling!
It’s interesting b/c I read a paper a while ago on the use of light oxygen proxies to describe the amt of water vapor locked up in the more-arid ice ages. The release of water vapor tracked more consistently with the paleo temperature record than CO2 levels.
( superimposition of the light oxygen chart over the vostok chart).

Mike Bryant
June 14, 2008 8:47 am

This bit of knowledge is so incredible. I had to go touch a leaf here in 85 degree south Texas…. cool….

June 14, 2008 12:17 pm

This discussion brings a whole new meaning to the “old” song. “In the shade of the old Apple tree”.
Can a tree now be considered a “natural” air conditioner, in addition to providing shade when it is hot out?????
Just wondering!!!!!!

retired engineer
June 14, 2008 3:49 pm

“I think that I shall never see
a poem so lovely as a tree”
With apologies to both poetSam and Kilmer

Pamela Gray
June 15, 2008 4:10 pm

Mike, I am out in the woods regularly at logging sites (my boyfriend works on broken equipment). I love your post about all the things that can affect tree rings. I always study the tree rings when I am at logging sites. You can easily see what you are talking about by looking at the cut ends in each stack. There are similarities there that are not necessarily present in the next stack at another logging site. Older sites, cherry picked sites, clear cuts, young sites, lower vs higher elevations, fire, dryer location, wetter location, shade, sun, etc, can be deduced by looking at the tree rings at each site. Fascinating.

David Segesta
June 15, 2008 4:16 pm

This is certainly an unexpected result. A tree leaf has a very high area to volume ratio. That’s the way you would design something if you wanted it to transfer heat to or from the air, like a cooling fin. Its not the ideal shape for maintaining a temperature that is different from the air. Also, unlike the polar bear the leaf does not have a coat of insulating fur.
Anthony you should be able to test this with your infra-red camera. If you get the same temperature reading then maybe I’ll have to change my opinion. In the mean time I’m going to go out in the back yard and feel some leaves.
My neighbors are going to think I’m crazy.
REPLY: Way ahead of you. (the camera, not the crazy behavior)

Pamela Gray
June 15, 2008 4:51 pm

To duplicate this experiment, you would need the same 39 species at the same altitudes, measured in like manner. Doing less may increase your odds of hitting an odd data point that may not match the conclusions. I realize what I am saying is that anecdotal evidence proves nothing. Which I agree with, and you never use anecdotal evidence after your initial observation of said anecdote to prove or disprove the hypothesis.

Mike Bryant
June 15, 2008 5:14 pm

I’m gonna put a fan in the branches of my pecan tree and point it at my patio…

Evan Jones
June 15, 2008 7:38 pm

I’m going to go out in the back yard and feel some leaves. 🙂
My neighbors are going to think I’m crazy.

Better watch out or you could be the next star on YouTube come Monday.

June 15, 2008 9:12 pm

Leaves that are green are in the air seen
leaves that are brown are oft on the ground.
But leaves that are cool
seems the general rule.

June 16, 2008 6:55 am

What happens at night? Do the leaves assume ambient temperature?

Gary Gulrud
June 16, 2008 7:19 am

Adding to Mike D.:
Most dendro series include trees as young as 60 years, that can be ‘uniquely fit’ into the chronology at a number of places.
Most are proprietary and subject only to review by trusted experts. Wegman showed the interdependancies of such networks to be imprudent.
Keenan at ‘informath’ has a paper or two on 14C dating that also point up the very limited geographic scope of the dendro series when used to ‘calibrate’ other proxies.
Better than SWAG, but unrepeatably so.

David Segesta
June 16, 2008 8:31 am

Anthony, I think the outdoor air temp right now is probably quite a bit hotter than 21.4C in your neck of the woods. So you should be able to read that difference easily. My efforts at feeling leaves was rather unproductive. It seems my fingers are not calibrated that accurately. So I await the results of your tests with bated breath …or is it bad breath? Well whatever.
I guess you’ll have to wait until winter to see if the leaves can heat themselves.

June 16, 2008 9:36 pm

Poet sam…. LoL…

June 17, 2008 7:13 am

Takes bow, falls over 🙂

June 21, 2008 7:59 pm

[…] Tree Ring theory upon which Mann relied was proven wrong, as tree leaves maintain a constant internal temperature of […]

July 7, 2008 9:31 am

[…] links today to Anthony Watts’ excellent blog ‘Watts Up With that?’. The first, highlights new research with potentially huge implications for how past climates are measured, […]

October 30, 2008 7:08 am

well, i was just searching about leaves internal temperature then this page came out. jst wnna check cos this thing came out during the November biology o-level exam few days ago. i totally don’t know about the constant internal temperature thingy. it’s not even on the subject’s syllabus. crap. so i panicked at that time. but it looks like my answer was correct then. i mentioned about the evaporation and others. so yeah. whee~ ahaha.

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