In a recent post here at RealClimate, Simon Lewis wrote regarding a 2010 paper by Samanta et al. on the effect of single-year drought conditions on the Amazon. Samanta et al. claimed to have contradicted a 2007 paper by Scott Saleska et al., and to have thereby overturned some IPCC conclusions.
Lewis showed why Samanta’s paper did not contradict the IPCC, even if it may have correctly identified an error in Saleska et al. Now Saleska has written to say that, actually, Samanta et al.’s results do not identify any error in their work: the results agree completely. With our apologies for the journalistic whiplash, Simon Lewis and I are convinced he’s right. The more general point though, is that the the balance of evidence shows that the Amazon is sensitive to drought, and the IPCC’s statements about it remain valid.
Here is Saleska’s commentary in full
——-
Guest Commentary by Scott Saleska, University of Arizona
The title of the Lewis post (“Up is Down, Brown is Green”) is perhaps even more true than the insightful commentary by my colleague Simon Lewis indicates! The Samanta et al paper says brown, but in fact their own data (when you dig it out of the supplement) shows green, consistent with (and indeed virtually indistinguishable from) our original findings published in Science (Saleska et al., 2007).
Samanta et al. misrepresents our work on many levels (one of which is to assert, falsely, that we did not filter out atmosphere-corrupted observations when in fact we did), and we intend, of course, to present an appropriate response in the peer reviewed literature, where the technical details of our differences may be evaluated by anyone who wishes. But for the moment we will, for the sake of argument, accept their analysis at face value and ask: even if Samanta et al. are 100% correct in their critique of our methods (which we of course dispute), what are the implications? Does the alternative to our method which Samanta et al. advocate, or the recent update in the MODIS satellite data (to version 5 from version 4), make any difference for the main conclusion of our paper? With due respect to our friends and colleagues at Boston University, the answer is no, it does not.
First: the actual relevant Samanta et al data (which comes from their Supplement, Table S3) is this:
| Table S3 (Samanta et al. 2010, supplement) | |||||
|---|---|---|---|---|---|
| Year | Rain defecit (%) | Area Green (%) | Area Brown (%) | Area unchanged(%) | Area with valid pixels (%) |
| 2000 | 0.99 | 5.19 | 6.13 | 23.75 | 35.09 |
| 2001 | 6.09 | 5.15 | 5.68 | 24.24 | 35.09 |
| 2002 | 10.5 | 5.08 | 6.05 | 23.95 | 35.09 |
| 2003 | 5.34 | 8.05 | 4.12 | 22.90 | 35.09 |
| 2004 | 4.68 | 7.56 | 6.72 | 20.80 | 35.09 |
| 2005 | 87.04 | 10.80 | 3.89 | 18.98 | 33.68 |
| 2006 | 26.46 | 4.95 | 3.86 | 26.2735.09 | |
| 2007 | 41.59 | 4.76 | 6.43 | 23.88 | 35.09 |
| 2008 | 18.95 | 3.10 | 6.57 | 25.40 | 35.09 |
Note that the green area in the drought region increases to its maximum (10.8% of the total area = 10.8/33.68 = 32% of the valid area) in 2005. In other words, the Samanta et al data contradict the Samanta et al text and title (which states that Amazon forests did not green up): not only do forests in the drought region green up, they green up alot, more than any other year since the MODIS satellite sensor was launched.
Second, how does this compare to Saleska et al. (2007), which Samanta et al claim to rebut? Here are the numbers (again, taken directly from Samanta et al, Table S3 and Saleska et al., 2007):
Fraction of valid pixels in the 2005 drought region that are “green” (> + 1 Standard deviation)
Saleska et al. (2007): 34% (p<0.000001)
Samanta et al. (2010): 32% (p<0.004)
The bottom line is that their observed 2005 result (32% greenness) is indistinguishable from ours (34%). I.e. Samanta et al effectively reproduce the results of Saleska et al.
This summary response, of course, begs some very interesting questions about tropical forest function under climatic variability and change (indeed the most interesting questions of all!): what caused the anomalously disproportionate green-up in the drought region? And, even if satellite “green up” does in fact represent an increase in photosynthesis (as we think), could this in fact be a symptom of the trees compensating for the increased stress of the drought? The bottom line “carbon balance” of a tree depends on both photosynthetic uptake and respiratory losses, and it is almost certainly the case that those losses (which were not seen by the satellite) increased under the hotter and drier conditions of the drought as well.
Thus, the most intriguing idea to me is that the short-term satellite-detected green-up, and the longer term increase in net carbon loss reported in the Phillips et al paper (discussed by Simon Lewis) are not in conflict at all. It might well be that they represent different parts of a coherent forest response to drought, in which the longer term losses are larger than the satellite-detected attempt to compensate for them by increasing photosynthesis, and in the end, increased tree mortality is the result.
In conclusion I would like to reinforce Simon’s point about Samanta et al and the IPCC. More important than whatever they say about our one short paper, Samanta et al. truly and egregiously misrepresent the implications, of both their work and ours, when they claim that a single paper on short term vegetation response somehow rebuts the IPCC’s review of the large scientific literature on how Amazonia might respond to long-term shifts in the mean climate state. It is an illogical and misguided claim on many levels, one that is already and deservedly attracting the opprobrium of many of my colleagues, talented scientists who study Amazon forests and climate (see Scientists speak: Amazon “myths” are not debunked).
In sum:
— Samanta et al data show a drought region green up that is on average indistinguishable from Saleska et al (but they call it NO green up).
— Samanta et al data almost exactly reproduce Saleska et al’s most salient bottom-line result (but they say what we did was not reproducible).
— the Samanta et al paper, based on a three-month drought response, says not one word about long-term climate change scenarios reviewed in IPCC (but they advertise their analysis as “reject[ing] claims” put forward by the IPCC).
For those of you who want a bit more background on the research involved, here’s a story we published on the NASA Earth Observatory about the response of the Amazon to the wet season/dry season cycle:
Defying Dry: Amazon Greener in Dry Season than Wet
http://earthobservatory.nasa.gov/Features/AmazonEVI/amazon_evi.php
It’s also based on MODIS data, and involves many of the same researchers.
> using remotely sensed greenness
Not only remote data, they also report field results that matched it.
Here’s another verision of the recent press release (basically same text as the first version, not the current version now at the BU press site):
(hat tip to Eli, who said these easter eggs were out there to find)
http://www.prlog.org/10569878-amazon-forests-did-not-green-up-during-the-2005-drought.html
This one includes this comment about the Editors of Science:
“We actually submitted our evidence to the Editors at Science, but they decided not to seek peer-review, perhaps afraid of a controversy given all the hoopla surrounding IPCC” said Ranga Myneni, the senior author from Boston University.
A couple of interesting papers on the use of satellite imagery in ecology:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1689161/ Finds that it can predict areas of bird endemism very well, however, http://www.springerlink.com/content/q27nn14w3383710g/ sounds a note of caution after looking at the better-known British ecosystems.
I am puzzled by the amount of commentary on a single paper, well, pair of papers. It seems to me reminiscent of the media hype about some single fossil discovery which “proves Darwin wrong” or “completely rewrites the theory of evolution”.
Surely what is going on here is that studies are looking at whether you can use satellite imagery to analyse what is going on in forests as climate changes. Obviously if you can this will produce a lot more data than could be generated by lonely ecologists trudging around the Amazon measuring individual trees. But in order to use satellite imagery there has to be a correlation between climate/ecology interaction and what can be seen from space. There will obviously need to be a lot of satellite imagery, covering a range of different climatic events and forest types, and a lot of ground proofing to refine the correlation (if there is one). Surely the papers being discussed here are just a couple of early examples of the process, and have little if any direct information to offer on climate change in themselves?
[Response: Not a couple of the “early” ones. The relationship between directly sensed and remotely sensed data in ecological and earth system science is a very active research area with a nearly 40 year history dating to LANDSAT 1. The integration of biometrical, meteorological and remotely sensed information is absolutely crucial to the development of a mechanistic understanding of global change, and will only grow with time. An outstanding example of this integration is the North American Carbon Program.–Jim. correction–actually dating to much earlier than this if one includes aerial imagery–jb]
> David Horton
> surely …. Surely … little if any direct information ….
It’s not so much the paper, it’s the at least four different versions of the press release (old at EurekAlert; revised at BU; variations at Hannity and PRLog, all linked and noted above).
We’ve turned up _lots_ of papers on the subject using this same data set, by many of the same authors, in various combinations. This last one stands out to nonscientist readers I think because the PR done for it seems so out of line.
#54
David Horton is “puzzled by the amount of commentary on a single paper, well, pair of papers”.
But there’s nothing puzzling about it at all. Any paper issued with a press release that viciously attacks the IPCC is going to get a lot of right-wing media coverage, especially if the authors are available to support that message.
Well, colour me puzzled. I know satellite imagery per se has been used a long time in ecology. I thought we were into some brave new world method of analysing leaf colour or such like. What is the actual question here? It can’t just be whether forests get more green during a year of drought can it? I mean, that would be just silly, wouldn’t it? And it certainly can’t be that one forest got more green (apparently) during one year of one drought in one place therefore global warming isn’t going to be a problem for the Amazon forests can it? That would be really silly.
Ike’s ire is fully warranted: While the Samanta et al paper and research is science, the sweeping, unsubstantiated claims in the press release and Myneni’s hobnobbing with the conspiratorial radical right is anything but. I can think of no more egregious example of inappropriate politicization of science. Myneni makes Greenpeace appear downright objective in comparison.
wilt (44): Orwell would probably have used the following Newspeak line: Brown or Green, the IPCC is always right!
BPL: No, he wouldn’t. Orwell was very concerned with telling the truth.
David Horton, #57,
Why not? The obvious conjecture that the forest would brown turned out to be not only false, but reversed, because it greened. At a minimum, the experiment would have somewhat validated the use of satellite imagery as a tool to quickly, quantitatively measure drought effects, and instead it presents an unexpected new thing to study. It’s Science at it’s very best, where the Unexpected begs for further Investigation.
I previously (post #34) offered three scenarios as purposeful strategies for plants against drought (migration, hibernation, competition), of which the first and last might cause extra greening, but I’ll offer an idea for another explanation:
Drought stress kills off the older, weaker trees, which gives more room in the overstory and canopy for the younger, healthier trees, and also leaves them more of the scarce water to work with. These young, healthy trees are better at producing foliage even in a drought than the older trees were in a good year, so the drought (like natural forest fires) has just helped things along by finishing off the weaker competition. Their extra greening quickly covers the browning of the dying trees, as well as areas where the dead trees were already sparse. This process also lets slightly more mature, taller trees crowd out and shade shorter ones, which under the stress of the drought also die. As such, tree mortality is up but so is greening. So one drought every once in a while could be a good thing for a forest, by culling the herd.
FYI, this happens in the tiny forest beside my yard every year, as the oaks battle each other to see who can get taller first, while their older, lower branches die and drop off to litter my yard, and every year or two another short, loser tree just gives up and dies.
The point being… looking at the satellite imagery and color is a perfectly logical, simple experiment to perform. It was never meant to answer the ultimate question to life, the universe, and everything. It was just meant to confirm the conjecture that forests would brown in a drought and that satellites could detect this. Nothing more, nothing less, and as often happens, the answer didn’t match the hypothesis, which is exactly why experiments must actually be performed.
The only mistake here involves any scientist that chose to over reach with his conclusions, and to needlessly politicize a single, useful experiment in an already volatile environment.
[Response: Do we? I certainly don’t! Most of us at RC are pretty skeptical about these sorts of ‘solutions’.–eric]
[Response: Baloney–I’ve seen no one here proposing that, and I’ve explicitly commented that I’m against it in relation to other C mgt strategies.–Jim]
Ah, I think you need to sit down with Gavin’s book, my friends, which has several chapters explicitly boosting ludicrous CCS strategies. It’s called “climate change: picturing the science” – by Gavin Schmidt, Joshua Wolfe, Jeffrey D. Sachs. If that’s not explicit promotion of CCS, what is?
Why doesn’t RC resolve this issue and publish a post reviewing the technical issues involved in CCS? You did something similar with solar energy issues, correct?
[Response: Oh please. Now merely mentioning CCS is ‘explicit promotion’? How about you quote me ‘explicitly promoting’ anything of the sort? If you’d rather just pretend the concept or technology doesn’t exist, go ahead. – gavin]
“Their time scale is what it is.–Jim”
Yes, but it’s just not applicable to any estimation of the sensitivity of Amazon primary production to global warming.
[Response: You can’t conclude that. It may or may not be, depending on many unknowns at this point.–Jim]
> Drought stress kills off the older, weaker trees, which gives more
> room in the overstory and canopy for the younger, healthier trees
Depends on your specific situation. One of my hobbies is a forest fire restoration site that gets almost all its water as snowfall. Drought there hits the shallow-rooted younger trees while the older trees with deeper roots will survive a few dry years much better.
One of the papers you’re talking about mentions the lag time at their study site between precipitation and changes in groundwater as about a month, I recall.
Agreed, and I’m speaking from relative ignorance, but recognizing that a rain forest is very, very different from a temperate forest. Mostly, I’m just offering a possible scenario that would account for both increased tree mortality and increased greening. I do remember reading that roots in rain forests aren’t that deep, however, because they don’t need to be, and that’s one reason that rain forests are very susceptible to flipping to savanna.
It will be interesting to see, in future years, when they combine a study of more specific ground observations tied to corresponding satellite observations. I wonder if the data for that exists now (i.e. measured tree mortality rate vs. moisture level vs. greening rate for a set of pixels) so that someone just has to do the collating and thinking, or if a study needs to be designed from the ground up (so to speak).
I could also see looking at it from another perspective and comparing greening with “bluing” (i.e. “blue sky” measurements from the ground up over time). Each week, use a camera or light sensor to take pictures from the same spots, straight up, to determine the amount of blue sky visible (versus bright green, i.e. leaves with strong light passing through vs. dark green leaves which are shielded by higher leaves). I could also see this being done with cameras established above the canopy looking up into the overstory.
In a perfect world, I could even see a remote controlled or robot helicopter designed to cover a very wide area taking such measurements (predator drone and shopping mall toy RC helicopter technology put to good use).
Oh, well… it’s all just amateur musings. I do think there’s merit in the further pursuit of their “greening/browning” evaluation strategy, however. It doesn’t seem “silly” at all to me.
I response to my remark (#44): “Orwell would probably have used the following Newspeak line: Brown or Green, the IPCC is always right!”, Barton Paul Levenson wrote: “No, he wouldn’t. Orwell was very concerned with telling the truth.”
So I suppose he would have written: Brown or Green, the IPCC is always wrong ?!
Just joking.
[Response: The probability of sampling one third of your analysis area with imagery and getting a strong bias wrt to vegetation response to env. drivers is very low.–Jim]
Um, not trying to pick a fight or anything but this isn’t right, or at least it doesn’t apply to the case at hand. The 33/35% of valid pixels aren’t just a random sample. (Or at least, that’s not my understanding of what “valid pixels” means.) The key question is: is this 32% a *representative* sample of the total area? Is there some possible reason that, say, areas that react strongly to drought conditions might be less (or more) likely to turn out valid pixels (say by being situated on flat ground etc.)? This seems to me to be a fairly important question – apologies if it has already been dealt with in the comments.
[Response: I hope Scott responds, but my reading of it is that the 33.7% of the area with valid pixels in 2005 represents those locations that both fell within the drought-defined area and also were considered to be “intact” forest. 32% of that area (=10.8% of the total area) showed EVI-based greening. Some other (unstated) % of the entire area was also intact forest but did not fall within the drought-defined area, which was defined with relation to precipitation alone, not topography, vegetation condition etc. I see no evidence or reason to expect that this latter area would have a different % of it’s pixels showing greening–but it seems this could be easily checked by the authors.–Jim.
p.s.–correction, that last clause is wrong. Such a check wouldn’t address what we are discussing–any check would have to involve an analysis of the hydrological or vegetation characteristics of that area in relation to the drought affected area.]
> what “valid pixels” means
This information is available; try some of the links posted earlier.
There are a _lot_ of discussions and papers on this data set with much effort put into distinguishing pixels covering original forest, from those showing smoke, clouds, agriculture, etc. or those taken at too great an angle rather than from closer to straight down. They put a lot of work into explaining this, and different studies do it differently trying to get better information.
Here’s something (article is free):
It also shows effects on the Amazon region, though it looks to be more of a macro-study than the topic here.
#33 (OT but that comment needs a reply)
Brian Cox (and spouse?) on the atmosphere. Super photography,very interesting on Titan, some good teaching, but …..
The narrative missed yet another opportunity for the BBC to explain the CO2 mechanism properly. He ran at least three themes simultaneosly, the role of gravity in retaining (or not) the atmospheres of the planets, the roles of the atmosphere in protecting the surface from meteors and in creating the greenhouse (gh) effect.
For the gh effect he started by going to a desert in Mamibia where the temperature falls by about 30K after dark. This was attributed to the lack of water vapour; this is fair enough. His next example was Mercury which has no atmosphere at all and has a much bigger diurnal effect. Did he ask whether this might be caused by the fact that it had neither water vapour nor CO2.?. No. Perhaps that would have been too advanced (our Moon might have been simpler) Instead he switched abruptly to the other two themes mentioned above. Later he quoted the well known (rough) result that the globe would be about 30K colder without an atmosphere. The same 30 K as for the desert. The alert beginner might recognise this quantity from the earlier discussion and deduce that water vapour is the only gh gas that matters. (Sounds familar?)
To be fair he did assert that CO2 was another gh gas and that it was helping to warm Venus. But he never mentioned that much of the water vapour would condense or turn into ice without the CO2. So this version would not upset most contrarians like Lindzen, perhaps not his wife either, who comes to this ‘balanced’ if vague conclusion:
(I added the italics).
http://www.giagia.co.uk/category/science/global-warming/
[I don’t oppose everything in her article, but I draw the line when it comes to adding a link to Patrick Moore from the Great Global Swindle who uses his early association with Greenpeace to act as an extreme lobbyist for almost every anti-environmentalist campaign going.]
I remember some 40 years ago going to Seattle from California for a summer course, and being stunned by how lush and green it was. There didn’t seem to be much rain, and the summer was very sunny until very late in the day/early in the evening. (So why is Calif brown and this place green, I wondered). So I said to a resident, “There must be a lot of rain here during the winter,” and the person responded in the extreme affirmative. Later I learned that the Pacific NW Coast is a temperate rainforest area.
[Response: It has much more to do with the intensity of the summer drought and the vegetation types in the two places, than the winter precip, which can be enormous in parts of CA as well as WA (e.g. the winter snowfall record was in the central Sierra for many years before broken on Mt Baker (WA) ~10 yrs ago).–Jim]
So I can believe that there could be a greening during a higher drought summer in a tropical rainforest.
I’m also thinking that droughtish summers are going to “stress” the roots by forcing them to go deeper (a slow process over many years) — and such droughtish conditions would select for trees and plants with deeper roots. Maybe plant “stress” might be like animal exercise — and no pain, no gain.
There was an anthropological study that showed minor stress/pain on babies and children (such as bathing them in water a bit too hot for them, or other stresses/pain) makes them grow taller. Very counter intuitive. I know people are different from trees, but maybe some similar principles are at work.
Lynn Vincentnathan “I remember some 40 years ago going to Seattle from California for a summer course, and being stunned by how lush and green it was. There didn’t seem to be much rain, and the summer was very sunny until very late in the day/early in the evening.”
hahaha – That must have been the same year I moved to the PNW [from AZ], thought I’d died and gone to heaven. It rained every week or so, but only at night for about 4 months that summer, and while it’s still pretty common for summer rain to happen at night, the frequency and intensity seems much less. The change I’ve watched in this area of temperate rain forest is the western red cedars dying while the various species of fir carry on – the cedars require a more consistent high level of soil moisture and summers are getting drier. Not sure the change would be detectable from space with the mix of species, and part of the color effect would be due to increased cone production by some species when drought stressed.
It would seem to me that the Rainforests – and, perhaps, other types of vegetation – are saving water for the dry times; probably in both their own Roots, as well as in the various soil fungi and bacilli around them; and then taking advantage of the increased insolation of the (assumedly cloudless/reduced) drought time to do some real growing.
This would account for the seeming paradox of a ‘Rain’ Forest ‘greening up’ during a ‘drought’, nicely.
It’s not just rain that falls from the sky – not to a Tree.
James Staples (#72), I think you are right, this could nicely explain the paradox. In addition to the increased temperature that goes with the increased insolation, the increase of light intensity is probably at least as important (after all it’s called photosynthesis for a good reason!)
Thanks for the information — I always like to learn more.
Another thing I was thinking is that the harms we are doing to the rainforests and earth in general — to all the myriad of species that live in those habitats — is very very sad. How can we explain to the animals, for instance, what we are doing and why.
We have this standard of “pick on someone your own size [and I would add ‘your own intellect’].” I’m not suggesting that we should not use the biota of the world for our own sustanence and well-being, and spiritual uplift (of enjoying the beauty of nature). However, simply destroying it because we have to blow-dry our hair instead of let it dry naturally and a myriad of other frivolous actions, makes it all the sadder. How do we tell the animals as they die and go extinct at our hands? How do we tell the plants?
Pixel-based image analysis is the past, GEOBIA (geographic object-based image analysis) is the future.
[Response: I never said that image analysis had to be pixel based–but I very much agree with your statement as a broad generalization, and am very interested in object-based analysis, particularly w.r.t. image segmentation employing textural information and high spatial resolution (<= 5m) imagery, for structural analyses. R seems to be very lacking in existing methods although SciLab seems promising at first glance. GEOBIA looks interesting–thanks for the info.–Jim]
I must say, am truly impressed with the level of commentary here at RealClimate, there is a real engagement with the issues, including by Robert Simmon of NASA who generously refers Real Climate readers (in comment 51) to a rather nice piece he put together reporting on the work of my collaborator Alfredo Huete and I (http://earthobservatory.nasa.gov/Features/AmazonEVI/amazon_evi.php), and some of our prominent tropical ecologists like Steve Mulkey (comment 31), who cites his group’s important high-profile Graham et al 2003 paper in PNAS about how tropical trees are light-limited during the wet season. Incidentally, 2003 was a good year for advancing our understanding of controls on tropical forest productivity, seeing the publication not only of the just-mentioned Graham et al study, but of Nemani et al in Science (a remote sensing-based modeling study suggesting the Amazon was light llmited) and our previous Saleska et al, also in Science (an observational study that saw unexpected seasonality of CO2 fluxes in an Amazon forest). Of these, I would say the Graham et al was perhaps the most compelling, as it was based on actual experimental manipulation (they installed high-intensity lamps above the forest to augment the incoming radiation).
It would be difficult to address all the questions that have arisen here, so maybe the best thing would be if posted more of the details about this “controversy” (more like a tempest in a teapot) on my website, which I should be able to do in the next few days. In the meantime, let me just quickly say:
1. About “valid” v. “invalid” pixels, asked about by many: this is indeed one of the technical disputes between Samanta et al and Saleska et al. They say you have to throw out 60%, whereas we think you can actually keep a much larger fraction of the drought area (something like 90+%) than they do. There will be more details about this in our response paper, but as I indicated in my original post: it doesn’t make much difference in the results, because whatever you use as a criteria for “valid” pixels, you still see an anomalously green drought region.
2. About Ranga Myneni’s comment that “perhaps” Science didn’t publish their critique because Science was afraid of a controversy” about the IPCC (comment 52). We need not speculate about Science’s motives, because they told Ranga and me why they declined to publish his comment critical of our work. In the words of the Associate Editor who handled it: “the arguments presented in the comment, which were well addressed in [the Saleska et al] response, in the end did not pose a sufficiently robust challenge to the main conclusions of the original report.”
3. About what is perhaps the crux of the issue, highlighted in a colloquy (comment 49) between commenter Andy and RealClimate contributor Jim, where Andy quotes are 207 Science paper conclusions: “These findings suggest that Amazon forests, although threatened by human-caused deforestation and fire and possibly by more severe long-term droughts, may be more resilient to climate changes than ecosystem models assume.” The context to this conclusion is the Hadley Center model in particular, which has been used to make both short-term and long-term predictions, and made famous the now widely-replicated prediction of Amazon-forest die-off under climate change. The motivation and purpose of our 2007 Science paper was to test the short-term prediction of prompt slow-down in forest photosynthesis following drought. Our observations tended to falsify that short-term prediction (at least if you accept that satellite EVI correlates with photosynthesis, for which I think there is pretty good evidence).
Our observations implied to me that forests were more resilient than the model assumed, at least in the short term. But what does that imply about the long-term prediction? That’s a more complicated question. In the model world, the same mechanism (strong water limitation of forest productivity) is implicated in *both* the short-term prediction of photosynthetic slow-down and long-term die-off. In the model world, greater short-term resiliency also implied greater long-term resiliency. But it could be that in the real world (as opposed to the model world) that forests are more resilient in the short term but still vulnerable in the longer-term. In fact, I think the accumulating evidence from observations and experiments is consistent with this interpretation. Results from forest drying experiments (e.g. Nepstad et al 2007, cited in our Science paper) showed that forests did start to experience big mortality increases following drought – but it took several *years* of drought, not just a few weeks or months as suggested by the models. Likewise, the Phillips et al results discussed by Simon show elevated mortality during a several-year interval – not inconsistent with our observed 3-month green-up. Since several years is still very short compared to long-term climate changes, it seems that the balance of evidence now is that forests are more resilient in the very short-term, but still vulnerable on the timescale of years to decades that is relevant to global climate change.
[Response: Scott, many thanks for your contribution and detailed responses to questions/comments, as well as those by Simon and Steve (and also many good reader comments/questions). Topics like this really help educate everyone on the research process, as well as important results, of highly important investigations–exactly what we want to do here–Jim
p.s. this is not to be taken to mean the discussion is now over!]
“the increase of light intensity is probably at least as important (after all it’s called photosynthesis for a good reason!)”
Not necessarily. Plants have in general evolved to optimize their photosynthetic efficiency at some optimum, not peak, intensity that they experience in their particular ecological niche. I know more about algae and duckweed, not rain forests, which I am interested in for biofuels. Many species of biofuel useful algae saturate the photosynthetic mechanism at about 20-30% full sun. Stirring a raceway so that the algae move vertically in a water column or distributing them vertically in tubes in greenhouses so that they self shade is how 200 tonne dry matter yield per hectare can be achieved. See http://www.ncbi.nlm.nih.gov/pmc/articles/PMC541035/pdf/plntphys00359-0031.pdf for groth curves.
Duckweed has to float on the surface, so it is harder to manage for maximum yield. Frequently companion crops that partially shade the pond surface are grown to productively use otherwise excess photons.
“Bamboo, for example, grows well in a wet environment
and has market value as a structural material. Planted along the
perimeter of a duckweed culture plot, bamboo will diffuse the
wind and filter sunlight during hot, dry weather. When the more
moderate and cloudy monsoon season begins, the bamboo crop
can be thinned to allow more light on the duckweed crop and sold
to increase cash flow.” http://www-wds.worldbank.org/external/default/WDSContentServer/WDSP/IB/1993/03/01/000009265_3970128103342/Rendered/PDF/multi_page.pdf
Brian Dodge (#77), thank you for your interesting remarks. But wouldn’t you agree that when as a result of dying trees holes would form in the canopy this would provide more sunlight to the area below and bring sunlight intensity closer to the optimum for several plants and small trees? In other words some trees may die but overall the greening may remain more or less the same?
“In other words some trees may die but overall the greening may remain more or less the same?”
Sure, especially if the ecosystem is complex, and the changes are small. A shift in rainfall just large enough to kill a few susceptible members of the most sensitive species won’t (by definition) be large enough to disrupt the ecosystem; tolerant organisms will increase their growth in response to the removed competition. An ecosystem evolves to optimize resource utilization in response to both the average conditions and normal variability, just like a species evolves to the conditions in its niche. The ecosystem in my part of NC, where we get about 1 meter of rain per year, but only a 2:1 monthly variation, is very different from places in Northern CA with similar annual rainfall, but very dry summers. The change that I’ve seen in the climate in the 40 years I’ve lived here towards drier summers have shifted the forest balance towards more pines and fewer oaks and magnolias. In the clay soils here pines typically have more robust root systems, so they are more drought tolerant, and More evergreens might give a “greener” yearly average satellite image. There is a very diverse forest in my neck of the woods – 14 species native on my 2 acre lot – which limits the spread of pests like pine borers. In western NC and the sandhills, where there are larger percentages of pines, drought stress plus easy spread of the pine borer has resulted in larger mortalities in pines. In oak dominated woodlands (typically deep unsaturated loamy-sandy soils) drought has increased the effect of “oak decline” from a complex interaction between weather, insects, and fungus. One drought year is going to have a small effect on a robust, complex ecosystem that experiences occasional drought, and may have counterintuitive effects on the chlorophyll measured by satellite. 5 years of drought will have a much larger effect, and if 5 year droughts occur every 20 years instead of every 100 years, the effect will be greater still. If the droughts become severe and frequent enough civilization collapses – google anasazi+hopewell+drought; it’s never just the drought, because the drought doesn’t occur in a vacuum. There are always pine beetles, and fungi, and soil type, and warfare, and dependence on corn, and so on that vary spatially (the realtors are right – location, location, location!). Arguing about which is most important in an attempt to discredit the importance of something you like (burning fossil fuels) is as ridiculous as arguing “which is more important if you get shot in the head, the gunpowder or the lead?”
Brian Dodge (#77), thanks again, it’s a pity that your contributions did not appear earlier during the discussion because in my view your observations make sense. I had underestimated the ability of the ecosystem to adapt, certainly when the drought is short-lasting like in 2005. It suggests to me that none of the articles that try to extrapolate from the 2005 obeservations make much sense in relation to projections of long-term droughts, and the discussion in this thread about increase or decrease (or no change) in photosynthesis in 2005 is largely academic.
The “holes in the canopy” discussion triggered my memory from Ecological Anthropology. Swidden horticulture (also called “shifting,” and in some cases “slash & burn,” with the ash putting nutrients into the soil) is adapted to rainforests because apparently the soil is very poor, and the nutrients are in the plants & canopy. I can’t remember if it’s due to the heavy rain washing out the soil’s nutrients, or lack of decomposition. Anyway, to grow crops in one spot more than 1 or 2 seasons would degrade the soil, and continuous agriculture could turn it into moonscape. At least that’s what I learned.
Well, when tribal peoples practicing swidden plant a crop they cut down the trees in that area so sunlight can come through to let their crops grow (also to clear space). After the harvest, they then move on to another spot, and the rainforest in the original spot then takes from 6 to 25 years to grow back to how it was, after which they can again grow crops there. Also, I’ve seen films on how they plant — they do so without disturbing the soil and leaf litter very much.
The point is, from what I learned, rainforest soil is poor, and if some trees die out, then my guess is that the area there would not thrive for long. When the rains do come the soil would probably get degraded in those areas so that the rainforest there would not easily “pop back” to life. So perhaps (I’m no expert), that type of situation in which increasingly more dire and more frequent droughts (which might happen with global warming) would slowly kill off small stands of trees here & there in the rainforest, causing it to shrink and shrink in total area, and be replaced by savanah or something else less rich in life.
I also understand from someone who lives in the arctic that the soil there is also poor. So the idea that agriculture will just shift north with the onslaught of global warming may not be as rosy a picture as some make it out to be.
Now if you add all the other assaults on the soil and growing conditions in rainforests and elsewhere (I understand we Americans have used up over half of our rich, very deep topsoil in 200+ years), then the situation for the future looks still bleaker.
“I can’t remember if it’s due to the heavy rain washing out the soil’s nutrients, or lack of decomposition.”
From what I remember of physical geography, it’s the rainfall leeching.