In this podcast, I had the wonderful experience of talking with Douglas Sheil, professor at Wageningen University in the Netherlands, about forests. Douglas's academic adventures took him on a journey from his homeland to places like the rainforests of Indonesia, where he studied how local communities can help protect forests. He has studied forests in many forms of their complexity and wrote a well-regarded textbook on tropical rainforests. He became interested in questions of forest and water and helped clarify a big issue in the ecohydrology field about whether trees were contributing to or depleting the water in ecosystems. His results showed that when tree cover is done right, and not as dense invasive monocultures, then there would be an increase of water in the ecosystem.
As he worked on forest water issues, he discovered Victor Gorshkov and Anastasia Makarieva's work on the Biotic Pump, and collaborated with them on explanations of their theory. Here an explanation he wrote about the Biotic Pump:
“For centuries we’ve believed that temperature differences generate the pressure gradients that drive winds and carry moisture inland. The sun heats the land, warm air rises, drawing in winds. These winds carry moisture laden air inland, where it eventually warms, rises, and condenses as rain. There are complications, such as Earth's rotation and atmospheric cells, but overall temperature differences underpin our understanding of how wet inland areas of the planet stay wet. But there’s an alternative: the Biotic Pump. An idea developed and championed by colleagues Anastassia Makarieva and Victor Gorshkov. Given common misunderstandings, I thought I would try a simple intuitive summary without equations.
Here goes: Imagine two vast areas of atmosphere next to each other: one wet and one dry. Both are at equilibrium with the same surface temperature. Atmospheric pressure follows a roughly similar near exponential decline with altitude in both, but there are small differences. In the wet column, the presence of moisture means that air pressure is slightly lower (compared to the neighbouring dry column) in the lower atmosphere (up to 3-4 km height) and becomes greater above that. These pressure differences mean that if the columns are brought together, and the pressure differences maintained, a circulation is established: At the surface, air moves from the dry to the wet area. At higher altitudes, it flows in the opposite direction. This circulation causes air to rise over wetter regions and descend over drier regions. In the real world, “dry” areas may be just a little less moist. As long as it is sufficiently moist the moister region draws in air from surrounding drier areas. As this incoming air rises, it cools and reaches saturation, producing rainfall. This process maintains the moisture contrast with neighbouring areas, where the now-dry high-altitude air returns and descends. As long as the wet column stays wetter, and has sufficient moisture to maintain the pressure differences, the circulation continues.Forests are key here. Forests generate water vapour more effectively and rapidly than most other land cover types, maintaining a moist atmosphere that is effective at drawing in air from elsewhere and sustaining this circulation deep inside continents. Energy derives from the sun evaporating water vapour. Heat energy still features (because condensation releases latent heat), but in this case overall pressure differences depend on condensation and the removal of water molecules from the air.”
For more info here is a link to a paper where is a co-author with Makarieva & Gorshkov.
Douglas Sheil and I discussed a variety of topics, like how forests evolve, how to restore forests, and his work on the intermediate disturbance theory for forests. Here’s an abridged edited version of my interview with Douglas Sheil:
FOREST ECOLOGY AND WATER SYSTEMS
Douglas: My background is very much as a forest ecologist, somebody who's been looking at how we can do conservation, particularly in remote parts of the tropical rainforest. How can we actually work with communities, for example, to actually protect these incredibly rich communities that are often under threat from large-scale industrial transformation, etc? My preoccupation has often been with the biodiversity, the rich species richness of these forests, which of course is famous.
But if you're working with communities and stuff, talking about things like water, that's something that really matters to everybody. Everybody needs reliable water, even in wet parts of the world. If there's a big drought, that's a problem. So everybody cares about water. So I guess I've come into the water topic, partly because I see it really matters.
Alpha: You grew up in Ireland. Did you then get into forestry immediately, or was it a little bit of a winding route?
Douglas: It was winding. When I was young, I thought I was going to be an astronomer or a physicist, because I kind of like these elegant relationships. So I've always had a broad interest. But I think I got jealous of some of my colleagues when I was a student who went on these amazing field trips and stuff. So I thought, well, I really want to do that. That looks like way more fun than living under fluorescent light bulbs for the rest of my life. So I was kind of doing that in my holidays. And then I thought, how can I make people actually pay me at the same time to do what I do in my holidays? So that's kind of been my career trajectory. And I guess I've been super lucky, because it kind of worked out for me.
Alpha: Cool. So when you went to graduate school, which department were you in?
Douglas: At that point, I had done molecular biology. And then I actually went to a course called Forestry and its Relation to Land Use at the University of Oxford.
It doesn't exist anymore. But it was a pretty practical applied kind of research. And I thought, OK, this is a real change of direction, but really exciting. I'd done a bit of holiday work in various remote places. And I thought I really need a solid grounding, some kind of expertise that people are going to employ me for. And then I did a PhD also in Oxford on long-term change in tropical rainforest.
Alpha: Cool. And then you ended up writing a textbook about rainforest, right?
Douglas: Yeah, that's right. So that was about 15 years ago. I did it with Jaboury Ghazoul - he's at ETH in Switzerland. I'd known him previously. We had worked together a little bit. So that was a real opportunity to actually think more broadly and more widely about tropical rainforest and how they work and why they matter and what we can do to try and protect them in the future. So that was a really nice opportunity to really look around these topics. And that's actually also when I got very much more interested in this forest and water topic.
Alpha: As you go about your research, how do you select your problems and how do you then try and tackle them?
Douglas: I guess I've been lucky that it's a mixture of curiosity and practical opportunity. I guess a lot of my career early on, I wasn't actually in academia. I spent 10 years in Indonesia with a place called Center for International Forestry Research. That's very applied work, really trying to look at how we could protect forests and also protect the poor and vulnerable people who depend on them. So that was very much a goal for 10 years. I really appreciated that because I think that's so fundamental to what we're trying to do to protect forests.
It can't be ignoring the people, it has to be including the people. So it was a real opportunity to go and spend time with these communities and with teams of researchers. I was leading teams of researchers there and trying to look at how we could come up with ways to protect these forests in the long term, that the communities themselves would support. Maybe just to clarify that so much of the cost of conservation around the world is the squabbling and the fighting and the conflict with local communities because the communities themselves are not on board with this. And I think it's a huge opportunity and a huge mistake in the first place to have done that and a huge opportunity to turn that around.
Because actually if you work with the communities, particularly in forest-rich parts of the world, like Indonesia, often the communities themselves are also very keen that these forests are protected in the long term. So if we actually ask them how, where, you know, that they would be supported for that, that's such a valuable opportunity. And it sounds very simple.
BIOTIC PUMP
Alpha: And then how did you get into forest and water and what was the research question you were looking at when you started out?
Douglas: I had written this book on tropical rainforests. And one of the chapters that I had decided I was going to draft was about the various services that come from forests. So I was reviewing different articles and reviews particularly and interesting ideas that maybe should go into this chapter. And I found this article from Anastasia Makarieva and Victor Gorshkov. And I remember reading and thinking, oh, this is really interesting.
I hadn't heard of this. But I put it in a pile for once that I should talk to people who know these topics better. So I talked to various colleagues who had worked on trees and water-related topics. And yeah, it was kind of quickly clear to me that the different responses I got around those ideas. So I should say I'm talking about the biotic pump, the idea that forests actually can create these low pressure areas that draw in winds and moisture, that they actually play an active role in these atmospheric processes.
This idea was super interesting. But I hadn't really heard it talked about. So asking colleagues, I quickly got very different answers from different people.
Why I should dismiss it. You know, oh, if this was true, we would know already. Or yeah, it's true, but it's a small effect. And you quickly realized there's a lot of conflicting opinions out there which can't all be true. So it made me more curious.
So I took more and more about this and realized that really this was quite a novel idea. It wasn't really in the climate models, but that if it was true and it seemed to me plausible, if it was true, then it was super important. So I used to say to people, I don't know if this is true or not, but if it is true, this is super important, we should find out. So I spent quite a lot of time and effort in trying to promote that. And I wrote a popular article about it at the time in Bioscience. Trying to just promote how important this theory was if it was true, not saying it was true, but saying it looked fairly plausible and should be taken seriously.
Alpha: And you reached out to Makarieva and Gorshkov, right?
Douglas: Yeah, at that time, I also wrote to them. And I think they were away, you know, they spend a few months every year in Siberia offline, recuperating and brainstorming with each other, I understand. So at that time, they were offline.
So it was only some months later after the article was really impressed that they finally got back to me. And I think they were happy that these ideas were gaining traction. I think what I saw at that time was part of the problem for people was physicists, Anastasia and Victor, they're physicists.
So they were very used to writing in the way that physicists would approve of, you know, in terms of differential equations and making the approximations that physicists maybe would accept. And they found that a convincing argument for a lot of people who work in these topics, the hydrologists or whatever, of course, this is totally over their heads. We can't really say ourselves whether this is convincing. There is an assumption that the physics itself would be already well sorted. You know, it's in the climate models, it gives convincing answers. So obviously we don't need this additional insight.
We don't need these additional mechanisms. So I think that was interesting. But I also found that they have published these ideas in physics journals.
I think that's something important to highlight because people say, well, you know, if it's true, then they should publish it in physics journals. And the answer is they have. They haven't had any pushback from the physics community. They've had pushback from the climate community. And that was basically because these ideas are not properly represented in the models. I should say, I mean, it always sounds a little bit problematic for people when you start criticizing the climate models, because I know there's this narrative of the climate change deniers.
And I should say it's not about that at all. It's about whether the climate models are really capturing the key mechanism. It's not about denying climate change. I think often when I'm talking about this, I have to be very careful that the climate models themselves recognize there's all kinds of discrepancies and details missing in their models. You know, they have to simplify to make it viable. And when I talk to the people who make these models, they're quite sympathetic individually.
But they will say, well, you know, join the queue. There are 150 other things they're working on, whether it's sea ice representation or the proper resolution of vortices in the atmosphere or whatever. There are all these topics that other people have highlighted as needing more attention. And the Biotic Pump to them is just another one in this long list. But I think for the climate strategy people, the bigger picture stuff, it is challenging because if a lot of the models are wrong, then a lot of the predictions potentially are wrong.
Alpha: Can you describe the Biotic Pump for those listening?
Douglas: I guess it's good to start with the traditional idea of how wind circulation works on the planet is you have areas where air is warmer and air is colder. And if you can think of a hot air balloon, why does the hot air balloon rise? It rises because the space that's taken up by the hot air is larger than a similar volume of cold air. And that means it's lighter per unit volume and that air will rise. So the hot air balloon is capturing this low weight volume of air and using it to rise, that column of air, which is warmer rises.
So across the entire surface of the earth, the areas which are warmer, typically the air is rising and where the air is colder, the air is falling. There's this constant circulation going on. And that's pretty much how climate models understand circulation.
It's all about temperature. In the Biotic Pump, we say there's an additional feature, which is that when water evaporates and condenses, there's a change in the number of particles in the atmosphere. So when you have more humidity coming off a wet surface, you're adding to the number of molecules in the gas.
And when you have moisture, humidity, condensing back to water or to ice in the atmosphere, you have fewer particles. And what we're saying is that also has an effect. And of course, physical recognition of how things work. Of course, it has an effect. There is an effect. But traditionally and historically in the models, it's been considered to be a small and possibly negligible effect. And what Victor and Anastasia did in their studies was saying, actually no, because a lot of things cancel out in the expressions. Actually, it becomes a dominant effect in certain circumstances.
And it can actually explain certain kinds of patterns that are otherwise difficult to explain. And I guess, I don't know how much you want to go into the physics on this, but the point would be where the air is typically condensing more often, you'll get low pressure on average. And that means air is generally moving to those places. So very moist places, very wet places, where there's a lot of rainfall going on, will naturally draw in moist air from other places. We call that convergence, the idea that there's moisture coming in, the air will be rising, rain will be falling. So you have this active process of moisture being taken from certain areas to other areas, particularly towards wet areas. It's a very clear example of a positive feedback. Wet areas get more rain because of this Biotic Pump effect.
Alpha: Yeah, cool, well, that's a good summary. Yeah, and I think part of it was just this mechanism at first was a bit strange that the water vapor, when it condenses, was creating enough pressure that it was creating an effect. And yeah, so I think normal climate scientists thought it was much smaller than latent heat, but when you take out the whole circulation where the latent heat might impede the downward trajectory of the water, I mean, the air circulation, I think they were saying that if you take that into account, then actually the partial vacuum effect becomes important.
Douglas: But I think what was interesting, and maybe it's one of the details that often doesn't get so much attention, is one of the comments I got from colleagues earlier on was if you really want to show this matters, you have to show that the current models or the current understanding is somehow inadequate.
You have to show this gap. And that is something that we did, or I should say Anastasia and Victor really did, showing how the actual profile of air pressure in the current atmospheric circulation doesn't really fit with what the current theory, so the traditional heat-driven theories would tell you you should see, whereas their model, their understanding actually gives additional insight. So there are various cases like that, also in tropical cyclones, so these typhoons and tropical storms that are rotating systems, also showing that the physics that they're talking about, this condensation effect, actually makes a difference, which is actually allowing you to better fit what you observe. And I guess in science that's key. It's not enough to have good ideas that make sense or are plausible, but actually showing that it fits data. So I think that's really important to underline, because I think a lot of people who look at this, the hydrologists who look at this, they maybe read one or two articles, but there's a whole set of studies out there showing that there is a need for this. There are patterns where this idea actually is helpful, where you provide a better fit to what we observe.
Alpha: Right, yeah. So yeah, so they call this condensation-induced dynamics, or kinetics, I think, and so they were looking for other places to apply it to, right? So hurricanes also have this effect, because there's lots of water condensing there. And so they're saying that their calculations, if you use this effect, it gets more accurate experimental results of how the hurricanes actually move.
Douglas: Yeah. And initially when they were working on this, I said, you can keep me out of this, because I'm an ecologist. This is very technical physics, and it's very much about the meteorology of atmospheric systems. So I didn't play much role initially, but I did get involved later on just in helping clarify a lot of the arguments. And I do see the point, because it adds to this larger weight of evidence, which shows that these ideas make sense and are valid.
I mean, I think physically they already made sense. It was way back sometime in the 1950s when computing power was super expensive. People put a premium on making approximations and assumptions that were giving good answers. And they did that.
They made amazing models for what they had at the time. And it's only now that we come back and revisit some of these assumptions that people are super defensive about. It's like, hang on, but we don't have the same computing restrictions as we had way back in the 1950s. We don't have to make these assumptions and approximations.
What happens if we don't? And to me, this is something I've been really pushing for the last few years, really looking for people who'd be willing to sit with Anastasia now, because I should say Victor passed away a few years ago, so with Anastasia, and really work through and try and show that models that don't make these approximations and assumptions that they have made historically would actually give better results. And I think that makes total sense.
Alpha: And what's interesting, as I looked around, is that there are actually other condensation-induced dynamics that actually climate models do use. I mean, like microbursts is this phenomenon where the cold air underneath clouds condenses. And so it creates this really rapid acceleration. That's like, it could create almost like a tornado, like it hits the ground and all the vegetation gets wiped out. And so that is actually one of these powerful examples of this condensation-induced dynamics that...
Douglas: That's right. Now, when we see these things in the tropical forest, because they're amazing, this downburst, exactly what you're talking about, that they can flatten the forest over a monthly energy area. So they are occasional, but when they happen, they are incredibly impactful on the forest. Yeah. So in the Amazon, this has been known for, I think, a couple of decades. And we have something in press showing that it also happens in Africa. So, yeah, exactly that.
Alpha: And did you try to see more of this effect? So what is the impact of the biotic pump on ecology? So just looking at more the effect on the ecosystem, what would you say is the effect of this large-scale circulation?
Douglas: I guess it's tricky to summarize these things, because you take very different angles on it. I think an angle which is super important is these... What's the consequence of losing forests?
I mean, that's an obvious one, but there's this dependence, which is at a larger scale, perhaps, than most of us are traditionally thinking of. Often in tropical countries, we're happy to save any little fragment here or there, because there's important biodiversity. But what we see is that there's also real importance to maintaining really large-scale tree cover over large areas, because this is actually crucial for maintaining the climate system, which these systems have evolved under. So there is a danger when we lose too much forest, particularly in coastal areas, perhaps that we cut off a lot of the flow of moisture to the interior. So when I'm talking about interior, I'm thinking of continents like the Amazon basin or the Congo basin. So to actually get inland there, the water that's coming in has to pass over large areas of land, which is forested.
But if we lose that forest, we potentially break that circulation and could dry out those interior areas. So this dependence, I guess in a sense, people were aware of this already. This idea that rainfall is recycled, of course, is very much key. But what we're saying is it's not just the recycling, it's also these atmospheric pressure gradients that draw in the winds that are also potentially vulnerable to these processes. And I think we're seeing that already, I mean, both in the Amazon and in Africa, it's always a little bit speculative, because there are so many things going on in these landscapes. But the loss of tree cover, the loss of forest, forest. I guess something I would really want to highlight, because it often gets a bit, it starts sounding a bit depressing when I start talking through the conservation story, but there's so much we can fix with this as well, because the recognition that keeping these landscapes forested, keeping them green, particularly with native vegetation, this is a real opportunity. And just to highlight that it all is not lost in these systems, because we do have this insight how this large-scale tree cover can really be recovered and bring back these wetter and more productive systems. And that would also be true in many parts of the world, which are already dry and fairly unproductive. That you know, historically, when there was native vegetation, they were probably much more productive and wetter and moister. And a lot of this is an opportunity to bring this back. So yeah, I always want to temper the slightly tragic and pessimistic story with this optimistic one that there's a lot here we can actually fix.
Alpha: And people have raised awareness that Amazon rainforests and its impact on rainfall, but the Congo rainforest, it's not, you don't hear about it too much and its impact on rainfall, but you know, it's still a huge effect. And it's just, I think our attention hasn't been on Africa, right? You have work in Uganda.
Douglas: The data across the Congo is generally much poorer. So there's been a lot of attention to the Amazon historically and Congo by comparison, there's much less. And particularly if you want historical data series, there just isn't data over large parts of the Congo. So I don't quite know what the situation is now. But I do remember there was even a lack of clarity about where the rainfall gradients were. You know, is this area wetter than that area? It's like, well, we don't know, you know, the data is too noisy, it's too poor in general. So there was a big problem with that. I think it's much improved, but it's still only recent data now that we have.
And a lot of the satellite products are reasonably reliable. But yeah, I think we're starting to get a handle on these things better and people are paying more attention to Africa now.
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FORESTS AND THE WATERSHED
Alpha: There was a big debate in science around the forests, how they impact the watershed.
Douglas: There's been debate amongst those people who think that trees are good for water and people who think trees are bad for water. And what's really striking is it's either good or it's bad, it's never about the nuances, sometimes it's good, and sometimes it's bad.
There have been some studies with hundreds of examples, I think it was Jackson et al in Science, where they're basically showing, wherever you grew forest in their study, you had less water in those landscapes. But the problem was that even though they had hundreds of cases, these were all drylands, and they were all plantations of alien species or exotic species. And they were only looking at really dense forests.
But if you actually have native species, and often it's partial cover, particularly in drylands, with a certain amount of tree cover, it can be a good thing. We published a study in 2016, based on several years of really detailed work in Burkina Faso in the Sahel of West Africa. It's a drylands area where it's a landscape with scattered tree cover, people have little plots of vegetables, there are a lot of animals grazing around. And in this landscape, people are digging quite deep wells to access water. So a lot of work for people just in daily lives to find the water they need. Every now and then, they would have drought.
Water was a real part of the hardship of daily life. The study I was involved with publishing showed that a small amount of tree cover or some amount of tree cover has a dramatically positive effect on the ability of this landscape to retain the rainfall that falls and allow it to absorb into the soil profile and recharge the groundwater. So this is a landscape with hooved animals trampling around. The soil surface is slightly compacted by this. It creates a surface where when the rain does fall, it tends to run off very quickly and is lost from the landscape. Whereas if you actually have the right kind of vegetation, it doesn't run off as much. The vegetation doesn't have to be trees, but trees are good because trees provide other values and services as well. They are providing fuel, they're providing shade.
There are other reasons you might want trees in these landscapes, but trees make a dramatic improvement in allowing the water that's falling on these landscapes to be held in those landscapes. And what we were showing is it wasn't just a small percentage, it wasn't just like a few percent, we were talking several times, you know, several hundred percent. This is dramatic because this was something new, something new in such recent times that is such a big effect.
So the point here would be you don't necessarily need dense tree cover, you probably don't need exotic species, you just need some scattered tree cover, it has to suit the conditions. It's not like trees don't use water and re-emit it to the atmosphere, they do. This has always been what people say, oh, you always lose water, but it depends, you know, because also runoff is another way you lose water.
Alpha: So to summarize, if you have dense, invasive species, monocultures, then what happens is that the trees draw too much of the groundwater and soil water and transpire. And so then the landscape loses water. But if you have less dense polycultures, then the trees can actually allow the rainfall to infiltrate the ground. So you build up the groundwater. So actually, there's a net positive effect on the water.
Douglas: Yes, exactly.
If you plant a big plantation of Australian trees in Africa, it's often not good. It doesn't have the right phenology. Phenology is when the trees are actually leafing up when they're actually transpiring, when they're losing water into the atmosphere. If you look at native vegetation, everything is adapted to the local conditions. And particularly in these drylands, when the timing of when species actually produce leaves and how they transpire, how deep their roots are, all of these biological details, obviously, these are crucial to the survival of these trees, but they're also tuned to the local conditions over centuries, over millennia.
These communities have become very much tuned to this. There's a really interesting observation that I think highlights this tuning is in many parts of the drylands around the tropics, you have what we call monsoon weather. So a lot of the time it's dry, and then suddenly, generally around the same time of year, every year, it rains, right? And this is crucial when it rains and how much it rains is crucial. This is the monsoon systems. And what you find, in most dryland native systems is before the monsoon comes, before the rain falls, the plants are already greening up. And what we see is that greening up is using these last little reserves of moisture that have still survived all through this dry season, that the vegetation has been holding onto to allow them to produce these green leaves, these young green leaves, and they already start to transpire, even though it's been dry for maybe several months, maybe half a year, very dry conditions, but the plants are adapted to provide this pulse of moisture. And this actually helps trigger the monsoon.
Where is this happening? All across dryland Africa. So you have this very specific ecological evolutionary tuning of the vegetation, which is not only adapted to the monsoon, but also to bring the monsoon. And I think this is crucial. It's this two-way relationship that it's not just that the vegetation is a consequence of the weather, but it has interacted and evolved over time to have a big impact on these processes. So we very much believe that the monsoon is dependent on a certain level of moisture in the atmosphere to trigger it. And having a large extensive area of vegetation transpiring is very much helpful for triggering this start of the monsoon. If you don't have that, then you can delay the monsoon. Maybe when it comes, it doesn't even happen. It should have come.
‘having a large extensive area of vegetation transpiring is very much helpful for triggering this start of the monsoon’
Alpha: This is cool. I knew this was happening in the Amazon rainforest and the Congo rainforest. I didn't know it was also true of less tropical regions too.
Douglas: It's one of the reasons why we can't just replace native vegetation with pine trees or eucalyptus all over the world. I think this is going back to why some trees are better than others. There's a lot to be said for local vegetation just because we know it's worked in the past without human intervention. And I think there's a lot of, I mean, it's not so much a scientific statement, but maybe a respect statement, respecting nature has done this pretty well without human intervention.
So we shouldn't necessarily be so arrogant as to assume we can just replace this forest with that forest and it has no impact. People quite often ask me like with palm oil in Borneo. We've seen a huge decline in rainfall in Borneo, for example, with deforestation there. And then say, well, why doesn't the oil palm compensate? Well, it's a totally different kind of vegetation.
It is a rainforest palm, but the vegetation, it's behaving very differently. It won't have the same responses. It's not so deep-rooted. Yeah, it's not so deep-rooted. It won't have the same responses.
I can't say in any detail which of all these differences really matches in this case, but we're worried that, yeah, it's much less robust, much less resilient. And we have seen very dramatic rainfall decline in Borneo with deforestation, for example.
Alpha: Yeah, and so that's the other thing too, that when they talk about like this whole climate and water thing, because when the hydrologists look at the impact of forests on the water system, they're just saying you lose water because of transpiration, but that's forgetting the climate aspect where that transpiration comes back down as rain. And so it's because you're only looking at half the equation, you sometimes make this wrong assumption that transpiration is all loss.
Douglas: Yeah, so in a sense it is always a local loss, but it's a regional gain. Somebody is gaining your rainfall. In some parts of the world, like if you're a farmer in China, there are huge populations of people, your rainfall is almost always coming from somebody else's land to the west. So that's crucial.
And I think in some ways it's obvious with modern understanding, but I still don't think we've quite caught up with the implications. You know, the land cover upwind of you is obviously crucial to you. I should say with the Biotic Pump, it also matters downwind because that's the forest cover that's actually drawing in that moisture. So people often emphasize the upwind where the rain is coming from, but we also need that process that's drawing the wind right to you with the moisture, which is due to these low pressures that are also behind you as it were when you're facing where the rain is coming from.
So we need both. But it's these large-scale processes we're still in the early days of trying to understand. There's some work in some parts of the world where there are big river systems which cause conflict like with the Nile. Now all of these countries need more water or many of them need more water and there's a real potential for conflict as to who gets the water. I think it's going to be the same potentially in the future when we have a better understanding of how rainfall is translated from one area to another. Just thinking how we can fairly make sure we maintain these cycles that the farmers in China or in drylands Africa who are getting the rainfall from other land. There's a responsibility there which we haven't really engaged with yet.
Alpha: Yeah, like for instance in the Sahel there's a Great Green Wall of Africa being built, and people don't realize that they need to restore the Congo rainforest to help the Sahel, because a lot of the rainfall from the Congo rainforest actually flows northward to the Sahel. Just like the Amazon rainforest supplies rainfall for other South American countries.
Douglas: Yeah, yeah. It's a source of rain and also all these circulation processes. It affects these air currents.
Alpha: And regarding the Nile issues between Egypt and Ethiopia, they are not realizing the potential that if they help Ethiopians restore their land, they could generate a lot more rainfall which will increase the amount of water that goes into the Nile.
Douglas: Yeah, I think there are a lot of opportunities out there and I think some people at least are increasingly looking at these things and there's open-mindedness about it. I mean, I know a lot of colleagues are looking at that. What I do see in this realm, as I say, I'm not a hydrologist, I'm an ecologist by background. What I see is we all have our little window we're looking through, but these are big picture questions and it's really important, I think, to bring the different expertise together.
And if we disagree, then we argue and we think how we're going to clarify. What's frustrating me at the moment is there's a lot of dogmatism or outdated thinking. People who are just dismissive of certain things or not interested in certain things and that's a shame because these opportunities are real, the problems are real, and the opportunities to fix them, I think we need to come together and actually think how we can bring the different expertise and ideas together. So that includes what I would call the controversial ideas.
Alpha: And there are a bunch of climate scientists who are going around proposing that the different countries have to make these agreements. Just like with rivers, you have to deal with the atmospheric water because the downwind, the upwind countries are affecting the downwind countries' rainfall.
They're suggesting that they need to start making agreements across transboundary, transnational agreements around how you're affecting each other's rainfall. But it's still early days. But there are climate scientists going around trying to promote these ideas.
Douglas: Yeah, exactly. I think this is increasingly true. I think it was a bit of a fringe thing but it's beginning to get traction. I think the circulation process is everybody nods their head and agrees. There's nothing controversial about the fact that it happens. I guess the role of land cover in maintaining it, that's still controversial. And many details are still debated. Particularly what can you do locally and what are the effects locally?
FOREST RESTORATION
Alpha: There are a lot of tree planting and forest restoration projects, but some of these are not so good. As an ecologist, what would you say—what are some of the things we should be aware of when we're trying to plant trees or when we're trying to restore forests?
Douglas : I'm very much coming to this from a conservation perspective and background. So I'm always valuing the native vegetation as much as possible. The things that should be there because they were naturally evolved and adapted to the local conditions. And I think we make a mistake often in just using the word forest as if all forests are interchangeable.
We need more emphasis on working with communities to see what it is they're interested in having and willing to support, rather than kind of bringing in huge plantations over their heads, but actually what it is that they would like to see in their landscapes. Often they're totally on side with wanting richer and more diverse systems, pleasant places to live.
So there's a lot of opportunities there. Planting itself is even an issue, I would say, because planting often means you really need to do an intervention. But trees are pretty good at planting themselves in most cases. And if you create the conditions, I would say this is kind of a lesson for restoration projects in general.
People really like to show they're doing something. But often the most important thing to do is be able to step back and let nature repair. And I think we're very bad at that because we want targets, we want to actually have something physically we can show in the photograph with the local mayor or whatever, you know, where they're planting the tree.
And this is again, this is a sort of a human nature thing. But actually letting the forest replant themselves, they're pretty good at it, you know, as long as you create the right conditions. And to me, that's going to be a more interesting and more robust forest. It doesn't mean you never need planting. There will be cases where systems are too degraded, or where there's been too much lost or whatever. And you do need to kickstart.
And that can be valuable. But in general, once the system is up and running and self maintaining, yeah, we shouldn't have to be having to do these planting things. So every time I see all these big targets of that tree planting, I'm thinking, oh, no, this is a little bit misguided.
What we need to do is be able to think why the forest isn't there in the first place. You know, what were those forces? Have we managed to address those?
What's the reason the forest is gone? If it's goats, or if it's kind of uncontrolled harvesting, then obviously we need to address that. If it's fires or whatever, we need to address that. There's so much that we should be addressing. And these are not big, visible things that you can set easy targets.
Alpha: So yeah, I guess birds drop seeds and monkeys pass seeds around, squirrels, animals trample the seeds into the soil to help them grow. So you're saying that if we're not doing detrimental things, like maybe just cordoning off that area and allowing that forest to naturally reproduce and grow in larger areas, is that a good way?
Douglas : That would be perfect. I mean, you should be able to do this over large areas with much less work. But the point is in the long term, even if you plant, you're going to have to protect against these forces that cause deforestation in the first place. So we need to fix those things anyway. And if we fix those things anyway, I think the forest in most cases will come back. As long as I say, I mean, there are cases where it helps the plant because the soil is too degraded.
So you need to loosen up the soil, whatever, your mind reclamation or whatever, you probably need to give it a kickstart. But in most cases, nature is remarkably robust. I mean, I'm used as a conservationist to kind of also claiming the opposite.
It depends on the case. But nature is robust. If you give it the opportunity to come back, it'll come back. And I don't know if you've ever had a chance to go to like the islands of Krakatau in Indonesia, you know, this is this was sort of famously sterilized by this massive volcanic eruption back at the, you know, the end of the, what was it, 19th century. But yeah, this is lush forest now covered in fig trees, lots of birds and animals, you know, the forest comes back. And we have to think long term, because you're never going to grow a forest in two or three years. It's always going to be long term. But ultimately, if we want forests of native species, you know, all that richness, all that biodiversity, and I think most of us do, you're going to have much more interesting and much more robust forests if we allow nature to do its thing.
Alpha : And so what's the time scale we should be thinking a decade, two decades?
Douglas : To have a reasonably good forest? Yes, in most of the tropics, things are actually quite fast-growing, particularly in the wetter areas. You can see a good forest in two or three decades indeed. In drylands, it's less clear. It depends how dry and it depends how degraded. Often these landscapes, as I say, like in Africa, there are a lot of grazing animals.
So that's a whole different set of issues. But I think it's remarkable how quickly things will bounce back even then. I mean, there's this really positive story coming out of much of dryland Africa about the fact that it really is greening up at the moment. I think it's a combination of communities themselves, recognizing the value of tree cover and recognizing that some trees in your field is actually much better, and slightly wetter climate just naturally, just due to variation.
So it's largely greening up, but we are really seeing a lot of greening up in parts of dryland Africa without much outside intervention. So I think that's hugely exciting.
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Douglas Sheil’s Wageningen webpage
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