How do trees work?
October 24, 2023 5:09 AM Subscribe
I've been reading a bunch of writing for lay people about hydrology and permaculture.
Help me understand some seemingly contradictory assertions about how trees affect ground water....
Things I've read (that I may well have misunderstood):
1. Planting trees on hilltops around a water catchment area helps recharge aquifers in the valleys, by capturing water in roots and helping stop rapid runoff out of the catchment area during heavy rain.
2. Trees in grassland can make drought conditions worse since the trees transpire water out of the ground.
3. Forests help to move water long distances across continents, because they transpire fallen rain back into the atmosphere so it can go further inland (presumably improving the water situation inland, at the expense of ecosystems near the forest itself?)
4. Trees are good for drought conditions, because they transpire water vapour that causes nucleation of airborne water and makes it rain more (presumably improving the conditions in the forest, at the expense of land further inland?)
Does anyone know of a lay person explanation that integrates the available evidence and gives a better holistic picture of how trees can affect the water cycle, locally and continentally? I realise it's complicated and maybe not fully understood. Also I hear there's more one type of tree and more than one type of ecosystem - I'm happy to read about all of it.
Things I've read (that I may well have misunderstood):
1. Planting trees on hilltops around a water catchment area helps recharge aquifers in the valleys, by capturing water in roots and helping stop rapid runoff out of the catchment area during heavy rain.
2. Trees in grassland can make drought conditions worse since the trees transpire water out of the ground.
3. Forests help to move water long distances across continents, because they transpire fallen rain back into the atmosphere so it can go further inland (presumably improving the water situation inland, at the expense of ecosystems near the forest itself?)
4. Trees are good for drought conditions, because they transpire water vapour that causes nucleation of airborne water and makes it rain more (presumably improving the conditions in the forest, at the expense of land further inland?)
Does anyone know of a lay person explanation that integrates the available evidence and gives a better holistic picture of how trees can affect the water cycle, locally and continentally? I realise it's complicated and maybe not fully understood. Also I hear there's more one type of tree and more than one type of ecosystem - I'm happy to read about all of it.
It's been so long since I studied this as a university student that I can't find the literature, and also the science has developed a lot. But: all of these points have to be adapted for the local conditions. What makes sense in Arizona doesn't work in the arctic tundra. If you are interested in doing this, you'll need local experts. But when I did just that, my local expert was confounded by my tiny local ecosystem because it obviously worked differently from what he had been taught and had practiced. To be very specific, I have two types of pine, and one is supposed to be good for the land and water retention (and eventually precipitation), and the other is supposed to be bad. But they work exactly opposite the prediction and no-one knows why. This specifically adresses your question about trees in grassland. My consultant ended up basically saying I should keep on doing what I do. So I have also been reading a lot, and I can understand why you find the literature confusing, but I'll suggest that my local issue may come down to regional differences and also the complexity of the ecosystem that 50-something forestry people weren't taught and haven't updated later. I work a lot with the undergrowth, but that is more based on instinct than on knowledge.
People have been working to re-green deserts for more than 150 years, while others have been desertifying land with unsustainable practices in the exact same time-span. Sometimes the same people have been doing both. In most cases, the primary focus has been on profit, but even on protected land, there have been conflicts of interest. An obvious example is the desert land of the "desert belt" from North Africa into China . It does seem to be possible to re-green much of that land. But it would mean ending the nomad cultures in that whole huge region. AFAIK, Saudi Arabia and China are trying to do something to this effect with immense human rights violations as a consequence.
Specifically about this: 4. Trees are good for drought conditions, because they transpire water vapour that causes nucleation of airborne water and makes it rain more (presumably improving the conditions in the forest, at the expense of land further inland?) It depends a lot on local weather patterns and scale. To keep it simple, the rain doesn't have to fall back on the forest unless it is huge, like the Amazon. Smaller forests transpire water vapor as well, but the resulting clouds may move on to other areas depending on wind and pressure.
posted by mumimor at 10:00 AM on October 24, 2023 [1 favorite]
People have been working to re-green deserts for more than 150 years, while others have been desertifying land with unsustainable practices in the exact same time-span. Sometimes the same people have been doing both. In most cases, the primary focus has been on profit, but even on protected land, there have been conflicts of interest. An obvious example is the desert land of the "desert belt" from North Africa into China . It does seem to be possible to re-green much of that land. But it would mean ending the nomad cultures in that whole huge region. AFAIK, Saudi Arabia and China are trying to do something to this effect with immense human rights violations as a consequence.
Specifically about this: 4. Trees are good for drought conditions, because they transpire water vapour that causes nucleation of airborne water and makes it rain more (presumably improving the conditions in the forest, at the expense of land further inland?) It depends a lot on local weather patterns and scale. To keep it simple, the rain doesn't have to fall back on the forest unless it is huge, like the Amazon. Smaller forests transpire water vapor as well, but the resulting clouds may move on to other areas depending on wind and pressure.
posted by mumimor at 10:00 AM on October 24, 2023 [1 favorite]
The common thread among your examples is that they exist in different climate types. "Drought conditions" in a grassland (which is an arid continental climate) will be different that "drought conditions" in an arid coastal climate (where you get a lot of humidity from proximity to the ocean that trees could draw from). I wish I knew of a better resource, but I think delving into climate subtypes and the global water cycle could give you a little more context to frame your knowledge so far.
posted by DoubleLune at 10:59 AM on October 24, 2023 [1 favorite]
posted by DoubleLune at 10:59 AM on October 24, 2023 [1 favorite]
It’s very regional — down to being profoundly different on different sides of the same valley, sometimes. We visited a bunch of these in soil science field summers. I assume forestry field summers get an overlapping set of illuminating contrasts!
“What was here before human fire; the plough; and cars” is a good set of questions — the answers don’t necessarily still apply because even without climate change there can be sharp cusps in how the ecosystem works, but it’s the first thing to check.
posted by clew at 12:47 PM on October 24, 2023 [1 favorite]
“What was here before human fire; the plough; and cars” is a good set of questions — the answers don’t necessarily still apply because even without climate change there can be sharp cusps in how the ecosystem works, but it’s the first thing to check.
posted by clew at 12:47 PM on October 24, 2023 [1 favorite]
I'm unaware of anything that integrates the lot as it's a very diverse topic.
I blog to explain some landscape things including about water - you may find this useful Plants Really Work
Ecosystems eg desert, prairies, forest, freshwater wetland, cropping farms, pasture farms, abandoned land - these are all ecosystems, but each will have it's own context; position on planet (especially latitude), altitudes, distance from sea (salt affects many plants), climate, weather, microclimate, soils, history of human use...
Don't think just trees - plants include trees, shrubs, perennials, grasses, lichens..., all have different characteristics re water;
• How much they use
• What season they use it
• How much they hold back from groundwater
• How much they release to groundwater
• Where they take their water - some plants mainly use fog (eg giant redwoods, and many wetland plants), while red oak and English beech take from groundwater and ignore near-surface water. This is not yet being widely used in planting design but is known as the Two Water Worlds Hypothesis [wiley pdf link].
Water is processed by plants (and soil and microbes) at the molecular level and then these micro effects scale upwards and change in myriad ways.
I recently designed a garden where most plants were chosen for their ability to shift water to the sky. In the US there's a parking lot in MA at Walden Pond where plants pump upwards of 70% of rainfall to the sky - but not just any plants, and also some specific groundwork to help the plants work.
There's a new tribal project in Manitoba using tree shelterbelts to trap snow to recharge groundwater (as well as provide shelter and add much needed native biodiversity with the belts), again, all very specific plants.
The Waterwise Home by Laura Allen is a good book for using the right plants in different contexts (and climates) to accomplish specific outcomes.
I'm wary of much permaculture literature as there's too much opinion (and woo) rather than proven field works. Sepp Holzer's writings are reliable and I use some of his ideas in my work
Desert or Paradise. Holzer’s Permaculture now
posted by unearthed at 3:45 PM on October 24, 2023 [5 favorites]
I blog to explain some landscape things including about water - you may find this useful Plants Really Work
Ecosystems eg desert, prairies, forest, freshwater wetland, cropping farms, pasture farms, abandoned land - these are all ecosystems, but each will have it's own context; position on planet (especially latitude), altitudes, distance from sea (salt affects many plants), climate, weather, microclimate, soils, history of human use...
Don't think just trees - plants include trees, shrubs, perennials, grasses, lichens..., all have different characteristics re water;
• How much they use
• What season they use it
• How much they hold back from groundwater
• How much they release to groundwater
• Where they take their water - some plants mainly use fog (eg giant redwoods, and many wetland plants), while red oak and English beech take from groundwater and ignore near-surface water. This is not yet being widely used in planting design but is known as the Two Water Worlds Hypothesis [wiley pdf link].
Water is processed by plants (and soil and microbes) at the molecular level and then these micro effects scale upwards and change in myriad ways.
I recently designed a garden where most plants were chosen for their ability to shift water to the sky. In the US there's a parking lot in MA at Walden Pond where plants pump upwards of 70% of rainfall to the sky - but not just any plants, and also some specific groundwork to help the plants work.
There's a new tribal project in Manitoba using tree shelterbelts to trap snow to recharge groundwater (as well as provide shelter and add much needed native biodiversity with the belts), again, all very specific plants.
The Waterwise Home by Laura Allen is a good book for using the right plants in different contexts (and climates) to accomplish specific outcomes.
I'm wary of much permaculture literature as there's too much opinion (and woo) rather than proven field works. Sepp Holzer's writings are reliable and I use some of his ideas in my work
Desert or Paradise. Holzer’s Permaculture now
posted by unearthed at 3:45 PM on October 24, 2023 [5 favorites]
This recent news article titled More than carbon sticks in Nature Water looks like a pretty good overview of things, and is very readable with further citations to explore in more detail. There definitely are mechanisms that can certainly generate/capture the water and introduce it to ecosystems or retain it better in ways that mean they reduce drought, but they won't necessarily apply everywhere.
posted by ambrosen at 11:12 AM on October 26, 2023 [2 favorites]
posted by ambrosen at 11:12 AM on October 26, 2023 [2 favorites]
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As far as the 'contradiction' between 2 and 4, keep in mind that the word 'trees' can mean both 'trees in general' as well as 'more than one tree':
If you plant a few trees in a drought affected area, yes they will transpire water and take away available ground water from the area around their roots, starving the grass or whatever. But if you plant a LOT of trees in a drought affected area, as long as there is enough water for them to survive, they collectively will create enough nucleation to cause rain. It takes a critical mass of trees for this to happen, like full on tree farms. The vast fruit and nut tree farms in Fresno, CA are a good example of this. The tree farms completely changed the climate.
posted by ananci at 6:27 AM on October 24, 2023 [4 favorites]