China changed water cycles by planting billions of trees
Wout Hoff
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Wout Hoff
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Sheil, Douglas
"more trees is better" –often, but depends where you are and what trees. There are multiple effects involved that operate at different scale—for example, infiltration and atmospheric transport.
In dry areas more trees can make it drier and in wet areas it can make it wetter. See for one summary here:
https://onlinelibrary.wiley.com/doi/full/10.1111/gcb.16644
The point is that tree cover can deplete water too though this is often overemphasised (due to past studies focusing on plantations in dry regions that shouldn’t support dense tree cover), see https://www.sciencedirect.com/science/article/pii/S2197562025000855
On dry degraded soils intermediate tree cover is often much better than none even when dense cover is also not desirable: https://www.nature.com/articles/srep21930
Timing matters too. Introduced species can be maladaptive. Native species with locally adapted phenology (when they leaf and transpire) is key.
Ideally don’t plant when you can simply promote local regeneration/regrowth. Plan for a self-sustaining system
In general, I would say “follow nature” in terms of vegetation structures/covers/densities and compositions.
For some general overviews see: https://www.sciencedirect.com/science/article/pii/S0959378017300134 and my own, https://link.springer.com/article/10.1186/s40663-018-0138-y?fbclid=IwAR07VSVqp34OEJqHrwqMVncrVTfXWNJgrMXZzJxtDSKSQVcObEorAgsDLTw
“Can we predict how rainfall patterns will be affected if we restore degraded land by planting trees?” … yes, we can approximate this for small scale changes. For large scale changes (where the moisture tracking is not enough because circulation will also change) it depends on the confidence you put in the models … certainly you can get “a prediction” but different models give different predictions …
Douglas
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Sheil, Douglas
I took a little more time to refine the first part of my answer into a LinkedIn post that seems to be generating some discussion. See https://www.linkedin.com/posts/douglas-sheil-55330a9a_ecorestoration-naturalregeneration-watercycles-activity-7408271599025459202-fn8e?utm_source=share&utm_medium=member_desktop&rcm=ACoAABUMCo8BrLwsbmfOYxByVavYPlYIkGcQH9s
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Planting more trees … or perhaps we can work smarter?
Over on the Ecorestoration list-serve, someone who plants trees and is concerned about the water cycle asked: “ May I ask for peoples views on this?
My view was always 'more trees is better.' "
I share my response here:
A common assumption is that more trees means more water. But that's not true. It’s more complicated: we need to prioritize conservation, recovery, and smart restoration.
Outcomes depend on location, climate, and species. Trees emit moisture into the air. That loss depletes local water stores. If the air is dry, added moisture will generally end up elsewhere and drylands become drier.
In wet regions, in contrast, when we add moisture to the air the chance of rainfall increases and, if we look at a large enough scale, we can gain more moisture than we lose. We intensify moisture recycling and bolster moisture import. So drier areas get drier,
while wetter areas get wetter:
https://lnkd.in/esRU3jG7.
These patterns shift with the seasons. The role of vegetation in responding to, and contributing to, these changes is underappreciated. Phenology—when a plant uses water versus when it doesn't—is crucial.
In the past, many studies of trees and water have focused on dense tree plantations in relatively arid sites that would not naturally support dense tree cover. In such locations, this tree planting isn’t beneficial for local moisture. While such cases predominate
in much of the published literature, there are also cases where we see increased tree cover does lead to improved water yields.
Our recent research—focusing on soil-related processes and ignoring larger-scale changes in the atmospheric water cycle—shows that restoring dense tree cover can actually boost dry-season streamflow in 10-20% of viable sites:
https://lnkd.in/efJKDbv4. In these contexts, trees enhance soil permeability and storage capacity, reduce runoff, and sustain stream flows even during droughts.
On degraded dry soils, intermediate tree cover—scattered distributions—often outperforms both a bare landscape or a dense forest:
https://lnkd.in/eBWjFkuf. It depends on many factors, but often "scattered" trees are enough for the hydrological sweet spot:
https://lnkd.in/ejnvpCYz.
We shouldn’t conflate dense monocultures with natural forests. For hydrology the smart approach is to prioritize natural vegetation and processes. Native species carry key local adaptations.
People often consider planting the default, this is a mistake. Trees are good at planting themselves if given the opportunity. We need to enable that and plant only where natural regeneration fails.
Given what we know (and what we don't) the smart way forward is to conserve and repair nature and follow nature's template.
Let us know if you have your own principles, corrections or caveats.
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Ecosystems
Owen
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