Many scientific studies show that because trees draw water from the ground and transpire it into the atmosphere, greater forest cover at the regional level increases precipitation. Rainfall amounts also vary locally, but explanations for this are elusive.
Can forests also increase rainfall at the scale of a few city blocks? If so, how?
With increasing prevalence of drought — this year was bad — more rain would be a powerful argument for a big societal push to increase local tree cover.
A friend and I were talking about this while driving to a Halloween event. I said our rain gauge in Ottawa had registered 2.7 inches for October 30-31, but I didn’t know if a similar amount was recorded at the airport. It wasn’t. The “Weather Dashboard for Ottawa” website shows 47.9 mm for those two days: 1.89 inches.
We have a healthy urban canopy in our part of the city. The possibility that more tree cover increases rainfall at a local level can easily be tested with real data. We could install rain gauges and send rainfall data (and our geographic coordinates) to a local university. Scientists with access to tree cover data could do the necessary analyses.
Why might more trees mean more rain?
My wife suggested one possibility: the tree canopy slows down the passage of the rain clouds. I came up with another one: rain stimulates trees to release cloud condensation nuclei (CCN). These are tiny particles that provide surfaces on which water vapour, a gas, condenses into a liquid. This makes cloud droplets and rain.
Is there scientific evidence for this? I asked Google. AI Overview said “Yes, rain stimulates trees to release compounds that lead to formation of new cloud condensation nuclei.” It added, “this is a newly discovered paradigm.”
It didn’t take long to track down the source of this statement — a 2024 study by researchers from Germany, Brazil, Saudi Arabia, US, Sweden, and China in the journal Nature Geoscience: “Frequent rainfall-induced new particle formation within the canopy in the Amazon rainforest.” This study was done at the Amazon Tall Tower Observatory, using a 1066-foot-high sampling tower and two nearby 260-foot towers.
The prevailing theory is that sulfur-containing gases, from both natural sources and pollutants, are the main source of CCN. These gases are oxidized to sulfuric acid aerosols under the influence of sunlight in the atmosphere. But sulfuric acid CNN can also be a “condensation sink.” Water vapour condenses on so many sulfur aerosol particles that only small cloud droplets form, and they do not combine into the larger droplets than make rain: more clouds, but less rain.The new research study found that when rains come (as happens a lot in the Amazon) they wash out the sulfuric acid CCN. This reduces the condensation sink and allows trees to create their own, more effective CCN, directly over the forest canopy.
The scientists found that rain also injects another atmospheric component, ozone, into the canopy. Ozone stimulates the trees to release CCN precursor chemicals such as terpenes. This results in “particle bursts” of CCN that enhance local rainfall.
Rain lasts longer and occurs in greater amounts. But is this unique to the Amazon?
A study published in 2022 suggests this may happen in Canada as well.
Two German researchers travelled to the Fraser River basin in British Columbia with equipment used to measure aerosol particles. They studied two remote sites where there were no sources of sulfur gases that could create condensation sinks under the effect of sunlight.
The researchers observed substantial aerosol formation at both sites, both at night and during the day. They concluded that these aerosols — most of which act as CCN — came from organic compounds such as terpenes that were released by trees.
The researchers noted major differences between their two study sites. At their Eagle Lake (EL) site, aerosols formed each day in high amounts. At their Nazko River (NR) site, aerosols only formed six days out of eleven, and in five-fold lower amounts.
The researchers proposed “density and type of forest cover in the surrounding regions” as a possible explanation: “Whereas the landscape around EL is completely dominated by conifers, a large fraction of the area around NR has been deforested in recent years either by logging or wildfires.”
Neither the Amazon nor the BC study has attracted much interest from other scientists to date (8 and 17 citations, respectively). But they have important implications. Even in polluted cities, rain could wash away the sulfuric acid condensation sink, inject ozone into the urban canopy, and allow trees to make more CCN and more rain.
Nor is this effect limited to the forest canopy. Forest soil with an intact surface layer of decaying leaf litter is also a major source of the volatile organic compounds that form CCN, perhaps as important a source as living foliage. A study in southern France, published this year, had used a 15 m x 20 m automated roof over an oak forest canopy to exclude 35 per cent of the incoming rain for ten years. Production of CCN precursors declined by 43 per cent compared to an adjacent area that received natural rainfall.
CCN from decaying leaves might explain why it rained more in our Ottawa neighbourhood than at the airport just before Halloween. Many leaves had fallen, but most had not yet been raked, bagged, and taken away.
Scientists must be cautious in interpreting their findings. But, given the importance of natural water cycles in maintaining life and reducing fire, new findings on tree-derived CCN suggest an urgent need for policy reform to retain intact native forests, and to create additional forest cover in urban and agricultural settings.
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