Moisture Recycling: Roots of the Sky, Pt.1 - The Piedmont and Coastal Loblolly Forests of North Carolina
Moisture Recycling: Roots of the Sky, Pt.1 - The Piedmont and Coastal Loblolly Forests of North Carolina
Trees help make rain to re-water themselves and sometimes, it is a last ditch, hopeful attempt to stave off complete dehydration.
“I want to warn the white people before they wind up tearing the sky’s roots from the ground.” Davi Kopenawa and Bruce Alpert, The Falling Sky: Words of a Yanomami Shaman, 2013.
Hello - it’s been a minute. This post has been marinating a good, long while. I’ve been building towards this the whole year+. But this was hard to write for several reasons, not the least of which is because it is based in atmospheric science. This is not my forte and is a challenging topic to learn.
Also, while working on this I read the book Vibrant Matter by Jane Bennett. I had started a “book report” to post but didn’t finish it. For now, I’ve decided to set it aside. I hope to come back to it again at some point. That book was mind-blowing. I talk about plants as conscious individuals. Bennett talks about all matter as having a kind of agency, everything conspiring together, as Alfred North Whithead and others might say, to form each new, unique moment. Poof!
The other reason I had a hard time with this post is because it seems to be a hot topic on Substack right now. There is an extended, on-going conversation here about the combined effects of climate change and land use change on our planetary life support system. So I had to try and find my own way into this story. Before I get to that, I want to acknowledge work by others who wrote about this here well before me.
Firstly, there is the Climate Water Project by Alpha Lo that inspired me to start my own Substack. Alpha’s work is primarily devoted to the same general topic, but with his unique take. As a sampling of some of his excellent posts, I recommend his introduction to the field of water ecology, aka ecohydrology; his description here and here about the importance of plants to water recycling; his summary of the “small [transpiration-driven] water cycle” and what causes the rainy season to start early in the Amazon, Congo and Great Plains.
The aptly named Climate According to Life by Rob Lewis also explores much of the same ground. Rob brings poetry to his work. The posts most interesting to me include his part three-part series on Millan Millan and the Mystery of Disappearing Mediterranean Storms, pt. 1, pt. 2 and pt.3 and an introductory post that includes a set of resources that provide a great foundation for this topic.
Regenesis, by Ali Bin Shaheed is about regenerating the living planet, including posts on applying plant moisture recycling for site regeneration. He has a great post explaining water and water recycling for his daughters that is straightforward and helpful. Didi Pershouse in The Wisdom Underground also has a healing perspective. She has several classes and posts on what we can do to reconnect the natural water cycle, here, here and here. Ugo Bardi’s Living Earth has several posts on the Biotic Pump and dispatches from the March, 2024 conference “Embracing Nature’s Complexity” and from Anastasia Makareva herself, one of the originators of the Biotic Pump theory (more on this). I apologize to those I’m missing! The Substack community is large and diverse and I am just scratching the surface on content touching or devoted to this topic.
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I started my version of the moisture recycling story underground - focusing on the mutable, invisible networks of rooting zone communities built to manage environmental vagaries. These plant communities, strewn across hill and dale are continuously building their bodies and the rooting zones outside their bodies. They are building their lives and helping to sustain ours.
The other part of the moisture recycling story rises up over the canopy and courses downwind. This story is about plant stomata regulating the phase change of liquid water to water vapor at the leaf surface. Transpiration is not an inadvertent loss of water occurring during photosynthesis. Quite the contrary, somehow through the ultra-long process of evolution, plants have learned to structure the ground and the lower atmosphere to better suit their needs. This plant-mediated atmospheric structuring process is moisture recycling and is about plants purposely giving back water to the air. How they control that giving back is in large part, I think, the subject of my Substack.
Our planet is mostly rock hurtling around the sun. Gravity, energy and entropy tip and spin the earth and we get seasons, winds, day and night. The rock is coated with a living skin, rooted in the ground, head in the clouds. Plants may not control the spinning earth, but they do facilitate recycling water from sky to ground and back again. They suck up and blow out water, help build clouds and let loose rain.
Somehow with water recycling, plants know well enough how much water stays with them and how much goes while helping the lower atmosphere recycle moisture to and from the ground again and again. Research suggests that almost half the precipitation that falls on continental land masses has already been evaporated or transpired (plant-mediated evaporation), lofted back up, at least once, by someone else. (Keys, Wang-Eriandsson and Gordon, 2016).
But plant water recycling may be more about how and when a plant returns water back to the air rather than the quantity of water returned. The abiotic water cycling of ocean evaporation may be most of the story and plant water recycling an action to nudge waning water availability just enough to bring back water sufficiency or surfeit back to a water-starved ecosystem. The work that gets done by plants to work out at the edge of water availability convinces me it is a conscious decision. I mean another strategy would be to hold onto water more carefully and wait out dry weather.
I have broken this version of the moisture recycling story into two parts. This first part is about loblolly pine plantations in the piedmont and coastal regions of North Carolina. The second part will focus on the tropical rain forests of the Amazon. Both will show how forests are willing to work out at the edge of water availability in the service of a bigger payoff.
In the North Carolina study, researchers Gabrielle Manoli, Jean-Chistophe Domec, Kimberly Novick, Andrew Christopher Oishi, Asko Noormets, Marco Marania and Katul Gabriel (2016) were looking for a potential relationship between the water recycling of loblolly pine plantations and the transition of the atmospheric system from cloudy to cloudless conditions. They picked this condition because the difference between cloudy and cloudless is readily confirmed and clouds are the necessary pre-condition for rain, in this case, convective rain. Their two main objectives were to identify the free atmospheric states that biotic processes control that can lead to convective cloud formation and investigate how forest age might affect moisture recycling.
Convective rain is that late day summer shower that seems to come out of nowhere. This kind of convective rain happens mostly during the growing season, when plants are maxing out photosynthesis and transpiration, aka: moisture recycling. This moisture recycling is the dance of plants managing energy and water at the surface. While convective rainfall is not the only way rainfall occurs in the US Southeast, it contributes a significant proportion of summertime rainfall when the productivity of these pine plantations is at its highest.
The region of air directly influenced by energy and water fluxes over land is known as the atmospheric boundary layer (ABL). I think of the ABL like I think about the boundary layer at the bottom of a river. Air at the ground surface, like water at the bottom of a river, is essentially static. And, just like water pressure, the more air you stack up, the higher the pressure at the bottom of the stack. Once the air or water rises above the friction effects of the surface, it’s moving or is ready to move. The higher you get above the ground/bottom, the faster the potential air and water movement as friction is less able to hold you back.
The top of the ABL marks the basement of new clouds. Beneath the ABL the air is more prone to the dynamism of the earth’s skin than the sway of the sky. Above the ABL air movement is enthrall to the earth’s rotation rather the chaotic eddies and swirls of turbulence near the surface.
In the pre-dawn of a clear day, the ABL may be only a few hundred meters (or lower) from the ground, but as the sun traverses the sky the temperature goes up, air expands and water evaporates. An air mass can get lifted hundreds or thousands of meters into the sky as it absorbs heat. As it rises, the air mass inhabits a larger space and pressure and temperature decrease. As the temperature falls the water vapor content necessary to saturate the air falls; that is, the water vapor content that defines the dew point falls. Cooler air holds less water.
As the air cools, water vapor is more likely to turn back into water. This cooling air needs a surface to condense onto. Condensation nuclei like the aerially drifting detritus of sea salt, air pollution and biogenic particles like pollen and spores, become the centers of condensation. At a critical mass, this water eventually falls back to the ground as rain, sleet, hail or snow.
The ABL expands and contracts every day and night. That expansion rate and height depends on the heat and water balancing performed at the earth’s surface. That expansion and the contraction of the ABL at night is like the shaking out a beach blanket. The blanket is lofted up off the ground, pulling up air underneath it. you pull down on the rising blanket and the air descends and squeezes out from under the blanket. The atmosphere breathes.
The threshold of convective cloud formation is understood to be when the dew point of the rising air crosses the ABL elevation and breaches the threshold between the ABL and the free atmosphere above it. The ABL insinuates itself into the relative homogeneity of the air masses circling the earth in the free atmosphere. The dew point change with altitude in called the Lifting Condensation Level (LCL). The ABL crossing of the LCL is the precondition for shallow, convective cloud formation (see conceptual model figure).
These loblolly pine plantations are part of the Ameriflux monitoring network. AmeriFlux is a network of managed sites measuring ecosystem CO2, water, and energy fluxes in North, Central and South America. It was established to conduct research on sites representing major climate and ecological biomes, including tundra, grasslands, savanna, crops, and conifer, deciduous, and tropical forests. As a grassroots, investigator-driven network, the AmeriFlux community has tailored instrumentation to suit each unique ecosystem.
These Ameriflux sites gather extensive atmospheric monitoring data, including eddy covariance data. Eddy covariance is a micro-meteorological technique that measures the exchange of energy, gas, and momentum between the atmosphere and ecosystems. It's a statistically intensive method that analyzes high-frequency data on wind, gas, and energy atmospheric properties.
In addition, continuous rooting zone soil moisture (or Soil Water Content – SWC) was also measured over the period of study at these plantations. Nearby airport atmospheric sounding data – continuously measured air pressure, temperature and humidity from weather balloons, was used to characterize the ABL, it’s height and free atmosphere above it.
Moisture in the air is coupled to open water and soil water. The soil reservoir holds the water plants transpire. Changes in vegetation – age, canopy, tree density, health, species make-up, other plant or fungi competition or cooperation, all of it can affect the vegetative hydraulic control on transpiration and the partitioning of sensible and latent heat fluxes at the earth’s surface.
Sensible heat is the heat a body can feel. Latent heat is the energy absorbed by water that induces a change of state from liquid to vapor. Latent heat energy does not change the temperature of water but changes its form. A convenient index of heat partitioning between the making of water vapor and heat build-up is the Bowen ratio. It is simply sensible heat/latent heat.
The Bowen ratio at these sites rapidly increases as the soil water content gets drawn down (See Bowen ratio figure). This rapid increase in the Bowen ratio shows how quickly the heat that was going into evaporation is now becoming sensible heat. As the soil water reservoir is depleted the trees close their stomata and all the energy that was going into the transformation of liquid water to water vapor now goes into heating up the air at the surface.
The SWC figure below shows the distribution of relative soil water content at both plantations. The soil water content axis goes from 0 (completely dry) to about 30% to 40% of the soil rooting zone volume occupied by water. The distribution shows that the trees at both sites operate most often near the edge of water availability.
These results suggest that these trees maximize water use for photosynthesis and transpiration up to a point. At that point, of their choosing, they close the stomata and allow heat energy to warm up the air and increase its buoyancy. The air gets lifted, expands, and cools to find its new dew point. Once the ABL cross the LCL, the threshold for initiating convective cloud formation is met. These trees are purposely altering their energy and water use to get the response they want.
In terms of forest age, the researchers found that the young trees did not or could not take the same risks as the older trees (see forest age-cloudiness-SWC figure). With shallower and less extensive root systems, these trees consistently kept more water in their soil water reservoir to avoid potential damage to their interior plumbing (hydraulic architecture). I covered the possibility of hydraulic failure – cell death and plant death in my last post. The oldest trees were generally insensitive to soil water content and appear better able to manage the trade-offs between their soil water storage and the effort to recycle water for potential convective rainstorms.
The researchers caution that despite the active role vegetation plays in moisture recycling, the generation of convective rainfall depends on the ABL/LCL dynamics and regional moisture and pressure gradients. That is, vegetation-mediated rainfall feedback is only a secondary influence on rain.
When the researchers say trees are not a primary influence on rain, I think they are cautioning against saying ‘trees make rain’. I don’t believe trees control the weather, but that they influence it. These forests make decisions and take action in the hopes of getting more rain. It is a much more poignant story than a strictly mechanistic explanation, because they don’t always get it right. The transpire, then choose the moment to stop and from there can only hope it achieves their ends. They can fail and the consequences are dire. It is their rain dance.
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For part 2, the story in the Amazon is about moisture recycling, but more than just for convective rainfall. It is also about how at the end of the dry season, with their soil moisture reserves nearly empty, the rain forest trees replace dying leaves with new ones. This switch-out increases transpiration even though they are running low on water to transpire. Their shallow convective cycling creates the same kind of rising air masses and falling air pressures as in the North Carolina example. Here this process acts like a vacuum and can pull moist air from over the ocean across the continent. These forests can literally kick-start the rainy season and a deep convective water cycling from the ocean. This is known as the Biotic Pump. Stay tuned.
Really cool to read this. And honored I helped inspire you to write a substack newsletter. I think its really helpful to have more people writing about this, and giving their particular angle on it