Imagine the days before physicians recognized germ theory. Plagues and other diseases would spread mysteriously across the land. Few recognized the importance of washing their hands, even before delivering babies.
After one physician in Vienna observed fewer deaths among midwives delivering babies, he introduced hand washing among his hospital staff in 1847. The proportion of new mothers dying from “childbed fever” dropped five-fold among his patients.
Unfortunately, it took another two decades before the practice of hand washing gained popularity.
We may be in a similar situation today when it comes to the health of our planet—Mother Earth.
Many life scientists now recognize the ability of forests to generate rain and otherwise stabilize climate. However, a long-standing rift between foresters and climatologists has prevented the widespread application of this knowledge in policy and practice.
Yet knowledge is emerging that arguably could clear up observed contradictions and allow scientists to move past this rift.
Supply versus demand
Soon after I started graduate school in 1996, one of my University of Arizona professors assigned those of us in his Watershed Management course a homework question: How much would the “water yield” of a specified mountain forest increase if we cut down all the trees?
The question seemed like an affront to me. I responded with a lengthy answer about how the amount of water flowing from the mountain would see a short-lived increase, for maybe a few years—but so would local temperature. Water temperatures would rise too, without protective shade. Soil particles would erode away, darkening rivers and taking nutrients with them.
I would soon learn my professor, who specialized in modeling, wasn’t the only one eyeing trees as competitors for water. Yet research was already challenging this take, and suggesting trees actually supply water in some situations, and certainly regulate it.
It turns out the clash between foresters and climate modelers has a long history.
Foresters have long suspected that forests help generate rain, as Brett Bennett and Gregory Barton explain in a 2018 Forest Ecosystems paper outlining the history of this question. As they point out, even Christopher Columbus noticed subsequent drying on some of the Caribbean islands he and his ilk deforested.
Man and Nature, an influential 1864 book by former senator George Perkins Marsh, reviewed the literature to conclude, “a majority of the foresters and physicists who have studied the question are of the opinion that in many, if not all, cases, the destruction of the woods has been followed by a diminution of the annual quantity of rain and dew.”
Then climatologists entered the debate.
The 1883 Handbook of Climatology by Julius von Hann emphasized global drivers of climate, generally ignoring any complications from vegetation. Engineers joined the climatologists to cast shade on what some of them considered foresters’ “hazy conception” about how forests influenced watersheds.
Foresters adapted to the climatologists’ way of thinking for almost a century. Trees were generally considered to “demand” water rather than “supply” it.
To this day, climatologists, atmospheric scientists and engineers tend to ignore the role of forests on climate—to the point of sometimes arguing the planet would be cooler without them. This leads to proposals from this crew that we could cool the planet by adding sulfuric acid to “dim the sun,” with little attention given to how this additional pollution would affect forests and other life forms.
Rainforests supply water
In 1979, a Water Resources Research paper led by Eneas Salati analyzed water chemistry to conclude rainforest trees recycle rain. This opened the floodgates for more research on this topic. A general consensus has emerged that rainforests recycle at least half of incoming rainfall to share with downwind forests.
A larger-scale potential influence on rainfall involves the biotic pump theory by theoretical physicists Anastassia Makarieva and the late Victor Gorshkov, which I describe here.
In short, it maintains closed-canopy forests—those so full of trees their canopy mostly shades the ground—can generate a biotic pump that actually pulls in moisture-laden winds when suitably near the coast.
Given the century-long resistance by climatologists to credit forests for their climate-regulating roles, it should come as no surprise that these folks remain skeptical.
Still, many life scientists admire the elegant biotic pump theory and would like to see its influence considered as well as tested by observational science.
Dryland forests can demand water
Makarieva continues to test the biotic pump theory using existing data on forests and climate. In a 2023 paper in Global Change Biology, she and her colleagues applied the concepts to tackle the complex issue of whether forests demand or supply water. They used data for forests under variable climates in Amazonian rainforests and on China’s Loess Plateau.
As one might expect when life is involved, the 2023 paper concluded, “it depends.” The paper explains why there are different answers in different locations—and does so in a way that can apply to policy.
In effect, they conclude that under water-stressed conditions or in locations where rainfall amounts are near the edge of what forests need, planting trees or sustaining forests can cost water that otherwise might flow into rivers.
This indicates my former professor wasn’t totally off the mark by suggesting cutting down trees would, if only briefly, increase river flow in our semi-arid lands.
In contrast, in situations where rainfall and recycling processes provide enough moisture to support forests, growth of additional trees and forest expansion regulated water in a way that didn’t change the amount of water flowing into nearby rivers.
To suggest how this theory could apply to policy recommendation, planting trees in semi-arid lands such as grasslands would not be recommended—with possible exceptions for areas near a coast or directly downwind of thriving mature forests.
By the same token, protection of coastal forests should take priority, as the biotic pump suggests forests bordering water bodies help initiate and keep moisture-laden winds flowing toward them. Given that forests can require centuries to reach sufficient status to serve as biotic pumps, as Makarieva indicated in an interview, existing forests should take priority for protection.
Still, it would useful to allow forests to restore themselves in patches of land formerly occupied by forests to prevent the destabilization of the biotic pump.
Unnecessary deaths
In the Vienna hospital that saw a five-fold drop in deaths from childbed fever when doctors started washing their hands, physician Ignaz Semmelweis implemented the change based on observations of maternal survival. Germ theory remained controversial.
Unfortunately, his fellow physicians soon rejected him and his advice, not liking his pushy attitude or the implication that their unwashed selves were perpetrating the feverish disease. Semmelweis was fired.
Twenty years later Joseph Lister applied germ theory to promote the value of washing hands and sterilizing surgery instruments in an 1867 article in the influential Lancet. As physicians implemented his advice, childbirth and surgeries became safer.
We seem to be in a similar lag period between observation of the problem and implementation of policy to improve the situation when it comes to forest health.
We have knowledge picked up from observations showing downwind trees recycle the water used by other trees. We have a robust theory—albeit one that remains controversial, especially among climatologists—about how mature forests can pull in moisture-laden winds from nearby bodies of water.
Yet until everything comes together in a way deemed acceptable by those making decisions, more forests will die.
If only the actions needed to help Mother Earth were as simple as washing our hands.
On Thursday, I plan to come back to these concepts in the context of the current situation in the Amazon, with a focus on Bolivia.
Bravo! Great piece on a subject too long steeped in obscurity. It's time to move out of these old, nature-suspicious paradigms. If you haven't already read it, this piece by Bonan et al, goes into the "controversy" in some detail. https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023MS004017