Dirt Makes Forests Work
By Bennett Hrala, WellKind Forestry Intern
Bennett Hrala wasan intern for WellKind Forestry during our spring 2022 session, exploring soil health and other environmental issues.
Picture yourself in a coastal redwood forest. What do you see? Your eyes are likely drawn up by the sheer scale of the old trees and the various birds chirping among the branches. Maybe you’ve come early enough to see the fog swirling between the green needles or settling among ferns on a nearby incline. If you’re quiet and lucky, maybe there’s even a deer on the same incline, next to a gurgling creek. The whole ecosystem seems to be happening at eye level or above. But right below your feet is one of—if not the most—critical pieces of the puzzle.
Soil, and its health, is not something most think about while exploring. It’s an easy thing to miss when there’s plenty to keep you occupied above it. But soil fuels every aspect of a forest, or any land-based ecosystem.
For one, healthy soil has a whole variety of nutrients and organisms in it. The organisms are mostly bacteria and fungi, and even just one gram of soil can hold up to 10 billion bacteria cells of 50 thousand species (Raynaud, Nunan, 2014). They take dead organisms and turn them back into the nutrients that living things can use. The soil then holds the nutrients, and trees can take it to grow ever higher. Without these microbes, nitrogen, phosphorus, and other critical nutrients would never be in the right form for plants to use.
Soil also holds onto water so that trees’ roots can drink it up, which takes time. And when those plants are healthy, the herbivores eat them, birds can nest in them, and the whole ecosystem can thrive.
But what happens when the soil is disturbed? In the case of drought, soil dryness can obstruct many beneficial processes. But the most important process obstructed is carbon sequestration. Variations in soil moisture cause 90% of carbon sequestration fluctuations between years (Humphrey, V., Berg, A., Ciais, P. et al., 2021). This means that a drought can completely change how a forest would remove carbon, both increasing global warming and also stunting the forest, since trees need carbon to grow. And because warming temperatures cause drier soil, the whole cycle repeats in a feedback loop.
Sadly, this is not the only soil moisture feedback loop. As previously mentioned, lack of moisture stunts tree growth. It can even lead to tree death. Throughout California’s 2012-2015 drought, the soil became extraordinarily dry. This extraordinarily dryness killed trees with lack of water, which led to an increase of tree deaths by 55% (Goulden, M.L., Bales, 2019).
This, of course, leads to even less carbon sequestered, drier soil, and more tree die-off. For every 1 degree Celsius, California’s Sierra forests can expect tree deaths to increase by 15-20% with each drought (Goulden, M.L., Bales, 2019).
So how can we protect this soil and keep it moist? Obviously, we can’t be drilling pipelines all over the country to funnel water from Louisiana hurricanes to California redwoods. But what we can do is preserve and expand old-growth forests.
Interestingly, forests don’t actually make a place drier—they cool air via evapotranspiration, or the release of water from a plant's leaves (Ellison, Morris, Locatelli et al., 2017). That same moisture can create new clouds, and a forest’s ever-leeching moisture can have a profound impact on communities' weather thousands of miles downwind.
This means by preserving and expanding existing forests, we can counteract these feedback cycles with one of our own: more forests bring more rain, leading to moister soil, leading to more forests, and so on. We just have to act now.
Further Reading
Ellison, D., Morris, C. E., Locatelli, B., Sheil, D., Cohen, J., Murdiyarso, D., Gutierrez, V., Noordwijk, M. van, Creed, I. F., Pokorny, J., Gaveau, D., Spracklen, D. V., Tobella, A. B., Ilstedt, U., Teuling, A. J., Gebrehiwot, S. G., Sands, D. C., Muys, B., & Sullivan, C. A. (2017, February 9). Trees, forests and water: Cool insights for a hot world. Global Environmental Change 43, pages 51-61 . Retrieved March 13, 2022, from https://www.sciencedirect.com/science/article/pii/S0959378017300134?via%3Dihub
Goulden, M. L., & Bales, R. C. (2019, July 1). California forest die-off linked to multi-year deep soil drying in 2012–2015 drought. Nature News 12, pages 632–637 . Retrieved March 13, 2022, from https://www.nature.com/articles/s41561-019-0388-5
Humphrey, V., Berg, A., Ciais, P., Gentine, P., Jung, M., Reichstein, M., Seneviratne, S. I., & Frankenberg, C. (2021, March 31). Soil moisture–atmosphere feedback dominates land carbon uptake variability. Nature volume 592, pages 65–69. Retrieved March 13, 2022, from https://www.nature.com/articles/s41586-021-03325-5
Lal, R. (2016, November 16). Soil health and carbon management. Wiley Online Library. Retrieved March 13, 2022, from https://onlinelibrary.wiley.com/doi/full/10.1002/fes3.96
Raynaud, X., & Nunan, N. (2014). Spatial ecology of bacteria at the microscale in soil. PloS one, 9(1), e87217. https://doi.org/10.1371/journal.pone.0087217