What Coral Reefs Can Teach Us about California Forests
Out of sight, out of mind: the vast majority of logging in the US takes place on private, not public, land. The effects of timber harvest are not always at the forefront of our thoughts — or our eyes. But if it’s the unseen consequences of clearcut logging that are most easily overlooked, they are also some of the most far-reaching. Our environment is an interconnected fabric, and one thread undone can unravel the system in unexpected places.
A remarkable, if surprising, consequence of aggressive logging is its damage to tropical coral reefs. It’s not intuitive; inland forests can be miles from the coral in surrounding waters. Certainly at a macro level, extreme logging hurts the ocean by accelerating global climate change. (Read more about the connection between clearcutting and climate in our previous blog post!)
But tree harvests have a very direct and local impact on coral reefs as well, causing erosion which can smother corals. Shallow and warm-water reefs, like those in Madagascar or Australia, show the clearest signs of this dangerous ripple effect. What can we take away from reefs on distant islands? After all, California boasts multiple deep-sea coral species, but no tropical ones. And there is not yet a large body of research on how clearcutting itself affects California’s marine life.
However, we have deep knowledge of clearcutting’s harm to the water quality of streams. Clearcutting a forest adds sediment to the river basin around it, threatening aquatic wildlife. This is the same phenomenon we see in the case studies of reef decline. The similarities suggest that clearcutting’s impact on ocean health is a promising area for further research. Most importantly, the lessons from vibrant corals around the world were learned at a price. They serve as reminders that extreme logging has effects beyond the obvious. When we fail to protect our forests, it’s not only the trees that suffer.
For decades, scientists have researched the ways in which forest harvests change watersheds. Destructive logging practices, like clearcutting, are known to increase sediment in nearby rivers. A study of 28 coastal watersheds in northern California discovered that forestry management best practices were still not enough to prevent the erosion from heavy logging. The results found that the higher the rate of timber harvest, the higher the turbidity — essentially, a measure of how murky the water is — in neighboring streams. This kind of contamination poses problems even before it reaches the ocean, reducing clean drinking water and harming fish such as salmon or trout.
As sediment leaves the forests, it flows down to the sea. Human activity and land management are perhaps the most significant factors in the amount of sediment that reaches the ocean. In the San Francisco Bay Area, small but urbanized tributaries account for the majority of suspended sediment in the Bay, despite providing only 7% of average river flow. In Queensland, Australia, home to the Great Barrier Reef, land clearing and livestock use are the primary causes behind 90% of the sediment that enters coastal environments.
High sedimentation can pose a problem for any ocean with tropical reefs. In one review, coral experts compiled a comprehensive analysis of no less than 77 scientific studies, detailing how sediment and turbidity affect dozens of coral species. Shallow-water corals are formed by a symbiotic relationship between algae, called zooxanthellae, and coral polyps, tiny animals related to the jellyfish. The algae use sunlight to photosynthesize, providing the polyps with valuable oxygen and nutrients. When the water is murky, not enough light reaches these algae. As the zooxanthellae fail to photosynthesize effectively, many coral species see a corresponding drop in nutrition, growth, and reproduction. Eventually, the corals may starve to death. Case in point, analysis of data in Barbados suggested that a 28% increase in sediment levels caused a 20% drop in coral growth.
Scientists can, then, observe how logging smothers reefs and blocks the light that the coral need to thrive. For instance, a 2017 study in Biological Conservation measured the direct impact of logging on reef habitats. The team conducted underwater visual surveys over a span of multiple years, diving along reefs on the western end of the Solomon Islands. The researchers compared two islands: one subject to commercial logging, and another which was not. Specifically, they measured the presence of young bumphead parrotfish, which live under the protection of the reefs. The bumphead parrotfish, a threatened species, is a striking sight due to the iconic lump on its head and its large size. (It can grow to over four feet in length and up to 100 pounds in weight!)
While the researchers were collecting data, the local community had already noticed how logging was altering the reef ecosystem. The study authors write, “We heard numerous anecdotal accounts from local spearfishers of how sedimentation from logging had dramatically reduced the abundance of juvenile B. muricatum on fringing reefs adjacent to logged islands.”
The ultimate analysis yielded sobering results. The study estimates that 1,931 hectares (about 4,770 acres) of coral habitat were lost due to logging, over half of the area suitable for juvenile bumpheads in the Kia district. It further estimated that logging reduced the presence of juvenile bumpheads by 24 times, a combined effect from habitat loss and sediment harming the bumpheads directly.
A study in Nature Communications even found that deforestation eclipses climate change when it comes to sediment risk. Researchers modeled how forest harvests in Madagascar would affect local coral reefs. They found that while climate change in isolation might actually lower the amount of sediment washed out to the reefs, logging itself increased sedimentation to a much larger degree. The study model projected that sediment would increase by another 54–64% as the current natural forest was logged.
The researchers note that since humans arrived in Madagascar around 2000 years ago, 90% of the original forest cover has disappeared.
The scars that clearcuts leave behind are not only surface-level. They spread deep into our environment and into the water itself. For marine and riparian life, there are urgent consequences caused by aggressive logging, even before climate change transforms our soil, rivers, and oceans. The next time you hear of clearcut harvests, taking place not-so-close to home, you might stop to remember that they reach farther and strike closer than they seem.
Interested in learning more about the studies covered in this article?
For clearcutting and its effect on water quality, you can check out the California watershed study on logging and turbidity, an account of clearcutting in Malawi that reduced access to safe drinking water, and studies on salmon and trout decline after clearcutting.
For data around human causes of sedimentation in the ocean, take a look at these studies from the Bay Area as well as Queensland.
For more on how sediment affects corals, a comprehensive review (focused on ocean dredging) goes into depth on 89 coral species around the world. You can also read more about the research on bumphead parrotfish or why deforestation outweighs future climate change when it comes to sedimentation.
