A lush canopy is a defining characteristic of most forests on the planet. But canopy-forming species can be particularly vulnerable to disturbance and environmental change – even forests that lie under the sea. But what is a forest without its trees?
Researchers from the University of Virginia and the University of California, Santa Barbara are tackling this question by examining how the productivity, or growth, of kelp forests changes when its largest species, the one that forms its canopy, does not is no longer present.
“Primary productivity is the fuel that powers ecosystems and the basis for biodiversity, nutrient cycling and carbon sequestration, so understanding what controls primary productivity is one of the most important questions in ecology,” said Max Castorani, assistant professor in UVA’s Department of Environment. Sciences.
After a 10-year experiment that involved repeatedly removing giant kelp from their ecosystems, lead researcher Castorani and his colleagues found that although the smaller understory algae benefited from the brighter conditions resulted, they were not able to compensate for the loss of productivity. of giant kelp, a seaweed that scientists have dubbed the “redwoods of the sea”. The study of the team, published in Letters of ecologyconcludes that the giant canopy-forming kelp, the largest of its kind, plays a fundamental role in driving the productivity of the coastal ecosystems to which it belongs.
In terrestrial and aquatic forests, a canopy performs many functions. It forms habitat, creates microclimates, and forms the basis of entire food webs, from herbivores to carnivores to organisms that feed on decaying organic matter. The smaller plants that make up the understory also contribute to the primary production process or the creation of organic matter from sunlight, but before the study began it was unclear whether these plants could do the same job.
Speculating that understory vegetation could compensate for the loss of giant kelp, the team designed an experiment that would mimic the conditions created by natural disturbances, such as ocean storms and heat waves, by clearing the canopy of forest patches, then recording how much light reached the forest floor and what growth, or biomass, was produced by the exposed understory. Because giant kelp grows and dies quickly, the team was able to examine many cycles of disturbance and recovery over the course of 10 years; whereas a similar study could have taken decades or centuries in a terrestrial forest, said study co-author Dan Reed, a research biologist at UC Santa Barbara.
Located just offshore, the testbeds studied by the Santa Barbara Coast Long-Term Ecological Research Project, or SBC LTER, are part of the National Science Foundation’s LTER Network, set up to study ecological processes over longer periods. In order to draw stronger conclusions, the team conducted the study under a variety of conditions, conducting the experiment at five different locations in the Santa Barbara Channel. Some were ideal for algae growth, with plenty of hard surfaces for algae growth and few sea urchins that feed on kelp and other algae. Other sites provided less than ideal conditions for growth.
At each site, scientists delineated three plots: one to serve as a control, a second cleared of giant kelp annually to mimic the effects of winter swells and storm surges that can uproot forest cover, and a third cleared every year. quarters. to provide researchers with a picture of what happens when giant kelp is simply absent, a scenario that may become more common in the future as ocean conditions change.
Drawing on years of experience in collecting samples and taking measurements, the team used non-destructive methods to estimate algae growth and productivity, which allowed them to more accurately establish the relationships between biomass and measures such as canopy cover or algal size for about 30 different species.
To calculate the productivity of these efforts, it was necessary to rely on even more previous work. Study co-authors Robert Miller and Shannon Harrer, researchers at UC Santa Barbara, had previously developed a bio-optical model that predicts algae productivity under a particular amount of light. By feeding this model light and biomass data from field sites, the team was able to calculate the productivity of the kelp forest with greater accuracy.
During the study, Castorani and his colleagues found a gradual reduction in canopy productivity and an exponential increase in understory algae productivity, especially in high-quality habitats. As they had expected, they also found greater effects with more frequent canopy removal as additional light became available for the understory to grow. But overall, the understory proved unable to fully compensate for the productivity of the lost canopy, even under ideal conditions.
“It’s an important finding,” Harrer said. “This underscores the importance of considering site-level factors when we assess the impacts of disturbance on ecosystem function.”
Even though the understory may compensate for some of the lost productivity of the canopy, this does not mean that the total impact of this productivity will be the same. A large amount of giant kelp ends up washing up on beaches, for example, which also affects this ecosystem.
“Southern California has some of the greatest diversities of beach organisms in the world,” Miller said, “and that’s because of giant kelp, which is the most significant source of beach kelp.” Other algae simply cannot fulfill this role, he explained.
Understanding the long-term, broader effects of changes to kelp canopies is one of the key aspects of the study, explained Castorani, who was a researcher at UC Santa Barbara before coming to UVA.
“These changes were slow to develop, even in a very dynamic, fast-growing environment like a kelp forest,” he said. “Most field studies last less than three years. If we had conducted our experiment for only a few years, we would have missed these dramatic changes and come to different conclusions. »
The results could have important implications for the future as the climate continues to change. Stronger and more frequent storms, warmer waters, and changes in ocean chemistry can create unfavorable conditions for giant kelp. Climate change and human activities threaten terrestrial forests. This could alter the productivity and biomass of the ecosystem and the complex interactions between different species.
The team stopped removing kelp from the experimental plots in 2018, but they continue to monitor the sites and track changes in the ecosystem as it recovers, in addition to conducting a long-term experiment. which will examine competitive interactions between invertebrates and understory algae.
“Kelp forests play a vital role in coastal environments and provide many benefits to society,” Castorani said. “We need long-term experiments like these to understand how future ecosystems will function in a changing ocean.”