Featured Image Credit: Abby Lewis
Reference: Caspi, T., Hartz, L. A., Soto Villa, A. E., Loesberg, J. A., Robins, C. R., & Meyer III, W. M. (2019). Impacts of invasive annuals on soil carbon and nitrogen storage in southern California depend on the identity of the invader. Ecology and Evolution. https://doi.org/10.1002/ece3.5104
Unrealistic beauty standards… for plants
In Southern California, the seasons work backwards. Spring comes at the right time (as evidenced by this year’s super bloom), but then leaves fall and plants hunker down for a long, hot summer. December and January bring much-needed rain, allowing plants to come alive with bright green winter colors. For kids growing up with U.S. media and curricula that show a green summer and white winter, this can be a bit hard to understand.
Last year, I worked closely with an elementary school teacher in Southern California to plan a field trip for her students. The standardized curriculum at her school has the kids make models of trees in different seasons: pink in the spring, green in the summer, red and orange in the fall, and white in the winter, even though this is not at all representative of the area in which they live.
When we brought the students on a field trip to see native ecosystems that summer, they saw that many of the plants were brown and leafless, and they had trouble understanding that the plants were still alive and ecologically valuable. This doesn’t only apply to elementary students. Even with adults, it’s hard to advocate for the value of an ecosystem that looks dead for a substantial chunk of the year.
Carbon, climate, and California sage scrub
Without the ability to sell the value of these ecosystems based on looks alone, scientists have done a fair amount of research to assess the ecological importance of native ecosystems in Southern California. Tal Caspi, a student at Scripps College, and several of her student and faculty colleagues from the Claremont Colleges, wanted to answer a question that could add to this body of knowledge: they were interested in figuring out what role the native ecosystem plays in climate change and how things would change if native plants get replaced by non-native grasses.
Specifically, the team was interested in one particular type of native habitat: California sage scrub. California sage scrub used to cover most of the coastal region of Southern California, but development has taken away most of its range. In areas that survived development, nearly 50% of the sage scrub has been lost due to the invasion of non-native grasses that spread from people’s yards into natural habitats.
Caspi’s team studied nine sites across two years. At each site, they assessed the amount of carbon that is stored in the soil in native habitats, and how much carbon could be released as greenhouse gasses if the landscape became overrun by non-native grasses. They also measured several other characteristics of the soil at each site so that if they saw a difference between the two habitats, they would be able to analyze what chemical and physical factors might be causing this difference.
Caspi and her colleagues’ results underline how important these ecosystems are, making the loss of this important habitat even more tragic. According to the study, significantly more carbon is stored in the soil of native sage scrub habitats than in the non-native grassland. When land is invaded by non-native grasses, more than 940 tons of carbon are lost per square kilometer! This carbon is most likely released to the atmosphere in the form of greenhouse gases that contribute to global climate change. If you do the calculations, this means that each square kilometer that gets converted to non-native grassland causes the same amount of greenhouse gas emissions as driving nearly eight million miles in an average car.
Interestingly, when the land was invaded by non-native mustard plants researchers didn’t see the same decrease in carbon storage, indicating that the type of non-native plant matters a lot when you try to predict the effects of non-native species on carbon storage.
Why does this difference in carbon storage exist? One reason draws from the fact that the plants in the non-native grassland are much shorter and more sunlight is able to reach the ground. Ultraviolet radiation from the intense California sun helps break down dead leaves, leading to faster decomposition. Faster decomposition means that less organic matter gets buried in the soil, and more gets eaten by microbes. When microbes eat organic matter, they breathe out carbon dioxide (just like we do), and carbon dioxide goes up into the atmosphere where it traps heat near earth. So, more sunlight on the ground means more greenhouse gasses released by microbes.
Beautiful inside and out
California sage scrub, like many other ecosystems around the world, doesn’t really subscribe to the mainstream version of seasons. But the fact that it goes brown in summer absolutely does not mean that the plants are any less healthy or important. The few remaining tracts of sage scrub are home to a number of rare or endangered species, many of which would struggle to survive in any other habitat. Furthermore, as this study indicates, sage scrub habitat plays an important role in storing carbon in the ground, and the invasion of non-native grasses has the potential to disrupt that ability.
While some people enjoy perfectly manicured green lawns, these grasses can spread to the surrounding natural areas and have devastating effects. Regardless of where you live, it is important to take a minute and consider how your choice of landscaping plays into the wider ecology of the region. And who knows, you may discover some beautiful native plants in the process!