Article Information: Coldren, G.A., Langley, J.A., Feller, I.C., and Chapman, S.K. 2018. Warming accelerates mangrove expansion and surface elevation gain in a subtropical wetland. Ecology: doi: 10.1111/1365-2745.13049
Is “Blue Carbon” really blue?
Have you ever heard of “blue carbon”? Chances are, you probably haven’t, but I’m sure you have heard about how our coastal ecosystems are super important! Blue carbon sinks, such as mangrove forests and salt marshes (along with seagrass beds), store TONS of carbon, and you guessed it, are on the coast. No, blue carbon isn’t really blue, but because it comes from the ocean (which is “blue”) and is captured and stored by blue carbon sinks for long periods of time. You already know that carbon dioxide is in our atmosphere, so it might not come as a surprise to you we that need carbon sinks to keep the balance. Unfortunately, human activities are offsetting this balance, causing our planet to warm and impacting our blue carbon sinks.
Mangroves are shifting their ranges!
Mangroves are found in tropical and subtropical areas of the world. If you’ve ever been kayaking in the tropics like Florida or the Bahamas, you have probably seen these magnificent trees with their roots extending out of the water every which way! It’s pretty cool to see. Mangroves provide us with many services (“ecosystem services”): they protect us from storm surges, improve water quality, provide a habitat for a variety of wildlife, and of course, store carbon. Because of climate change (a.k.a. global warming), mangroves are expanding their ranges northward in the Northern Hemisphere, and southward in the Southern Hemisphere. They are on the move. But what are the implications for ecosystem services?
Where are mangroves going?
Mangroves are encroaching, or moving into, salt marsh ecosystems. This “woody encroachment” could change what’s going on above ground and below ground, like the microbial community, plant height, root production, surface elevation, and carbon storage! In general, warming can really impact how plants in these ecosystem grow, and their ability to keep up with sea level rise and build land! In turn, carbon storage might be impacted. So, what’s really going on above and below ground in these ecosystems?
Getting to the “root” of what’s going on!
A team in Florida led by Glenn Coldren, Samantha Chapman and others at Villanova University set out to answer this question: how is climatic warming changing plant growth (above- and below ground) and surface elevation gain? To answer this question, the team set up large warming chambers in Florida at the Kennedy Space Center inside the Merritt Island National Refuge (Figure 2). This unique location is actually right where mangroves are encroaching into salt marshes! That’s where the scientists set up their chambers: right on the transitional line between the two ecosystems.
Along with the warming chambers, they also had plots that weren’t warmed, so they could compare the warming effects on this transitional zone. In both warming and non-warming plots, they measured mangrove and salt marsh growth, the “percent cover” (how many mangrove and marsh species, percentage-wise, there are), and below ground plant mass (including roots and rhizomes). They also used something called surface elevation tables (SETs) to see how much soil is building up.
After two years of warming, the researchers found that warming definitely had an impact on these ecosystems. They found that mangrove plant height and abundance (how many mangroves or cover) increased, and that the expansion of mangroves into salt marshes increased faster in warming plots versus non-warming plots.
Why is this a big deal? As mangroves are moving into salt marshes, the plant communities are changing. We need to know if these ecosystems are going to keep up with climate change and sea level rise. As for surface elevation gain (soil-building), they found that it increased due to warming as well. This is a good thing! But, why is it happening?
The take home message: warming might not be so bad for mangroves
With our climate warming, there are less “freeze events” or cold snaps that keep mangroves in check. Now, mangroves are able to survive at high elevations and are moving poleward, encroaching into salt marshes.
The results of the study show two things: 1) warming can enable the plant communities to shift from a salt marsh-dominant community to a mangrove-dominant community and 2) higher surface elevation can help these coastal wetlands keep up with sea level rise. This is a big leap in the direction of finding out how coastal ecosystems are responding to global warming!
The most important message here, perhaps, is that mangrove expansion in a warming world may provide us with better resistance and resilience to storms. Higher surface elevation in these ecosystems may mean mangroves and marshes will sustain and survive an increasingly high sea level. Above all, blue carbon sinks like mangroves and marshes are important to conserve. They not only store carbon that would be in our atmosphere further warming our planet, but they provide us with many ecosystem services. Understanding how these ecosystems are impacted by climate change will further ready us for what a warming world will bring.