Getting to the Roots (and Fungi) of Carbon Sequestration

This week, I found a paper that I’m calling the best of both worlds. Well, for me at least. This paper combines my past (and lingering) interest in island biogeography with a current interest in climate change and carbon storage.

If you have been reading my blog long enough then you already know my love of islands. They are just so darn useful. In the past, I have focused on oceanic islands, but lake islands are also really neat. These types of islands typically form when lower lying land areas fill with water, effectively cutting off higher areas from the mainland and making them into islands. As such, these islands usually already contain forest as opposed to an oceanic island that emerges from the ocean and must be colonized. A new study, published in journal Science, looks at a fire-driven boreal forest chronosequence on forested lake islands in northern Sweden. Such a chronosequence allows the study of soil carbon sequestration over time scales of centuries to millennia.

This new study looks at roots and their associated fungi (mycorrhizae) as sources of this stored carbon. I’m not going to spend the space to describe mycorrhize, but will, instead, send you over to my Free Market Fungi post for more information, if you need it. It is known that 16 percent of the global carbon stock is sequestered in soils. To date, most carbon studies of this type focus on aboveground leaf litter as the fundamental determinants of this carbon accumulation. But a large portion of photosynthetically fixed carbon is actually directed belowground to the roots and, subsequently, the mycorrhizae. Now, let’s add fire. It has been shown that when a forest doesn't burn, the soil and ecosystem carbon accumulate unabated, and in a linear fashion. Add this information together and it becomes a big deal when it comes to correctly allocating carbon, calculating the long term sequestration rates, and predicting how forests will respond to climate change and other environmental shifts.

The study sites were in two adjacent lakes, Lake Hornavan and Lake Uddjaure, in northern Sweden. The islands in these lakes were formed after the most recent glaciation and come in a variety of sizes. In terms of fire, larger islands burn more frequently because they are larger targets for lightning strikes. As a result, several of the large islands in these lakes have burned in the last century, whereas some of the small islands haven’t burned in at least 5000 years. This lack of fire leads to very thick humus layers (or organic layers towards the top of the soil column) on smaller islands, up to 1 meter thick!

The researchers divided islands into three size classes of 10 islands each: large (over 1 ha), medium (0.1-1.0 ha), and small (less than 0.1 ha). They took soil samples from these islands to look at the organic soil profiles and found that large islands accumulated 6.2 kg of C per square meter belowground with a mean time since fire of 585 years, medium islands accumulated 11.2 kg of C per square meter with a mean time since fire of 2180 years, and small islands 22.5 kg of C per square meter with a mean time since fire of 3250 years. Then they looked at the carbon dynamics across this chronosequence by analyzing bomb 14C. This allowed them to determine the age since fixation of soil carbon. Then they fitted a mathematical model to measurements of carbon mass and age distribution across the soil profiles for six of the islands (3 large, 3 small). This model revealed that the distribution of carbon mass and age could only be predicted when they included carbon from roots. This root-derived carbon accumulation was found to be larger on small islands (70 percent, that's a LOT!) than large islands (47 percent). They were able to explain the entire carbon sequestration difference on small islands from these root-derived inputs. The model also showed that small islands store a major proportion of their soil carbon in the deeper soil layers, those over 100 years old. However, below 20 cm depth, the root-derived carbon inputs were shown to be low and to decompose slowly. So the root-derived carbon input into the upper layers probably contributes to the long-term buildup of humus that is seen on these islands. But, as usual, that's not the end of the story.

We know that fungi play very important roles in forest ecosystems, both as decomposers and in root-assoicated carbon transport and respiration. So the researchers also profiled the relative abundance of major groups of fungi by depth in the soil profiles. They found that the upper soil layers are dominated by free-living saprotrophs (fungi that obtain their nutrition heterotrophically from non-living organic materials), and greater depths were dominated by mycorrhizal and other root-associated fungi. Their model suggests that these mycorrhizae live at the spots where the largest difference in carbon sequestration between the island size classes exists, which also tends to be the areas of highest root mass. When they ran tests for fungal biomass throughout each soil profile they found greater mycelial (the vegetative part of a fungus, consisting of a mass of branching, threadlike hyphae) production on large islands, but less mycelial necromass (dead stuff) on small islands. This suggests that the large production is counterbalanced by faster decomposition of mycelial remains. “Correspondingly, the 14C model indicated faster decomposition of root-derived [carbon] on large islands, despite inputs being conservatively constrained to be equal across all islands.”

I found these conclusions to be interesting because of the amount of soil carbon from roots and mycorrhizal fungi, especially on small islands. And although they saw less carbon accumulation on large islands, these islands have a greater root density and so should have more carbon allocation to roots and the associated fungi. Did you catch the contradiction? Well, in response to increased carbon dioxide, there will be an increase of carbon inputs to the roots which will accelerate the turnover of soil organic matter. This counteracts carbon accumulation and enhances nitrogen cycling through the microbial pools, an effect these researchers observed when they tested the C:N-ratios in the humus of large islands. This is much lower on small islands, possibly because of impared mycorrhizal nitrogen mobilization and the accumulation of nitrogen in fungal remains. This leads to progressive nutrient limitations, then leads to changes plant productivity, leading to changes in community composition, which leads to changes in total belowground carbon allocation, that leads to changes in fungi.

Definately starting to grasp the importance of the belowground dirty stuff. There’s a whole lot of carbon down there that we need to start looking at, accounting for, and seeing where it goes. We know that changes in the environment such as climate change, soil fertilization, fire suppression, and forest management make big differences to the aboveground stuff. It only makes sense that the belowground stuff is impacted as well.


Clemmensen, K., Bahr, A., Ovaskainen, O., Dahlberg, A., Ekblad, A., Wallander, H., Stenlid, J., Finlay, R., Wardle, D., & Lindahl, B. (2013). Roots and Associated Fungi Drive Long-Term Carbon Sequestration in Boreal Forest Science, 339 (6127), 1615-1618 DOI: 10.1126/science.1231923

If you would like some follow-up reading I suggest:

Treseder, K. K. (2013-03-29) Fungal Carbon Sequestration. Science, 339(6127), 1528-1529. (DOI: 10.1126/science.1236338) 

Also check out the write-up in Nature "Fungi and roots store a surprisingly large share of the world's carbon"

(image via Forest Keepers)

Tales from the Road: St. John, USVI

The discovery of St. John is the same as that of St. Thomas which is not all that surprising considering that they are right next to each other. Christopher Columbus is credited for discovering it in 1493 but the island had been visited and inhabited by indigenous peoples long before that, as evidenced by the petroplyphs found on the island. Early European settlers established sugar plantations worked mostly by African slaves. In the late 1600's the British and the Danes disputed over ownership of the island. On March 25, 1718 Danish planters from St. Thomas raised their flag over the first permanent settlement on the island at Estate Carolina in Coral Bay. The British continued to overtake the Danes on St. John until 1762 when they relinquished their claims to keep up good relations. Sugar, and also cotton, became the major industries with 109 plantations that covered almost the entire island. The adoption of a harsh slave code, the arrival of an elite group of African tribal rulers who preferred death to life as slaves, and a harsh summer of natural disasters lead to the Slave Revolt of 1733 when the island's slaves rose up and took control of the island. They held the island for seven months before the Danes enlisted the help of the French Marines to put down the revolt. The Danes built a prison, known as the Battery, in Cruz Bay intended to improve the treatment of slaves by making justice a government issue rather than leaving it to individual planters. They abolished slavery in 1848, prompted by a revolt on St. Croix. After this the main economy was mostly small scale subsistence farming, a hard life that cost the island much of its population. St. John was purchased along with St. Thomas to become part of the U.S. Virgin Islands. One of the things St. John is best known for is its large National Park which started as 5000 acres donated to the Federal Government by Laurence Rockefeller in 1956.

The Virgin Islands National Park includes 7200 acres of land and 5600 acres of underwater lands. It encompasses over half the island of St. John and almost all of Hassel Island. All together it is one of the most undisturbed and comprehensive Caribbean landscapes. There are several significant historical sites including archaeological sites that date as early as 840 BC. The large span of the park means that it covers a variety of ecosystems and forest types from dry to moist forests, salt ponds, beaches, mangroves, seagrass beds, coral reefs, and algal plains. Much of the vegetation on the island is second generation growth as the original vegetation was clear-cut to make way for sugar cane production. Some native species of tyre palm remain, but much of today's growth consists of introduced The animal species on St. John are abundant with 140 species of birds, 302 species of fish, 7 species of amphibians, 22 species of mammals, and many species of insects. The National Park's marine areas are also quite diverse with hundreds of species of fish and beautiful coral reefs  that are open to visitors.

Traveled up to Trunk Bay and the Underwater Snorkling Trail which is part of the Virgin Islands National Park.

Along the way was some beautiful scenery.

We would be driving along and then the trees would part to show beautiful views!

Most of the drive through the Park was surrounded by lush vegetation.

After a morning of snorkling, back in Cruz Bay we found the only ice cream place. Yum.

Part of Cruz Bay.

St. John as we leave on the ferry to St. Thomas.

Unfortunately all of my underwater pictures while snorkling Trunk Bay came out in a horrible blue-green washed color. Photoshop and I will be endeavoring to correct the color but the prospect looks dim. Bummer.


Some websites to find out more about the history of St. John:
St. John Historical Society
VInow's St. John History Page
The Beach's St. John History Page

And I really recommend looking around the National Park Service's Virgin Islands National Park website. It includes some great natural science information as well as links to other great websites, field guides, and animal and plant checklists.

Tales from the Road: St. Thomas, USVI

St. Thomas is one of the islands that make up the U.S. Virgin Islands (USVI). Credit for the island's discovery is given to Christopher Columbus during his second voyage in 1493, but was likely first inhabited by the Arawak and then Carib Indians. After that it became the home of pirates including legendary figures such as Blackbeard and Bluebeard. The deep natural harbor of what is now Charlotte Amalie was a particular favorite. In 1666, the Danes made their first colonization attempts for agricultural development, building Fort Christian at this harbor. The city of Taphus was the name given to what is now known as Charlotte Amalie and remained so until 1691 when it was renamed after the wife of King Christian V. The city was very important as a trading port in the late 1600's into the mid-1700's as a place where slavers, pirates, and merchants could anchor and trade. It became a free port in 1764. When steamships became popular in the 1840's St. Thomas became an important coaling station for ships running between North and South America. On  March 31, 1917, the island went from Danish rule to a territory of the United States to keep it from being taken by the Germans in World War I. They remained under U.S. Navy Rule until 1931, during which many major public works and social reform projects were undertaken The island has an elected governor since 1969 and holds a non-voting seat in the U.S. Congress. Currently, tourism is one of the biggest industries, the island is a popular vacation spot and cruise ship destination.

The view from my hotel room on the eastern edge of St. Thomas, near Red Hook.

*sigh* Just had to lay on this beach all day. Life is rough.

One of the docks in Charlotte Amalie.

Arrrr...pirates be anchorin' in Charlotte Amalie.

Doing some shopping in the historic district of Charlotte Amalie.

On the road back to Red Hook from Charlotte Amalie. What you can't see is me holding on for dear life as the driver whips around hairpin mountain turns.

Here are some good websites on the history of St. Thomas:
VInow's History Page
The Beach, USVI History Page
St. Thomas Visitor's Center Culture and History Page
Trip Advisor's St. Thomas History Page

Tales from the Road: The Florida Keys

The Florida Keys are located in Monroe County, the southernmost county in Florida and the United States. It is composed of a string of islands connected by U.S. Highway 1 (US1) which ends at mile marker 0 in Key West, 150 miles south of Miami. There are 1700 islands in the Florida Keys and they have been broken up into the Upper, Middle, and Lower Keys. The Upper Keys are defined as Key Largo south to Lower Matecume Key. This area is characterized by tropical hardwood hammock habitat and well developed and protected Atlantic-side reefs. The Middle Keys are from Long Key southwestward to the end of the Seven Mile Bridge. They are characterized by grass beds and hard-bottom communities with a good diversity of fish. The islands are relatively far apart which allows for rapid water flows and big tides resulting in turbid, underdeveloped or absent Atlantic-side coral reefs. The Lower Keys include all of the rest of the islands south and west of the end of the Seven Mile Bridge. They are relatively separate from the other islands but have a good amount of land area and well developed, protected Atlantic-side coral reefs.

Key Largo, the first and largest island in the Florida Keys, is focus of these pictures. Because of its reefs and fish diversity it is a popular spot for snorkelers and sport fishermen. Due to storms to the north and south we had quite a bit of chop, and more than usual turbidity, out on the reef but still managed to see some great stuff. After snorkeling we took a look around the mangroves.

A view from the first bridge driving into the Keys.

The above are some underwater pics from reefs off of John Pennekamp State Park.

Magroves in Key Largo, Florida

The Florida Keys is in (or makes up) Monroe County. Visit their website for great information, news and history:
http://www.monroecounty-fl.gov/

When it comes to the history of the Florida Keys this is one of the best websites I've found. It has some really great info and some amazing photographs:
http://www.mile-markers.org/

Visit NOAA's Florida Keys National Marine Sanctuary website:
http://floridakeys.noaa.gov/

If you are looking for some more info but especially if you are looking to visit the Keys then check out this site:
http://www.fla-keys.com/
Florida's Department of Enviromnmental Protection has a page on the watershed of this area:
http://www.protectingourwater.org/watersheds/map/florida_keys/

Learn about the history of the Conch Republic:
http://www.conchrepublic.com/history.htm

Frommer's Travel Guide takes you to the Florida Keys:
http://www.frommers.com/destinations/thekeys/0385010001.html

John Pennekamp State Park has some of the best snorkeling and diving in the Keys:
http://www.pennekamppark.com/

Phosphorically Speaking

A picture of the Everglades from my recent trip.

 Since I just visited the Florida Everglades I thought I would present you with a paper on the topic. This paper is from the journal Plant Ecology and discusses phosphorus distribution in the Everglades as it relates to landscape patterns and environmental gradients.

So you first need to think of how phosphorus naturally cycles through a system. An important part of this cycle involves the redistribution of nutrients and detritus at the landscape level. Basically plant and animal litter, dissolved compounds, soil particles, dry fallout (dust), and feces can be moved in and out of a system by various transport mechanisms - think wind, water, etc. The transfer of these compounds through a system and into or out of a food web can have consequences in terms of enhancing or limiting biological productivity. You can also end up with nutrient gradients that lead to landscape heterogeneity and geomorphological changes (large changes in a particular area that may impact movements of a material within the landscape - like the growth of certain plants or the attraction of groups of animals).

The Florida Everglades is a phosphorus limited, oligotrophic (lacking nutrients and having large amounts of oxygen) freshwater ecosystem. As such, it is composed of vast amounts of land that is covered by shallow water interspersed with tree islands and some other non-forested areas (usually sawgrass areas) that this paper refers to as marshes. Tree islands can be relatively uncommon and are usually about 10-70ha and stand about 0.2-2.5m above the surrounding area. These tree islands can be divided into zones based on the direction of water flow and elevation. Once trees become established in these areas the redistribution of nutrients, including phosphorus, begins. This paper takes a look at the redistribution of phosphorus from marshes to tree islands, hypothesizing that phosphorus levels should be higher on tree islands, particularly because of their elevation.

Figure 1 from the article showing a logitudinal cross section of a tree island.

Alright. So how to go about measuring this? Well, first, like all good scientists, they did a literature search, gathering all of the existing data on phosphorus measurements from fixed tree islands. Then soil cores were collected from the head, near tail, and far tail along the island. These cores were then analyzed for TP (total soil phosphorus) and bulk density.

For higher soil depths (10cm), on the heads of the tree islands TP was found to be higher than in marshes, but it was slightly variable. The near tail TP was about 20 times lower than that of the heads, and the far tail TP were similar to the near tail means. For lower soil depths (10-20 and 20-30cm) the TP concentrations were higher for heads and near tails than the 10cm cores, but were lower on far tails. The head TP ratios were found to be positively correlated with elevation, but as there is little data for high elevation spots the authors point out that this could be an artifact. They also point out that high elevation areas could simply be older and as such have larger trees. Older equals more time and more accumulation of phosphorus, but as the age of these tree islands can only be speculated at it is hard to know. Or maybe it is that higher elevation equals drier conditions. Dryer equals no washing away and more accumulation. Anyway, you get the point right?

The researchers took a look at this accumulation rate specifically. They found that the rate of annual phosphorus accumulation was higher in the heads. When comparing the TP of the heads to the TP ascribed to mean annual inputs associated with wet and dry fallout (remember the constituents of the cycle?) they found that less than 10% of the annual TP input in the heads can be associated with the fallout. This means that just 1m^2 of a head is sequestering the same amount of phosphorus as 10m^2 of the marsh.

Think about that in terms of distribution and heterogeneity. Also think about it in terms of Everglades restoration. Looking at the history of the Everglades you see a drastic decline in the number of tree islands over time - up to 60% since 1940! Now lets throw some more numbers into your thinking. If tree islands sequestered around 67% of the TP entering the Everglades and you factor in the decline of tree islands (up to 90% locally), the decline of wading birds (up to 90% over the last 100 years), and various draining practices by agricultural, mining, and urban developers, what do you get? Not necessarily less phosphorus (some models indicate a possible increase), but rather a more homogeneous ecosystem, phosporically (and likely biotically) speaking. This homogeneous effect can be seen in areas where agricultural runoff (that has bunches and bunches of phosphorus in it) are high - the landscape patterns have disappeared. You get less tree islands and more marshes and wet prairies dominated by sawgrass.

Is there a take-home message? Well, considering the Everglades restoration is a multi-million dollar investment, I would hope so. Perhaps the message is to use tree islands not as just a performance measure but to actually designate them as part of the restoration itself in order to ensure diversity and long-term survival in the system.

Here's the paper:
Wetzel, Paul R. et al. (2009) Heterogeneity of phosphorus distribution in a patterned landscape, the Florida Everglades. Plant Ecology: 200(1), 83-90. (DOI: 10.1007/s11258-008-9449-3)

Collecting Debt

Have I said "yay for islands!" lately? Well, yay for islands!

This paper, published in the journal Ecography, looks at "extinction debt." So when we look at extinction we see that a majority of the documented extinctions of species are of those that occurred on oceanic islands. Typically these extinctions are a result of habitat destruction and fragmentation associated with human colonization, invasive species, etc. But extinction isn't an instantaneous event. A species can be reduced to a small population but it can take several generations for extinctions to take full effect. The time lag represents an "extinction debt," a 'future ecological cost.' I posted a story called A Little Patch of Home which goes over some of the theory of island biogeography, including the species-area relationship. The methods commonly used for estimating future extinctions are extrapolated from this.

This study takes place in the Azores, a system that has lost >95% of its original native forest during the last 6 centuries in addition to being very isolated and having a significant number of endemic species. The authors used the well-documented historical sequence of deforestation to calculate realistic and ecologically-adjusted species-area relationships. The results showed drastic levels of extinction debt. Over half of the arthropod (insects, spiders, etc.) species might eventually go extinct due to habitat loss. The severity of the deforestation has reduced the opportunities for forest-dependent species to cope with environmental change. The analysis shows that the taxa Araneae (spiders) and Coleoptera (beetles) are at greater risk of extinction than the third studied taxa, Hemiptera (true bugs). This could be because spiders and beetles have more species that are isolated to single islands rather than being found on multiple islands, although this is probably not the only cause. The results suggest the need for caution when thinking about conservation. Generalizing across species based on data for ecologically different taxa is inadvisable. For this reason the authors state that "large-scale conservation efforts need to be implemented if the high extinction debt we have identified is to be deferred or avoided" and that "the conservation of the Azorean natural heritage...will largely depend on establishing an integrated large-scale strategy to manage both indigenous and non-indigenous species while simultaneously protecting the remnants of native habitat and...increasing their extent."

Read more here:
Triantis, Kostas A., et al. (2010) Extinction debt on oceanic islands. Ecography: 33, 285-294. (DOI: 10.1111/j.1600-0587.2010.06203.x)

A Little Patch of Home

Yep, I love me some stories about islands. Have I said that before? Probably, especially considering that's what my masters degree was about. Anyway, this is a new study published in the journal Ecology looking at how land birds colonized the Lesser Antilles.

I can, and have, written long and in depth on this topic, but I'll try to quickly sum up the Equilibrium Theory of Island Biogeography (ETIB). First you need an isolated habitat, island or otherwise -- they are great for study because they are relatively small, replicated, and isolated. The whole formation of a theory thing started when scientists noticed that as the size of a patch increased the number of species increased (not always necessarily the number individuals of a species), this is known as the Species-Area Relationship. Yeah, pretty logical right? Let's keep going with the concept though. Let's say you have a brand new island, species will immigrate to the island -- how many, which ones, and who stays? All that depends on the island itself -- how big it is and how isolated it is from the source population (usually the mainland but also other islands). An island far from the mainland is harder to get to and so has fewer species. A small island is harder to survive on and so also has fewer species. Get where I'm going with this? Now the ETIB says that there is an equilibrium point between immigration and extinction -- a balance between these two processes that determines the number of species on an island. Over time immigration increases the number of species on the island, but as you get more species on an island there are more species that have the possibility of going locally extinct (see the embedded graph).

This study looks at not-so-much the equilibrium point itself, but rather the colonization of island by bird species.

As you might guess, birds are pretty good colonizers, considering that they can fly. But also think about bird species themselves, some are widespread and undifferentiated while others are pretty limited in their distributions and can be rather specific. On islands, because of their size and isolation, this can can mean some species will exhibit morphological or genetic differentiation between individual island populations and/or the mainland source population. This study looks at ecological and geographic distributions of source population to see how recent/continuing colonizers, old colonizers, and non-colonizing/mainland species are different from each other. When you are thinking about these categories, think about them in terms of time and how long it takes a population to genetically differentiate.

In terms of ecological and geographic distribution, the author found that recent colonists tend to inhabit open habitats and are pretty abundant and widely distributed on the islands they are found. Conversely, old colonists tend to inhabit forests and their abundance and distribution depends on the species and on the island. If an old colonist has recently spread to other islands then they act like recent colonists in that they have relatively greater abundance and broader habitat distributions. In general, most of these data showed that the distributions of source populations is broader for recent colonizers than for old colonizers.

To read much more on this topic, here's the article:
Ricklefs, Robert E. (2010) Colonization of the Lesser Antilles by land birds. Ecology: Vol. 91(6), 1811-1821. (DOI: 10.1890/09-0682.1)

(image from birdquest.co.uk and algebralab.org respectively)