Applying equations to forests- carbon storage across environmental gradients in northeastern Puerto Rico

If you’ve ever read about carbon storage or sequestration (and if you’ve read my blog before, there’s a good chance you have), have you ever wondered how scientists come up with the numbers they love to throw around? How do we “know” that there are 684-724 petagrams of carbon to a depth of 30 cm in all the soils across the whole wide world? How could anyone possibly know that?

We don’t. And we can’t. But there are those crazy enough to try, and trying essentially boils down to  sampling, sampling, sampling, then a whole lot of extrapolating. If you imagine a landscape in which there is a continuous but highly variable distribution of carbon, the only way we’re gong to get any sort of meaningful estimate of the total is to dig a lot of holes. The points you see below represent a lot of holes that I actually helped dig! (I’m one of those crazies). I’ve thrown them up over this topographical surface to give you a sense of just how variable carbon can be across space.

Rather than subject you to a discussion of a ecosystem carbon storage, I’d like to share some pretty pictures I’ve been putting together. I’m attempting to create a model for carbon storage across a roughly 10 km area of forest in northeastern Puerto Rico that is underlain by two bedrock types, contains three distinct forests dominated by different tree species, and has broad gradients in temperature and rainfall across the topographically varied landscape.

Bluer regions represent areas of greater carbon storage while yellower regions store less carbon. Essentially I’ve created an image made of a series of pixels -here, the resolution is coarse enough that you can distinguish individual pixels around the edges. For each pixel, a predicted carbon value has been calculated from a very simple equation that takes bedrock, forest type and elevation into account.

Breaking it down by forest type…

 

 

That’s all for now, hopefully I’ll have more and more interesting pictures to show in the future.

Tree girdling reveals that photosynthates drives soil activity late in the growing season

The role of trees in nourishing the soil has long been understood by ecologists, but the extent to which trees control belowground processes remains unclear. This is due in large part to the difficulties associated with measuring root activity and the dynamic interactions between roots, mycorrhizal fungi, and the soil environment.

To develop a better understanding of how photosynthates, the sugars produced by photosynthesis, influence patterns of growth and metabolic activity in the soil, a group of ecologists in Sweden decided to conduct a large scale girdling experiment. Tree girlding involves stripping tree bark all the way to the depth of the tree’s xylem, the long tubes that transport water from the soil to the leaves using negative pressure and the difference in water potential between the soil and the atmosphere. Xylem is located well inside the living tissue of a tree, but most importantly for a girdling experiment, it is located directly inside the phloem. Phloem is the piping that runs down the tree and transports photosynthates produced in the leaves via gravity into the rest of the plant. Thus by stripping a tree down to its xylem, the researchers are removing phloem and cutting off the supply of photosynthates to the roots and the soil. This allows them to study the effect of photosynthate removal on the soil without influencing roots, water transport or any other soil parameters.

In this large-scale experiment conducted in a boreal Scots pine forest in northern Sweden, six 900m² plots were chosen and rouglhy 120 trees per plot were girdled.  In each plot, half of the trees were girdled early in June and half in late August. Early and late girdling were used to detect whether phenological (seasonal) differences affected root photosynthate production.

The results of this experiment were striking. In the early-girdled plots, soil respiration declined by 27% compared to control plots within the first five days of girdling. (Remember, “soil respiration” refers to the amount of CO₂ released by the soil. It is a proxy for total soil metabolic activity, including the respiration of microbes, fungi, nematodes and other small soil invertebrates, and even roots themselves! Thus more respiration = healthier, more metabolically active soil.) By the end of the growing season, roughly 50% less respiration had occurred in the early-girdled plots. The occurrence of ectomychorizal fungi, an essential nutrient-acquiring symbiote for most plants, was also dramatically reduced. The late-girdled plots responded even faster, with respiration declining almost 40% within the first 5 days of girdling. Interestingly, the researchers found less response to girdling toward the edge of the girdled plots, indicating that soil organisms here were acquiring some photosynthates from neighboring, ungirdled trees.

The more rapid declines of soil respiration in the late girdling plots fit with our current understanding of how trees use their photosynthates. Early in the growing season, and especially in conifer forests, trees allocate most new photosynthates towards shoot, bud and needle production. Simultaneously, trees begin tapping their root carbon stores to enhance above ground growth. Later in the growing season more photosynthates are sent belowground to nourish the soil community. By this point root carbon stores are also diminished.  Girdling trees later in the growing season cuts off carbon supply to the soil at precisely the moment when it is being ramped up.

The flux of photosynthates to the roots has a big impact on belowground processes, with a clear seasonal component. In moving forward in our understanding of whole ecosystem carbon balances, understanding that trees are conduit, connecting the earth to the atmosphere and transforming both in the process, will, I believe, be an essential paradigm to adopt.

 

Reference:

Ho¨gberg P, Nordgren A, Buchmann N et al. (2001) Large-scale forest girdling shows that photosynthesis drives soil respiration. Nature, 411, 789–792.

 

 

 

deep sea carbon cycling: more dynamic than we thought?

For years, scientists have speculated that deep sea carbon may have played an essential role in past climate change episodes. Specifically, it has been suggested that the C bound in seafloor sediment has undergone thermodynamic alterations in the past to due upwellings of molten magma from the mantle. Magma may have triggered the release of CO2 and methane into the upper ocean and eventually the atmosphere. Evidence suggests that the Paleocene-Eocene thermal maximum, which lasted approximately 100,000 years, may have been triggered in part by the release of greenhouse gases from the seaflooor.

Despite these speculations about deep sea carbon influencing past climate, little research has been done on the role of seafloor carbon in the present day C cycle. In some ways, this is surprising given the enormous amount of attention being payed to global C budgets and possible means of C sequestration. It is generally assumed that the deep sea represents a huge “carbon sink”, to which organic C from the upper ocean enters and does not emerge again for thousands to millions of years. This would suggest that whatever carbon-cycling processes are occuring at the seafloor are not powerful enough to cause a net carbon release.

Recent research published in Nature Geoscience suggests otherwise. Several case studies have demonstrated dynamic processes occurring on the ocean floor can in fact lead to a net release of greenhouse gases. Spreading seafloor centers- regions where oceanic plates pull apart-  are a site of magma activity and hydrothermal venting. Hydrothermal vents release a variety of hot, mineral-rich fluids that can support a diverse microbial and invertebrate community. At one such spreading center in the Gulf of California, magma is intruding into thick organic basin sediments. These sediments have long been thought to sequester C, however, it now appears tht their heating is causing the release of methane into the upper ocean.

In the Northeast Pacific, another intriguing deep ocean C cycling system has been discovered. Here, microbes are converting ancient inorganic C into dissolved organic C, which is subsequently released to the overlying ocean. This discovery contradicts the general belief that ancient deep-sea C is highly stable and not accessible to microbes.

Other distinct seafloor C sources are rapidly emerging around the world, as improved technology and a heightened interest in seafloor processes are accelerate the pace of discovery. However, the contribution of such “point sources” to global C budgets is still highly uncertain and far more research is needed to come up with even a rough estimate of global deep sea C sources. Nonetheless, it would seem that we can no longer consider the deep ocean a black box of C sequestration, and that we should think carefully about the ramifications of introducing more carbon- either accidentally through the introduction of dissolved greenhouse gases to the ocean, or intentionally as part of a climate change mitigation strategy- to a system that is clearly more dynamic than we once thought.,

Reference : “Deep Sea Discoveries.” 2011. Nature Geoscience: Letters. Volume 4, Page 1.

Carbon sequestration in soils: sources, sinks and pitfalls

When we talk about “sequestering” carbon in the soil to mitigate anthropogenic climate change, what are we really saying as scientists, and what is the public getting from it?  And perhaps even more importantly, what implicit assumptions are we as scientists making that may affect the validity of the widely held belief that increased C storage in soils is a good thing, and can offset our CO2 emissions?

In a recent soil carbon series in the European Journal of Soil Science, one review paper attempts to address these issues. “Carbon sequestration” has become a popular idea among policy makers and scientists alike. As anyone who keeps up on ecosystem science literature will know, C sequestration has also become an explicit motivation for numerous studies of soil and ecosystem properties.

Current best estimates claim that worldwide, soils store 684-720 Pg of C within the upper 30 cm, and 1462-1548 Pg to a depth of 1m. (For anyone not familiar with these terms, 1Pg = 1×10ˆ15 gram). The upper 30% of the soil profile to 1m clearly stores a disproportionate amount of C; this ecologists know to be the zone where continuous inputs from roots and organic matter replenish C. To put these numbers in an ecosystem context, the amount of C stored globally in soils to 30cm is twice the amount  of C stored as CO2 in the atmosphere and three times the amount of C stored in above ground vegetation. Clearly, soils represent a huge pool in global C budgets and should be managed with the knowledge that changes may represent a powerful feedback on the global C cycle. This vast storage potential is what has popularized the idea of sequestering even more C in soils and has spurred a myriad of different research approaches, from ecosystem-scale reforestation efforts to molecular studies of C-mineral binding interactions.

Given the amount of hope that has already been invested in this powerful idea, it is important for scientists interested in soil C to keep several caveats in mind when planning and conducting their research, and when communicating that research to a broader audience at a science or policy conference.

Caveat #1) Carbon sequestration does not necessarily mean climate change mitigation.

Increasing the amount of C stored in a particular ecosystem’s soil does not by default decrease the amount of CO2 released to the atmosphere. This is partially an issue of scale. If policy makers choose to set aside a certain amount of pasture land for reforestation (with the assumption that this will increase the net C storage in these systems), it is often the case that new land will have to be cleared somewhere else to compensate for the lost agricultural production.  The clearest way around this problem is to be selective in land use changes: land that is more fertile should be exploited for farming, and land that is less fertile should be reforested to boost C storage. However, an increasing global population poses obvious constraints to this line of reasoning. Ultimately, the fact remains that in the upcoming decades, we will need to boost our agricultural yields to meet global demand, and furthermore, climate change is predicted to decrease the fertility of some of the world’s most productive regions.

Moreover, allowing land to go back to its “natural” state does not necessarily lead to a net accumulation of C. A forest respires far more CO2 per acre than a wheat field. Due to the high water demands of forests relative to crops, it can sometimes be the case that trees dry up otherwise inundated subsurface soils, and in doing so create a soil environment that accelerates the decomposition of C.  One must have a refined understanding of the sources and sinks of CO2 in any particular ecosystem to claim that it will or will not “sequester” C.

Finally, management changes leading to increased C storage may increase or decrease the flux of other greenhouse gases, most notably N2O and methane. Given that these gases have 298 and 25 times the global warming potential of CO2, respectively, they are not a trivial consideration. Forests often hold less nitrogen than pastures (due to the different C:N demands of woody vs. non-woody tissue), and consequently release more N2O through the microbial process known as denitrification.

Caveat #2) The amount of carbon that can be locked up in soil in finite

Our understanding of how C is actually “stabilized” in soil on a molecular scale is still evolving. Scientists are using powerful technologies such as x-ray crystallography to understand in detail the chemical interactions between soil minerals and carbon-rich compounds that cause C to be tightly bound and inaccessible to microbes. As a general rule, the more mineral surface area available for C binding, the more tightly C is bound. In subsurface soils lower levels of C mean higher (mineral surface area: C) ratios and thus tighter binding of the C that is present.

However, mineral stabilization has its limits. Laboratory experiments are finding that as C is added to soils, “steady states” are sequentially reached. Careful manipulation of the  chemical and thermodynamic parameters of a soil may bump that soil from a lower steady state to a higher one, but whether such techniques can be applied on a whole-ecosystem scale remains to be seen.

To repeat, caveat #2 is that the amount of carbon that can be locked up in soil is finite, which leads to caveat #3.
At present, the amount of CO2 we put in the atmosphere does not seem to be.

Powlson et al. 2011. Soil carbon sequestration to mitigate climate change: a critical re-examination to identify the true and the false. European Journal of Soil Science 62: 42-55.

urban jungles

a study published recently in Global Change Biology found that rainforests have been displaced as ecosystems that store the most carbon- by cities! cities store more carbon in their trees, buildings and dirt, than the densest and most productive tropical rainforests.

according to researcher Galina Churikina, who led the study, US cities store about 20 billion tons of organic carbon. most of this carbon is held in soils, though a sizable fraction is also contained within buildings constructed with wood. ironically, the key to city’s’ remarkable capacity to store carbon seems to be their artificial nature. buildings and asphalt “bury” soils, locking away carbon that was once part of a dynamic forest, grassland, or other natural ecosystem.

Shanghai, one of the world's largest cities, is an enormous carbon sink!

this is not to discount the importance of urban trees in both storing carbon and providing numerous ecosystem services. trees and other urban plants ameliorate temperatures, providing a cooling effect in summers that reduces the need for air conditioning. trees also directly take up CO2 emitted from cars, reducing the amount of pollution that enters the atmosphere from cities in the first place.

Urban trees such as those in Central Park, NYC, keep buildings cool, capture CO2, reduce stormwater runoff, and improve quality of life.