- Author: Rob York
Article reviewed: A Large and Persistent Carbon Sink in the World’s Forests
By Y. Pan and many others. 2011. Published in Science. Vol. 333 pp. 988-983.
The plot line: This team of scientists gathered all of the data they could find from across the world to quantitatively describe the roll that forests have had in influencing the global carbon balance over the past two decades. They distinguished between temperate, tropical, and boreal forests in terms of which forests were sinks versus sources, and in terms of how recent management, land use, and climate factors have caused forests to be sources versus sinks. They found that, despite huge variation between and within the three different forest types, forests have been a net sink of carbon over the past two decades. They have been sequestering about one “Pg C yr-1” (one “Petagram” of carbon is the equivalent of 1 billion metric tonnes). Of all types of vegetation on land (i.e. versus grasslands or shrublands), forests dominate the carbon balance equation. They found that tropical forests by far have the largest influence, both in terms of sinks and sources. They conclude that, while the carbon sequestered in forests can be at risk of loss from future climate change and human actions, forests nonetheless have and may continue to be important sinks of carbon originating from fossil fuel consumption.
Relevant quote: “Clearly, forests play a critical role in the Earth’s terrestrial C sinks, and exert strong control on the evolution of atmospheric CO2. Drivers and outlook of forest carbon sink.”
Relevance to landowners and stakeholders:
Forests are robust when it comes to sequestering carbon. They are usually carbon sinks (they remove CO2 from the atmosphere), and only become distinct sources when they are dramatically changed by actions that convert them from forests into something else. Converting a forest into agricultural or residential land, for example, takes away the carbon-sequestering power that a forest has. So the first step in conserving the beneficial roll of forests in sequestering carbon is to keep forests as forests. This does not mean forests shouldn’t be disturbed (by sustainable harvests or prescribed fires, for example). In fact, young forests can be significant carbon sinks if they are growing fast. In terms of carbon sequestration, effective conservation efforts are those that aim to limit long-term forest conversion from occurring. Because of the huge losses of carbon from tropical forests deforestation, the authors of this paper cite the REDD program (Reducing Emissions from Deforestation and Degradation) as having good potential to mitigate climate change impacts.
General public and cocktail-party audiences are often surprised when I tell them that the forest area in the United States has increased over the past half-century because of abandoned agricultural lands that are returning to forests. Now I have a reference to provide, as this paper cites this increase in forest area as being one reason why U.S. forests have been a net sink of carbon (other reasons being that many forests are young and fast-growing, and that there is likely some fertilization happening from CO2 enrichment and Nitrogen deposition (see this previous post about N deposition).
As briefly pointed out by the authors, forests are sequestering large amounts of carbon that can off-set some fossil fuel production of carbon, but forests are not a complete bail-out when it comes to climate change. When it comes to forests reducing impacts of climate change, they are like seat belts. They might save us from dying in minor crashes, but if a head-on collision occurs, we're still going to die.
Relevance to managers:
The primary relevance to managers that I interpret from this article is that it suggests that existing young forests, in particular, can be managed to increase their capacity to be carbon sinks. Cultural or commercial treatments that reduce stem density while allocating growing space to larger, fast-growing trees can have the carbon-related benefit of reducing the potential for massive carbon loss from wildfire or insect epidemics, while also contributing to long-term carbon storage by producing forest products.
The factors that influenced whether forests were sources or sinks over the past two decades mentioned in this article are:
- Wildfires – high severity fires in Russia and Western US were recent C sources
- Insect outbreaks – caused forests to be C sources in Western US and Canada
- Forest age – immature tropical forests recovering from disturbance were cited as C sinks
- Deforestation – long-term conversion of tropical forests have been huge C sources
- Afforestation – planting has been effective in creating C sinks in China forests
- Soil management – the draining of water-logged soils in Europe has caused forests to be a C source
- Fertilization- CO2 enrichment and N deposition in US may be increasing productivity and hence forests as a C sink.
Critique (I always have one, no matter how good the article is):
It was often unclear what the authors meant when using the term “harvesting.” In some cases, they implied that harvesting was a carbon source (in European Russia), but they also present data that suggest that, globally, harvested wood products were a carbon sink. Similarly, it was unclear what they meant when referring to “managed” versus “unmanaged” forests.
- Author: Rob York
Article reviewed: Subsurface carbon contents: Some case studies in forest soils
By D.W. Johnson, J.D. Murphy, B.M. Rau, and W.W. Miller, published in the journal Forest Science, Vol 57, 3-10
The plot line: This is an article that was part of a special issue of Forest Science that was born from a conference on the importance of carbon in deep forest soils (it is not surprising that, yes, soil scientists think it is important!). This article emphasizes the need for understanding the pattern of carbon content as one goes deeper into forest soils. The pattern is highly relevant because deep soils are rarely sampled because of the physical difficulty involved in getting to deep soils (it’s a lot of digging). If one knows the pattern pretty well, then one can sample shallow soils and then estimate how much carbon is deeper if they are confident of the pattern. They found two basic shapes- linear (total carbon increases at a constant rate with depth) and asymptotic (total carbon increases at lower rates as you go deeper). The linear soils tended to have about 50% of their carbon below 20 cm (the common depth of sampling), while asymptotic tended to have about 35% below 20cm. The conclusions seemed to be that, it is reasonable to sample only part of the soil horizon and then extrapolate for estimating lower depths, but the correct extrapolation equation (i.e. the mathematical representation of the pattern) has to be used.
Relevant quote: “…there is a tendency to either ignore C and nutrient stores in deeper soil horizons, perhaps producing significant bias in soil C in global scale modeling efforts or to resort to modeling soil C contents of deeper soil horizons. ”
Relevance to landowners and stakeholders:
This article is largely for academics, so it is difficult to find direct relevance for landowners and stakeholders. Instead, I refer readers to the previous discussion of carbon accounting on September 4, 2009. The main point of that discussion- that we are a long ways off from being confident in estimating carbon in forests with great accuracy- still seems to be the case today. Studies like this, however, move us closer to the “gold standard” in carbon accounting that will someday be needed for true carbon markets to develop.
Relevance to managers:
I recently tried to bone up on the issue of carbon in forest soils because foresters are now required to address impacts of forest treatments upon carbon when conducting environmental impact reports. I liked this article because it focused on missing data- specifically data estimating the carbon in the deep soil profiles that are usually not sampled. For managers, the entire amount of carbon in soils is often virtually unknown compared to above ground carbon. It has always been part of a foresters job to understand how much wood volume (i.e. carbon) is standing above-ground in the forest, but it has only recently become part of our job to also understand how much carbon is below-ground.
One item of relevance from this article seems to be that there are two major sources of variability when it comes to carbon in soils: one is the pattern at which carbon content changes as one gets deeper in soils; the other is the total amount of carbon that exists from location to location. The latter can be estimated, but only if the former is understood with relatively good certainty. When applying estimates of belowground carbon to total forest carbon budgets, it probably makes sense to check to see how deep soil carbon is estimated, if at all.
Another relevant point is that there is likely to be carbon in very deep horizons that is unaccounted for when below ground carbon is estimated. In this article, soil carbon was estimated down to between ½ and 1 meter. Where deeper soils exist, a significant amount of carbon is not being estimated if carbon amounts are extrapolated to only ½ or 1 meter. This is less of a problem for soils with asymptotic patterns of soil carbon (the amount of carbon declines with depth). But even for these soils, a true asymptote was not reached within ½ to 1 meter depth so carbon would still remain unaccounted.
Critique (I always have one, no matter how good the article is):
My only critique is that the selection of the two models used to represent the pattern of how carbon changes with soil were not justified in a statistical sense. They fit patterns to either a logarithmic or asymptotic (a “Langmuir” equation) model, but don’t explain how one or the other model was selected. Often it is the correlation coefficient or a model selection criteria that is used to pick the “best” model. It is only a minor critique since both models are relatively parsimonious and do not have as much need for a model selection approach.
