Tuesday, April 2, 2013

Carbon sequestration in boreal forests: below-ground interactions matter

One of the most important developments in plant ecology over the last 20 or so years is the inclusion of belowground interactions with fungi into traditional studies of plant diversity, productivity, and ecosystem functions. Results like those from van der Heijden (1998)--which showed experimentally that the assumed link between ecosystem function and plant diversity was actually driven by arbuscular mycorrhizal fungal diversity (through their effects on plant communities)—must alter how we see plant community dynamics. Not only does this reinforce the importance of complexity in ecology, but more specifically it suggests that if fungi are a necessary component of plant community identity and function, they must be explicitly considered in management and conservation plans.

For example, an important current issue is the question of which ecosystems will be carbon sinks as part of a focus on atmospheric CO2 levels. Understanding the mechanisms by which carbon is stored is therefore an important topic. Boreal forests sequester net amounts of carbon in soil and it is generally assumed that this is as a result of plant litter and organic matter accumulating in soil. Clemmensen et al. (2013) examined soil chronosequences for forested islands in Sweden to test whether this hypothesis held. These islands differed in the frequency of fire occurrences, between large and frequently burnt islands and smaller, infrequently burnt islands.

The authors identified the age since fixation of C found in the chronosequences and used models of C source to look at the relative contribution of the two possible processes: either fixation of C through aboveground plant litter or below-ground inputs through root-associated fungi. Carbon input tended to be higher on the small islands that were burnt less often, and this was associated entirely with root-derived input. Further, DNA barcoding showed that on these small islands, there were mycorrhizal fungi associated with the soil depths where the root-derived inputs were occurring. On islands which burned more frequently, and had lower carbon input, fungi were absent at these depths (figure below). This difference in fungal profile was related to the fact that infrequently burnt islands had older mycelium with low turnover, hence greater carbon sequestration.
From Clemmensen et al (2013). A) Fungal functional groups associated with soil depths on large, frequently burnt islands (panel 1) and small, infrequently burnt islands (panel 2).

The authors convincingly show that, at least in some ecosystems, the view that decomposition of litter primarily drives humus accumulation (and the accompanying carbon sequestration) must be tempered with the knowledge that organic layers also accumulate from below by roots and root-associated fungi. This suggests that there is a need to consider fungal communities as well as plant communities for when managing forests and making inventories of global carbon stores. And probably a need to consider fungi much more often in general.


D.T. de Kerckhove said...

Great post Caroline. This reminds me of the amazing nutrient / micro-organism cycles in lake sediments which in terms of energy makes the plankton, insects and fish dynamics seem so small. Some less publicized great work from the Experimental Lakes Area was being conducted on those types of interactions.

Erik Verbruggen said...

Indeed, great post! What I also particularly like from the paper, apart from the perceived "importance" of mycorrhizae (I also study these fungi so feel "proud"), is that with time, instead of levelling off, the rate of carbon accumulation actually seems to increase. Note also that in the figure shown here, the way I interpret is that there was no "soil" in the large islands deeper than 30 cm (from there there was bedrock), not that just that fungi were absent.

Caroline Tucker said...

I agree Erik - less disturbed islands had deeper soil. I think one of the interesting management implications from this paper is that disturbance disrupts carbon sequestration in boreal forests (which we knew, but now we know better some of the underlying mechanisms (i.e. fungi) and that rates of carbon accumulate may increase with time). BUT disturbance also has important positive effects like diversity maintenance in boreal forests, so there would be conflicting outcomes from changes in disturbance regimes..

Erik verbruggen said...

That is indeed important to take into account when considering management implications. I suppose to really address that we should analyze in the exact same way also mainland boreal forest. According to the study I guess these should be younger (since even more lightning will be intercepted), but I would be curious how they scale on carbon sequestration and biodiversity compared to the islands. Also, there are other less severe management implementations of "disturbance" than total fire-mayhem :-)