Life, death and what sphagnum does afterwards

If you ever get the opportunity to see a good peat bog then you should. It’s one of those things that can sound amazingly dull until you actually see one. It’s like nothing else. The way a peat bog builds means that it literally has hidden depths, but not all bogs are the same.

Bengtsson et al. have recently published Photosynthesis, growth, and decay traits in Sphagnum – a multispecies comparison in Ecology and Evolution, an open access journal. They’ve measured 15 Sphagnum species for rates of photosynthesis and decay. The idea is that there’s a relationship between the two, but they have found this isn’t entirely true.

In the lab it’s certainly true. Species that can grow fast also decay fast, but the lab misses some environmental factors, and sphagnum bog is a very special environment.

I’ve walked out into the middle of a bog, but only on a raised wooden walkway. I was warned that stepping off the walkway ‘to check that that bog really was boggy’ was a Very Bad Idea. This is because of the high water table, and it’s this water that makes the real world so different to the laboratory. That’s because sphagnum bogs are anoxic at shallow depths. The lack of oxygen means that inherent differences between the decay of sphagnum mosses matter a lot less because there’s so little oxygen to help break them down anyway.

So any study not only has to measure the behaviour of the mosses, it also has to make sure that the measurements are done in a sensible environment to make what is being measured worth measuring. Part of the study was, therefore, a comparison of their controlled conditions for decay in the laboratory against real-world decomposition.

What they found is that in wetter years hollow species tended to grow faster than hummock species – but not always and slower growing species could do better in dry years. They also found that hollow species tend to decompose faster than hummock species. However, it wasn’t just about differences in species.

They point to work (in Ann. Bot.) on how shade matters in biomass production for Sphagnum. Mosses in the open need protection and that affects their photosynthetic efficiency. So in some cases, shaded plants may be better photosynthesisers. Sure enough, they found that shade did have an effect on photosynthetic capacity. It’s not just a matter of what is growing, but also what is growing around it.

The research matters not merely because it’s interesting, but also because it has an urgent application. I knew peat bogs store a lot of carbon, but Bengtsson et al.’s paper surprised me with how much. They cite Rydin & Jeglum’s The Biology of Peatlands that states that boreal peatlands cover 2-3% of the earth’s surface but store around a third of the world soil carbon. They also point out that getting an understanding of functional traits has a direct application in Earth system modelling, as mentioned in Wullschleger et al.’s paper from 2014. However, it’s not just a matter of traits. What Bengtsson et al. show is that habitats and context are important too, in particular through shading. Integrating habitat and phylogenetic data are going to be essential to understanding peatlands in the future.