Tag Archives: ecosystem function

Seasonal not annual rainfall determines grassland biomass response to carbon dioxide

After a brief hiatus, journal club is back and this time we’re discussing a paper by Hovenden et al from Nature in May exploring the interaction between carbon dioxide and rainfall on plant biomass.

At this point it seems that we ecologists have a reasonably good idea of what effect many environmental variables – like water, temperature, and carbon dioxide – have on certain ecosystem parameters, though always with a few caveats and exceptions thrown in to keep it interesting.  However, our understanding of how these variables interact and the effects of these interactions, especially over various temporal and spatial scales, is still pretty woeful.

For example, we know that plants need water to grow, and when there isn’t enough water they stop growing – very straightforward. Plants also need CO2 to grow, and in general higher CO2 levels lead to higher plant biomass.  This is because increased CO2 allows for higher rates of photosynthesis and greater water use efficiency.  Due to the greater water use efficiency, we also expect that the effect of elevated CO2 (eCO2) on plant growth will be greater when water scarce.  Basically, with higher CO2­, plants can photosynthesize more per unit available water, so will be able to grow more before the water runs out compared to plants grown at lower CO2 levels.

As with so many things in ecology, what we predict is exactly what we see … except when we don’t.  If the relationship between water availability, eCO2 and plant biomass is so straightforward, biomass responses to eCO2 would always be positive and we would see the strongest responses in the driest years.  I bet you can see where this is going…

TasFACE ring

Photo credit: TasFACE website

Hovenden et al looked at data from a nine year FACE experiment in Tasmania (TasFACE) and found that the eCO2 effect was far from consistent across years.  Some years there was no discernable eCO2 effect on biomass, some years it was positive (like we’d expect) and one year it was actually strongly negative; and these responses were not correlated with annual rainfall or soil water availability.

Instead, Hovenden et al found that the biomass responses to eCO2 were strongly correlated with seasonal rainfall variability.  Higher rainfall in the summer resulted in a positive effect of eCO­2 on biomass, as we would expect.  Summer rain at the site tends to come in short, sharp bursts, so the increased water use efficiency would allow the plants to maintain growth for longer between rain events.  However, increased rain during the spring and autumn were correlated with a negative effect of eCO2 on biomass.  During these cooler, wetter periods plants don’t grow as much and it is likely that increased rain would leach nutrients from the soil.  This was supported by a strong negative relationship between spring rain and soil nitrogen availability.

It seems probable that such a relationship between seasonal rainfall and eCO2 effects on biomass could be seen throughout temperate and seasonally wet systems, and that this could have big implications for global carbon models.  It also highlights the importance of looking beyond plants to fully understand the mechanisms that drive responses to climate change.

I would love to see similar analyses of other FACE datasets to see if these trends are replicated in other systems.  It’s an important finding, but opens up lots of other interesting questions: How does vegetation type or soil type effect the relationship between seasonal rainfall and eCO2 effects on biomass?  Does seasonal temperature variability affect the relationship significantly?  What about increased nitrogen pollution or fertilisation – would increased nitrogen deposition overturn the negative relationship between high spring/autumn rain and the eCO2 effect on biomass?

As always, we’d love to hear what you think about the paper.  Is it the best paper you’ve ever read or do you think it contains some fundamental flaw? Does it raise interesting questions or link well with something else you’ve read recently?  Would you use similar methods or could you propose a better protocol?  Let us know in the comments or on twitter with hashtag #psejclub!

Finally, don’t forget about our joint meeting with the Plant Environmental Physiology group coming up in October.  All the details, including links for registration and abstract submission, are available here.  It’s going to great!

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Endemism and functional convergence across the North American soil mycobiome

I’m really interested in scale. The world we live in is full of things that are doing things, sometimes to other things, and how we see those things (doing things to things) depends fundamentally on how close we are to them. Take soil as an example: from about 170 cm up, it looks brown, reasonably inert, and good for growing plants in. However, assume that you’re about a thousand times smaller, and the soil becomes a much more interesting, and probably quite frightening, place. Everywhere you look, there are mites, larvae, worms, beetles, and sticky white chords, clinging to vast, pipe-like plant roots. And it’s those strange, white chords that are the topic of today’s #psejclub paper.

Ectomycorrhizal mycelium with some white spruce roots (André-Ph. D. Picard, CC BY-SA 3.0)

The paper, by Jennifer Talbot, Kabir Peay, and several others, appears in PNAS. The authors wanted to study the community structure of soil fungi and their contribution to ecosystem functions, like soil nutrient cycling, across the continental USA. In order to capture differences in functioning at a variety of scales, soil sampling in the study was nested (see Supplementary Figure S1): at the broadest level, sites were chosen from three different regions of the USA (1000 km); within regions, different landscapes were sampled (100 km); in each plot within a landscape unit, thirteen soil samples were taken at increasing distances apart along three transects (40 km). To look at the effect of scale without the confounding influence of plant community, the study focuses on a single plant family, the Pinaceae, which occur across North America. The authors determined fungal community composition from soil DNA by sequencing the internal transcribed spacer (ITS) region using primers ITS1f and ITS4, clustering the sequences into taxonomic units. To assess the functioning of the soil fungi, the authors used the activity of seven extracellular enzymes involved in carbon and nutrient cycling. Finally, each soil core was split into an organic and a mineral horizon, which were analysed separately.

I was attracted to this paper initially by the implication of scale in the title, and the fact that I misread ‘mycobiome’ as ‘microbiome’. After skim-reading the paper and wondering ‘But what about the bacteria, mites, nematodes, etc.?’ I realised my mistake. The results are interesting: while fungi were highly endemic, the activity of their enzymes was broadly similar across large scales, varying with soil chemistry at smaller scales. The authors suggest that this provides evidence for a high level of functional redundancy in fungal communities at large spatial scales; function has little to do with structure. They argue that efforts to include the soil fungal community in biogeochemical models would be better focused trophic groups rather than identity, which is good news for modellers!

While the study does ‘only’ consider fungi in stands of Pinacaea, it does so at a range of scales, encompassing the continental to plot-level. The way that scale was incorporated into the sampling design is probably the best thing about this study, for me, because it provides a way of examining how ecosystem structure is related to function across increasing scales, in a way that I can imagine applying to other groups of organisms. On that note, it would be very interesting to see this approach applied to other functional groups, particularly those with contrasting degrees of mobility, to see if the same conclusions can be drawn: what about bacteria, mites, or earthworms? Might we expect to see the same degree of endemism in organisms that move around more? How does endemism belowground relate to ‘lifestyle’?

Another interesting angle to pursue could include disturbance. The fungi and fungal functions characterised in the study were from predominantly natural ecosystems; how does the disturbance embodied in, for example, conversion of grassland to agriculture, affect the functioning of soil communities, at a range of scales? I wonder how feasible it would be to combine the sequential sieving approach from the previous #psejclub paper with the scale methodology presented in this one.

This is a really interesting paper, suggesting that no matter which soil you zoom into, all the fungi (those strange white chords from earlier) are clubbing together to basically the same end, in Pinaceae forest anyway. I enjoyed reading it, and liked the figures, particularly the use of colour to show different regions. I wonder why the authors didn’t use different shapes the represent the different soil horizons – it’s very difficult to tell the difference between a small circle and a slightly larger one – but that’s a minor gripe. And now it’s over to you: the #psejclub readers and contributors. What did you think? Are there elements you think could have been handled better? Where would you go from here? Tweet your thoughts using #psejclub, post them in the Facebook group, or comment on this post – I’m looking forward to hearing from you!