Author Archives: Sarah

There’s no place like home? An exploration of the mechanisms behind plant litter–decomposer affinity in terrestrial ecosystems

This week’s post is by PSE member and star #psejclub commenter Relena Ribbons. If you’d like to write a guest post, please get in touch!

I was really excited to see a new article synthesizing recent publications on decomposition and litter affinity effects in New Phytologist by Austin et al. One of the most well-known examples of litter affinity effects is the Home Field Advantage (HFA) hypothesis, whereby litter decomposition is accelerated in its home environment. Given the importance of nutrient cycling in terrestrial systems the patterns observed in litter decomposition have attracted a large audience, and expansions and new hypotheses have been postulated including the substrate-matrix interaction (SMI) or the phenology-substrate match (PSM) hypotheses. The authors go on to briefly describe each of these hypotheses, while also providing examples that don’t align with HFA, and conclude the article by raising additional research questions and future directions to elucidate the mechanisms behind observed litter affinity affects. Following the yellow brick road on home field advantage leaves us asking more questions, which is a good thing.

I really liked that the authors provided the background on HFA, expanded on to SMI and PSM, as it serves as a logical trajectory of how these hypotheses have been developed, and provides a good introduction for readers who may not be familiar with litter affinity effects. The authors state that the most cited explanations for litter affinity effects result from a specialization of microbial community, which is due to a constant and chronic input of similar litter quality over time which results in a microbial community that is optimized to degrade those particular litter inputs.  However, the authors go on to assert that standard indices of lignin, C : N ratios, and nutrient content alone, may not be sufficient to explain an observed HFA. For example, recalcitrant litter decomposes more quickly in a less favourable home environment and simply as a function of its litter quality or site fertility (see  Vivanco and Austin 2008 for further insights to this particular point and figure 1- copied below). Furthermore, high-quality litter may decompose faster in a fertile microsite as a result of higher resource availability for decomposers independent of its origin, which is known as a priming effect.

pseRR1

What I found especially insightful was the section of this article discussing green leaf hitchhikers, and discussion incorporating the legacy of the phyllosphere community. I see the term rhizosphere used quite frequently in journal articles, but I have not had the same exposure to the term phyllosphere (which is used to refer to the above-ground parts of plants). The microbiology of the rhizosphere (refers to the belowground habits of plants, e.g. plant roots) has received more attention than the microbiology of the phyllosphere (see Vorholt’s 2012 Nature Review Microbiology publication for further discussion), which I found interesting as historically scientists have first explored above-ground components.

Austin et al. went on to discuss how green leaves can serve as a conduit for introducing not only the litter nutrients, but also serve as a vector for carrying fungi and bacteria from green leaves into the decomposer community. In effect, microbial community might be hitching a ride from green leaves, down to the leaf litter (as leaves senesce) and could jump-start decomposition, alter the microbial decomposer community, and ultimately play an important mechanistic role in explaining observed HFA effects. The authors further explored the potential connections between volatile organic compounds used in plant defences, as a means for attracting a specific decomposer community. This got me thinking about all those caterpillars I have seen crawling towards my garden this summer, and what their role might be in decomposition. The authors highlight that further exploration of macrofauna decomposers is also warranted (see figure 2 from their paper below as you contemplate whether there is more to the story of Eric Carle’s Very Hungry caterpillar).

pseRR2

I found this particularly interesting as I think about my own experimental work, and the potential intersection between pathogens and pests and observed patterns in litter decomposition and nutrient cycling. Has this paper jump-started any new lines of inquiry in your research? Are there any future directions you think the authors should have considered in this paper? Any points where you are confused, or wish the authors explained the concepts in more details? Are any of your looking at caterpillars differently after reading this article? Share your thoughts on Twitter and Facebook using the #psejclub tag.

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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!

Interactions with soil biota shift from negative to positive when a tree species is moved outside its native range

Pinus contorta

Pinus contorta. Image by Rudi Riet, Washington, DC, United States [CC-BY-SA-2.0], via Wikimedia Commons

This week’s paper is a Rapid Report by Gundale et al which appeared in New Phytologist in late January.  It also happens to be the paper we discussed at the Hawkesbury Institute for the Environment’s student journal club this week.

This study was a really good example of a home-away growth experiment, and showed some really striking results.  Seeds from Pinus contorta trees of known provenance were collected in Sweden, and then grown in soil from their native Canadian sites and their introduced

Swedish sites to look for differences in aboveground and belowground biomass.  Sounds simple?  It would be, but they didn’t just compare fresh soils containing whatever biota could contend with the 2mm sieve.  They also used sterilised soils from both countries, sterilised soils which had been cross-inoculated with soils from the opposite country, and included a fertilisation element to make it more interesting.

So what was the take-home message?  The clue is in the title, but the short version is that it’s all about the soil biota.  Seedlings grown in Swedish soils had much higher biomass (around 43% higher) than those grown in native their native Canadian soils, despite the Swedish soils having lower pH and nutrient availability.  When seedlings were grown in sterilised soils, the soil origin had no effect on biomass.  When seedlings were grown in sterilised soil from either country, but inoculated with soil biota from Sweden, biomass was again much greater than when sterilised soils were inoculated with Canadian soil biota.  The effect of soil biota was even greater than the effect of fertiliser on biomass.

Considering these different trials together, it becomes clear that not only did the Swedish biota have a strong positive effect on P. contorta seedling biomass, but that the Canadian biota actually had a negative effect on biomass. This provides evidence that better growth in soils from outside of the native range is probably down to a combination of pathogen release and positive biotic associations.

However, this evidence also highlights the gap in this paper – analysis of the soil biota itself.  We see the effects, but what are the actual differences in the bacterial, fungal, and/or mesofaunal communities?  Are there differences in community structure? Abundances or biomass? Activity?  All of these?  It would be great to see data on this, especially regarding known pathogens or ectomycorrhizal fungi associates.  I hope/suspect there may be another paper on the way exploring some this.

Overall, this was a really nicely designed project that asked interesting questions and addressed them in a very straightforward way.  The paper itself was well written.  It flows nicely and is easy to follow and understand even on a quick reading.  The methods made no attempt to disguise the logistical issues associated with transporting soils half way around the world, and the data analysis and presentation was simple and direct with no unnecessary frills or risk of misinterpretation.  All of these are elements that we should expect of any paper, but sometimes experiments are complex and difficult to describe, methodological detail gets glossed over in an attempt to meet word counts, and data are not easy to interpret.  While it is not possible to answer all questions with a glasshouse study, or to present all data with bar graphs, it is a nice reminder of the clarity with which we should try to communicate our work.

So now that I’ve had my say, exposing my soil ecology and science communication biases, we’d love to hear what you thought about the paper.  Was there anything you would have done differently?  What do you think the next steps should be?  What did you think of the tree provenance element of the study?  It didn’t show a significant effect here, but would it be worth pursuing in other systems?  How well do you think the results of this glasshouse study represent what’s happening in the natural communities?

You can let us know your take on this either in the comments or via twitter using the hashtag #psejclub.  We’d also love to hear from you If you have a suggestion for a paper you’d like us to discuss, or if you want to write a post yourself!

Pinus contorta native range.  Image by  Elbert L. Little, Jr., of the U.S. Department of Agriculture, Forest Service, and others http://esp.cr.usgs.gov/data/little/

Pinus contorta native range. Image by Elbert L. Little, Jr., of the U.S. Department of Agriculture, Forest Service, USGS

P. contorta range in Sweden.  Image by Philippe Rekacewicz, UNEP/GRID-Arendal

P. contorta range in Sweden. Image by Philippe Rekacewicz, UNEP/GRID-Arendal