Cool new Amazon studies: thinkin’ bout resilience

Three very cool new Amazon studies came out in quick succession this December and when viewed in combo they mostly remind me how all the raddest, baddest questions in Amazonia right now ask about how resilient the Amazon might be to environmental change.  The answer, based on these three articles: ummm, maybe not very resilient?

Exhibit A:  In 2005 there was a pretty epic drought during the Amazon’s dry season, especially in the southwest of the Amazon.  A group of remote sensing scientists used the backscatter from microwave data (gathered by satellites – coooollll!!!!) to look at how the drought affected the forest canopy (Saatchi et al. 2012).  They found that the forest hadn’t recovered from the ’05 drought by the time there was another major drought, in 2010.  Wah wah wah wahhhhhhh.

What’s super awesome about this approach is the microwave backscatter data they looked at only penetrates a few meters into the canopy, i.e. the backscatter coming back from bouncing off the Amazon that the satellites record only measures water information (leaf water content, etc.) about how the tops of trees are reacting to a big drought.  Hence, reduced uncertainty from things like how wet or dry the soil was.  It’s a really clever way of using remote sensing to look at how resilient these trees are to droughts (and particularly their productive canopy structure).  The future is now, folks.

Cool-ness of approach aside, the Saatchi et al. paper is a big reminder that drought regimes in the Amazon really, really matter for the state of the ecosystem.  As droughts get more common, if Amazonian trees in very drought-affected areas haven’t managed to recover by the time the next drought hits, that’s bad news bears for long-term ecosystem resilience.  Some trees become persistently drought-stressed, maybe what kinds of trees do well in the forest changes, the biodiversity in these forests might change over time, and we end up with an Amazonian plant community that’s different than the community we started with.  Good thing some of my friends are working on these exact relationships (Paulo Brando, whaaaaat!).

OK, Exhibit B: Another group of scientists (Rodrigues et al. 2012) checked out what happens to bacteria biodiversity in the soil of land that used to be forest but is now cattle pasture.  They found that while in a single soil sample soil microbe biodiversity went up, over a larger spatial scale (e.g. a big pasture field) the communities were much more related than they were over a similar landscape in Amazon rainforest – meaning that the system’s soil microbe biodiversity went down.  The authors call this “homogenization of microbial communities” and we have no idea if (a) the soil microbe community will bounce back if forest regrows on the deforested landscape or (b) how having homogenized soil microbe communities might matter for things like stabilizing soil carbon, promoting fertility in the soil, or fostering a biodiverse set of plants when/if regrowth does occur.  More evidence that the Amazon isn’t as resilient as we might hope.

Exhibit C’s counterpoint?  A third group of scientists (so many scientists!  we’re taking over!!!!) wanted to see whether a significant number of Amazonian tree species had been around back when the Earth was much warmer, e.g. during the Pliocene (2.6–5 Ma) and late-Miocene (8–10 Ma) geological epochs when it was hella warm (like, as warm as we’re projecting it might be in 2100) (Dick et al. 2012).  They found that 9 or 12 of the species they looked at were more than 8 million years old and so are likely good to go when it comes to warm temperatures.  It’s a controversial idea, though, since a lot of other studies have provided evidence that higher temperatures is going to be a problem for Amazon plant species.  The authors themselves note that just because these species could survive higher temps doesn’t mean that they’ll be able to survive higher temps in combination with forest fragmentation and changing rainfall patterns (like, for instance, the ’05 and ’10 droughts).  [Quick, un-data-substantiated plug: my hunch is that rainfall matters a lot more than mean annual temperature for a lot of these plant communities.  Questions to consider for a post doc.]

So what’s my take away?  (Other than the fact that tropical ecologists are a bunch of exciting and smart scientists?)  I suppose that the jury’s still out, but that we’re seeing more and more evidence that the very large-scale global changes being inflicted on the Amazon have a high likelihood of causing cascading, interrelated effects (lower canopy functioning, changes in soil habitats) that could lower plant community resilience to the point where ecosystems start to change in ways that we won’t be able to predict or manage.  What the magnitude of those changes might be is a totally open question.

PS.  Sorry no pictures in this one, guys!  I’m just super excited about these three studies.  Plus, I’m on my parents sofa in DC, so not a lot of plants to photograph.

PPS.  Check out the Mongabay reporting on all three of these articles.

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