Tropical forest carbon storage is related to tree richness and traits. Or is it?

There’s been a lot said about relationships between species diversity and ecosystem function over the last two decades. The general view of these relationships is that diverse ecosystems are more productive, use resources more efficiently and are more stable.

But, and it’s a big but, almost none of this work has been done in forests and even less in mega-diverse tropical forests. Because of this diversity productivity relationships can’t really be described as general. How do we know what is general across the globe if we have only concentrated on temperate grasslands?

This is something a new paper in Global Ecology and Biogeography by Kyle Cavanaugh and colleagues hopes to set straight. Their study drew on a dataset of carbon storage and tree biodiversity from 59 plots across the tropics produced by members of the Terrestrial Ecosystem Assessment Monitoring  (TEAM) Network.

The great thing about this work is that all plots were surveyed using the same methods, meaning they should be reasonably comparable. All sites collected measures of aboveground carbon storage, genus diversity, functional diversity – by measuring wood density of trees and potential maximum diameter, and the mean value for wood density and maximum diameter for each plot. All of this was then analysed while trying to account for climatic differences between sites.

The general findings of the study were that both genus diversity and the mean potential maximum diversity of species appear to be  positively related to aboveground carbon storage.

Relationships between
Relationships between site (a) elevation and genus richness, (b) precipitation and functional diversity, (c) carbon storage and genus richness and (d) carbon richness and mean maximum diameter of trees. Stolen from the paper.

This enforces the view that diverse ecosystems are more productive and that large species may contribute a disproportionate amount of biomass – as I have written before. Very few studies have shown a relationship between diversity and biomass in tropical systems before, so this is exciting stuff.

And yet, I still have a few queries about some findings. The study failed to find any relationship between climate and carbon storage – a connection that is fairly well established. Also it uses stepwise model selection, which is beginning to become one of my (and others’) pet peeves . I am of the feeling that testing all possible models and then averaging amongst them based on the ones that have greatest support is the best way to do things, and this often comes up with very different findings to stepwise selection.

Previous similar work has suggested that carbon – biodiversity relationships are scale-dependant, with positive relationships in small plots and mixed results at larger plot sizes. Given the increasing number of tropical forest research networks I am sure this study will not be the last of its type. Once these get published we will have a better idea of how general these findings are.  At the moment I am not completely convinced.

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