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Hey! We have a new tree ring isotope paper out in preprint and this one’s on tricky iWUE, so I thought I’d do a quick background thread on trees ,co2 and water: the ultimate complicated threesome! 1/ https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.16811
Trees change the size, geometry, density, & functioning of their stomata when there’s more CO2 available. This changes their water use & photosynthetic rate. There's a lot of trees, so this impacts the C & h20 cycles via plant physiological forcing. 2/ https://onlinelibrary.wiley.com/doi/full/10.1111/j.1365-2486.2010.02273.x?casa_token=gOl-NKVrHXcAAAAA%3AqHAmd31V_oKkNVeqlo5JWfw9xDW6-OTInQp9JwpI_CLcOm-l7Pa_vmvGtLMvwUYGYrQg3ezzkXB2Uw8
Some of the changes are physical & some are changes in the action of tree’s super clever two-way carbon-water system. Summary-Tree stomata; hypersensitive to CO2 since the Devonian https://www.sciencedirect.com/science/article/abs/pii/S0305736485711122.
adapted frm McCarroll & Loader 2004 3/
adapted frm McCarroll & Loader 2004 3/
The stomata seesaw (with CO2 [in] on one end and water [out] on the other) can be studied over time by measuring d13c in tree rings and converting the values to intrinsic Water Use Efficiency, as a stomata-seesaw proxy. When we do that we see, unsurprisingly, a complex pic 4/
![The stomata seesaw (with CO2 [in] on one end and water [out] on the other) can be studied over time by measuring d13c in tree rings and converting the values to intrinsic Water Use Efficiency, as a stomata-seesaw proxy. When we do that we see, unsurprisingly, a complex pic 4/](https://pbs.twimg.com/media/EeKt5v8XYAYPxAr.jpg "The stomata seesaw (with CO2 [in] on one end and water [out] on the other) can be studied over time by measuring d13c in tree rings and converting the values to intrinsic Water Use Efficiency, as a stomata-seesaw proxy. When we do that we see, unsurprisingly, a complex pic 4/")
The iWUE proxy has always shown a complex story. With some trees showing rising iWUE with rising CO2, but reaching a plateau, some showing no response, and some a reduction in iWUE. https://onlinelibrary.wiley.com/doi/full/10.1111/j.1365-2486.2010.02273.x?casa_token=PkcxLROD0fgAAAAA%3ArkU0ULlsewDk8O8MCk_qritcosaYVI9662oS4ukJTclaJQaKoR72kY4xdgOcP65wBO4CQ_tUjyMuZP0 & evidence that the CO2 response is weakening https://onlinelibrary.wiley.com/doi/full/10.1111/gcb.15166?af=R 5/
So why don’t trees just grow more with more CO2? Most evolved when there was less CO2 so they’re already maxed out & living in a raised CO2 world ( https://besjournals.onlinelibrary.wiley.com/doi/full/10.1046/j.1365-2745.2003.00742). A standard hypothesis was that they would show a mix of passive and active responses. 6/
But, more importantly, rising iWUE, and even capturing more C, doesn’t mean more biomass. https://www.nature.com/articles/s41559-019-1090-0 and https://link.springer.com/article/10.1007/s10021-018-0309-2 and https://www.pnas.org/content/116/7/2749 Summary: More CO2 doesn’t mean more tree. 7/
Other factors limit tree growth when there’s enough CO2. Like moisture and nutrients. https://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.14428#.W4XjjgzCiXg.twitter Although trees may increase their WUE in droughts, https://www.nature.com/articles/s41561-018-0212-7 that doesn’t mean they’re laying down more carbon. 8/
And actually, climate change, and the way we manage forests, is making them dominated by trees that are younger and smaller, over time https://science.sciencemag.org/content/368/6494/eaaz9463 9/
and the latest climate models are aligning around things getting drier as temps rise further https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020GL087820 “Future drought changes are larger and more consistent in CMIP6 compared to CMIP5“. So rising iWUE in trees, sure, more tree & robust forest, probably not. 10/
So all this makes for a tricky set of variables to consider when it comes to modeling the likely physiological forcing response to future CO2 levels. 11/ https://pubmed.ncbi.nlm.nih.gov/23504999/
So what did we explore in our new paper? https://nph.onlinelibrary.wiley.com/doi/abs/10.1111/nph.16811 We sought to contribute to moving 'iWUE from d13c’ forwards to a point where we can explore how dynamic the response of tree carbon-water balance is to rising CO2 - via their tree ring histories. 12/
The trees we studied had adjusted their water-gas exchange dynamically in response to CO2, moisture, stand dynamics, site conditions. They shifted iWUE between passive/active (and neutral-ish) ci/ca behaviors in response to multiple variables, not in a simple one-way shift. 13/
Such physiologically complex responses are going to pose an ongoing challenge to modellers (sorry modellers). AND, current tree planting strategies need to plant the right trees, in the right places, to work with likely iWUE changes. More CO2 doesn't mean more tree. /
