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My new paper and first of my #PhD was published today in @PlantCellEnvir. We show small #Amazon trees are better at responding to long-term than larger canopy trees. Full article here: https://doi.org/10.1111/pce.13838
1. The Amazon rainforest is experiencing more frequent, prolonged and extreme drought events. These are predicted to intensify over the next century.
2. Drought can kill many large trees because of failure to their water transport systems. This can cause the canopy to open, increasing light availability in the understory. https://doi.org/10.1038/nature15539
3. Small trees are typically adapted to dark understory conditions. As the canopy becomes more open, small trees must adapt to changes in both water and light environments during drought events.
4. A long-term drought experiment was established in 2002 in the remote Caxiuanã forest in the Eastern Amazon. The aim was to investigate the impacts of drought on tropical forest function. Plastic panels impose drought by excluding half of the total rainfall.
5. So far, the experiment has revealed changes in biomass, mortality rates, litterfall, flowering, water cycling and respiration. https://doi.org/10.1093/biosci/biv107, https://doi.org/10.1111/1365-2745.12931, https://doi.org/10.1111/gcb.13851
6. In this study, we measured leaf traits of 66 small trees and compared them to 61 surviving canopy trees. We measured maximum photosynthetic capacity, leaf respiration and leaf morphological traits on trees exposed to 15 years of drought and compared them to control trees.
7. Small trees positively responded to the drought treatment by increasing photosynthetic capacity, respiration and leaf mass per area. Drought did not affect leaf nitrogen or phosphorus concentrations.
8. Small trees modified how they invested their resources under drought conditions. They increased their maximum rates of electron transport (Jmax – the light capture reactions) more than their maximum rates of carboxylation (Vcmax – the dark reactions that fix CO2 into sugars).
9. Small trees were able to increase the nutrient use efficiency of the light capture reactions. Neither nitrogen nor phosphorus limited these reactions. Higher nutrient costs of carboxylation and respiration reactions likely prevented them from becoming more efficient.
10. Greater responses of the light capture reactions suggest small trees are responding to changes in their light environment. They are able to make the most of the extra available light.
11. Unlike small trees, large trees did not respond to the drought treatment. This suggests small trees are less stressed and have greater phenotypic plasticity, increasing their potential to respond to changes in their environment.
12. Small trees may be more able to avoid drought stress by stronger stomatal regulation, greater resistance to xylem embolism or from reduced architectural constraints. Alternatively, small trees may be able to tolerate drought by maximising growth during the wet season.
13. Some genera were more responsive than others. This may give some species an advantage over others and could ultimately shift the structure of the community. Eschweilera and Swartzia were most responsive, whilst Inga and Protium showed little response.
14. Overall, we show small trees can respond to drought and changes in canopy structure following the death of canopy trees. This may allow a more resilient forest to establish and moderate some of the negative effects of climate change on tropical forests.
15. This work was only possible because of a great team of researchers, field assistants and tree climbers. A thank you also to @NERCscience, @arc_gov_au, @CNPq_Oficial, @AgencyFAPESP, @Microsoft, @CAPES_Oficial & @royalsociety for funding this work.
