Leaf Development and Demography Explain Photosynthetic Seasonality in Amazon Evergreen Forests
Cameras show how synchronized birth and death of leaves in the dry season drive increases in photosynthesis and reconcile ground- and satellite-based observations.
Scientists used special tower-mounted cameras to discover that synchronization of leaf birth and death in evergreen forest trees across broad areas of the Brazilian Amazon is the cause of strong dry season increases in tropical forest photosynthesis. Furthermore, careful re-analysis of satellite data shows that, contrary to previous reports indicating that dry season increases in Amazon forest greenness may be an artifact of sun-sensor geometry problems, satellite observations do in fact show statistically significant dry-season greenup.
These findings about how forests regulate their seasonal “breathing in” of atmospheric carbon dioxide help reconcile the seeming discrepancy between large seasonal changes in photosynthesis seen from towers on the ground versus the smaller changes in “greenness” seen from satellites in space. These findings will also help scientists better understand how climate influences these forests and more accurately predict how they will respond to future climate change.
In evergreen tropical forests, the extent, magnitude, and controls on photosynthetic seasonality are poorly resolved and inadequately represented in Earth system models. Combining camera observations with ecosystem carbon dioxide fluxes at forests across rainfall gradients in the Amazon, this work shows that aggregate canopy phenology, not seasonality of climate drivers, is the primary cause of photosynthetic seasonality in these forests. Specifically, synchronization of new leaf growth with dry season litterfall shifts canopy composition toward younger, more light-use efficient leaves, explaining large seasonal increases (~27%) in ecosystem photosynthesis. Coordinated leaf development and demography thus reconcile seemingly disparate observations at different scales and indicate that accounting for leaf-level phenology is critical for accurately simulating ecosystem-scale responses to climate change.
University of Arizona
Funding was provided by the National Science Foundation’s Partnerships for International Research and Education (0730305); National Aeronautics and Space Administration’s Terra-Aqua Science program (NNX11AH24G); and GOAmazon project, funded jointly by the U.S. Department of Energy (DE-SC0008383) and Brazilian state science foundations in Sao Paulo state (FAPESP) and AmazÃ´nas state (FAPEAM).
Wu, J., et al. “Leaf development and demography explain photosynthetic seasonality in Amazon evergreen forests.” Science 351, 972–76 (2016). DOI:10.1126/science.aad5068.
Saleska, S. R., et al. “Dry–season greening of Amazon forests.” Nature 531, E4–E5 (2016). DOI:10.1038/nature16457.