@article{896483ebc6a94c34b2323d9f4f2e0fd5,
title = "No evidence of canopy-scale leaf thermoregulation to cool leaves below air temperature across a range of forest ecosystems",
abstract = "Understanding and predicting the relationship between leaf temperature (Tleaf) and air temperature (Tair) is essential for projecting responses to a warming climate, as studies suggest that many forests are near thermal thresholds for carbon uptake. Based on leaf measurements, the limited leaf homeothermy hypothesis argues that daytime Tleaf is maintained near photosynthetic temperature optima and below damaging temperature thresholds. Specifically, leaves should cool below Tair at higher temperatures (i.e., > ∼25–30°C) leading to slopes <1 in Tleaf /Tair relationships and substantial carbon uptake when leaves are cooler than air. This hypothesis implies that climate warming will be mitigated by a compensatory leaf cooling response. A key uncertainty is understanding whether such thermoregulatory behavior occurs in natural forest canopies. We present an unprecedented set of growing season canopy-level leaf temperature (Tcan) data measured with thermal imaging at multiple well-instrumented forest sites in North and Central America. Our data do not support the limited homeothermy hypothesis: canopy leaves are warmer than air during most of the day and only cool below air in mid to late afternoon, leading to Tcan/Tair slopes >1 and hysteretic behavior. We find that the majority of ecosystem photosynthesis occurs when canopy leaves are warmer than air. Using energy balance and physiological modeling, we show that key leaf traits influence leaf-air coupling and ultimately the Tcan/Tair relationship. Canopy structure also plays an important role in Tcan dynamics. Future climate warming is likely to lead to even greater Tcan, with attendant impacts on forest carbon cycling and mortality risk.",
keywords = "canopy temperature, homeothermy, leaf temperature, leaf traits, photosynthesis",
author = "Still, {Christopher J.} and Gerald Page and Bharat Rastogi and Griffith, {Daniel M.} and Aubrecht, {Donald M.} and Youngil Kim and Burns, {Sean P.} and Hanson, {Chad V.} and Hyojung Kwon and Linnia Hawkins and Meinzer, {Frederick C.} and Sanna Sevanto and Dar Roberts and Mike Goulden and Stephanie Pau and Matteo Detto and Brent Helliker and Richardson, {Andrew D.}",
note = "Funding Information: ACKNOWLEDGMENTS. C.J.S. acknowledges support from the National Science Foundation (NSF) via the Macrosystems Biology program (EF award 1241953) and the USDA McIntire Stennis program. A.D.R. acknowledges support from the NSF, through the LTER (DEB 1237491, DEB 1832210) and Macrosystems Biology (EF 1241616, EF 1702697) programs. B.H. acknowledges support from the NSF (awards EF 1241873, and IOS 0950998). M.D. thanks BP for its support of Princeton{\textquoteright}s Carbon Mitigation Initiative. S.S. was supported by Los Alamos Center of Space and Earth Sciences. H.K. and C.V.H. acknowledge support from the US Department of Energy{\textquoteright}s Office of Science for AmeriFlux core sites. We thank Dave Bowling for providing GPP data for Niwot Ridge. Funding Information: C.J.S. acknowledges support from the National Science Foundation (NSF) via the Macrosystems Biology program (EF award 1241953) and the USDA McIntire Stennis program. A.D.R. acknowledges support from the NSF, through the LTER (DEB 1237491, DEB 1832210) and Macrosystems Biology (EF 1241616, EF 1702697) programs. B.H. acknowledges support from the NSF (awards EF 1241873, and IOS 0950998). M.D. thanks BP for its support of Princeton{\textquoteright}s Carbon Mitigation Initiative. S.S. was supported by Los Alamos Center of Space and Earth Sciences. H.K. and C.V.H. acknowledge support from the US Department of Energy{\textquoteright}s Office of Science for AmeriFlux core sites. We thank Dave Bowling for providing GPP data for Niwot Ridge. Publisher Copyright: Copyright {\textcopyright} 2022 the Author(s). Published by PNAS.",
year = "2022",
month = sep,
day = "20",
doi = "10.1073/pnas.2205682119",
language = "English (US)",
volume = "119",
journal = "Proceedings of the National Academy of Sciences of the United States of America",
issn = "0027-8424",
publisher = "National Academy of Sciences",
number = "38",
}