No evidence of canopy-scale leaf thermoregulation to cool leaves below air temperature across a range of forest ecosystems

Still, Christopher J.; Page, Gerald; Rastogi, Bharat; Griffith, Daniel M.; Aubrecht, Donald M.; Kim, Youngil; Burns, Sean P.; Hanson, Chad V.; Kwon, Hyojung; Hawkins, Linnia; Meinzer, Frederick C.; Sevanto, Sanna; Roberts, Dar; Goulden, Mike; Pau, Stephanie; Detto, Matteo; Helliker, Brent; Richardson, Andrew D.

doi:10.1073/pnas.2205682119

Title: No evidence of canopy-scale leaf thermoregulation to cool leaves below air temperature across a range of forest ecosystems

Journal Article · Mon Sep 12 00:00:00 UTC 2022 · Proceedings of the National Academy of Sciences of the United States of America

DOI: https://doi.org/10.1073/pnas.2205682119 · OSTI ID:1889968

^[1];

^[2]; Rastogi, Bharat ^[3];

^[4]; Aubrecht, Donald M. ^[5]; Kim, Youngil ^[6]; Burns, Sean P. ^[7]; Hanson, Chad V. ^[1]; Kwon, Hyojung ^[1]; Hawkins, Linnia ^[1]; Meinzer, Frederick C. ^[8];

^[9]; Roberts, Dar ^[10];

^[11]; Pau, Stephanie ^[12];

^[13];

^[14];

^[5]

Oregon State University, Corvallis, OR (United States)
Department of Biodiversity, Conservation and Attractions (DBCA), Bentley (Australia); CSIRO Land and Water, Wembley (Australia)
University of Colorado, Boulder, CO (United States); National Oceanic and Atmospheric Administration (NOAA), Boulder, CO (United States)
Oregon State University, Corvallis, OR (United States); Wesleyan University, Middletown, CT (United States)
Northern Arizona University, Flagstaff, AZ (United States)
CFB Trenton, Canadian Forces (Canada)
University of Colorado, Boulder, CO (United States); National Center for Atmospheric Research, Boulder, CO (United States)
USDA Forest Service PNW Research Station, Corvallis, OR 97331; USDA Forest Service, Corvallis, OR (United States)
Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
University of California, Santa Barbara, CA (United States)
Univ. of California, Irvine, CA (United States)
Florida State University, Tallahassee, FL (United States)
Princeton University, NJ (United States); Smithsonian Tropical Research Institute, Balboa (Panama)
University of Pennsylvania, Philadelphia, PA (United States)

Understanding and predicting the relationship between leaf temperature (T_leaf) and air temperature (T_air) 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 T_leafis maintained near photosynthetic temperature optima and below damaging temperature thresholds. Specifically, leaves should cool below T_air at higher temperatures (i.e., > ~25–30°C) leading to slopes <1 in T_leaf/T_air 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 (T_can) 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 T_can/T_air 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 T_can/T_air relationship. Canopy structure also plays an important role in T_can dynamics. Future climate warming is likely to lead to even greater T_can, with attendant impacts on forest carbon cycling and mortality risk.

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Research Organization:: Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)

Sponsoring Organization:: National Science Foundation (NSF); USDOE Office of Science (SC); United States Department of Agriculture

Grant/Contract Number:: 89233218CNA000001

OSTI ID:: 1889968

Report Number(s):: LA-UR-20-24466

Journal Information:: Proceedings of the National Academy of Sciences of the United States of America, Journal Name: Proceedings of the National Academy of Sciences of the United States of America Journal Issue: 38 Vol. 119; ISSN 0027-8424

Publisher:: National Academy of SciencesCopyright Statement

Country of Publication:: United States

Language:: English

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59 BASIC BIOLOGICAL SCIENCES
canopy temperature
functional traits
homeothermy
leaf temperature
leaf traits
photosynthesis

Title: No evidence of canopy-scale leaf thermoregulation to cool leaves below air temperature across a range of forest ecosystems

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