Overstory and Understory Leaves Warm Faster Than Air in Evergreen Needleleaf Forests

Ganz, Keenan; Still, Christopher J.; Rastogi, Bharat; Moskal, L. Monika

Citeable URL: https://ir.library.oregonstate.edu/concern/articles/9593v4136

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Creator	Ganz, Keenan Still, Christopher J. Rastogi, Bharat Moskal, L. Monika
Abstract	The limited homeothermy hypothesis states that leaves maintain their temperature within an optimal range for photosynthesis by increasing transpiration during warm conditions. Under limited homeothermy, plants may offset thermal stress caused by climate change. If this hypothesis is true, we should observe: 1) leaf temperature increasing slower than air temperature and 2) leaves cooler than air during warm conditions. We tested these predictions with an energy balance model for evergreen needleleaf forest sites in the National Ecological Observatory Network. A key feature of our model was its vertical stratification of the canopy, which allowed us to analyze vertical gradients in canopy temperature. This feature is especially important given that prior work has focused on the tops of forest canopies. Our results do not support limited homeothermy at any canopy position. In all canopy strata, leaf temperature increased faster than air and periods with leaves cooler than air were rare. In such cases, cooling was due to emitted radiation, not transpiration. But, when water was abundant, transpiration could produce mildly homeothermic behavior. We attribute these results to the needle-like shape of leaves in our study sites. This leaf shape increases boundary layer conductance and causes heat gain from surrounding air to overpower heat loss from transpiration when leaves are cooler than air. Our results indicate that needleleaf forests cannot avert thermal stress in a warming world. Thermal limits on photosynthesis and non-linear increases in respiration with temperature may weaken the role of evergreen forests as a global carbon sink.
License	All rights reserved
Resource Type	Article
DOI	https://doi.org/10.1016/j.agrformet.2025.110456
Date Issued	2025-02-22
Journal Title	Agricultural and Forest Meteorology
Journal Volume	364
Academic Affiliation	Forest Ecosystems and Society
权利声明	In Copyright
Funding Statement (additional comments about funding)	This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE-2140004. Field work and computational needs were supported by the Precision Forestry Cooperative at the University of Washington. We thank Christian Torgersen and David Butman for helpful comments on earlier versions of this manuscript. We thank Anthony Stewart and Rachel Deininger (UW RSGAL) for collecting the lidar point clouds from which the ABBY, DEJU, and RMNP tree graphics in Fig. 1 were derived. The National Ecological Observatory Network is a program sponsored by the National Science Foundation and operated under cooperative agreement by Battelle. This material is based in part upon work supported by the National Science Foundation through the NEON Program. Funding for the Ameriflux data portal was provided by the U.S. Department of Energy Office of Science. Specific acknowledgement of Ameriflux and NEON data products is provided in Supplementary Information.
Publisher	Elsevier
Peer Reviewed	Yes
Language	English [eng]
ISSN	1873-2240
Accessibility Feature	unknown

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Overstory and Understory Leaves Warm Faster Than Air in Evergreen Needleleaf Forests

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Overstory and Understory Leaves Warm Faster Than Air in Evergreen Needleleaf Forests

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