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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 leaf is 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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The Duality of Reforestation Impacts on Surface and Air Temperature
Abstract Evidence is mounting that temperate‐zone reforestation cools surface temperature (Tsurf), mitigating deleterious effects of climate warming. WhileTsurfdrives many biophysical processes, air temperature (Ta) is an equally important target for climate mitigation and adaptation. Whether reductions inTsurftranslate to reductions inTaremains complex, fraught by several nonlinear and intertwined processes. In particular, forest canopy structure strongly affects near‐surface temperature gradients, complicating cross‐site comparison. Here the influence of reforestation onTais assessed by targeting temperature metrics that are less sensitive to local canopy effects. Specifically, we consider the aerodynamic temperature (Taero), estimated using a novel procedure that does not rely on the assumptions of Monin‐Obukhov similarity theory, as well as the extrapolated temperature into the surface layer (Textrap). The approach is tested with flux tower data from a grass field, pine plantation, and mature hardwood stand co‐located in the Duke Forest (North Carolina, USA). During growing season daytime periods,Tsurfis 4–6 °C cooler, andTaeroand near‐surfaceTextrapare 2–3 °C cooler, in the forests relative to the grassland. During the dormant season, daytime differences are smaller but still substantial. At night, differences inTaeroare small, and near‐surfaceTextrapis warmer over forests than grasslands during the growing season (by 0.5 to 1 °C). Finally, the influence of land cover onTextrapat the interface between the surface and mixed layer is small. Overall, reforestation appears to provide a meaningful opportunity for adaption to warmer daytimeTain the southeastern United States, especially during the growing season.
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- PAR ID:
- 10375717
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Biogeosciences
- Volume:
- 125
- Issue:
- 4
- ISSN:
- 2169-8953
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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