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

   https://doi.org/10.1073/pnas.2205682119  

   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 ; et al ( September 2022 , Proceedings of the National Academy of Sciences)

   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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2. Experimental warming across a tropical forest canopy height gradient reveals minimal photosynthetic and respiratory acclimation

   https://doi.org/10.1111/pce.14134  

   Carter, Kelsey R. ; Wood, Tana E. ; Reed, Sasha C. ; Butts, Kaylie M. ; Cavaleri, Molly A. ( July 2021 , Plant, Cell & Environment)

   Tropical forest canopies cycle vast amounts of carbon, yet we still have a limited understanding of how these critical ecosystems will respond to climate warming. We implemented in situ leaf‐level + 3°C experimental warming from the understory to the upper canopy of two Puerto Rican tropical tree species,Guarea guidoniaandOcotea sintenisii. After approximately 1 month of continuous warming, we assessed adjustments in photosynthesis, chlorophyll fluorescence, stomatal conductance, leaf traits and foliar respiration. Warming did not alter net photosynthetic temperature response for either species; however, the optimum temperature ofOcoteaunderstory leaf photosynthetic electron transport shifted upward. There was noOcotearespiratory treatment effect, whileGuarearespiratory temperature sensitivity (Q₁₀) was down‐regulated in heated leaves. The optimum temperatures for photosynthesis (T_opt) decreased 3–5°C from understory to the highest canopy position, perhaps due to upper canopy stomatal conductance limitations.Guareaupper canopyT_optwas similar to the mean daytime temperatures, whileOcoteacanopy leaves often operated aboveT_opt. With minimal acclimation to warmer temperatures in the upper canopy, further warming could put these forests at risk of reduced CO₂uptake, which could weaken the overall carbon sink strength of this tropical forest.

    

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3. Suppressed Daytime Convection Over the Amazon River

   https://doi.org/10.1029/2020JD033627  

   Wu, M. ; Lee, J. ‐E. ; Wang, D. ; Salameh, M. ( July 2021 , Journal of Geophysical Research: Atmospheres)

   We investigated the interaction between surface conditions and precipitating convection by comparing the Amazon River against the surrounding forest. Despite similar synoptic conditions within a few tens of kilometers, the river surface is substantially cooler than the surrounding forest during the day and warmer at night. We analyzed 20 years of high‐resolution satellite precipitation data and confirmed previous findings of daytime rainfall reduction over the river for the whole Amazon Basin. The percentage reduction is strongest during the dry‐to‐wet transition season. In addition, the percentage reduction of individual tributary is significantly correlated with the Laplacian of surface temperature, which causes thermally driven surface divergence and suppresses local convection. Additionally, nighttime rainfall is enhanced over tributaries near the Atlantic coast during the wet season. A regional climate model then simulates the local rainfall anomalies associated with the river. Above the river, moisture diverges near the surface and converges above the surface before the daytime rainfall, partially driven by the horizontal gradient of humidity. Unlike the river, moisture convergence within the boundary layer is more critical for the rainfall above the forest region. Our studies suggest that strong thermal contrast can be important in deriving heterogeneous convection in moist tropical regions.

    

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4. Reforestation and surface cooling in temperate zones: Mechanisms and implications

   https://doi.org/10.1111/gcb.15069  

   Zhang, Quan ; Barnes, Mallory ; Benson, Michael ; Burakowski, Elizabeth ; Oishi, A. Christopher ; Ouimette, Andrew ; Sanders‐DeMott, Rebecca ; Stoy, Paul C. ; Wenzel, Matt ; Xiong, Lihua ; et al ( April 2020 , Global Change Biology)

   Land‐use/cover change (LUCC) is an important driver of environmental change, occurring at the same time as, and often interacting with, global climate change. Reforestation and deforestation have been critical aspects of LUCC over the past two centuries and are widely studied for their potential to perturb the global carbon cycle. More recently, there has been keen interest in understanding the extent to which reforestation affects terrestrial energy cycling and thus surface temperature directly by altering surface physical properties (e.g., albedo and emissivity) and land–atmosphere energy exchange. The impacts of reforestation on land surface temperature and their mechanisms are relatively well understood in tropical and boreal climates, but the effects of reforestation on warming and/or cooling in temperate zones are less certain. This study is designed to elucidate the biophysical mechanisms that link land cover and surface temperature in temperate ecosystems. To achieve this goal, we used data from six paired eddy‐covariance towers over co‐located forests and grasslands in the temperate eastern United States, where radiation components, latent and sensible heat fluxes, and meteorological conditions were measured. The results show that, at the annual time scale, the surface of the forests is 1–2°C cooler than grasslands, indicating a substantial cooling effect of reforestation. The enhanced latent and sensible heat fluxes of forests have an average cooling effect of −2.5°C, which offsets the net warming effect (+1.5°C) of albedo warming (+2.3°C) and emissivity cooling effect (−0.8°C) associated with surface properties. Additional daytime cooling over forests is driven by local feedbacks to incoming radiation. We further show that the forest cooling effect is most pronounced when land surface temperature is higher, often exceeding −5°C. Our results contribute important observational evidence that reforestation in the temperate zone offers opportunities for local climate mitigation and adaptation.

    

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5. A practical exploration of land cover impacts on surface and air temperature when they are most consequential

   https://doi.org/10.1088/2752-5295/accdf9  

   Novick, Kimberly A. ; Barnes, Mallory L. ( May 2023 , Environmental Research: Climate)

   Widespread shifts in land cover and land management (LCLM) are being incentivized as tools to mitigate climate change, creating an urgent need for prognostic assessments of how LCLM impacts surface energy balance and temperature. Historically, observational studies have tended to focus on how LCLM impacts surface temperature (T_surf), usually at annual timescales. However, understanding the potential for LCLM change to confer climate adaptation benefits, or to produce unintended adverse consequences, requires careful consideration of impacts on bothT_surfand the near-surface air temperature (T_a,local) when they are most consequential for ecosystem and societal well-being (e.g. on hot summer days). Here, long-term data from 130 AmeriFlux towers distributed between 19–71 °N are used to systematically explore LCLM impacts on bothT_surfandT_a,local, with an explicit focus on midday summer periods when adaptive cooling is arguably most needed. We observe profound impacts of LCLM onT_surfat midday, frequently amounting to differences of 10 K or more from one site to the next. LCLM impacts onT_a,localare smaller but still significant, driving variation of 5–10 K across sites. The magnitude of LCLM impacts on bothT_surfandT_a,localis not well explained by plant functional type, climate regime, or albedo; however, we show that LCLM shifts that enhance ET or increase canopy height are likely to confer a local mid-day cooling benefit for bothT_surfandT_a,localmost of the time. At night, LCLM impacts on temperature are much smaller, such that averaging across the diurnal cycle will underestimate the potential for land cover to mediate microclimate when the consequences for plant and human well-being are most stark. Finally, during especially hot periods, land cover impacts onT_a,localandT_surfare less coordinated, and ecosystems that tend to cool the air during normal conditions may have a diminished capacity to do so when it is very hot. We end with a set of practical recommendations for future work evaluating the biophysical impacts and adaptation potential of LCLM shifts.

    

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