More Like this

1. 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.

    

   more » « less
2. Does plant ecosystem thermoregulation occur? An extratropical assessment at different spatial and temporal scales

   https://doi.org/10.1111/nph.18632  

   Guo, Zhengfei ; Still, Christopher J. ; Lee, Calvin K. F. ; Ryu, Youngryel ; Blonder, Benjamin ; Wang, Jing ; Bonebrake, Timothy C. ; Hughes, Alice ; Li, Yan ; Yeung, Henry C. H. ; et al ( December 2022 , New Phytologist)

   To what degree plant ecosystems thermoregulate their canopy temperature (T_c) is critical to assess ecosystems' metabolisms and resilience with climate change, but remains controversial, with opinions from no to moderate thermoregulation capability.

   With global datasets ofT_c, air temperature (T_a), and other environmental and biotic variables from FLUXNET and satellites, we tested the ‘limited homeothermy’ hypothesis (indicated byT_c&T_aregression slope < 1 orT_c < T_aaround midday) across global extratropics, including temporal and spatial dimensions.

   Across daily to weekly and monthly timescales, over 80% of sites/ecosystems have slopes ≥1 orT_c > T_aaround midday, rejecting the above hypothesis. For those sites unsupporting the hypothesis, theirT_c–T_adifference (ΔT) exhibits considerable seasonality that shows negative, partial correlations with leaf area index, implying a certain degree of thermoregulation capability. Spatially, site‐mean ΔTexhibits larger variations than the slope indicator, suggesting ΔTis a more sensitive indicator for detecting thermoregulatory differences across biomes. Furthermore, this large spatial‐wide ΔTvariation (0–6°C) is primarily explained by environmental variables (38%) and secondarily by biotic factors (15%).

   These results demonstrate diverse thermoregulation patterns across global extratropics, with most ecosystems negating the ‘limited homeothermy’ hypothesis, but their thermoregulation still occurs, implying that slope < 1 orT_c < T_aare not necessary conditions for plant thermoregulation.

    

   more » « less
3. Thermal sensitivity across forest vertical profiles: patterns, mechanisms, and ecological implications

   https://doi.org/10.1111/nph.18539  

   Vinod, Nidhi ; Slot, Martijn ; McGregor, Ian R. ; Ordway, Elsa M. ; Smith, Marielle N. ; Taylor, Tyeen C. ; Sack, Lawren ; Buckley, Thomas N. ; Anderson‐Teixeira, Kristina J. ( December 2022 , New Phytologist)

   Rising temperatures are influencing forests on many scales, with potentially strong variation vertically across forest strata. Using published research and new analyses, we evaluate how microclimate and leaf temperatures, traits, and gas exchange vary vertically in forests, shaping tree, and ecosystem ecology. In closed‐canopy forests, upper canopy leaves are exposed to the highest solar radiation and evaporative demand, which can elevate leaf temperature (T_leaf), particularly when transpirational cooling is curtailed by limited stomatal conductance. However, foliar traits also vary across height or light gradients, partially mitigating and protecting against the elevation of upper canopyT_leaf. Leaf metabolism generally increases with height across the vertical gradient, yet differences in thermal sensitivity across the gradient appear modest. Scaling from leaves to trees, canopy trees have higher absolute metabolic capacity and growth, yet are more vulnerable to drought and damagingT_leafthan their smaller counterparts, particularly under climate change. By contrast, understory trees experience fewer extreme highT_leaf's but have fewer cooling mechanisms and thus may be strongly impacted by warming under some conditions, particularly when exposed to a harsher microenvironment through canopy disturbance. As the climate changes, integrating the patterns and mechanisms reviewed here into models will be critical to forecasting forest–climate feedback.

    

   more » « less
4. The Duality of Reforestation Impacts on Surface and Air Temperature

   https://doi.org/10.1029/2019JG005543  

   Novick, Kimberly A. ; Katul, Gabriel G. ( March 2020 , Journal of Geophysical Research: Biogeosciences)

   Evidence is mounting that temperate‐zone reforestation cools surface temperature (T_surf), mitigating deleterious effects of climate warming. WhileT_surfdrives many biophysical processes, air temperature (T_a) is an equally important target for climate mitigation and adaptation. Whether reductions inT_surftranslate to reductions inT_aremains 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 onT_ais assessed by targeting temperature metrics that are less sensitive to local canopy effects. Specifically, we consider the aerodynamic temperature (T_aero), 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 (T_extrap). 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,T_surfis 4–6 °C cooler, andT_aeroand near‐surfaceT_extrapare 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 inT_aeroare small, and near‐surfaceT_extrapis warmer over forests than grasslands during the growing season (by 0.5 to 1 °C). Finally, the influence of land cover onT_extrapat the interface between the surface and mixed layer is small. Overall, reforestation appears to provide a meaningful opportunity for adaption to warmer daytimeT_ain the southeastern United States, especially during the growing season.

    

   more » « less
5. High Leaf Respiration Rates May Limit the Success of White Spruce Saplings Growing in the Kampfzone at the Arctic Treeline

   https://doi.org/10.3389/fpls.2021.746464  

   Griffin, Kevin L. ; Schmiege, Stephanie C. ; Bruner, Sarah G. ; Boelman, Natalie T. ; Vierling, Lee A. ; Eitel, Jan U. ( November 2021 , Frontiers in Plant Science)

   Arctic Treeline is the transition from the boreal forest to the treeless tundra and may be determined by growing season temperatures. The physiological mechanisms involved in determining the relationship between the physical and biological environment and the location of treeline are not fully understood. In Northern Alaska, we studied the relationship between temperature and leaf respiration in 36 white spruce ( Picea glauca ) trees, sampling both the upper and lower canopy, to test two research hypotheses. The first hypothesis is that upper canopy leaves, which are more directly coupled to the atmosphere, will experience more challenging environmental conditions and thus have higher respiration rates to facilitate metabolic function. The second hypothesis is that saplings [stems that are 5–10cm DBH (diameter at breast height)] will have higher respiration rates than trees (stems ≥10cm DBH) since saplings represent the transition from seedlings growing in the more favorable aerodynamic boundary layer, to trees which are fully coupled to the atmosphere but of sufficient size to persist. Respiration did not change with canopy position, however respiration at 25°C was 42% higher in saplings compared to trees (3.43±0.19 vs. 2.41±0.14μmolm −2 s −1 ). Furthermore, there were significant differences in the temperature response of respiration, and seedlings reached their maximum respiration rates at 59°C, more than two degrees higher than trees. Our results demonstrate that the respiratory characteristics of white spruce saplings at treeline impose a significant carbon cost that may contribute to their lack of perseverance beyond treeline. In the absence of thermal acclimation, the rate of leaf respiration could increase by 57% by the end of the century, posing further challenges to the ecology of this massive ecotone. 

   more » « less