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Only sun-lit leaves of the uppermost canopy exceed both air temperature and photosynthetic thermal optima in a wet tropical forest
Agricultural and Forest Meteorology ( IF 5.6 ) Pub Date : 2021-02-06 , DOI: 10.1016/j.agrformet.2021.108347
Benjamin D. Miller , Kelsey R. Carter , Sasha C. Reed , Tana E. Wood , Molly A. Cavaleri

Tropical forests have evolved under relatively narrow temperature regimes, and therefore may be more susceptible to climatic change than forests in higher latitudes. Recent evidence shows that lowland tropical forest canopies may already be exceeding thermal maxima for photosynthesis. Height can strongly influence both the microclimate and physiology of forest canopy foliage, yet vertical trends in canopy micrometeorology are rarely examined in tropical forests. To improve our understanding of how climatological and micrometeorological conditions affect tropical tree function, we assessed vertical gradients of photosynthetic photon flux density, vapor pressure deficit, air temperature, leaf temperature, and the difference between leaf and air temperature (ΔT) in a Puerto Rican tropical wet forest. Both air temperature and vapor pressure deficit increased linearly with height. Leaf temperature, however, did not significantly differ across the shaded foliage from 0-16 m, while the uppermost layer (20 m) was up to 4°C hotter than the rest of the foliage and up to 5°C hotter than air temperature at the highest radiation intensity. As a result, leaf temperatures in the shaded middle canopy and understory showed nearly poikilothermic behavior (i.e., leaf temperatures = air temperature), while the uppermost canopy strata showed megathermic behavior (i.e., leaf temperatures greater than air temperature), revealing different thermoregulation strategies for sun-lit versus shaded foliage. In addition, the uppermost canopy was the only stratum to exceed mean photosynthetic temperature optima for this site (Topt = 30.2 ± 1.1°C). Because the upper canopy plays a disproportionately large role in whole-forest photosynthesis, continued warming could potentially weaken the tropics’ carbon sink capacity. However, the shaded leaves may be able increase carbon uptake with further warming because they appear to be able to maintain temperatures below photosynthetic optima, possibly with the help of radiation shielding provided by the uppermost canopy layer.



中文翻译:

在潮湿的热带森林中,只有最顶冠层的阳光照射的叶片超过空气温度和光合作用的最佳温度

热带森林是在相对窄的温度范围内演化的,因此,与高纬度地区的森林相比,热带森林更容易受到气候变化的影响。最近的证据表明,低地热带森林的冠层可能已经超过光合作用的热最大值。高度可以强烈影响森林冠层叶片的微气候和生理,但是在热带森林中很少检查冠层微气象学的垂直趋势。为了增进我们对气候和微气象条件如何影响热带树木功能的了解,我们评估了波多黎各人的光合光子通量密度,蒸气压赤字,气温,叶片温度以及叶片与气温之间的差异(ΔT)的垂直梯度热带湿林。空气温度和蒸汽压差均随高度线性增加。然而,从0到16 m的阴影,叶温在整个遮荫的叶子上没有显着差异,而最上层(20 m)的温度比其余温度高4°C,比空气温度高5°C在最高的辐射强度。结果,在遮荫的中冠层和下层的叶片温度表现出几乎热化的行为(即叶片温度=气温),而最上层的冠层表现出大热行为(即叶片温度大于空气温度),揭示了不同的温度调节策略适用于日光照射或遮荫的树叶。此外,最上层的冠层是唯一超过该部位平均光合温度最佳值的地层(T 然而,从0到16 m的阴影,叶温在整个遮荫的叶子上没有显着差异,而最上层(20 m)的温度比其余温度高4°C,比空气温度高5°C在最高的辐射强度。结果,在遮荫的中冠层和下层的叶片温度表现出几乎热化的行为(即叶片温度=气温),而最上层的冠层表现出大热行为(即叶片温度大于空气温度),揭示了不同的温度调节策略适用于日光照射或遮荫的树叶。此外,最上层的冠层是唯一超过该部位平均光合温度最佳值的地层(T 然而,从0到16 m的阴影,叶温在整个遮荫的叶子上没有显着差异,而最上层(20 m)的温度比其余温度高4°C,比空气温度高5°C在最高的辐射强度。结果,遮荫的中冠层和下层的叶片温度显示出几乎热化行为(即叶片温度=气温),而最上层的冠层显示出高温行为(即叶片温度大于空气温度),揭示了不同的温度调节策略适用于日光照射或遮荫的树叶。此外,最上层的冠层是唯一超过该部位平均光合温度最佳值的地层(T 在辐射强度最高的情况下,最上层(20 m)的温度比其余树叶高4摄氏度,比气温高5摄氏度。结果,在遮荫的中冠层和下层的叶片温度表现出几乎热化的行为(即叶片温度=气温),而最上层的冠层表现出大热行为(即叶片温度大于空气温度),揭示了不同的温度调节策略适用于日光照射或遮荫的树叶。此外,最上层的冠层是唯一超过该部位平均光合温度最佳值的地层(T 在辐射强度最高的情况下,最上层(20 m)的温度比其余树叶高4摄氏度,比气温高5摄氏度。结果,在遮荫的中冠层和下层的叶片温度表现出几乎热化的行为(即叶片温度=气温),而最上层的冠层表现出大热行为(即叶片温度大于空气温度),揭示了不同的温度调节策略适用于日光照射或遮荫的树叶。此外,最上层的冠层是唯一超过该部位平均光合温度最佳值的地层(T 遮荫的中冠层和下层的叶片温度几乎表现出高温热行为(即叶片温度=空气温度),而最上层的冠层表现出高热行为(即叶片温度大于空气温度),揭示了日光照射下不同的温度调节策略与阴影的树叶。此外,最上层的冠层是唯一超过该部位平均光合温度最佳值的地层(T 遮荫的中冠层和下层的叶片温度几乎表现出高温热行为(即叶片温度=空气温度),而最上层的冠层表现出高热行为(即叶片温度大于空气温度),揭示了日光照射下不同的温度调节策略与阴影的树叶。此外,最上层的冠层是唯一超过该部位平均光合温度最佳值的地层(Topt  = 30.2±1.1°C)。由于上层冠层在整个森林的光合作用中起着不成比例的重要作用,持续的变暖可能会削弱热带地区的碳汇能力。但是,阴影的叶子可能会随着进一步的变暖而增加碳的吸收,因为它们似乎能够将温度保持在光合最适温度以下,这可能是借助于最上层的冠层提供的辐射屏蔽来实现的。

更新日期:2021-02-07
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