No evidence of canopy-scale leaf thermoregulation to cool leaves below air temperature across a range of forest ecosystems.
No evidence of canopy-scale leaf thermoregulation to cool leaves below air temperature across a range of forest ecosystems.
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DOI：
10.1073/pnas.2205682119
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发表时间：
2022-09-20
影响因子：
11.1
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中科院分区：
文献类型：
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长期以来，叶温一直被认为是植物功能的重要因素，而气候变暖可能会对叶温和功能产生过大的影响。这包括碳同化，因为许多研究表明，各种生态系统正在达到或接近温度阈值。然而，在一系列生态系统和条件下持续、高频地测量冠层尺度的叶温是罕见的。我们发现白天树冠叶的温度并不像树叶恒温假说所预测的那样在空气中冷却。树叶通常比空气温暖，这种偏离的程度因树叶大小和树冠结构而异。几乎所有生态系统的光合作用都发生在树叶温度超过气温时。未来的变暖不太可能通过树叶降温来缓解。了解和预测树叶温度和气温之间的关系对于预测对气候变暖的反应至关重要，因为研究表明，许多森林接近碳吸收的温度阈值。基于叶片测量，有限叶片恒温假说认为，白天叶片维持在光合作用最适温度附近，低于有害温度阈值。具体地说，在更高的温度下(例如，∼25-30°C)，树叶应该冷却到塔尔以下，从而在树叶/塔尔关系中产生斜率<；1，当树叶比空气凉爽时，它会大量吸收碳。这一假设意味着，气候变暖将通过补偿性的树叶降温反应来缓解。一个关键的不确定性是了解这种温度调节行为是否发生在天然森林树冠上。我们提供了一组前所未有的生长季冠层叶温(TCAN)数据，这些数据是在北美和中美洲的多个仪器良好的森林地点用热成像测量的。我们的数据不支持有限的恒温假说：树冠树叶在一天的大部分时间里比空气温暖，只有在下午中后期在空气下冷却，导致tcan/tair坡度和滞后行为。我们发现，大多数生态系统的光合作用发生在树冠叶比空气温暖的时候。利用能量平衡和生理模型，我们证明了关键的叶片性状影响叶-气耦合，并最终影响tcan/tair关系。冠层结构在TCAN动力学中也起着重要作用。未来气候变暖可能导致更大的总碳排放，随之而来的是对森林碳循环和死亡风险的影响。
Leaf temperature has long been recognized as important for plant function, and climate warming may lead to outsized impacts on leaf temperature and function. This includes carbon assimilation, as numerous studies suggest that a variety of ecosystems are operating at or near thermal thresholds. However, sustained, high-frequency measurements of canopy-scale leaf temperature across a range of ecosystems and conditions are rare. We show that daytime canopy leaf temperatures do not cool below air as predicted by the leaf homeothermy hypothesis. Leaves are typically warmer than air and the magnitude of this departure varies with leaf size and canopy structure. Almost all ecosystem photosynthesis occurs when leaf temperature exceeds air temperature. Future warming is unlikely to be mitigated by leaf cooling. Understanding and predicting the relationship between leaf temperature (Tleaf) and air temperature (Tair) 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 Tleaf is maintained near photosynthetic temperature optima and below damaging temperature thresholds. Specifically, leaves should cool below Tair at higher temperatures (i.e., > ∼25–30°C) leading to slopes <1 in Tleaf/Tair 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 (Tcan) 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 Tcan/Tair 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 Tcan/Tair relationship. Canopy structure also plays an important role in Tcan dynamics. Future climate warming is likely to lead to even greater Tcan, with attendant impacts on forest carbon cycling and mortality risk.
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影响因子：
5.2
作者：
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