Canopy temperature (T) plays an important role in regulating the rates of mass and energy fluxes at the leaf surface. Better understanding of the relationship between T and water availability may enable more accurate monitoring of ecosystem functioning in a changing climate. Here, we used high spatiotemporal resolution thermal infrared cameras deployed at three eddy covariance flux tower sites along a water- to energy-limited gradient – including a predominately water-limited grassland/shrubland site, a seasonally water-limited evergreen needleleaf forest, and a predominantly energy-limited deciduous broadleaf forest – to determine T seasonality and its relationship with gross primary productivity (GPP) and environmental drivers. We found midday T was generally warmer than air temperature (T) during the growing season (T:T slope: 1.14–1.27) for all sites. Water-limited sites exhibited higher positive T deviations from T (2.30 ± 0.06 °C) compared to the energy-limited site (1.29 ± 0.09 °C) partly due to their reduced latent heat fluxes during water-limited periods. We further found that the T:T slope increased with site aridity, namely for 1.14 slope for the grassland, 1.15 for the evergreen forest, and 1.27 for the broadleaf forest. Peak GPP occurred when T was higher than T across all sites, with peak GPP at the grassland site occurring at +1.1 °C (T-T) and peak GPP at the broadleaf evergreen site occurring at +2.2 °C (T-T). T-T dynamics were mostly associated with soil water content at water-limited sites where canopies undergo a substantial cooling during the transition from dormancy to the peak GPP, while net radiation played a crucial role at the energy-limited site where the canopy heats up compared to T over the same phenological transition. Our findings provide novel insights into T-ecosystem water availability links, highlighting the drivers of T-T across diverse ecosystems in various phenological stages, which has implications for ecosystem management in a changing climate.

中文翻译：

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冠层温度动态与水与能量有限梯度上的生态系统可用水量密切相关


冠层温度 (T) 在调节叶表面质量和能量通量速率方面发挥着重要作用。更好地了解 T 和可用水量之间的关系可能有助于更准确地监测气候变化中的生态系统功能。在这里，我们使用了高时空分辨率热红外摄像机，部署在沿着水到能量有限梯度的三个涡流协方差通量塔地点——包括主要受水限制的草原/灌木丛地点、季节性受水限制的常绿针叶林和主要是能源有限的落叶阔叶林 - 确定 T 季节性及其与总初级生产力 (GPP) 和环境驱动因素的关系。我们发现所有地点的生长季节中午 T 普遍比气温 (T) 温暖（T:T 斜率：1.14–1.27）。与能量限制地点 (1.29 ± 0.09 °C) 相比，水限制地点表现出更高的 T 值正偏差 (2.30 ± 0.06 °C)，部分原因是它们在水限制期间潜热通量减少。我们进一步发现，T:T斜率随着场地干旱程度的增加而增加，即草地为1.14，常绿林为1.15，阔叶林为1.27。当 T 高于所有地点的 T 时，GPP 峰值出现，草地地点的 GPP 峰值发生在 +1.1 °C (TT)，阔叶常绿地点的 GPP 峰值发生在 +2.2 °C (TT)。 TT 动态主要与水限制地点的土壤含水量有关，在这些地点，冠层在从休眠期到 GPP 峰值的过渡期间经历了大幅冷却，而净辐射在能量限制地点发挥了至关重要的作用，与T 经历相同的物候转变。 我们的研究结果为 T 生态系统水资源可用性联系提供了新颖的见解，强调了不同物候阶段不同生态系统中 TT 的驱动因素，这对气候变化下的生态系统管理具有影响。