Canopy temperature dynamics are closely aligned with ecosystem water availability across a water- to energy-limited gradient

Abstract

Canopy temperature (Tc) plays an important role in regulating the rates of mass and energy fluxes at the leaf surface. Better understanding of the relationship between Tc 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 Tc seasonality and its relationship with gross primary productivity (GPP) and environmental drivers. We found midday Tc was generally warmer than air temperature (Tair) during the growing season (Tc:Tair slope: 1.14–1.27) for all sites. Water-limited sites exhibited higher positive Tc deviations from Tair (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 Tc:Tair 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 Tc was higher than Tair across all sites, with peak GPP at the grassland site occurring at +1.1 °C (Tc-Tair) and peak GPP at the broadleaf evergreen site occurring at +2.2 °C (Tc-Tair). Tc-Tair 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 Tair over the same phenological transition. Our findings provide novel insights into Tc-ecosystem water availability links, highlighting the drivers of Tc-Tair across diverse ecosystems in various phenological stages, which has implications for ecosystem management in a changing climate.