Plant canopies exhibit stronger thermoregulation capability at the seasonal than diurnal timescales

Abstract

Plant canopy temperature (Tc) plays a crucial role in regulating plant growth and metabolism. Although dominant controls on Tc are observed to differ across timescales, whether this would cause differences in plant thermoregulation capability (PTC) remains unclear, raising concerns about extrapolating findings on plant thermoregulation from one timescale to another. Here we constructed diurnal and seasonal datasets of Tc, air temperature (Ta), and other biotic and abiotic factors from global hourly flux data, and explored diurnal and seasonal variations in PTC (indicated by Tcvs. Ta regression slope, with lower slopes indicating higher Tc stability and stronger thermoregulation). Our result revealed significantly lower Tcvs. Ta slopes (i.e. stronger PTC) at seasonal than diurnal timescales, primarily due to different transpiration cooling at high Ta between the two timescales. At the diurnal timescale, transpiration rates initially increase before decreasing with Ta after reaching a specific temperature threshold (∼85th percentile of Ta; related to midday depression of stomatal activities); Conversely, at the seasonal timescale, transpiration rates consistently increase with Ta (related to the coincidence among high water availability and the peak annual Ta). PTC also displays considerable spatial variability, with latent heat vs. net radiation relationship and water availability being the dominant regulators. Collectively, we recommend caution when extrapolating thermoregulation-relevant conclusions drawn from short-term observations to longer-term predictions, and vice versa, since they have different patterns and underlying mechanisms.