Air and Surface Temperatures Differently Drive Terrestrial Carbon and Water Cycles in the High Latitudes.

High‐latitude vegetation experience different temperatures than the ambient air temperature. While lacking a regional plant temperature product, we drove the dynamic ecosystem model, LPJ‐GUESS, with widely used ERA5‐land surface temperature (Tsurf, at radiative equilibrium) and air temperature to understand ecosystem process responses to these two temperatures. We show that tundra plants' growth is stimulated by warmer Tsurf in the summer, but in the boreal forests, colder Tsurf in the non‐summer months constrains leaf development and enzyme activity the following growing season. Tsurf drives higher productivity of tundra plant individuals but leads to less productive individuals in the boreal forest, although with compensatory changes (almost 68%) in vegetation structure. We demonstrate the importance of forcing temperature in simulating high‐latitude ecosystem processes and call for a community effort to measure plant temperatures across canopy heights and seasons to reduce uncertainties in estimating high‐latitude plant responses and feedback to climate. Plain Language Summary: The temperature experienced by plants often differs from the surrounding air temperature. This plant temperature plays a large role in how plants grow and function. By using two common observation‐based temperature data sets (air and surface temperatures) to drive an ecosystem model, we found that warmer summer surface temperatures stimulate tundra plant growth, but colder temperatures in other seasons than summer can limit growth in the boreal forest. We highlight how important it is to use actual temperatures plants in high‐latitude ecosystems experience to improve our understanding of how these plants interact with their environment under the changing climate. Key Points: Plant growths in tundra are stimulated by surface temperature which is higher than air temperature in summer monthsForcing temperature is crucial in determining high‐latitude vegetation structure and productivityNor surface or air temperature can sufficiently capture plant temperature dynamics [ABSTRACT FROM AUTHOR]