Comparison of forest under-canopy and regional warming over 37 years (1986 - 2023)

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Climate warming is expected to outpace some species' adaptive capacities. Though forests buffer understory microclimates by moderating temperature and humidity, it is unknown if forest understories will warm at similar rates to open environments under climate warming to affect potential forest refugia and climate resilience. We found that in temperate forests, located in the Pacific Northwest of the USA, under-canopy late spring and summer temperatures warmed at slower rates (i.e., were decoupled) from regional increases in maximum daily temperatures over 37 years. Decoupling resulted in a 43% reduction in the rate of warming under forest canopies (0.38 ± 0.06 °C decade -1) relative to open sites (0.67 ± 0.06 °C decade-1) in the warmest months (July-August). Warming in the early growing season (May-June) was similarly decoupled by 63% in under-canopy (0.16 ± 0.08 °C decade-1) relative to open sites (0.41 ± 0.08 °C decade-1). We hypothesize that changes in evapotranspiration over time may be a mechanism underpinning these decoupling trends, where temperature-driven increases in vapor pressure deficit may have increased forest evapotranspiration and long-term trends in drying may have led to decreased evapotranspiration in open environments. Summer (July-August) decoupling of summer maximum temperatures delayed extirpation of simulated populations by 117 - 153 years, suggesting that current terrestrial biodiversity predictions may overestimate temperature-driven extinction risks. Thermal decoupling therefore provides an additional climate-related incentive for forest conservation and restoration.We used 35 years of daily temperature data collected from meteorological stations west of the Cascade Mountain range crest in Oregon, USA, including a globally unique set of closed-canopy stations in the H.J. Andrews Experimental Forest (HJA). We selected 13 closed-canopy and 13 open-canopy stations along an elevational gradient from 75 m to 1609 m that met our standards for surface substrate (i.e., not concrete) and low canopy cover change over the years 1985 - 2021(range < 35%; Fig. S1). Because elevation is known to affect temperatures and warming rates (33), we then paired a subset (n = 18) of the 26 stations based on elevation, constraining open- and closed-canopy pairs to be less than 130 m apart in elevation. To test the sensitivity of our results to our paired station assumptions, we replicated our analysis 1000 times using data constructed by resampling the full 26 stations, randomly holding out eight stations (four open- and four closed-canopy) in each iteration. Curated climate data from the HJ Andrews Experimental Forest can also be found at the Environmental Data Initiative: Daly, C., & McKee, W. A. (2025). Air and soil temperature data from the Reference Stand network at the Andrews Experimental Forest, 1971 to present [Dataset]. Environmental Data Initiative. https://doi.org/10.6073/PASTA/D0ABE716146004268BB5F876EE42C992Daly, C., & McKee, W. A. (2025). Meteorological data from benchmark stations at the Andrews Experimental Forest, 1957 to present (Version ver 37) [Dataset]. Environmental Data Initiative. https://doi.org/10.6073/pasta/5d4ab4b210165d6e860ebe58e0579e4e

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