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Biogeosciences, 22, 1135–1147, 2025https://doi.org/10.5194/bg-22-1135-2025© Author(s) 2025. This work is distributed underthe Creative Commons Attribution 4.0 License.Research articleA shift in circadian stem increment patterns in a Pyrenean alpinetreeline precedes spring growth after snow meltingHelen Flynn 1,2 , J. Julio Camarero 2 , Alba Sanmiguel-Vallelado 3 , Francisco Rojas Heredia 2 ,Pablo Domínguez Aguilar 2 , Jesús Revuelto 2 , and Juan Ignacio López-Moreno 21 Department of Geosciences, Colorado State University, Fort Collins, Colorado 80521, USA2 CryoPyr, Instituto Pirenaico de Ecología (IPE-CSIC), Zaragoza, 50059, Spain3 iuFOR, EiFAB, Universidad de Valladolid, Campus Duques de Soria, Soria, 42004, SpainCorrespondence: Helen Flynn (helen.f l ynn@colostate.edu) and Juan Ignacio López-Moreno (nlopez@ipe.csic.es)Received: 30 October 2024 –Discussion started: 25 November 2024Accepted: 13 January 2025 –Published: 28 February 2025Abstract. Changing snow regimes and warmer growing sea-sons are some climate factors inf l uencing the productivityand growth of high-elevation forests and alpine treelines.In low-latitude mountain regions with seasonal snow anddrought regimes such as the Pyrenees, these climate fac-tors could negatively impact forest productivity. To addressthis issue, we assessed the relationships between climate,snow, and inter- and intra-annual radial growth and stem in-crement data in an alpine Pinus uncinata treeline ecotonelocated in the central Spanish Pyrenees. First, we devel-oped tree-ring-width chronologies of the study site to quan-tifyclimate–growthrelationships.Second,radialgrowth,treewater def icit, and shrinking–swelling cycles were quantif iedand identif ied at monthly to daily scales using f ine-resolutiondendrometer data. These variables were extracted for threeclimatically different years, including one of the hottest sum-mers on record in Spain (2022), and they were related to soilwater content, soil and air temperature, and the dates of snowduration across the treeline ecotone. Warmer February andMay temperatures enhanced tree radial growth, probably be-cause of an earlier snow meltout, the start of the growingseason, and the higher growth rates in spring, respectively.The characteristic circadian cycle of stem increment, def inedby night swelling and day shrinking, was detected in summerand fall. However, this pattern was inverted during the snowseason from November through April, suggesting a transitionphase characterized by wet soils and swollen stems preced-ing the spring onset of growth. Air temperature, soil tem-perature and moisture, and the presence of snow are strongindicators of how much and for how long mountain trees cangrow. Shifts in daily stem increment patterns reveal changesin early growth phenology linked to snow melting.1 IntroductionIn mountain areas, warming rates are much higher than inlowlands (Pepin et al., 2015), leading to changes in seasonalsnow regimes and the soil moisture available for tree growth(Harpold and Molotch, 2015). Forest productivity and treegrowth in high-elevation, subalpine forests and alpine tree-lines are especially sensitive to the warming effects of cli-mate change (Albrich et al., 2020). However, drier conditionscould also negatively impact low- to mid-latitude mountainforests, making the late-winter soil moisture coming fromsnow melting critical for tree growth. This has been observedin mountain areas subjected to strong snow and drought sea-sonality, such as the southern Rockies, the central Andes, andMediterranean mountain ranges like the Pyrenees (Andrus etal., 2018; Saavedra et al., 2018; Vicente-Serrano et al., 2021;Villalba et al., 1994). High-elevation treeline forests are lim-ited in productivity by climate variables like air and soil tem-perature (Peterson, 1998; Sanmiguel-Vallelado et al., 2021).Such forests are especially sensitive to the warming effectsof climate change (Albrich et al., 2020) and can experiencelonger and, thus, more productive growing seasons (Yang etal., 2024).Snow distribution and processes are clearly impacted byforest structures, but forests are also inf l uenced by climateand snow. For instance, productivity was found to decreasePublished by Copernicus Publications on behalf of the European Geosciences Union.