Cold and low irradiation shape Polylepis reticulata's seasonal growth and water use dynamics at the Ecuadorian Andean tree line

Aldemar Carabajo-Hidalgo^1,2,3*Daniel Nadal-Sala^1,3Byron Poma²Heidi Asbjornsen⁴Patricio Javier Crespo²Santi Sabaté^1,3

• ¹Departamento de Biología Evolutiva, Ecología y Ciencias Ambientales, Universitat de Barcelona Facultat de Biologia, Barcelona, Spain
• ²Departamento de Recursos Hídricos y Ciencias Ambientales, Universidad de Cuenca, Cuenca, Ecuador
• ³Centre de Recerca Ecologica i Aplicacions Forestals, Bellaterra, Spain
• ⁴Department of Natural Resources and the Environment and Earth Systems Research Center, University of New Hampshire, Durham, United States

Polylepis reticulata is a tree species that grows in the Ecuadorian Andean páramo at 4500 m a.s.l., an environment characterized by low temperatures, elevated cloudiness, and recurrent fog. These environmental stressors result in scarce periods when both photosynthesis and stem growth can occur. Particularly interesting are P. reticulata transpiration, stem growth, and water use efficiency in growth (WUEBAI) dynamics, which may provide valuable insights into the carbon source-sink growth limitation conundrum. Additionally, little is known about P. reticulata's sensitivity to the different environmental drivers, and its related life traits. In this study we quantify the seasonal transpiration, stem growth and WUEBAI patterns of P. reticulata from continuous field measurements on sap flow and stem growth during an entire year. We obtained mean annual values of 1.06 ± 0.3 cm2 day-1 for stem growth, 0.88 ± 0.5 m3 day-1 for transpiration and 1.24 ± 0.1 cm2 m-3 for WUE. Nevertheless, we found a seasonal pattern for these variables. Cold and cloudy conditions led to a reduction in stem growth, with mean values of 1.67 ± 1.2 cm2 day-1 during this season. Concomitantly, transpiration also declined, with mean values of 1.20 ± 0.8 m3 day-1, reducing WUEBAI, with mean values of 1.33 ± 0.4 cm2 m-3. On the contrary, during the warmer season, when photosynthesis and cambium cell differentiation occurred simultaneously, all the values were higher, with mean stem growth values of 2.58 ± 0.6 cm2 day-1, mean transpiration values of 1.85 ± 1.2 m3 day-1 and mean WUE values of 1.39 ± 1.0 cm2 m-3, suggesting a sink-driven limitation of tree growth during the cold season. Hence, our results suggest that P. reticulata's growth and transpiration patterns are limited by energy availability, rather than water availability. So, expected temperature increases for the next years may enhance P. reticulata's growth, should water stress not increase.