Seasonal Ecophysiology of Two Páramo Species: The Dominance of Light Over Water LimitationsDry-and rainy-season ecophysiology in two contrasting páramo species: incident sunlight outweighs water limitations

Adriana Sanchez ^1* Lina M Mercado ^2,3 Juan Manuel Posada ¹ William Kirby Smith ⁴

• ¹ Rosario University, Bogotá, Colombia
• ² University of Exeter, Exeter, England, United Kingdom
• ³ UK Centre for Ecology and Hydrology (UKCEH), Wallingford, Oxfordshire, United Kingdom
• ⁴ Wake Forest University, Winston-Salem, North Carolina, United States

Dry versus rainy seasons in many locations across the globe often involve major differences in cloudiness, precipitation, and incident sunlight. Thus, this study examines whether light availability or water stress is the primary limiting factor for photosynthesis in páramo plants across dry and rainy seasons. Somewhat surprisingly, mean net photosynthetic carbon gain per unit leaf area (An) in two dominant páramo species Espeletia grandiflora and Chusquea tessellata, was higher in the dry season despite increased water stress, suggesting that light availability plays a more dominant role in carbon assimilation. Photosynthetic light-response curves performed during both seasons suggest larger potential carbon uptake by E. grandiflora during the dry season and by C. tessellata during the rainy season, indicating seasonal differences in carbon assimilation. Statistical analyses showed that during the dry season, light is a more influential factor on carbon gain. During the rainy season, light is the most determinant factor on An for E. grandiflora, but for C. tessellata, light, temperature and vapor pressure deficit all have a significant effect. We conclude that the combination of high solar radiation and high leaf temperatures provide the conditions for larger carbon assimilation (especially in E. grandiflora) during the dry season. The cloudier and low incident sunlight conditions during the rainy season reduce carbon assimilation and growth, despite a lack of water stress during this season. Although C. tessellata had the highest mean An values during the dry season, it appeared vulnerable to intense radiation and subsequent desiccation. This study highlights that cloud cover and light availability, rather than water stress alone, are key drivers of páramo plant carbon uptake, with important implications for predicting future climate change effects.