Photosynthesis of tropical Andean tree species: insights from mechanistic modelling and sensitivity analysis

Andean tropical montane forests are highly biodiverse ecosystems with a carbon storage capacity comparable to lowland forests. However, their response to climate change remains uncertain, as species photosynthesis depends on their thermal acclimation capacity. This study evaluates the variability of photosynthetic traits across montane and lowland tree species using a leaf level photosynthesis model and data from a transplant experiment across three elevations (14 °C, 22 °C, and 26 °C) in the tropical Andes. Six montane species and two lowland species were analyzed to assess photosynthetic responses to environmental conditions. We find that intraspecific variability in photosynthetic parameters, such as the apparent maximum carboxylation capacity (Vcmax) and the apparent maximum electron transport rate (Jmax), is key to accurately model photosynthesis in these ecosystems. Apparent Vcmax was identified as the primary determinant of diurnal variations in photosynthesis, especially under varying thermal environments. Additionally, stomatal conductance (g1) was highly variable and responded to vapor pressure deficit (VPD), suggesting that stomatal regulation is crucial for adaptation to environmental changes. Sensitivity analysis revealed that at higher altitudes (14 °C), photosynthetically active radiation (PAR) and temperature were the main limiting factors for photosynthesis, while at lower altitudes (22 °C), VPD was the dominant factor. Finally, the study demonstrates that the common use, within global vegetation models, of average parameters from lowland species to simulate montane forest is inadequate as such parameterizations tend to underestimate montane forest photosynthesis by up to 65 %. It is also recommended that vegetation models incorporate both intra- and interspecific variability to improve predictions of the carbon cycle in tropical Andean forests and their response to climate change.