Water Use Efficiency content under fertilization schemes in an elevational gradient– Preliminary results from Nutrient Manipulation Experiment in Ecuador

Water-use efficiency (WUE) is a key physiological trait for understanding forest adaptation to rising atmospheric CO₂.   However, under conditions of elevated CO₂ and increasing atmospheric deposition of nitrogen (N) and phosphorus (P), the responses of vegetation in tropical montane forests in terms of WUE remain understudied. This study investigates WUE, estimated from stable carbon (δ¹³C), and also analyzes oxygen (δ¹⁸O) isotope compositions across six tree species in a long-term Nutrient Manipulation Experiment (NUMEX) located along an elevational gradient (1000, 2000, and 3000 m a.s.l.) in Ecuador. We assessed whether nutrient addition influences tree physiological responses in terms of iWUE and how these responses vary across elevations.

Preliminary results indicate species-specific and elevation-dependent differences in isotopic composition and intrinsic water-use efficiency (iWUE). Nutrient addition treatments (N and P) did not result in statistically significant changes in iWUE compared to control plots, suggesting that nutrient availability is not the primary driver of iWUE variability in the studied species. However, δ¹⁸O appeared more sensitive to nutrient inputs than iWUE. Pouteria torta, Alchornea lojanensis, and Myrcia sp., exhibited significant δ¹⁸O responses under nutrient addition, reflecting contrasting physiological strategies among taxa. These responses may indicate compensatory mechanisms that help maintain relatively stable iWUE across treatments. However, this interpretation remains preliminary and requires further analysis.

Isotopic composition and iWUE varied consistently along the elevational gradient. Both iWUE and δ¹³C followed an inverted U-shaped pattern, with slightly higher values at 2000 m. On the other hand, δ¹⁸O values were more enriched and similar at 1000 m and 2000 m, consistent with higher temperatures and evaporative demand at lower elevations. Overall, climatic factors associated with elevation exert stronger control over iWUE and isotopic signatures than nutrient availability.

Although preliminary, these findings provide new insights into water-use strategies of tropical montane tree species and highlight the importance of long-term nutrient manipulation experiments for addressing knowledge gaps in tropical forest eco-physiology under environmental change.