In tropical mountains, trees are the dominant life form from sea level to above 4,000-m altitude under highly variable thermal conditions (range of mean annual temperatures: <8 to >28°C). How light-saturated net photosynthesis of tropical trees adapts to variation in temperature, atmospheric CO concentration, and further environmental factors, that change along elevation gradients, is not precisely known. With gas exchange measurements in mature trees, we determined light-saturated net photosynthesis at ambient temperature (T) and [CO] ( A) of 40 tree species from 21 families in tropical mountain forests at 1000-, 2000-, and 3000-m elevation in southern Ecuador. We tested the hypothesis that stand-level averages of A and leaf dark respiration ( R) per leaf area remain constant with elevation. Stand-level means of A were 8.8, 11.3, and 7.2 μmol CO m s; those of R 0.8, 0.6, and 0.7 μmol CO m s at 1000-, 2000-, and 3000-m elevation, respectively, with no significant altitudinal trend. We obtained coefficients of among-species variation in A and R of 20-53% ( n = 10-16 tree species per stand). Examining our data in the context of a pan-tropical A data base for mature tropical trees (c. 170 species from 18 sites at variable elevation) revealed that area-based A decreases in tropical mountains by, on average, 1.3 μmol CO m s per km altitude increase (or by 0.2 μmol CO m s per K temperature decrease). The A decrease occurred despite an increase in leaf mass per area with altitude. Local geological and soil fertility conditions and related foliar N and P concentrations considerably influenced the altitudinal A patterns. We conclude that elevation is an important influencing factor of the photosynthetic activity of tropical trees. Lowered A together with a reduced stand leaf area decrease canopy C gain with elevation in tropical mountains. [ABSTRACT FROM AUTHOR]