Altitudinal Change in the Photosynthetic Capacity of Tropical Trees: A Case Study from Ecuador and a Pantropical Literature Analysis

Abstract

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 2 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 2] (A sat) 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 sat and leaf dark respiration (R D) per leaf area remain constant with elevation. Stand-level means of A sat were 8. 8, 11. 3, and 7. 2 μmol CO 2 m -2 s -1; those of R D 0. 8, 0. 6, and 0. 7 μmol CO 2 m -2 s -1 at 1000-, 2000-, and 3000-m elevation, respectively, with no significant altitudinal trend. We obtained coefficients of among-species variation in A sat and R D of 20-53% (n = 10-16 tree species per stand). Examining our data in the context of a pan-tropical A sat data base for mature tropical trees (c. 170 species from 18 sites at variable elevation) revealed that area-based A sat decreases in tropical mountains by, on average, 1. 3 μmol CO 2 m -2 s -1 per km altitude increase (or by 0. 2 μmol CO 2 m -2 s -1 per K temperature decrease). The A sat 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 sat patterns. We conclude that elevation is an important influencing factor of the photosynthetic activity of tropical trees. Lowered A sat together with a reduced stand leaf area decrease canopy C gain with elevation in tropical mountains. © 2012 The Author(s).