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 CO2 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 [CO2] (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 CO2 m−2 s−1; those of R D 0.8, 0.6, and 0.7 μmol CO2 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 CO2 m−2 s−1 per km altitude increase (or by 0.2 μmol CO2 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.
Highlights
With an estimated total of 37,000 woody plants (Odegaard 2000), tropical forests possess by far more tree species, and exist under a broader spectrum of environmental conditions, than any other biome on earth
A notable exception is the altitudinal transect study in Metrosideros polymorpha in the tropical island forests of Hawaii between 100 and 2,500 m a.s.l. (Cordell and others 1998, 1999). In this transect study in southern Ecuadorian Tropical mountain forests (TMFs), we examined patterns of altitudinal variation in light-saturated net photosynthesis of mature trees along a transect from 1,000 to 3,000 m a.s.l. covering a large number of tree species (40)
The 21 families investigated are arranged in Figure 2 according to their mean photosynthetic capacity
Summary
With an estimated total of 37,000 woody plants (Odegaard 2000), tropical forests possess by far more tree species, and exist under a broader spectrum of environmental conditions, than any other biome on earth. Tropical forests exhibit large changes in structure, physiognomy, and species composition as one ascends from the lowlands to high elevation. Tropical mountain forests (TMFs) replace lowland forests at approximately 1,000-m elevation, where the climate becomes cooler and often moister, and radiation is frequently reduced due to cloudiness (Hamilton and others 1995; Bruijnzeel and others 2010). In many altitudinal transects in tropical mountains, soil moisture tends to increase and the plant availability of nutrients, in particular of nitrogen and phosphorus, to decrease with altitude (Soethe and others 2008; Benner and others 2010; Bruijnzeel and others 2010; Moser and others 2008)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
