Carbon release from woody parts of trees along an elevation gradient in a tropical montane moist forest of Southern Ecuador

Abstract

Tropical montane forests (TMF), which account for 21.2% of tropical forests worldwide, are among the least studied ecosystems with respect to their C balance. The present study aimed to quantify CO2 losses from above- and belowground woody organs of representative tree species of a tropical montane moist forest in southern Ecuador. We used a portable CO2 measurement system to monitor the respiratory CO2 release from stems (RS) and coarse roots (RR) along a 2000 m-elevation transect with three study sites at 1050, 1890 and 3050 m a.s.l. In an intensive 1-year-measurement period we determined the impact of altitude and of seasonal climate variations on patterns of CO2 release from woody organs. We found substantial variation in the stem respiratory activity among different species and different tree individuals at all three study sites. Mean RS declined significantly from the premontane forest (1050 m) to the upper montane site (3050 m). Mean RR did not change significantly with altitude, though showing a decreasing tendency. Comparing dry and wet season patterns in stem CO2 release we found RS to be largely uncoupled from changes in the dial temperature regime under humid season conditions. During the dry season, the respiration-temperature relationship was generally stronger, though temperature sensitivity of RS differed greatly in degree and even in the direction of response among individual trees. Integrating additional influencing abiotic factors (vapour pressure deficit, wind speed and solar radiation) could not enhance the ability to explain the variability of RS. We assumed maintenance respiration to dominate under humid conditions unfavourable for photosynthetic carbon gain of the tree, whereas the dry season conditions principally favoured stem respiratory activity, and most likely energy acquisition. We found RS, but not RR to show a clear seasonality within the measurement year. Highest rates were measured during the dry season, though the increase in RS could not be simply related to changes in the temperature regime. On the other hand, the high degree of climate sensitivity of RS of the studied montane forest trees could also indicate C losses via the alternative pathway (cyanide-resistant oxidase), since the higher respiratory activity could not satisfactorily be related to stimulated cell growth. Along the elevation transect, annual carbon efflux from stems decreased from 167.1 g C m-2 yr-1 at 1050 m to 37.7 g C m-2 yr-1 at 3050 m, while coarse root carbon release changed little from 1050 m (40.9 g C m-2 yr-1) to 3050 m (36.8 g C m-2 yr-1). Stem growth respiration accounted for a comparatively small fraction of total stem respiration at all three sites, whereas coarse root growth respiration was of increasing importance with increasing altitude.