Faculty of Mathematics and Natural Sciences, University of Göttingen, 37073 Göttingen, GERMANY.
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. TMFs are mainly characterized by high atmospheric humidity and frequent cloudiness driving the structure and functionality of these highly diverse ecosystems across a large altitudinal extension. One of the most marked changes with increasing elevation is the shift in C allocation from above- to belowground plant organs leading to a considerable decline in average tree height towards higher altitudes. Information on the key processes controlling the plant internal C use, the assimilatory C gain and the respiratory CO2 loss from single plant organs, is missing.
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. Moreover, we wanted to extrapolate rates of woody tissue respiration to the stand level and bring them in the frame of a carbon balance. 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. The results corroborated the remarkable shift in the relative importance of above- to belowground plant organs with increasing altitude as it has already been found for the biomass allocation patterns along the elevation gradient.
Temperature has long been known to be the most important abiotic driver of plant respiration. In the field, however, a consistent relationship between temperature and respiratory CO2 efflux is often not found. 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. Differences in species distribution centres and hence in the level of climatic adaptation of co-existing moist forest tree species could provide an alternative tool to explain diverging temperature responses of RS during the dry season.
Information about seasonal patterns of woody tissue CO2 release from evergreen tropical montane trees has received little attention. 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.
A first C balance of the three study sites confirmed the expected decline in the total canopy carbon gain from premontane to upper montane forests. However, with respect to the altitudinal changes in above- and belowground C allocation patterns, discrepancies between flux measurements and direct biomass assessments emerged. The enormous C translocation to the root system at 3050 m as previously found could not be matched by the amount of C available as derived from C influx and efflux estimates. On the other hand, the C balances were incomplete and uncertainties still exist with regard to the magnitude of important flux components such as the canopy C gain and the canopy respiratory C losses (including branches and twigs).