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Much uncertainty exists about the magnitude of woody tissue respiration and its environmental control in highly diverse tropical moist forests. In a tropical mountain rain forest in southern Ecuador, we measured the apparent diurnal gas exchange of stems and coarse roots (diameter 1-4 cm) of trees from representative families along an elevational transect with plots at 1050, 1890 and 3050 m a.s.l. Mean air temperatures were 20.8, 17.2 and 10.6 degrees C, respectively. Stem and root CO(2) efflux of 13 to 21 trees per stand from dominant families were investigated with an open gas exchange system while stand microclimate was continuously monitored. Substantial variation in respiratory activity among and within species was found at all sites. Mean daily CO(2) release rates from stems declined 6.6-fold from 1.38 micromol m(-2) s(-1) at 1050 m to 0.21 micromol m(-2) s(-1) at 3050 m. Mean daily CO(2) release from coarse roots decreased from 0.35 to 0.20 micromol m(-2) s(-1) with altitude, but the differences were not significant. There was, thus, a remarkable shift from a high ratio of stem to coarse root respiration rates at the lowest elevation to an apparent equivalence of stem and coarse root CO(2) efflux rates at the highest elevation. We conclude that stem respiration, but not root respiration, greatly decreases with elevation in this transect, coinciding with a substantial decrease in relative stem diameter increment and a large increase in fine and coarse root biomass production with elevation.

Half‐lives of 50 to 100 yr have been reported for native soil C in temperate Mollisols taken under cultivation, whereas some C fractions have shown mean residence times of hundreds of years. In the Tropics, C turnover is much more rapid. This lower C stability may be attributable to climate or to a mineral suite with low C stabilization potential typical of many tropical soils. We present data on the mineralization and accretion pattern of organic C under land use conversion in illitic loessial loams of the semiarid, warm temperate Province of La Pampa, Argentina. These soils should have a low C stabilization potential, and the semiarid temperate climate should provide for relatively slow turnover compared with tropical conditions. Using 13C‐natural abundance and soil fractionation, we determined soil C derived from C3 and C4 plants under land use changes between native Calden savannah, pastures, and arable fields, specifically: (i) the conversion of C3 natural savannah to agriculture that includes (C4) maize in the rotation, (ii) the conversion of a 40‐yr‐old C4 pasture into arable land under C3 crops, and (iii) the reclamation of highly degraded C3 cultivated land with C4 pastures. Although none of the land cover changes represented 100% conversions between plants of C4 and C3 metabolism, C turnover could still be determined. All sites provided evidence for rapid C losses with C half‐lives just above 10 yr and no evidence for long‐term stabilized C in any soil fractions. Soil under long‐term pasture or the natural vegetation of the region, with initial C contents between 24 and 33 mg C g−1 bulk soil, lost 33 to 57% of this original bulk soil C within 12 to 18 yr of continuous cultivation. On degraded soils under restoration with C4 pasture, C accretion was also rapid but leveled off well below the original C levels. Theses results provide evidence for potentially irreversible soil degradation and corroborate local practical experience that these temperate, coarse‐textured, illitic soils are highly susceptible to degradation and should be managed carefully.

It is commonly presumed that plant respiratory CO 2 release increases with increasing temperature. However, we report on very contrasting stem CO 2 release (R S )-temperature relationships of trees in a species-rich tropical montane forest of southern Ecuador under dry and wet season conditions. Rates of R S were low and completely uncoupled from the dial temperature regime during the humid season. In contrast, during the dry season, R S was generally higher and temperature sensitivity of R S differed greatly in degree and even in the direction of response, indicating that temperature might not be the only determinant of R S . In order to explain the heterogeneity of R S , we related R S to vapour pressure deficit, wind speed and solar radiation as important abiotic drivers influencing transpiration and photosynthesis. Stepwise multiple regression analyses with these meteorological predictors either were biased by high collinearity of the independent variables or could not enhance the ability to explain the variability of R S . We assume maintenance respiration to dominate under humid conditions unfavourable for energy acquisition of the tree, thus explaining the pronounced uncoupling of R S from atmospheric parameters. In contrast, the drier and hotter climate of the dry season seems to favour R S via enhanced assimilatory substrate delivery and stem respiratory activity as well as elevated xylem sap CO 2 imports with increased transpiration. In addition, tree individual differences in the temperature responses of R S may mirror diverging climatic adaptations of co-existing moist forest tree species which have their distribution centre either at higher or lower elevations.