Contribution of transpiration to forest ambient vapour based on isotopic measurements

Abstract

Using a simple isotope mixing model, we evaluated the relative proportion of water vapour generated by plant transpiration and by soil evaporation at two sites in the Amazon basin. Sampling was carried out at two different soil covers (forest and pasture), in a seasonal tropical rainforest at eastern Amazon where major deforestation is the result of land‐use change, and compared to a less seasonal central Amazon forest. In both forests, vapour from transpiration was responsible for most, if not all, of the water vapour generated in the forest, while it could not be detected above the grassy pastures. Thus the canopy transpiration may be a major source of water vapour to the forest and perhaps to the atmosphere during the dry season. The results are discussed in relation to predictive models based on net radiation that usually are not able to distinguish between transpiration and evaporation.

Huascaran δ18O as an indicator of tropical climate during the Last Glacial Maximum

“…To match the annual average composition of -17 o /oowe fix the precipitation height at the mean of the seasonal extremes (6300m) and take r = .4 . This is close to runoff estimates based on streamflow and precipitation data [Moreira et al, 1997;Salati et al, 1979].…”

“…With a surface temperature of 298K, the thermodynamic model matches the observed present day freeze line. The stated δ0 is supported by coastal vapor data quoted in [Moreira et al, 1997]. r is constrained by the observed annual cycles of precipitation in the foothills of the Andes, and of recent snow deposited in Andes glaciers.…”

“…Some of the apparent increase in runoff may actually reflect a shift from transpiration to evaporation. Transpiration doesn't fractionate [Moreira et al, 1997], while evaporation preferentially releases isotopically light water. If water is 100% recycled this doesn't matter, but if recycling is partial a shift to evaporation increases pre-fractionation even if the runoff ratio is fixed.…”

“…If water is 100% recycled this doesn't matter, but if recycling is partial a shift to evaporation increases pre-fractionation even if the runoff ratio is fixed. Significantly, forest is dominated by transpiration, while grasslands are dominated by evaporation [Moreira et al, 1997]. The interpretation of isotopic fluctuations in snowfall would then be dependent on time scale, since short-lived climate fluctuations might not allow the turnover in vegetation needed to increase the pre-fractionation.…”

Decreased water flowing from a forest amended with calcium silicate

“…T ranspiration by trees represents a major loss of water from forested watersheds (1)(2)(3)(4) that is sensitive to forest nutrition (5)(6)(7)(8)(9). Plot-scale chemical amendments to forest soils can result in 16-155% increases in transpiration during the first 60 d to 11 mo (5-9), yet it is unclear how these responses are expressed at the whole-watershed scale over longer periods.…”