The isotopic signature of transpiration in mixed conifer forest
Abstract
Isotopic measurements of water vapor in forest air provide insights into ecosystem level responses to variability in surface hydrology. Studies of isotopic water balance of ecosystems should account for variation in the isotopic signature of transpired vapor in the non-steady state as well as isotopic differences in source water among dominant forest plants. Generally low rates of leaf gas exchange in some tree species, especially conifers, are associated with low leaf-water turnover rates, which greatly extends the time required to achieve transpiration at isotopic steady state as environmental conditions change over diurnal periods. Leaf longevity is also quite high (ca. 6-10 yrs.) in many evergreen coniferous species and stomatal conductance and rates of leaf gas exchange tend to decline as leaves age in conifer stands. We measured hydrogen and oxygen stable isotope ratios of water in leaves of different ages and from different canopy positions in sub-alpine fir (Abies lasiocarpa) and Engelmann spruce (Picea englemannii) over two separate diurnal periods during the growing season of 2008 in high elevation, mixed coniferous forest in the Snowy Range of southeastern Wyoming, USA. We collected needles every 3-4 h over 24-h periods to investigate the dynamics leaf water isotopic enrichment. Concurrent measurements of leaf gas exchange and leaf water content allowed us to model the leaf water turnover rate and the isotope ratio of transpiration in the non-steady state. Daily patterns of transpiration determined from sapflux measurements agreed with patterns of leaf conductance and transpiration determined from leaf-level by gas exchange measurements. Afternoon stomatal closure reduced leaf and tree level transpiration and leaf water turnover rates. Atmospheric water vapor was collected also every 3-4 h for isotopic analysis at multiple heights within and above the canopy. The isotopic composition of water vapor in the forest canopy became enriched in the heavy isotopes of hydrogen and oxygen during midday periods relative to that during early morning and nighttime periods reflecting an increased contribution of transpired water vapor to canopy air at midday. Keeling plot analyses suggested that the isotopic composition of the evapotranspiration flux was not constant during daytime periods, potentially reflecting changes in the isotope ratio of forest transpiration associated with transpiration at isotopic non-steady state. We conclude that modeling the isotope composition of forest transpiration should take into account the variation in leaf water turnover rates associated with leaf age and canopy position.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.H24A..03W
- Keywords:
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- 1813 Eco-hydrology;
- 1818 Evapotranspiration