Flanagan, L.B., J.R. Brooks, G.T. Varney, and J.R. Ehleringer. 1997. Discrimination against C18O16O during photosynthesis and the oxygen isotope ratio of respired CO2 in boreal forest ecosystems. Global Biogeochemical Cycles 11:83-98.

Carbon and oxygen isotope ratios (d13C, d18O) were studied in evergreen and deciduous forest ecosystems in semi-arid Utah (Pinus contorta, Populus tremuloides, Acer negundo and Acer grandidentatum). Measurements were taken in four to five stands of each forest ecosystem differing in overstory leaf area index (LAI) during two consecutive growing seasons. d13Cleaf of understory vegetation in the evergreen stands (LAI 1.5 - 2.2) did not differ among canopies with increasing LAI, whereas understory in the deciduous stands (LAI 1.5 - 4.5) exhibited strongly decreasing d13Cleaf values (increasing carbon isotope discrimination) with increasing LAI. d13C of needles and leaves at the top of the canopy were relatively constant over the entire LAI range, indicating no change in water-use efficiency with overstory LAI. In all canopies, d13Cleaf decreased with decreasing height above the forest floor due to physiological changes affecting ci/ca, and d13C of canopy air. This intra-canopy depletion was lowest in the open stand (1 ) and greatest in the denser stands (4.5 ). Although overstory LAI did not affect d13Cleaf of the upper canopy trees, d13C of soil organic carbon increased with increasing LAI in P. contorta and P. tremuloides ecosystems. This might be an indication that stand structure affects soil microorganism communities that differ in their decay rates of cellulose versus lignin. d13C of decomposing organic carbon became increasingly enriched over time (by 1.7 to 2.9 ) for all deciduous and evergreen forest ecosystems.

d13Ccanopy and d18Ocanopy of CO2 in canopy air varied temporally and spatially in all forest stands. Comparing vertical height profiles of d13Ccanopy and [CO2]canopy in a P. tremuloides and a P. contorta stand with similar LAI, revealed larger canopy gradients in the deciduous than in the evergreen canopy. In the very wet and cool year of 1993, canopy discrimination (Dcanopy) was similar for both deciduous P. tremuloides (16.9 ± 0.65 ) and evergreen P. contorta (17.2 ± 0.93 ) stands. Gradients of d13Ccanopy and [CO2]canopy were larger in denser Acer spp. stands than those in the open stand. However, 13C enrichment above and photosynthetic draw-down of [CO2]canopy below background values of the turbulent surface layer were larger in the open than in the dense stands, due to the presence of a vigorous understory vegetation. Seasonal patterns of d13Ccanopy versus 1/[CO2]canopy were strongly influenced by precipitation and air temperature during the growing season. Estimates of Dcanopy for Acer spp. were 16.1 ± 0.45 in 1993 and 16.3 ± 0.65 in 1994, and varied seasonally with small fluctuations for the open stand (± 0.3 ), but more pronounced changes for the dense stand (± 0.7 ).