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Carbon isotope ratios (delta(13)C) 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. The delta(13)C(leaf) (and carbon isotope discrimination) 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 delta(13)C(leaf) values (increasing carbon isotope discrimination) with increasing LAI. The delta(13)C values of needles and leaves at the top of the canopy were relatively constant over the entire LAI range, indicating no change in intrinsic water-use efficiency with overstory LAI. In all canopies, delta(13)C(leaf) decreased with decreasing height above the forest floor, primarily due to physiological changes affecting c(i)/c(a) (> 60%) and to a minor extent due to delta(13)C of canopy air (< 40%). This intra-canopy depletion of delta(13)C(leaf) was lowest in the open stand (1 parts per thousand) and greatest in the denser stands (4.5 parts per thousand). Although overstory delta(13)C(leaf) did not change with canopy LAI, delta(13)C of soil organic carbon increased with increasing LAI in Pinus contorta and Populus tremuloides ecosystems. In addition, delta(13)C of decomposing organic carbon became increasingly enriched over time (by 1.7-2.9 parts per thousand) for all deciduous and evergreen dry temperate forests. The delta(13)C(canopy) of CO2 in canopy canopy air varied temporally and spatially in all forest stands. Vertical canopy gradients of delta(13)C(canopy), and [CO2](canopy) were larger in the deciduous Populus tremuloides than in the evergreen Pinu contorta stands of similar LAI. In a very wet and cool year, ecosystem discrimination (Delta(e)) was similar for both deciduous Populus tremulodies (18.0 +/- 0.7 parts per thousand,) and evergreen Pinus contorta (18.3 +/- 0.9 parts per thousand,) stands. Gradients of delta(13)C(canopy) and [CO2](canopy) were larger in denser Acer spp. stands than those in the open stand. However, C-13 enrichment above and photosynthetic draw-down of [CO2](canopy) below tropospheric baseline values were larger in the open than in the dense stands, due to the presence of a vigorous understory vegetation. Seasonal patterns of the relationship delta(13)C(canopy) versus 1/[CO2](canopy) were strongly influenced by precipitation and air temperature during the growing season. Estimates of Delta(e) for Acer spp. did not show a significant effect of stand structure, and averaged 16.8 +/- 0.5 parts per thousand in 1933 and 17.4 +/- 0.7 parts per thousand in 1994. However, Delta(e) varied seasonally with small fluctuations for the open stand (2 parts per thousand), but more pronounced changes for the dense stand (5 parts per thousand).