title: Influence of stand structure on carbon-13 of vegetation, soils, and canopy air within deciduous and evergreen forests in Utah, United States.
abstract: Carbon isotope ratios (δ13C) 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 δ13Cleaf(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 δ13Cleafvalues (increasing carbon isotope discrimination) with increasing LAI. The δ13C 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, δ13Cleafdecreased with decreasing height above the forest floor, primarily due to physiological changes affecting ci/ca(> 60%) and to a minor extent due to δ13C of canopy air (< 40%). This intra-canopy depletion of δ13Cleafwas lowest in the open stand (1‰) and greatest in the denser stands (4.5‰). Although overstory δ13Cleafdid not change with canopy LAI, δ13C of soil organic carbon increased with increasing LAI in Pinus contorta and Populus tremuloides ecosystems. In addition, δ13C of decomposing organic carbon became increasingly enriched over time (by 1.7-2.9‰) for all deciduous and evergreen dry temperate forests. The δ13Ccanopyof CO2in canopy air varied temporally and spatially in all forest stands. Vertical canopy gradients of δ13Ccanopy, and [CO2]canopywere 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 (Δe) was similar for both deciduous Populus tremulodies (18.0 ± 0.7‰) and evergreen Pinus contorta (18.3 ± 0.9‰) stands. Gradients of δ13Ccanopyand [CO2]canopywere larger in denser Acer spp. stands than those in the open stand. However,13C enrichment above and photosynthetic draw-down of [CO2]canopybelow 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 δ13Ccanopyversus 1/[CO2]canopywere strongly influenced by precipitation and air temperature during the growing season. Estimates of Δefor Acer spp. did not show a significant effect of stand structure, and averaged 16.8 ± 0.5‰ in 1933 and 17.4 ± 0.7‰ in 1994. However, Δevaried seasonally with small fluctuations for the open stand (2‰), but more pronounced changes for the dense stand (5‰).
authors.affiliation: Department of Biology, University of Utah, Salt Lake City, UT 84112, USA, , , , , , US.
authors.forename: N
authors.initials: N
authors.lastname: Buchmann
authors.affiliation: Botany Institute, Academia Sinica, Nankang, Taipei, Taiwan, Republic of China, , , , , , TW.
authors.forename: Wen-Yuan
authors.initials: WY
authors.lastname: Kao
authors.affiliation: Department of Biology, University of Utah, Salt Lake City, UT 84112, USA, , , , , , US.
authors.forename: Jim
authors.initials: J
authors.lastname: Ehleringer
keywords: Key words CO2 ;   Soil respiration ;  Ecosystem discrimination ;  Temporal and spatial variation ;  δ13C 