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The Effects of Environmental Variables on Gas Exchange of Several Tree Species Under Boreal Field Conditions
Author(s): Yang Litao, Timothy J. Arkebauer, Elizabeth A, Walter-Shea
Pages: 1-
11
Year: 1999
Issue:
1
Journal: GUANGXI SCIENCES
Keyword: tree species; gas exchange; net CO2 assimilation; stomat al conductance; photosynthesis; photosynthetic photon flux density; air temperature; vapor pressure deficit;
Abstract: The influence of environmental conditions on gas exchange of aspen (Populus tremuloides Michx.), balsam poplar (Populus balsamifera L.), hazelnut (Corylus cornuta Marsh.), jack pine (Pinus banksiana Lamb.) and black spruce (Picea mariana (Mill) BSP) was examined during the summers of 1994 and 1995 in the boreal forest near Prince Albert, Saskatchewan, Canada.Gas exchange rates were measured with a portable gas exchange system.The results showed that the re were significant effects of photosynthetic photon flux density (PPFD), air temperature (Ta), vapor pressure deficit (VPD) and internal CO2 concentration (Ci) on gas exchange. The effects differed among the different species. Under ambient CO2 concentrations (Ca), and relatively constant Ta and VPD, aspen showed a higher light saturation point, quantum yield of assimilation (Φ), and maximum assimilation at saturating PPFD (Pm), and a lower light compensation point (LCP) than either jack pine or black spruce. In aspen, the photosynthetic capacity was higher in young er trees than in older trees, and was higher in the earlier growing season than in the later growing season. Stomatal conductance (gs) increased with increasing PPFD for both aspen and jack pine. There was no obvious effect of PPFD on gs for black spruce. Under high light (PPFD>1 000 μmol m-2 s-1), young aspen leaves from shaded canopy positions showed lower net CO2 assimilation (A), gs and dark respiration (Rd) than sun leaves from exposed canopy positions. There were no statistical difference s in Pm, Φ and Rd between jack pine and black spruce. Under high light, as Ta increased from 15℃ to 35℃, A and gs increased for aspen, decreased for black spruce and were not obviously influenced for jack pine.Net assimilation rate and gs were highest in the Ta range of 24℃~29℃ for aspen, 22℃~28℃ for jack pine and 21℃~27℃ for black spruce.The effect of VPD on gas exchange was significant for aspen, jack pine and black spruce which showed that A and gs increased with decreasing VPD. The current-year shoots usually showed lower A than those of 1- and 2-year old shoots for jack pine and black spruce. There was no difference in gas exchange between 1- and 2-year old shoots in either conifer species. The initial slopes of the A-Ci responses (i.e., carboxylation efficiency (CE)) were PPFD-dependent and differed among species and tree ages. In general, CE followed the pattern:aspen>balsam poplar>hazelnut>jack pine, an d young aspen>old aspen. The overall results indicated that light was the major determinant of photosynthetic rate, although the rate was modified by Ci, Ta and VPD. Under high light, the major environmental influences on gas exchan ge were Ta and VPD.
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