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CO 2 enrichment in a maturing pine forest: are CO 2 exchange and water status in the canopy affected?
Author(s) -
ELLSWORTH D. S.
Publication year - 1999
Publication title -
plant, cell and environment
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.646
H-Index - 200
eISSN - 1365-3040
pISSN - 0140-7791
DOI - 10.1046/j.1365-3040.1999.00433.x
Subject(s) - transpiration , canopy , stomatal conductance , photosynthesis , chemistry , carbon dioxide , soil water , water content , water use efficiency , xylem , isotopes of carbon , botany , horticulture , agronomy , environmental chemistry , biology , ecology , total organic carbon , geotechnical engineering , organic chemistry , engineering
Anet , leaf net CO 2 assimilation
c a , CO 2 concentration of air surrounding a leaf
c i , leaf intercellular CO 2 concentration
Δ , 13 C isotope discrimination
δ 13 C, relative stable carbon isotope content
ɛ , ratio of A net at c a = 560 μ mol mol –1 to A net at c a = 360 μ mol mol –1
FACE, free‐air CO 2 enrichment
g w , stomatal conductance to water vapour
Π i , initial leaf osmotic potential
R t , relative water content at incipient turgor loss
Ψ l , xylem water potential of leaves
Ψ m , soil matric potential
Elevated CO 2 is expected to reduce forest water use as a result of CO 2 ‐induced stomatal closure, which has implications for ecosystem‐scale phenomena controlled by water availability. Leaf‐level CO 2 and H 2 O exchange responses and plant and soil water relations were examined in a maturing loblolly pine ( Pinus taeda L.) stand in a free‐air CO 2 enrichment (FACE) experiment in North Carolina, USA to test if these parameters were affected by elevated CO 2 . Current‐year foliage in the canopy was continuously exposed to elevated CO 2 (ambient CO 2 +200 μ mol mol –1 ) in free‐air during needle growth and development for up to 400 d. Photosynthesis in upper canopy foliage was stimulated by 50–60% by elevated CO 2 compared with ambient controls. This enhancement was similar in current‐year, ambient‐grown foliage temporarily measured at elevated CO 2 compared with long‐term elevated CO 2 grown foliage. Significant photosynthetic enhancement by CO 2 was maintained over a range of conditions except during peak drought. There was no evidence of water savings in elevated CO 2 plots in FACE compared to ambient plots under drought and non‐drought conditions. This was supported by evidence from three independent measures. First, stomatal conductance was not significantly different in elevated CO 2 versus ambient trees of P. taeda . Calculations of time‐integrated c i / c a ratios from analysis of foliar δ 13 C showed that these ratios were maintained in foliage under elevated CO 2 . Second, soil moisture was not significantly different between ambient and elevated CO 2 plots during drought. Third, pre‐dawn and mid‐day leaf water potentials were also unaffected by the seasonal CO 2 exposure, as were tissue osmotic potentials and turgor loss points. Together the results strongly support the hypothesis that maturing P. taeda trees have low stomatal responsiveness to elevated CO 2 . Elevated CO 2 effects on water relations in loblolly pine‐dominated forest ecosystems may be absent or small apart from those mediated by leaf area. Large photosynthetic enhancements in the upper canopy of P. taeda by elevated CO 2 indicate that this maturing forest may have a large carbon sequestration capacity with limiting water supply.