Abstract
Isotopic measurements on junipers growing in southern California during the last glacial, when the ambient atmospheric [CO2] (ca) was ~180 ppm, show the leaf-internal [CO2] (ci) was approaching the modern CO2 compensation point for C3 plants. Despite this, stem growth rates were similar to today. Using a coupled light-use efficiency and tree growth model, we show that it is possible to maintain a stable ci/ca ratio because both vapour pressure deficit and temperature were decreased under glacial conditions at La Brea, and these have compensating effects on the ci/ca ratio. Reduced photorespiration at lower temperatures would partly mitigate the effect of low ci on gross primary production, but maintenance of present-day radial growth also requires a ~27% reduction in the ratio of fine root mass to leaf area. Such a shift was possible due to reduced drought stress under glacial conditions at La Brea. The necessity for changes in allocation in response to changes in [CO2] is consistent with increased below-ground allocation, and the apparent homoeostasis of radial growth, as ca increases today.
Highlights
Isotopic measurements on junipers growing in southern California during the last glacial, when the ambient atmospheric [CO2] was ~180 ppm, show the leaf-internal [CO2] was approaching the modern CO2 compensation point for C3 plants
The average ci/ca ratio of the fossil wood samples from La Brea dated to the glacial sensu stricto (55–22 ka BP) is 0.51 ± 0.02, while the value for the sample closest to the glacial maximum is 0.53 ± 0.01 (22 ka BP sample), which is comparable to the value of 0.53 ± 0.05 for modern samples from six southern Californian sites[3,4]
The modern comparison sites are at higher elevations (630 to 2830 m a.s.l), at locations that are more similar in climate to the glacial climate of La Brea
Summary
Isotopic measurements on junipers growing in southern California during the last glacial, when the ambient atmospheric [CO2] (ca) was ~180 ppm, show the leaf-internal [CO2] (ci) was approaching the modern CO2 compensation point for C3 plants. Reduced photorespiration at lower temperatures would partly mitigate the effect of low ci on gross primary production, but maintenance of present-day radial growth requires a ~27% reduction in the ratio of fine root mass to leaf area Such a shift was possible due to reduced drought stress under glacial conditions at La Brea. Mean annual precipitation was 100–300 mm more than today, as a result of circulation changes due to southward deflection of the Westerlies by the Laurentide Ice Sheet[9,10,11], and relative humidity was greater than present as shown using 18O data[3] These changes in climate could potentially have compensated for the impact of ci on GPP and growth, through the effect of lower temperature in reducing photorespiration ( lowering the compensation point), and/or through reducing the potential loss of photosynthetic activity due to drought stress. There is some evidence for structural changes and decreased below ground allocation in modern C3 woody species grown at glacial [CO2]25,26
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