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Graham J Hymus

Publications and source records attributed to Graham J Hymus.

4 recordsLinked to original sources
PubMed2005

Large daily variation in 13C-enrichment of leaf-respired CO2 in two Quercus forest canopies.

The use of the 13C : 12C isotopic ratio (delta13C) of leaf-respired CO2 to trace carbon fluxes in plants and ecosystems is limited by little information on temporal variations in delta13C of leaf dark-respired CO2 (delta13Cr) under field conditions. Here, we explored variability in delta13Cr and its relationship to key respiratory substrates from collections of leaf dark-respired CO2, carbohydrate extractions and gas exchange measurements over 24-h periods in two Quercus canopies. Throughout both canopies, delta13Cr became progressively 13C-enriched during the photoperiod, by up to 7%, then 13C-depleted at night relative to the photoperiod. This cycle could not be reconciled with delta13C of soluble sugars (delta13Css), starch (delta13Cst), lipids (delta13Cl), cellulose (delta13Cc) or with calculated photosynthetic discrimination (Delta). However, photoperiod progressive enrichment in delta13Cr was correlated with cumulative carbon assimilation (r2 = 0.91). We concluded that there is considerable short-term variation in delta13Cr in forest canopies, that it is consistent with current hypotheses for 13C fractionation during leaf respiration, that leaf carbohydrates cannot be used as surrogates for delta13Cr, and that diel changes in leaf carbohydrate status could be used to predict changes in delta13Cr empirically.

Carbon Dioxide↗
PubMed2003

Respiratory oxygen uptake is not decreased by an instantaneous elevation of [CO2], but is increased with long-term growth in the field at elevated [CO2].

Averaged across many previous investigations, doubling the CO2 concentration ([CO2]) has frequently been reported to cause an instantaneous reduction of leaf dark respiration measured as CO2 efflux. No known mechanism accounts for this effect, and four recent studies have shown that the measurement of respiratory CO2 efflux is prone to experimental artifacts that could account for the reported response. Here, these artifacts are avoided by use of a high-resolution dual channel oxygen analyzer within an open gas exchange system to measure respiratory O2 uptake in normal air. Leaf O2 uptake was determined in response to instantaneous elevation of [CO2] in nine contrasting species and to long-term elevation in seven species from four field experiments. Over six hundred separate measurements of respiration failed to reveal any decrease in respiratory O2 uptake with an instantaneous increase in [CO2]. Respiration was found insensitive not only to doubling [CO2], but also to a 5-fold increase and to decrease to zero. Using a wide range of species and conditions, we confirm earlier reports that inhibition of respiration by instantaneous elevation of [CO2] is likely an experimental artifact. Instead of the expected decrease in respiration per unit leaf area in response to long-term growth in the field at elevated [CO2], there was a significant increase of 11% and 7% on an area and mass basis, respectively, averaged across all experiments. The findings suggest that leaf dark respiration will increase not decrease as atmospheric [CO2] rises.

Carbon Dioxide↗
PubMed2002

Elevated CO2 lowers relative and absolute herbivore density across all species of a scrub-oak forest.

The unabated increase in global atmospheric CO(2) is expected to induce physiological changes in plants, including reduced foliar nitrogen, which are likely to affect herbivore densities. This study employs a field-based CO(2 )enrichment experiment at Kennedy Space Center, Florida, to examine plant-herbivore (insect) interactions inside eight open-topped chambers with elevated CO(2) (710 ppm) and eight control chambers with ambient CO(2). In elevated CO(2) we found decreased herbivore densities per 100 leaves, especially of leaf miners, across all five plant species we examined: the oak trees Quercus myrtifolia, Q. geminata, and Q. chapmanii, the nitrogen-fixing vine Galactia elliottii and the shrub Vaccinium myrsinites. Both direct and indirect effects of lowered plant nitrogen may influence this decrease in herbivore densities. Direct effects of lowered nitrogen resulted in increased host-plant related death and an increase in compensatory feeding: per capita herbivore leaf consumption in elevated CO(2) was higher than in ambient CO(2). Indirectly, compensatory feeding may have prolonged herbivore development and increased exposure to natural enemies. For all leaf miners we examined, mortality from natural enemies increased in elevated CO(2). These increases in host-plant induced mortality and in attack rates by natural enemies decreased leaf miner survivorship, causing a reduction in leaf miner density per 100 leaves. Despite increased leaf production in elevated CO(2) from the carbon fertilization effect, absolute herbivore abundance per chamber was also reduced in elevated CO(2). Because insects cause premature leaf abscission, we also thought that leaf abscission would be decreased in elevated CO(2). However, for all plant species, leaf abscission was increased in elevated CO(2), suggesting a direct effect of CO(2) on leaf abscission that outweighs the indirect effects of reduced insect densities on leaf abscission.

Animals↗