Graham J. Hymus

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.

NITROGEN CYCLING DURING SEVEN YEARS OF ATMOSPHERIC CO2 ENRICHMENT IN A SCRUB OAK WOODLAND

Experimentally increasing atmospheric CO2 often stimulates plant growth and ecosystem carbon (C) uptake. Biogeochemical theory predicts that these initial responses will immobilize nitrogen (N) in plant biomass and soil organic matter, causing N availability to plants to decline, and reducing the long-term CO2-stimulation of C storage in N limited ecosystems. While many experiments have examined changes in N cycling in response to elevated CO2, empirical tests of this theoretical prediction are scarce. During seven years of postfire recovery in a scrub oak ecosystem, elevated CO2 initially increased plant N accumulation and plant uptake of tracer 15N, peaking after four years of CO2 enrichment. Between years four and seven, these responses to CO2 declined. Elevated CO2 also increased N and tracer 15N accumulation in the O horizon, and reduced 15N recovery in underlying mineral soil. These responses are consistent with progressive N limitation: the initial CO2 stimulation of plant growth immobilized N in plant biomass and in the O horizon, progressively reducing N availability to plants. Litterfall production (one measure of aboveground primary productivity) increased initially in response to elevated CO2, but the CO2 stimulation declined during years five through seven, concurrent with the accumulation of N in the O horizon and the apparent restriction of plant N availability. Yet, at the level of aboveground plant biomass (estimated by allometry), progressive N limitation was less apparent, initially because of increased N acquisition from soil and later because of reduced N concentration in biomass as N availability declined. Over this seven-year period, elevated CO2 caused a redistribution of N within the ecosystem, from mineral soils, to plants, to surface organic matter. In N limited ecosystems, such changes in N cycling are likely to reduce the response of plant production to elevated CO2.

The effects of elevated CO2 on nutrient distribution in a fire-adapted scrub oak forest

Elevated carbon dioxide (CO2) caused greater accumulation of carbon (C) and nutrients in both vegetation and O horizons over a 5-yr sampling period in a scrub oak ecosystem in Florida. Elevated CO2 had no effect on any measured soil property except extractable phosphorus (P), which was lower with elevated CO2 after five years. Anion and cation exchange membranes showed lower available nitrogen (N) and zinc (Zn) with elevated CO2. Soils in both elevated and ambient CO2 showed decreases in total C, N, sulfur (S), and cation exchange capacity, and increases in base saturation, exchangeable Ca2+, and Mg2+ over the 5-yr sampling period. We hypothesize that these soil changes were a delayed response to prescribed fire, which was applied to the site just before the initiation of the experiment. In the ambient CO2 treatment, the increases in vegetation and O horizon C, N, and S were offset by the losses of soil total C, N, and S, resulting in no statistically significant net changes in ecosystem C, N, or S over t...

Long term response of photosynthesis to elevated carbon dioxide in a Florida scrub oak ecosystem

The response of photosynthesis was analyzed during canopy closure in a Florida scrub-oak ecosystem exposed to elevated [CO2] (704 μmol CO2/mol air; concentration of CO2). The species were measured on six occasions, covering different seasons, during the third and fourth year of exposure to elevated [CO2]. The entire regrowth cycle of this community has been under elevated [CO2], providing a rare opportunity to assess the differential responses of species during the critical phase of canopy closure. Measurements were taken in order to determine both season-specific and species-specific differences in the response of photosynthesis to elevated [CO2]. Photosynthesis was measured with an open-gas exchange system, and in vivo rates of Rubisco carboxylation (Vc,max) and electron transport (Jmax) were derived to assess changes in the photosynthetic capacity in the codominant, evergreen oak species. Quercus myrtifolia did not show any change in photosynthetic capacity with prolonged exposure to elevated [CO2] dur...

Inter- and intraspecific variation in the response of photosynthesis to elevated [CO2] in a Florida scrub oak community

Understanding inter- and intraspecific responses to rising [CO2] is critical for assessing possible future shifts in competition and species dominance, and for accurately predicting future carbon exchange in forest ecosystems. Two clonal, co-dominant evergreen oaks within a Florida scrub-oak ecosystem have regrown entirely under elevated [CO2], providing a rare opportunity to assess inter- and intraspecific variation during canopy closure. Photosynthesis was measured with an open-gas exchange system to assess acclimation of photosynthetic capacity in two co-dominant oaks. There was no acclimatory loss of photosynthetic capacity in Quercus myrtifolia; therefore, with decreased photorespiration in elevated [CO2], a 72% stimulation of net photosynthetic rate was observed. Quercus geminata showed a large acclimatory loss of photosynthetic capacity sufficient to offset any stimulation of light-saturated net photosynthesis due to decreased photorespiration. The photosynthetic rate of Q. geminata leaves grown and measured at elevated [CO2] was no higher than leaves grown at ambient [CO2]. Q. myrtifolia also exhibited an increase in photosynthetic nitrogen use efficiency, which was still present at canopy closure. Intraspecific variation in the response of photosynthesis to elevated [CO2] exists in both species. The role that individual genotypes play in that variation is currently unknown. DNA microsatellite analysis of Q. myrtifolia and Q. geminata is being used to estimate genetic distances between individuals. The results show that at canopy closure in a woody community, elevation of [CO2] causes a substantial difference in the capacity of co-dominants to acquire carbon and energy, with important implications for fitness and future composition of this community.