J. Lloyd

Interannual growth rate variations of atmospheric CO2 and its δ13C, H2, CH4, and CO between 1992 and 1999 linked to biomass burning

[1]xa0High-precision, multispecies measurements of flask air samples since 1992 from CSIROs global sampling network reveal strong correlation among interannual growth rate variations of CO2 and its δ13C, H2, CH4, and CO. We show that a major fraction of the variability is consistent with two emission pulses coinciding with large biomass burning events in 1994/1995 and 1997/1998 in tropical and boreal regions, and observations of unusually high levels of combustion products in the overlying troposphere at these times. Implied pulse strengths and multispecies emission ratios are not consistent with any other single process, but do not exclude possible contributions from covarying processes that are linked through climatic forcing. Comparison of CO2 with its δ13C indicates that most of the CO2 variation is from terrestrial exchange, but does not distinguish forcing by biomass burning from imbalance in photosynthesis/respiration of terrestrial ecosystems. Partitioning of terrestrial CO2 fluxes is constrained by H2, CH4, and CO, all of which are products of biomass burning but which have no direct link to net respiration of CO2. While CO is a strong indicator of biomass burning, its short lifetime prevents it from usefully constraining the magnitude of CO2 emissions. If the H2 and CH4 variations were dominated by biomass burning, they would imply associated carbon emissions in excess of mean annual levels of other years, of 0.6–3.5 and 0.8–3.7 Pg C for 1994/1995 and 1997/1998, respectively. The large range in emission estimates mainly reflects uncertainty in H2/CO2 and CH4/CO2 emission ratios of fires in these years.

Effects of rising temperatures and [CO2] on the physiology of tropical forest trees

Using a mixture of observations and climate model outputs and a simple parametrization of leaf-level photosynthesis incorporating known temperature sensitivities, we find no evidence for tropical forests currently existing ‘dangerously close’ to their optimum temperature range. Our model suggests that although reductions in photosynthetic rate at leaf temperatures (TL) above 30°C may occur, these are almost entirely accountable for in terms of reductions in stomatal conductance in response to higher leaf-to-air vapour pressure deficits D. This is as opposed to direct effects of TL on photosynthetic metabolism. We also find that increases in photosynthetic rates associated with increases in ambient [CO2] over forthcoming decades should more than offset any decline in photosynthetic productivity due to higher D or TL or increased autotrophic respiration rates as a consequence of higher tissue temperatures. We also find little direct evidence that tropical forests should not be able to respond to increases in [CO2] and argue that the magnitude and pattern of increases in forest dynamics across Amazonia observed over the last few decades are consistent with a [CO2]-induced stimulation of tree growth.

Stability of elemental carbon in a savanna soil

We have investigated the stability of oxidation-resistant elemental carbon (OREC) in a sandy savanna soil at the Matopos fire trial site, Zimbabwe. The protection of some soil plots from fire for the last 50 years at this site has enabled a comparison of OREC abundances between those plots which have been protected from fire and plots which have continued to be burnt. The total 0–5 cm OREC inventory of the soil protected from fire is estimated to be 2.0±0.5 mg cm−2; approximately half the “natural” OREC inventory at the study site of 3.8±0.5 mg cm−2 (the mean for plots burnt every 1–5 years). The associated half-life for natural OREC loss from the 0–5 cm interval of the protected plots is calculated to be 2000 μm) in the soil being considerably <50 years. These results suggest that at least in well-aerated tropical soil environments, charcoal and OREC can be can be significantly degraded on decadal to centennial timescales. OREC abundance and carbon-isotope data suggest that OREC in coarse particles is progressively degraded into finer particle sizes, with a concomitant increase in resistance to oxidative degradation of OREC in the finer particle sizes due to the progressive loss of more readily degraded OREC. It remains unclear whether the OREC that is degraded is oxidized completely to CO2 and subsequently emitted from the soil, reduced to a sufficiently small particle size to be illuviated to deeper parts of the soil profile, solubilized and lost from the profile as dissolved organic carbon or transmuted into a chemical form which is susceptible to attack by the acid-dichromate reagent. The conclusion that a significant proportion of OREC can undergo natural degradation in well-aerated environments on decadal/centennial timescales suggests that only a fraction of the total production of OREC from biomass burning and fossil fuel combustion is likely to be sequestered in the slow-cycling “geological” carbon reservoir.

Effect of fire and soil texture on soil carbon in a sub-humid savanna (Matopos, Zimbabwe)

Abstract We investigated the effects of changing fire regime on the stocks and isotopic composition of soil organic carbon (SOC) in a tropical savanna ecosystem at Matopos, Zimbabwe. Vegetation plots from both sandy and clay-rich soil types at this location have been subjected to fire frequencies ranging from annual burn to complete protection for the last 50 years. Gross variations in 0–5 cm SOC stocks and the δ 13 C value of SOC were predominantly related to soil texture, with carbon densities at the sandy sites being consistently 35–50% lower than those at comparable clay sites. Average 0–5 cm carbon densities for all the burnt plots were approximately 100 mg/cm 2 and 50 mg/cm 2 , at the clay site and the sandy site, respectively. In both cases, lower fire frequencies had resulted in a ∼10% increase, while higher fire frequencies had resulted in a ∼10% decrease from these average values. Plots from which fire had been excluded experienced a 40% to 50% increase in carbon stocks in the 0–5 cm interval, compared with the average for the burned plots. There was a linear relationship between carbon density and δ 13 C value at both sandy and clay sites. This is controlled by the rate of delivery of C 3 - and C 4 -derived carbon to the SOC pool, by the differences in residence time for C 3 - and C 4 -derived carbon in the SOC pool (in turn controlled largely by fire frequency), and by soil texture. The distribution of carbon and 13 C between size fractions is also controlled by soil texture and fire frequency. Increasing fire frequency results in a relative increase in fine particulate SOC and an increase in the δ 13 C value of SOC in all size fractions. Soil texture, on the other hand, controls the magnitude of the increases in both the abundance and the δ 13 C value of SOC in all size fractions.

Response of central Siberian Scots pine to soil water deficit and long‐term trends in atmospheric CO2 concentration

[1]xa0Twenty tree ring 13C / 12C ratio chronologies from Pinus sylvestris (Scots pine) trees were determined from five locations sampled along the Yenisei River, spaced over a total distance of ∼1000 km between the cities of Turuhansk (66°N) and Krasnoyarsk (56°N). The transect covered the major part of the natural distribution of Scots pine in the region with median growing season temperatures and precipitation varying from 12.2°C and 218 mm to 14.0°C and 278 mm for Turuhansk and Krasnoyarsk, respectively. A key focus of the study was to investigate the effects of variations in temperature, precipitation, and atmospheric CO2 concentration on long- and short-term variation in photosynthetic 13C discrimination during photosynthesis and the marginal cost of tree water use, as reflected in the differences in the historical records of the 13C / 12C ratio in wood cellulose compared to that of the atmosphere (Δ13Cc). In 17 of the 20 samples, trees Δ13Cc has declined during the last 150 years, particularly so during the second half of the twentieth century. Using a model of stomatal behaviour combined with a process-based photosynthesis model, we deduce that this trend indicates a long-term decrease in canopy stomatal conductance, probably in response to increasing atmospheric CO2 concentrations. This response being observed for most trees along the transect is suggestive of widespread decreases in Δ13Cc and increased water use efficiency for Scots pine in central Siberia over the last century. Overlying short-term variations in Δ13Cc were also accounted for by the model and were related to variations in growing season soil water deficit and atmospheric humidity.

Should phosphorus availability be constraining moist tropical forest responses to increasing CO2 concentrations

Publisher Summary Moist tropical forests account for a substantial amount of global plant productivity. There are indications that the productivity of many of these forests is limited by low phosphorus availability. This has led to suggestions that moist tropical forests may be constrained in their ability to increase their growth rates in response to an increase in atmospheric carbon dioxide concentrations. This notion is examined in this chapter. Several factors prevent low levels of available phosphorus (P) significantly constraining moist tropical forest [CO 2 ]/growth responses. One of the main reasons for low soil-solution P concentrations in many tropical soils is the adsorption of most of the phosphate ions onto iron and aluminum oxides and clay minerals. The nature of P mineralization in soils is a second factor mediating towards phosphorus availability not constraining tropical forest [CO 2 ] responses. Third, most of the available evidence suggests that at a given soil P concentration plants growing at elevated [CO 2 ] are capable of maintaining their tissue phosphorus concentrations. Humic molecules and organic acids actively compete with phosphorus for soil fixation sites. This means that increases in soil carbon density at higher [CO 2 ] may serve to displace phosphate ions from sorption sites and into the soil solution, where they can then be utilized by plants.

Observations of O2:CO2 exchange ratios during ecosystem gas exchange

[1]xa0We determined O2:CO2 exchange ratios of ecosystem fluxes during field campaigns in different forest ecosystems (Harvard Forest/United States, Griffin Forest/United Kingdom, Hainich/Germany). The exchange ratios of net assimilation observed in chamber experiments varied between 0.7 and 1.6, with averages of 1.1 to 1.2. A measurement of soil gas exchange yielded an exchange ratio of 0.94. On the other hand, the observed canopy air O2:CO2 ratios, derived from the concurrent variations of O2 and CO2 abundances in canopy air, were virtually indistinguishable from 1.0 over the full diurnal cycle. Simulations with a simple one-box model imply that the combined processes of assimilation, respiration, and turbulent exchange yield canopy air O2:CO2 ratios that differ from the exchange ratios of the separate fluxes. In particular, the simulated canopy air O2:CO2 ratios (1.01 to 1.12) were clearly lower than the exchange ratios of net turbulent fluxes between the ecosystem and the atmosphere (1.26 to 1.38). The simulated canopy air ratios were also sensitive to changes in the regional O2:CO2 ratio of air above the canopy. Offsets between the various exchange ratios could thus arise if the component ecosystem fluxes have different diurnal cycles and distinct exchange ratios. Our results indicate that measurements of O2 and CO2 abundances in canopy air may not be the appropriate method to determine O2:CO2 exchange ratios of net ecosystem fluxes.

Diurnally variable δ18O signatures of soil CO2 fluxes indicate carbonic anhydrase activity in a forest soil

[1]xa0Oxygen isotopes are valuable tools for studying the gas exchange between terrestrial ecosystems and the atmosphere. We determined the δ18O signatures of soil CO2 fluxes from soil chamber measurements over the diurnal cycle in September 2000, May 2001 and July 2001 in a Sitka spruce plantation in Scotland. Concurrent estimates of the δ18O composition of soil water were obtained from soil samples collected in the vicinity of the chambers. The observed δ18O signatures of net soil CO2 fluxes were diurnally variable and strongly depleted compared to those expected from a simple evasion of respired CO2 at isotopic equilibrium with soil water. We then simulated the δ18O signatures of soil CO2 fluxes using a model of soil gas exchange that includes atmospheric invasion of CO2 with concurrent isotopic equilibration with soil water and evasion of the equilibrated CO2. This brought the modeled δ18O signatures closer to the observations, but complete agreement was only achieved when acceleration of isotopic exchange between CO2 and soil water by carbonic anhydrase activity was included. We hypothesize that carbonic anhydrase is present in the litter or surface soil layers. This introduces a feedback that can result in diurnally variable δ18O signatures of net soil CO2 fluxes. Such effects can only be captured in models that have an explicit description of the canopy air space with a variable δ18O signature of CO2.

Seasonal and inter-annual photosynthetic response of representative C4 species to soil water content and leaf nitrogen concentration across a tropical seasonal floodplain

We examined the seasonal and inter-annual variation of leaf-level photosynthetic characteristics of three C-4 perennial species, Cyperus articulatus, Panicum repens and Imperata cylindrica, and their response to environmental variables, to determine comparative physiological responses of plants representing particular microhabitats within a seasonal tropical floodplain in the Okavango River Delta, Botswana. Five measurement campaigns were carried out over a period of 2 y which covered two early rainy seasons, two late rainy seasons and one dry season. For all three species, light-saturated net photosynthetic rates (A(sat)) and stomatal conductance (9,at) decreased with decreasing soil water content with a seasonal range for A(sat) of approximately 5-45 mu mol m(-2) s(-1), and for g(sat) of 0.03-0.35 mol m-2 s(-1). The species representing the wettest microhabitat (Cyperus) had the highest g(sat) at low leaf-to-air vapour pressure deficits (D-l), the highest ratio of intercellular to ambient CO2 concentration (C-i/C-a), as well as the highest degree of variation in C-l/C-a from season to season. We interpret this as being indicative of its adaptation to a moist growth environment allowing for non-conservative water use strategies as soil moisture is usually abundant. For all three species there was significant variation in photosynthetic fluxes from one year to another that was related to variation in leaf nitrogen and phosphorus. This study shows that when assessing the role of savanna stands in large-scale carbon balance models, the remarkable inter-annual variation in leaf photosynthesis reported in this study should be taken into account. (Less)