Nocturnal accumulation of CO2 underneath a tropical forest canopy along a topographical gradient

Widespread inhibition of daytime ecosystem respiration

The global land surface absorbs about a third of anthropogenic emissions each year, due to the difference between two key processes: ecosystem photosynthesis and respiration. Despite the importance of these two processes, it is not possible to measure either at the ecosystem scale during the daytime. Eddy-covariance measurements are widely used as the closest 'quasi-direct' ecosystem-scale observation from which to estimate ecosystem photosynthesis and respiration. Recent research, however, suggests that current estimates may be biased by up to 25%, due to a previously unaccounted for process: the inhibition of leaf respiration in the light. Yet the extent of inhibition remains debated, and implications for estimates of ecosystem-scale respiration and photosynthesis remain unquantified. Here, we quantify an apparent inhibition of daytime ecosystem respiration across the global FLUXNET eddy-covariance network and identify a pervasive influence that varies by season and ecosystem type. We develop partitioning methods that can detect an apparent ecosystem-scale inhibition of daytime respiration and find that diurnal patterns of ecosystem respiration might be markedly different than previously thought. The results call for the re-evaluation of global terrestrial carbon cycle models and also suggest that current global estimates of photosynthesis and respiration may be biased, some on the order of magnitude of anthropogenic fossil fuel emissions.

Evidence for magmatic carbon bias in 14C dating of the Taupo and other major eruptions

Prehistoric timescales, volcanic hazard assessment, and understanding of volcanogenic climate events rely on accurate dating of prehistoric eruptions. Most late Quaternary eruptions are dated by ¹⁴C measurements on material from close to the volcano that may be contaminated by geologic-sourced infinite-age carbon. Here we show that ¹⁴C ages for the Taupo (New Zealand) First Millennium eruption are geographically arrayed, with oldest ages closer to the vent. The current eruption wiggle match date of 232 ± 5 years CE is amongst the oldest. We present evidence that the older, vent-proximal ¹⁴C ages were biased by magmatic CO₂ degassed from groundwater, and that the Taupo eruption occurred decades to two centuries after 232 CE. Our reinterpretation implies that ages for other proximally-dated, unobserved, eruptions may also be too old. Plateauing or declining tree ring cellulose δ¹³C and Δ¹⁴C values near a volcano indicate magmatic influence and may allow forecasting of super-eruptions.

Correlations between prehistoric eruptions and other phenomena depend on accurate dating of the eruption. Here the authors show that magmatic CO₂ in groundwater can bias radiocarbon ages for eruptions and that plateaux of carbon isotopic values in tree ring sequences biased by magmatic CO₂ foreshadow major eruptions.

Cyclic occurrence of fire and its role in carbon dynamics along an edaphic moisture gradient in longleaf pine ecosystems

Fire regulates the structure and function of savanna ecosystems, yet we lack understanding of how cyclic fire affects savanna carbon dynamics. Furthermore, it is largely unknown how predicted changes in climate may impact the interaction between fire and carbon cycling in these ecosystems. This study utilizes a novel combination of prescribed fire, eddy covariance (EC) and statistical techniques to investigate carbon dynamics in frequently burned longleaf pine savannas along a gradient of soil moisture availability (mesic, intermediate and xeric). This research approach allowed us to investigate the complex interactions between carbon exchange and cyclic fire along the ecological amplitude of longleaf pine. Over three years of EC measurement of net ecosystem exchange (NEE) show that the mesic site was a net carbon sink (NEE = −2.48 tonnes C ha⁻¹), while intermediate and xeric sites were net carbon sources (NEE = 1.57 and 1.46 tonnes C ha⁻¹, respectively), but when carbon losses due to fuel consumption were taken into account, all three sites were carbon sources (10.78, 7.95 and 9.69 tonnes C ha⁻¹ at the mesic, intermediate and xeric sites, respectively). Nonetheless, rates of NEE returned to pre-fire levels 1–2 months following fire. Consumption of leaf area by prescribed fire was associated with reduction in NEE post-fire, and the system quickly recovered its carbon uptake capacity 30–60 days post fire. While losses due to fire affected carbon balances on short time scales (instantaneous to a few months), drought conditions over the final two years of the study were a more important driver of net carbon loss on yearly to multi-year time scales. However, longer-term observations over greater environmental variability and additional fire cycles would help to more precisely examine interactions between fire and climate and make future predictions about carbon dynamics in these systems.

Responses of Humid Tropical Trees to Rising CO2

Although no mature tropical tree had ever been exposed to an experimentally CO2-enriched atmosphere, evidence from works with seedlings and saplings, theory, models, and proxy data suggest effects on biodiversity and enhanced forest dynamics. Tropical forest carbon stocking is unlikely to increase, given that carbon pool size is driven by tree and patch demography, with tree longevity unlikely to increase. Unlike epiphytes, tropical lianas are likely to play a more important role in a CO2-rich future.

An introduction to flux measurements in difficult conditions

The origins of the aerodynamic techniques now widely used at sites around the world to measure continuous biosphere-atmosphere exchange of carbon and energy are briefly reviewed. A survey of the current state of this approach concludes that the technique often fails when standard analysis routines are applied to data from single towers in complex flows. In the daytime, problems are signaled by failure to close the surface energy balance because turbulent energy fluxes are routinely underestimated. Complex flows are more prevalent at night when they lead to failure to measure all the respired CO2. At such times, the aerodynamic methodology is commonly supplemented by biological models. A set of papers from a workshop on "Flux Measurements in Difficult Conditions" held at the National Center for Atmospheric Research (NCAR) in January 2006 are introduced next. Two papers review the causes and magnitude of these flow-based problems. Four papers describe intensive field experiments that detail the mechanisms that cause problematic complex flows. These experiments show, inter alia, that the technique of replacing nighttime eddy flux measurements by biological models can also be systematically biased. Finally, two model studies are used both to illustrate the physics behind these complex flows and to motivate an approach to systematic correction of single-tower results.