• Energy Research

In this study, we explore how a hurricane disturbance influenced carbon allocation for the production of new fine roots. Before and after a hurricane, we measured the age of carbon (time since fixation from the atmosphere) in fine root structural tissues using natural abundance radiocarbon (14C) measured by accelerator mass spectrometry. Roots were sampled from five seasonally dry tropical forests ranging in age from 6 yr to a mature forest. Structural carbon in combined live + dead roots picked from soil cores sampled 1 month before the hurricane had mean ages ranging from 4 to 11 yr, whereas live roots alone had ages of 1-2 yr. Structural carbon in new live fine roots produced over a period lasting from 3 wk before the hurricane to 2 months after the event had mean ages of between 2 and 10 yr. Contrary to expectations, our results showed that plants allocate long-lived storage carbon pools to the production of new fine roots after canopy defoliation and root mortality. The age of the carbon allocated for new roots increased with forest age and forest above-ground biomass, suggesting an adaptation of plants to survive and recover from severe disturbances.

Forest ecosystems can act as sinks of carbon and thus mitigate anthropogenic carbon emissions. When forests are actively managed, treatments can alter forests carbon dynamics, reducing their sink strength and switching them from sinks to sources of carbon. These effects are generally characterized by fast temporal dynamics. Hence this study monitored for over a decade the impacts of management practices commonly used to reduce fire hazards on the carbon dynamics of mixed-conifer forests in the Sierra Nevada, California, USA. Soil CO2 efflux, carbon pools (i.e. soil carbon, litter, fine roots, tree biomass), and radial tree growth were compared among un-manipulated controls, prescribed fire, thinning, thinning followed by fire, and two clear-cut harvested sites. Soil CO2 efflux was reduced by both fire and harvesting (ca. 15%). Soil carbon content (upper 15 cm) was not significantly changed by harvest or fire treatments. Fine root biomass was reduced by clear-cut harvest (60-70%) but not by fire, and the litter layer was reduced 80% by clear-cut harvest and 40% by fire. Thinning effects on tree growth and biomass were concentrated in the first year after treatments, whereas fire effects persisted over the seven-year post-treatment period. Over this period, tree radial growth was increased (25%) by thinning and reduced (12%) by fire. After seven years, tree biomass returned to pre-treatment levels in both fire and thinning treatments; however, biomass and productivity decreased 30%-40% compared to controls when thinning was combined with fire. The clear-cut treatment had the strongest impact, reducing ecosystem carbon stocks and delaying the capacity for carbon uptake. We conclude that post-treatment carbon dynamics and ecosystem recovery time varied with intensity and type of treatments. Consequently, management practices can be selected to minimize ecosystem carbon losses while increasing future carbon uptake, resilience to high severity fire, and climate related stresses.

AbstractForests have substantial potential to help mitigate climate change. Private finance channeled through carbon credits is one way to fund that mitigation, but market‐based approaches to forest carbon projects have been fraught to date. Public skepticism of forest carbon markets signals a need to closely scrutinize the system for certifying carbon credits. We rigorously reviewed and scored new and existing protocols for the voluntary and North American compliance carbon markets. We included protocols for forest projects engaging in improved forest management, afforestation/reforestation, and avoided planned forest conversion. Most protocols score poorly overall, and none were assessed as robust. Only one new protocol that had yet to issue credits at the time of our evaluation was assessed as satisfactory, owing to improvements in the approach to additionality demonstration. We conclude that existing protocols do not ensure carbon credits are consistently real, high‐quality, and accurately represent 1 tonne of avoided, reduced, or removed emissions. We offer recommendations for how protocols can be strengthened using existing data and new tools to promote reliably high‐quality credits. Continuing to rely on the status quo without such investments is a serious risk to climate change mitigation, and in our estimation, these proposed improvements would increase the likelihood that forests carbon projects can deliver their promised climate mitigation benefits.

AbstractThe terrestrial carbon cycle is a major source of uncertainty in climate projections. Its dominant fluxes, gross primary productivity (GPP), and respiration (in particular soil respiration, RS), are typically estimated from independent satellite-driven models and upscaled in situ measurements, respectively. We combine carbon-cycle flux estimates and partitioning coefficients to show that historical estimates of global GPP and RS are irreconcilable. When we estimate GPP based on RS measurements and some assumptions about RS:GPP ratios, we found the resulted global GPP values (bootstrap mean $${149}_{-23}^{+29}$$ 149 − 23 + 29 Pg C yr−1) are significantly higher than most GPP estimates reported in the literature ($${113}_{-18}^{+18}$$ 113 − 18 + 18 Pg C yr−1). Similarly, historical GPP estimates imply a soil respiration flux (RsGPP, bootstrap mean of $${68}_{-8}^{+10}$$ 68 − 8 + 10 Pg C yr−1) statistically inconsistent with most published RS values ($${87}_{-8}^{+9}$$ 87 − 8 + 9 Pg C yr−1), although recent, higher, GPP estimates are narrowing this gap. Furthermore, global RS:GPP ratios are inconsistent with spatial averages of this ratio calculated from individual sites as well as CMIP6 model results. This discrepancy has implications for our understanding of carbon turnover times and the terrestrial sensitivity to climate change. Future efforts should reconcile the discrepancies associated with calculations for GPP and Rs to improve estimates of the global carbon budget.

We use eddy covariance measurements of net ecosystem productivity (NEP) from 21 FLUXNET sites (153 site-years of data) to investigate relationships between phenology and productivity (in terms of both NEP and gross ecosystem photosynthesis, GEP) in temperate and boreal forests. Results are used to evaluate the plausibility of four different conceptual models. Phenological indicators were derived from the eddy covariance time series, and from remote sensing and models. We examine spatial patterns (across sites) and temporal patterns (across years); an important conclusion is that it is likely that neither of these accurately represents how productivity will respond to future phenological shifts resulting from ongoing climate change. In spring and autumn, increased GEP resulting from an ‘extra’ day tends to be offset by concurrent, but smaller, increases in ecosystem respiration, and thus the effect on NEP is still positive. Spring productivity anomalies appear to have carry-over effects that translate to productivity anomalies in the following autumn, but it is not clear that these result directly from phenological anomalies. Finally, the productivity of evergreen needleleaf forests is less sensitive to phenology than is productivity of deciduous broadleaf forests. This has implications for how climate change may drive shifts in competition within mixed-species stands.

AbstractGlobal modeling efforts indicate semiarid regions dominate the increasing trend and interannual variation of net CO2 exchange with the atmosphere, mainly driven by water availability. Many semiarid regions are expected to undergo climatic drying, but the impacts on net CO2 exchange are poorly understood due to limited semiarid flux observations. Here we evaluated 121 site‐years of annual eddy covariance measurements of net and gross CO2 exchange (photosynthesis and respiration), precipitation, and evapotranspiration (ET) in 21 semiarid North American ecosystems with an observed range of 100 – 1000 mm in annual precipitation and records of 4–9 years each. In addition to evaluating spatial relationships among CO2 and water fluxes across sites, we separately quantified site‐level temporal relationships, representing sensitivity to interannual variation. Across the climatic and ecological gradient, photosynthesis showed a saturating spatial relationship to precipitation, whereas the photosynthesis–ET relationship was linear, suggesting ET was a better proxy for water available to drive CO2 exchanges after hydrologic losses. Both photosynthesis and respiration showed similar site‐level sensitivity to interannual changes in ET among the 21 ecosystems. Furthermore, these temporal relationships were not different from the spatial relationships of long‐term mean CO2 exchanges with climatic ET. Consequently, a hypothetical 100‐mm change in ET, whether short term or long term, was predicted to alter net ecosystem production (NEP) by 64 gCm−2 yr−1. Most of the unexplained NEP variability was related to persistent, site‐specific function, suggesting prioritization of research on slow‐changing controls. Common temporal and spatial sensitivity to water availability increases our confidence that site‐level responses to interannual weather can be extrapolated for prediction of CO2 exchanges over decadal and longer timescales relevant to societal response to climate change.