Heather Keith

Re-evaluation of forest biomass carbon stocks and lessons from the world's most carbon-dense forests

From analysis of published global site biomass data (n = 136) from primary forests, we discovered (i) the worlds highest known total biomass carbon density (living plus dead) of 1,867 tonnes carbon per ha (average value from 13 sites) occurs in Australian temperate moist Eucalyptus regnans forests, and (ii) average values of the global site biomass data were higher for sampled temperate moist forests (n = 44) than for sampled tropical (n = 36) and boreal (n = 52) forests (n is number of sites per forest biome). Spatially averaged Intergovernmental Panel on Climate Change biome default values are lower than our average site values for temperate moist forests, because the temperate biome contains a diversity of forest ecosystem types that support a range of mature carbon stocks or have a long land-use history with reduced carbon stocks. We describe a framework for identifying forests important for carbon storage based on the factors that account for high biomass carbon densities, including (i) relatively cool temperatures and moderately high precipitation producing rates of fast growth but slow decomposition, and (ii) older forests that are often multiaged and multilayered and have experienced minimal human disturbance. Our results are relevant to negotiations under the United Nations Framework Convention on Climate Change regarding forest conservation, management, and restoration. Conserving forests with large stocks of biomass from deforestation and degradation avoids significant carbon emissions to the atmosphere, irrespective of the source country, and should be among allowable mitigation activities. Similarly, management that allows restoration of a forests carbon sequestration potential also should be recognized.

Effects of soil phosphorus availability, temperature and moisture on soil respiration in Eucalyptus pauciflora forest

Rates of soil respiration (CO2 efflux) were measured for a year in a mature Eucalyptus pauciflora forest in unfertilized and phosphorus-fertilized plots. Soil CO2 efflux showed a distinct seasonal trend, and average daily rates ranged from 124 to 574 mg CO2 m−2 hr−1. Temperature and moisture are the main variables that cause variation in soil CO2 efflux; hence their effects were investigated over a year so as to then differentiate the treatment effect of phosphorus (P) nutrition.Soil temperature had the greatest effect on CO2 efflux and exhibited a highly significant logarithmic relationship (r2 = 0.81). Periods of low soil and litter moisture occurred during summer when temperatures were greater than 10 °C, and this resulted in depression of soil CO2 efflux. During winter, when temperatures were less than 10 °C, soil and litter moisture were consistently high and thus their variation had little effect on soil CO2 efflux. A multiple regression model including soil temperature, and soil and litter moisture accounted for 97% of the variance in rates of CO2 efflux, and thus can be used to predict soil CO2 efflux at this site with high accuracy. Total annual efflux of carbon from soil was estimated to be 7.11 t C ha−1 yr−1. The model was used to predict changes in this annual flux if temperature and moisture conditions were altered. The extent to which coefficients of the model differ among sites and forest types requires testing.Increased soil P availability resulted in a large increase in stem growth of trees but a reduction in the rate of soil CO2 efflux by approximately 8%. This reduction is suggested to be due to lower root activity resulting from reduced allocation of assimilate belowground. Root activity changed when P was added to microsites within plots, and via the whole tree root system at the plot level. These relationships of belowground carbon fluxes with temperature, moisture and nutrient availability provide essential information for understanding and predicting potential changes in forest ecosystems in response to land use management or climate change.

Nocturnal carbon efflux: reconciliation of eddy covariance and chamber measurements using an alternative to the u*-threshold filtering technique

Micrometeorological measurements made on single towers often underestimate nighttime respiration of terrestrial ecosystems because they cannot account for vertical and horizontal advection, thereby causing systematic errors in estimates of net ecosystem carbon exchange. We show that there is a maximum in the sum of the turbulent flux and change in storage of CO2 in the early evening, Rmax, that is in close agreement with concurrent and independent estimates of net carbon exchange from soil and plant chambers.We hypothesize that the peak occurs because there is a time delay between the onset of radiative cooling and the development of temperature gradients that are strong enough to initiate thermally-driven horizontal and vertical flows that remove the stored CO2. We propose taking advantage of this time delay to develop relationships between Rmax and soil temperature and moisture. The new parameterization leads to realistic values of nighttime respiration, and therefore to improved estimates of net ecosystem exchange.

δ13C of organic matter transported from the leaves to the roots in Eucalyptus delegatensis: short-term variations and relation to respired CO2

Post-photosynthetic carbon isotope fractionation might alter the isotopic signal imprinted on organic matter (OM) during primary carbon fixation by Rubisco. To characterise the influence of post-photosynthetic processes, we investigated the effect of starch storage and remobilisation on the stable carbon isotope signature (δ13C) of different carbon pools in the Eucalyptus delegatensis R. T. Baker leaf and the potential carbon isotope fractionation associated with phloem transport and respiration. Twig phloem exudate and leaf water-soluble OM showed diel variations in δ13C of up to 2.5 and 2‰, respectively, with 13C enrichment during the night and depletion during the day. Damped diel variation was also evident in bulk lipids of the leaf and in the leaf wax fraction. δ13C of nocturnal phloem exudate OM corresponded with the δ13C of carbon released from starch. There was no change in δ13C of phloem carbon along the trunk. CO2 emitted from trunks and roots was 13C enriched compared with the potential organic substrate, and depleted compared with soil-emitted CO2. The results are consistent with transitory starch accumulation and remobilisation governing the diel rhythm of δ13C in phloem-transported OM and fragmentation fractionation occurring during respiration. When using δ13C of OM or CO2 for assessing ecosystem processes or plant reactions towards environmental constraints, post-photosynthetic discrimination should be considered.

Allocation of carbon in a mature eucalypt forest and some effects of soil phosphorus availability

Pools and annual fluxes of carbon (C) were estimated for a mature Eucalyptus pauciflora (snowgum) forest with and without phosphorus (P) fertilizer addition to determine the effect of soil P availability on allocation of C in the stand. Aboveground biomass was estimated from allometric equations relating stem and branch diameters of individual trees to their biomass. Biomass production was calculated from annual increments in tree diameters and measurements of litterfall. Maintenance and construction respiration were calculated for each component using equations given by Ryan (1991a). Total belowground C flux was estimated from measurements of annual soil CO2 efflux less the C content of annual litterfall (assuming forest floor and soil C were at approximate steady state for the year that soil CO2 efflux was measured). The total C content of the standing biomass of the unfertilized stand was 138 t ha-1, with approximately 80% aboveground and 20% belowground. Forest floor C was 8.5 t ha-1. Soil C content (0–1 m) was 369 t ha-1 representing 70% of the total C pool in the ecosystem. Total gross annual C flux aboveground (biomass increment plus litterfall plus respiration) was 11.9 t ha-1 and gross flux belowground (coarse root increment plus fine root production plus root respiration) was 5.1 t ha-1. Total annual soil efflux was 7.1 t ha-1, of which 2.5 t ha-1 (35%) was contributed by litter decomposition.The short-term effect of changing the availability of P compared with C on allocation to aboveground versus belowground processes was estimated by comparing fertilized and unfertilized stands during the year after treatment. In the P-fertilized stand annual wood biomass increment increased by 30%, there was no evidence of change in canopy biomass, and belowground C allocation decreased by 19% relative to the unfertilized stand. Total annual C flux was 16.97 and 16.75 t ha-1 yr-1 and the ratio of below- to aboveground C allocation was 0.43 and 0.35 in the unfertilized and P-fertilized stands, respectively. Therefore, the major response of the forest stand to increased soil P availability appeared to be a shift in C allocation; with little change in total productivity. These results emphasise that both growth rate and allocation need to be estimated to predict changes in fluxes and storage of C in forests that may occur in response to disturbance or climate change.

Green Carbon : The role of natural forests in carbon storage

The colour of carbon matters. Green carbon is the carbon stored in the plants and soil of natural ecosystems and is a vital part of the global carbon cycle. This report is the first in a series that examines the role of natural forests in the storage of carbon, the impacts of human land use activities, and the implications for climate change policy nationally and internationally. REDD (“reducing emissions from deforestation and degradation”) is now part of the agenda for the “Bali Action Plan” being debated in the lead-up to the Copenhagen climate change conference in 2009. Currently, international rules are blind to the colour of carbon so that the green carbon in natural forests is not recognized, resulting in perverse outcomes including ongoing deforestation and forest degradation, and the conversion of extensive areas of land to industrial plantations. This report examines REDD policy from a green carbon scientific perspective. Subsequent reports will focus on issues concerning the carbon sequestration potential of commercially logged natural forests, methods for monitoring REDD, and the long term implications of forest policy and management for the global carbon cycle and climate change.

Modelling the potential for prescribed burning to mitigate carbon emissions from wildfires in fire-prone forests of Australia

Prescribed fire can potentially reduce carbon emissions from unplanned fires. This potential will differ among ecosystems owing to inherent differences in the efficacy of prescribed burning in reducing unplanned fire activity (or ‘leverage’, i.e. the reduction in area of unplanned fire per unit area of prescribed fire). In temperate eucalypt forests, prescribed burning leverage is relatively low and potential for mitigation of carbon emissions from unplanned fires via prescribed fire is potentially limited. Simulations of fire regimes accounting for non-linear patterns of fuel dynamics for three fuel types characteristic of eucalypt forests in south-eastern Australia supported this prediction. Estimated mean annual fuel consumption increased with diminishing leverage and increasing rate of prescribed burning, even though average fire intensity (prescribed and unplanned fires combined) decreased. The results indicated that use of prescribed burning in these temperate forests is unlikely to yield a net reduction in carbon emissions. Future increases in burning rates under climate change may increase emissions and reduce carbon sequestration. A more detailed understanding of the efficacy of prescribed burning and dynamics of combustible biomass pools is required to clarify the potential for mitigation of carbon emissions in temperate eucalypt forests and other ecosystems.

Managing temperate forests for carbon storage: impacts of logging versus forest protection on carbon stocks

Management of native forests offers opportunities to store more carbon in the land sector through two main activities. Emissions to the atmosphere can be avoided by ceasing logging. Removals of arbon dioxide from the atmosphere can be increased by allowing forests to continue growing. However, the relative benefits for carbon storage of managing native forests for wood production versus protection are contested. Additionally, the potential for carbon storage is impacted upon by disturbance events, such as wildfire, that alter the amount and longevity of carbon stocks. Using a case study of montane ash forests in southeastern Australia, we demonstrated that the total biomass carbon stock in logged forest was 55% of the stock in old growth forest. Total biomass included above- and belowground, living and dead. Biomass carbon stock was calculated spatially as an average across the landscape, accounting for variation in environmental conditions and forest age distribution. Reduction in carbon stock in logged forest was due to 66% of the initial biomass being made into products with short lifetimes (,3 years), and to the lower average age of logged forest (,50 years compared with 100 years in old growth forest). Only 4% of the initial carbon stock in the native forest was converted to sawn timber products with lifetimes of 30-90 years. Carbon stocks are depleted in a harvested forest system compared with an old growth forest, even when storage in wood products and landfill are included. We estimated that continued logging under current plans represented a loss of 5.56 Tg C over 5 years in the area logged (824 km2), compared with a potential gain of 5.18-6.05 TgC over 5 years by allowing continued growth across the montane ash forest region (2326 km2). Avoiding emissions by not logging native forests and allowing them to continue growing is therefore an important form of carbon sequestration. The mitigation value of forest management options of protection versus logging should be assessed in terms of the amount, longevity and resilience of the carbon stored in the forest, rather than the annual rate of carbon uptake.

Ecosystem accounts define explicit and spatial trade-offs for managing natural resources

Decisions about natural resource management are frequently complex and vexed, often leading to public policy compromises. Discord between environmental and economic metrics creates problems in assessing trade-offs between different current or potential resource uses. Ecosystem accounts, which quantify ecosystems and their benefits for human well-being consistent with national economic accounts, provide exciting opportunities to contribute significantly to the policy process. We advanced the application of ecosystem accounts in a regional case study by explicitly and spatially linking impacts of human and natural activities on ecosystem assets and services to their associated industries. This demonstrated contributions of ecosystems beyond the traditional national accounts. Our results revealed that native forests would provide greater benefits from their ecosystem services of carbon sequestration, water yield, habitat provisioning and recreational amenity if harvesting for timber production ceased, thus allowing forests to continue growing to older ages.Ecosystem accounts quantify trade-offs between the economy and the environment. Here, the authors apply this approach to a regional case study of native forest use to show how it can be used to inform policy about complex land management decisions.

Accounting for Biomass Carbon Stock Change Due to Wildfire in Temperate Forest Landscapes in Australia

Carbon stock change due to forest management and disturbance must be accounted for in UNFCCC national inventory reports and for signatories to the Kyoto Protocol. Impacts of disturbance on greenhouse gas (GHG) inventories are important for many countries with large forest estates prone to wildfires. Our objective was to measure changes in carbon stocks due to short-term combustion and to simulate longer-term carbon stock dynamics resulting from redistribution among biomass components following wildfire. We studied the impacts of a wildfire in 2009 that burnt temperate forest of tall, wet eucalypts in south-eastern Australia. Biomass combusted ranged from 40 to 58 tC ha−1, which represented 6–7% and 9–14% in low- and high-severity fire, respectively, of the pre-fire total biomass carbon stock. Pre-fire total stock ranged from 400 to 1040 tC ha−1 depending on forest age and disturbance history. An estimated 3.9 TgC was emitted from the 2009 fire within the forest region, representing 8.5% of total biomass carbon stock across the landscape. Carbon losses from combustion were large over hours to days during the wildfire, but from an ecosystem dynamics perspective, the proportion of total carbon stock combusted was relatively small. Furthermore, more than half the stock losses from combustion were derived from biomass components with short lifetimes. Most biomass remained on-site, although redistributed from living to dead components. Decomposition of these components and new regeneration constituted the greatest changes in carbon stocks over ensuing decades. A critical issue for carbon accounting policy arises because the timeframes of ecological processes of carbon stock change are longer than the periods for reporting GHG inventories for national emissions reductions targets. Carbon accounts should be comprehensive of all stock changes, but reporting against targets should be based on human-induced changes in carbon stocks to incentivise mitigation activities.