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

Australian Eucalyptus regnans Forests Have the World's Highest Biomass Carbon Density

Evergreen temperate forest dominated by E. regnans (F. Muell.) (Mountain Ash) in the moist temperate region of the Central Highlands of Victoria, southeastern Australia has the highest known biomass carbon density in the world. We found that E. regnans forest in the O'Shannassy Catchment of the Central Highlands (53 sites within a 13,000-ha catchment) contains an average of 1,053 tonnes carbon (tC)·ha⁻¹ in living above-ground biomass and 1,867 tC·ha⁻¹ in living plus dead total biomass in stands with cohorts of trees >100 years old sampled at 13 sites. We examined this catchment in detail because it had been subject to minimal human disturbance, either by Indigenous people or from post-European settlement land use. We compared the biomass carbon density of the E. regnans forest with other forest sites globally by using the collated site data (Table S1). No other records of forests have values as high as those we found for E. regnans.

Our field measurements and calculations revealed that maximum biomass carbon density for a E. regnans-dominated site was 1,819 tC·ha⁻¹ in living above-ground biomass and 2,844 tC·ha⁻¹ in total biomass from stands with a well-defined structure of overstory and midstory trees (see Fig. 1) consisting of multiple age cohorts with the oldest ≈250+ years (19). There was substantial spatial variability in total biomass carbon density across the sites in the catchment within an ecologically mature forest type, ranging from 262 to 2,844 tC·ha⁻¹. Unexpectedly, we found the highest values were from areas experiencing past partial stand-replacing natural disturbances.

• Download figure
• Open in new tab
• Download powerpoint

Fig. 1.

E. regnans forest with midstory of Acacia and understory of tree ferns. The person in the bottom left corner provides a scale.

In February 2009, extensive areas of the O'Shannassy Catchment and elsewhere in the Central Highlands of Victoria were burned in a major conflagration. We will be undertaking a major survey of the network of permanent field sites in the catchment (20) to assess changes in postfire carbon stocks. It will be important that these sites are not subject to postfire salvage logging over the coming years to prevent the extensive removal of dead biomass carbon (21).

Some Temperate Moist Forest Types Can Have Higher Biomass Carbon Density Than Both Boreal and Tropical Forests

Average values of the collated global site biomass data from largely mature or primary forests were much higher for the sampled temperate moist forests (n = 44) than they were for the sampled tropical (n = 36) and boreal (n = 52) forests, where n is the number of sites in each forest biome (Table S1) (Fig. 2). The locations of the global site biomass data are shown in Fig. S1. They do not represent all forest types or environmental conditions within a given biome (reflecting the difficulty of finding published field data) and therefore are insufficient to calculate biome spatial averages. We related site values of above-ground living biomass carbon (tC·ha⁻¹) and total biomass carbon (tC·ha⁻¹) to temperature and precipitation (Fig. 3).

• Download figure
• Open in new tab
• Download powerpoint

Fig. 2.

Global forest site data for above-ground biomass carbon (tC·ha⁻¹) in relation to latitude (north or south). Points are values for individual or average of plots, and bars show the range in values at a site. The O'Shannassy Catchment has a mean of 501 tC·ha⁻¹ and ranges from 104 to 1,819 tC·ha⁻¹. The highest biomass carbon occurs in the temperate latitudes.

• Download figure
• Open in new tab
• Download powerpoint

Fig. 3.

Global forest site data for above-ground living biomass carbon (tC·ha⁻¹) (A) and total biomass carbon (tC·ha⁻¹) (B), in relation to mean annual temperature and mean annual precipitation for the site. Site data are shown in relation to their distribution among biomes of boreal (dark green), temperate (midgreen), and tropical (light green) forests. The highest biomass carbon density occurs in cool, moderately wet climates in temperate moist forest biomes. Some sites had values for above-ground living biomass carbon but not dead biomass, so there was no value for total biomass carbon.

Fig. 3 shows that temperate moist forests occurring where temperatures were cool and precipitation was moderately high had the highest biomass carbon stocks. Temperate forests that had particularly high biomass carbon density included those dominated by Tsuga heterophylla, Picea sitchensis, Pseudotsuga menziesii, and Abies amabilis in the Pacific Northwest of North America [range in living above-ground biomass of 224−587 tC·ha⁻¹ and total biomass of 568–794 tC·ha⁻¹ (22–25)]. A synthesis of site data for the Pacific Northwest gave an average for evergreen needle leaf forest of 334 tC·ha⁻¹ (26), and this is used as the continental biome value by the IPCC (4). An upper limit of biomass accumulation of 500–700 tC·ha⁻¹ in the Pacific Northwest of the United States has been derived from an analysis of global forest data of carbon stocks and net ecosystem productivity in relation to stand age (11, 27). In New Zealand, the highest biomass carbon density reported is for Agathis australis [range in living above-ground biomass of 364–672 and total biomass of 400–982 tC·ha⁻¹ (28)]; and a synthesis based on forest inventory data gave a mean of 180 tC·ha⁻¹ with a range in means for forest classes of 105–215 tC·ha⁻¹ (29). In Chile, the highest biomass carbon densities reported are for Nothofagus, Fitzroya, Philgerodendron, and Laureliopsis [range in living above-ground biomass 142–439 and total biomass of 326–571 tC ha⁻¹ (30–33)].

IPCC Tier-1 Biome Default Values

IPCC biome default values are shown in Table 1 alongside the published global site biomass data (Table S1). The site data were averaged for each biome but they are not equivalent to a spatial average for each biome. The comparison helps identify biomes where site averages differ significantly from default values. The biome-averaged values of the global site biomass carbon data were 2.5–3 times higher than the IPCC biome default values for warm and cool temperate moist forests (Table 1). The IPCC default values were <1 SD from the averaged site values. Average site data were comparable with IPCC default values for tropical and boreal biomes. However, the IPCC biome default value for tropical moist forest was marginally <1 SD from the averaged site values. Also, the site data for the boreal biome reflected higher above-ground living biomass carbon values but lower below-ground plus dead biomass carbon values compared with the IPCC default values (Table 1).

The differences between the collated global site biomass data and IPCC biome default values for temperate moist forests reflect the diversity of forest ecosystem types considered under the temperate biome category. Biome default values likely under-represent Southern Hemisphere evergreen temperate moist forest types and do not distinguish forest condition caused by land-use history (5). The differences between site biomass data and IPCC default values for boreal forests could reflect the effect of land-use history and fire on carbon stocks at the site level.

Toward a Predictive Framework for High Biomass Carbon Forests

We developed a framework for identifying forests with high biomass carbon stocks based on an understanding of underlying mechanisms and using the E. regnans forests as an example. The factors in the framework include (i) environmental conditions, (ii) life history and morphological characteristics of tree species, and (iii) the impacts of natural disturbance such as fire and land-use history. It is the interactions and feedbacks among these factors that influence vegetation community dynamics and ultimately lead to very high carbon densities.

Derivation of Carbon Stocks.

Stock of carbon represents the net exchange of carbon fluxes in an ecosystem (net ecosystem exchange). In living biomass, the carbon stock is determined by the balance between the fluxes of carbon gain by photosynthetic assimilation by the foliage [gross ecosystem production (GEP)] and carbon loss by autotrophic respiration, which results in net primary productivity (NPP). In the total ecosystem (living plus dead biomass plus soil), the carbon stock is determined by the balance between the fluxes of carbon gain by NPP and carbon loss by decomposition of dead biomass and heterotrophic respiration. Ecosystem carbon stocks vary because environmental conditions influence the carbon fluxes of photosynthesis, decomposition, and autotrophic and heterotrophic respiration differently (34).

Environmental Conditions.

The key climatic variables of precipitation, temperature, and radiation are broadly correlated with vegetation structure and function (35, 36), although such empirical correlations do not necessarily reveal underlying biochemical processes or the dependence of these processes on environmental factors (37). Climatic influences on photosynthesis include effects of (i) irradiance and temperature on carboxylation rates, (ii) temperature and soil water status on stomatal conductance and thus diffusion of CO₂ from the atmosphere into the intercellular air spaces, and (iii) temperature-dependent nitrogen uptake (37). The climatic conditions and relatively fertile soils of the Central Highlands of Victoria favor rapid growth of E. regnans (>1 m·yr⁻¹ for the first 70 years), and these trees eventually become the world's tallest flowering plant (up to 130 m) (38).

Both dark respiration and maintenance respiration are temperature dependent (37). Soil respiration is correlated with temperature and water availability, although substrate also has an important influence (34). Rates of coarse woody biomass decomposition have been found to decrease with lower temperatures in temperate forests (39) and are also related to wood density, chemistry, and size (40–42).

Climatic conditions that favor higher rates of GEP relative to rates of respiration and decomposition should, other factors being equal, lead to larger biomass carbon stocks. Table 2 gives the average and range in climatic conditions (annual precipitation and temperature) for the global site data from Table S1 and compares estimates of GEP (34) and decomposition rates (k) (42). Estimates of the climate conditions and derived variables are also shown for E. regnans forests in the Central Highlands of Victoria. Temperate forests are characterized by higher rates of GEP than boreal forests but lower decomposition rates than tropical forests. There is considerable variation evident in rates of carbon fluxes within each forest biome, along with overlap between biomes.

Life History and Morphological Characteristics of Tree Species.

E. regnans can live for ≈450 years, with stem diameters up to 6 m (38, 43). In our analysis, the stands of E. regnans with high values of biomass carbon density were at least 100 years old. E. regnans wood density is high (450–550 g·cm⁻³) (44), so that biomass is greater for a given volume. Limited crown development in E. regnans (through crown shyness or reduced crown area caused by abrasion of growing tips by neighboring crowns) and the isolateral leaf form of this species enable high levels of light to penetrate the forest floor, allowing luxuriant understory layers to grow (45). Eucalypt foliage is evergreen and minimum winter temperatures in the Central Highlands are moderate, so E. regnans trees can grow all year. Similarly, evergreen temperate forests of the Pacific Northwest of North America with high biomass have been found to photosynthesize throughout the year (46).

Natural Disturbance Such as Fire.

Fire affects vegetation structure and biomass carbon stocks at multiple spatial scales, such as the landscape, stand, and individual tree levels. Fire can kill but not combust all of the material in trees, leading to much of the biomass carbon changing from the living biomass pool to the standing dead and fallen dead biomass pools. The amount of carbon lost from the forest floor and the soil profile may vary depending on ecosystem type, fire regimes, and postdisturbance weather conditions (47). The dead biomass then decays as the stand grows (48). Slow decomposition rates can therefore result in large total carbon stocks of dead biomass and regrowing living biomass. A study of temperate forests along a subalpine elevation gradient in the United States estimated coarse woody debris turnover time to be 580 ± 180 years (39). Large amounts of coarse woody debris biomass are also typical of old-growth forests of the Pacific Northwest of North America (40).

Unlike the majority of eucalypt species, E. regnans does not regenerate by epicormic growth or sprouting from lignotubers after a wildfire. Rather, a tree is killed if its canopy is completely scorched by fire. It then sheds seeds that germinate in the postfire ash-bed conditions (49). In the Central Highlands of Victoria, wetter sites on lower slopes and shaded aspects support longer fire intervals and less intense fires, leading to a greater probability of multiaged stands (50). Whether environmentally controlled or the result of stochastic processes, past partial stand-replacing wildfires produce younger cohorts of fast-growing E. regnans trees, mixed with an older cohort of living and dead trees, together with rejuvenating the understory of Acacia spp. and other tree species (Fig. 1).

Land-Use Activity.

The final reason for high biomass carbon densities in E. regnans forests is a prolonged absence of direct human land-use activity. The O'Shannassy Catchment has been closed to public access for >100 years to provide water for the city of Melbourne. It had an almost complete absence of Indigenous land use before European settlement. Natural disturbances have included wildfire, windstorms, and insect attacks. Logging has been excluded, including postwildfire salvage logging that removes large amounts of biomass in living and dead trees (thus preventing the development of multiple age cohorts) (21, 51, 52).

Some types of temperate moist forests that have had limited influence by human activities can be multiaged and do not necessarily consist exclusively of old trees, but often have a complex multiaged structure of multiple layers produced by regeneration from natural disturbances and individual tree gaps in the canopy (53). Net primary production in some types of multiaged old forests has been found to be 50–100% higher than that modeled for an even-aged stand (54). Both net primary production and net ecosystem production in many old forest stands have been found to be positive; they were lower than the carbon fluxes in young and mature stands, but not significantly different from them (55). Northern Hemisphere forests up to 800 years old have been found to still function as a carbon sink (11). Carbon stocks can continue to accumulate in multiaged and mixed species stands because stem respiration rates decrease with increasing tree size, and continual turnover of leaves, roots, and woody material contribute to stable components of soil organic matter (56). There is a growing body of evidence that forest ecosystems do not necessarily reach an equilibrium between assimilation and respiration, but can continue to accumulate carbon in living biomass, coarse woody debris, and soils, and therefore may act as net carbon sinks for long periods (12, 57–59). Hence, process-based models of forest growth and carbon cycling based on an assumption that stands are even-aged and carbon exchange reaches an equilibrium may underestimate productivity and carbon accumulation in some forest types.

Large carbon stocks can develop in a particular forest as a result of a combination and interaction of environmental conditions, life history attributes, morphological characteristics of tree species, disturbance regimes, and land-use history. Very large stocks of carbon occur in the multiaged and multilayered E. regnans forests of the Central Highlands of Victoria. The same suite of factors listed above operate, to varying degrees, across other evergreen temperate forests, particularly in the northwestern United States, southern South America, New Zealand, and elsewhere in southeastern Australia. Collectively, they provide the basis of a generalized framework for predicting high biomass carbon density forests. However, construction of a quantitative predictive model inclusive of all factors is complicated by a lack of process understanding (37), knowledge of species life history characteristics and dynamics, and many interactions and feedback effects (60).

Climate Change Policy Implications

Our results about the magnitude of carbon stocks in forests, particularly in old forests that have had minimal human disturbance, are relevant to negotiations under the United Nations Framework Convention on Climate Change (UNFCCC) concerning reducing emissions from deforestation and forest degradation. In particular, our findings can help inform discussions regarding the roles of conservation, sustainable management of forests and enhancement of forest carbon stocks (ref. 61; http://unfccc.int/resource/docs/2007/cop13/eng/06a01.pdf#page=8). 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 negotiated through the UNFCCC for the post-2012 commitment period. Similarly, where practical, management that allows restoration of a forest's carbon sequestration potential should be a recognized mitigation activity.

Our insights into forest types and forest conditions that result in high biomass carbon density can be used to help identify priority areas for conservation and restoration. The global synthesis of site data (Fig. 3 and Table 2) indicated that the high carbon densities of evergreen temperate forests in the northwestern United States, southern South America, New Zealand, and southeastern Australia should be recognized in forest biome classifications.

Concluding Comments

Our findings highlight the value of field-based site measurements in characterizing forest carbon stocks. They help reveal the variability within forest biomes and identify causal factors leading to high carbon densities. Further analyses of existing site data from forests around the world, along with new field surveys, are warranted to improve understanding of the spatial distribution of biomass carbon inclusive of land-use and fire history.