Soil respiration and carbon balance in a subtropical native forest and two managed plantations

From 1999 to 2003, a range of carbon fluxes was measured and integrated to establish a carbon balance for a natural evergreen forest of Castanopsis kawakamii (NF) and adjacent monoculture evergreen plantations of C. kawakamii (CK) and Chinese fir (Cunninghamia lanceolata, CF) in Sanming Nature Reserve, Fujian, China. Biomass carbon increment of aboveground parts and coarse roots were measured by the allometric method. Above- and belowground litter C inputs were assessed by litter traps and sequential cores, respectively. Soil respiration (SR) was determined by the alkaline absorbance method, and the contribution from roots, above- and belowground litters was separated by the DIRT plots. Annual SR averaged 13.742 t C ha⁻¹ a⁻¹ in the NF, 9.439 t C ha⁻¹ a⁻¹ in the CK, and 4.543 t C ha⁻¹ a⁻¹ in the CF. For all forests, SR generally peaked in later spring or early summer (May or June). The contribution of root respiration ranged from 47.8% in the NF to 40.3% in the CF. On average, soil heterotrophic respiration (HR) was evenly distributed between below- (47.3∼54.5%) and aboveground litter (45.5%–52.7%). Annual C inputs (t C ha⁻¹ a⁻¹) from litterfall and root turnover averaged 4.452 and 4.295, 4.548 and 2.313, and 2.220 and 1.265, respectively, in the NF, CK, and CF. As compared to HR, annual net primary production (NPP) of 11.228, 13.264, and 6.491 t C ha⁻¹ a⁻¹ in the NF, CK, and CF brought a positive net ecosystem production (NEP) of 4.144, 7.514, and 3.677 t C ha⁻¹ a⁻¹, respectively. It suggests that native forest in subtropical China currently acts as an important carbon sink just as the timber plantation does, and converting native forest to tree plantations locally during last decades might have caused a high landscape carbon loss to the atmosphere.     

Biometric-based estimation of net ecosystem production in a mature Japanese cedar (<Emphasis Type="Italic">Cryptomeria japonica</Emphasis>) plantation beneath a flux tower

Quantification of carbon budgets and cycling in Japanese cedar (Cryptomeria japonica D. Don) plantations is essential for understanding forest functions in Japan because these plantations occupy about 20% of the total forested area. We conducted a biometric estimate of net ecosystem production (NEP) in a mature Japanese cedar plantation beneath a flux tower over a 4-year period. Net primary production (NPP) was 7.9 Mg C ha⁻¹ year⁻¹ and consisted mainly of tree biomass increment and aboveground litter production. Respiration was calculated as 6.8 (soil) and 3.3 (root) Mg C ha⁻¹ year⁻¹. Thus, NEP in the plantation was 4.3 Mg C ha⁻¹ year⁻¹. In agreement with the tower-based flux findings, this result suggests that the Japanese cedar plantation was a strong carbon sink. The biometric-based NEP was higher among most other types of Japanese forests studied. Carbon sequestration in the mature plantation was characterized by a larger increment in tree biomass and lower mortality than in natural forests. Land-use change from natural forest to Japanese cedar plantation might, therefore, stimulate carbon sequestration and change the carbon allocation of NPP from an increment in coarse woody debris to an increase in tree biomass.     

Biomass and carbon dynamics of a tropical mountain rain forest in China

Biometric inventories for 25 years, from 1983 to 2005, indicated that the Jianfengling tropical mountain rain forest in Hainan, China, was either a source or a modest sink of carbon. Overall, this forest was a small carbon sink with an accumulation rate of (0.56±0.22) Mg C ha⁻¹yr⁻¹, integrated from the long-term measurement data of two plots (P9201 and P8302). These findings were similar to those for African and American rain forests ((0.62±0.23) Mg C ha⁻¹yr⁻¹). The carbon density varied between (201.43±29.38) Mg C ha⁻¹ and (229.16±39.2) Mg C ha⁻¹, and averaged (214.17±32.42) Mg C ha⁻¹ for plot P9201. Plot P8302, however, varied between (223.95±45.92) Mg C ha⁻¹ and (254.85±48.86) Mg C ha⁻¹, and averaged (243.35±47.64) Mg C ha⁻¹. Quadratic relationships were found between the strength of carbon sequestration and heavy rainstorms and dry months. Precipitation and evapotranspiration are two major factors controlling carbon sequestration in the tropical mountain rain forest.     

Postfire carbon pools and fluxes in semiarid ponderosa pine in Central Oregon

Forest fire dramatically affects the carbon storage and underlying mechanisms that control the carbon balance of recovering ecosystems. In western North America where fire extent has increased in recent years, we measured carbon pools and fluxes in moderately and severely burned forest stands 2 years after a fire to determine the controls on net ecosystem productivity (NEP) and make comparisons with unburned stands in the same region. Total ecosystem carbon in soil and live and dead pools in the burned stands was on average 66% that of unburned stands (11.0 and 16.5 kg C m⁻², respectively, P<0.01). Soil carbon accounted for 56% and 43% of the carbon pools in burned and unburned stands. NEP was significantly lower in severely burned compared with unburned stands (P<0.01) with an increasing trend from −125±44 g C m⁻² yr⁻¹ (±1 SD) in severely burned stands (stand replacing fire), to −38±96 and +50±47 g C m⁻² yr⁻¹ in moderately burned and unburned stands, respectively. Fire of moderate severity killed 82% of trees <20 cm in diameter (diameter at 1.3 m height, DBH); however, this size class only contributed 22% of prefire estimates of bole wood production. Larger trees (> 20 cm DBH) suffered only 34% mortality under moderate severity fire and contributed to 91% of postfire bole wood production. Growth rates of trees that survived the fire were comparable with their prefire rates. Net primary production NPP (g C m⁻² yr⁻¹, ±1 SD) of severely burned stands was 47% of unburned stands (167±76, 346±148, respectively, P<0.05), with forb and grass aboveground NPP accounting for 74% and 4% of total aboveground NPP, respectively. Based on continuous seasonal measurements of soil respiration in a severely burned stand, in areas kept free of ground vegetation, soil heterotrophic respiration accounted for 56% of total soil CO₂ efflux, comparable with the values of 54% and 49% previously reported for two of the unburned forest stands. Estimates of total ecosystem heterotrophic respiration (R_h) were not significantly different between stand types 2 years after fire. The ratio NPP/R_h averaged 0.55, 0.85 and 1.21 in the severely burned, moderately burned and unburned stands, respectively. Annual soil CO₂ efflux was linearly related to aboveground net primary productivity (ANPP) with an increase in soil CO₂ efflux of 1.48 g C yr⁻¹ for every 1 g increase in ANPP (P<0.01, r²= 0.76). There was no significant difference in this relationship between the recently burned and unburned stands. Contrary to expectations that the magnitude of NEP 2 years postfire would be principally driven by the sudden increase in detrital pools and increased rates of R_h, the data suggest NPP was more important in determining postfire NEP.     

Invasion by a Perennial Herb Increases Decomposition Rate and Alters Nutrient Availability in Warm Temperate Lowland Forest Remnants

We determined the impact of the invasive herb, Tradescantia fluminensis Vell., on litter decomposition and nutrient availability in a remnant of New Zealand lowland podocarp–broadleaf forest. Using litter bags, we found that litter beneath mats of Tradescantia decomposed at almost twice the rate of litter placed outside the mat. Values of k (decomposition quotient) were 9.44±0.42 yrs for litter placed beneath Tradescantia and 5.42±0.42 yrs for litter placed in native, non-Tradescantia plots. The impact of Tradescantia on decomposition was evident through the smaller forest floor mass in Tradescantia plots (2.65±1.05 t ha⁻¹) compared with non-Tradescantia plots (5.05±1.05 t ha⁻¹), despite similar quantities of annual leaf litterfall into Tradescantia plots (6.85±0.85 t ha⁻¹ yr⁻¹) and non-Tradescantia plots (7.45±1.05 t ha⁻¹ yr⁻¹). Moreover, there was increased plant nitrate available, as captured on resin bags, in Tradescantia plots (25.77 ± 8.32 cmol(−)/kg resin) compared with non-Tradescantia plots (9.55±3.72 cmol(−)/kg resin). Finally, the annual nutrient uptake by Tradescantia represented a large proportion of nutrients in litterfall (41% N, 61% P, 23% Ca, 46% Mg and 83% K), exceeded the nutrient content of the forest floor (except Ca), but was a small proportion of the topsoil nutrient pools. Taken together, our results show that Tradescantia increases litter decomposition and alters nutrient availability, effects that could influence the long-term viability of the majority of podocarp–broadleaf forest remnants affected with Tradescantia in New Zealand. These impacts are likely mostly due to Tradescantia's vegetation structure (i.e., tall, dense mats) and associated microclimate, compared with native ground covers. This revised version was published online in July 2006 with corrections to the Cover Date.     

Carbon cycling and storage in world forests: biome patterns related to forest age

Forest age, which is affected by stand‐replacing ecosystem disturbances (such as forest fires, harvesting, or insects), plays a distinguishing role in determining the distribution of carbon (C) pools and fluxes in different forested ecosystems. In this synthesis, net primary productivity (NPP), net ecosystem productivity (NEP), and five pools of C (living biomass, coarse woody debris, organic soil horizons, soil, and total ecosystem) are summarized by age class for tropical, temperate, and boreal forest biomes. Estimates of variability in NPP, NEP, and C pools are provided for each biome‐age class combination and the sources of variability are discussed. Aggregated biome‐level estimates of NPP and NEP were higher in intermediate‐aged forests (e.g., 30–120 years), while older forests (e.g., >120 years) were generally less productive. The mean NEP in the youngest forests (0–10 years) was negative (source to the atmosphere) in both boreal and temperate biomes (?0.1 and –1.9 Mg C ha^?1 yr^?1, respectively). Forest age is a highly significant source of variability in NEP at the biome scale; for example, mean temperate forest NEP was ?1.9, 4.5, 2.4, 1.9 and 1.7 Mg C ha^?1 yr^?1 across five age classes (0–10, 11–30, 31–70, 71–120, 121–200 years, respectively). In general, median NPP and NEP are strongly correlated (R²=0.83) across all biomes and age classes, with the exception of the youngest temperate forests. Using the information gained from calculating the summary statistics for NPP and NEP, we calculated heterotrophic soil respiration (R_h) for each age class in each biome. The mean R_h was high in the youngest temperate age class (9.7 Mg C ha^?1 yr^?1) and declined with age, implying that forest ecosystem respiration peaks when forests are young, not old. With notable exceptions, carbon pool sizes increased with age in all biomes, including soil C. Age trends in C cycling and storage are very apparent in all three biomes and it is clear that a better understanding of how forest age and disturbance history interact will greatly improve our fundamental knowledge of the terrestrial C cycle.     

Carbon cycling and sequestration in a Japanese red pine (Pinus densiflora) forest on lava flow of Mt. Fuji

Biometric-based carbon flux measurements were conducted in a pine forest on lava flow of Mt. Fuji, Japan, in order to estimate carbon cycling and sequestration. The forest consists mainly of Japanese red pine (Pinus densiflora) in a canopy layer and Japanese holly (Ilex pedunculosa) in a subtree layer. The lava remains exposed on the ground surface, and the soil on the lava flow is still immature with no mineral soil layer. The results showed that the net primary production (NPP) of the forest was 7.3 ± 0.7 t C ha^?1 year^?1, of which 1.4 ± 0.4 t C ha^?1 year^?1 was partitioned to biomass increment, 3.2 ± 0.5 t C ha^?1 year^?1 to above-ground fine litter production, 1.9 t C ha^?1 year^?1 to fine root production, and 0.8 ± 0.2 t C ha^?1 year^?1 to coarse woody debris. The total amount of annual soil surface CO₂ efflux was estimated as 6.1 ± 2.9 t C ha^?1 year^?1, using a closed chamber method. The estimated decomposition rate of soil organic matter, which subtracted annual root respiration from soil respiration, was 4.2 ± 3.1 t C ha^?1 year^?1. Biometric-based net ecosystem production (NEP) in the pine forest was estimated at 2.9 ± 3.2 t C ha^?1 year^?1, with high uncertainty due mainly to the model estimation error of annual soil respiration and root respiration. The sequestered carbon being allocated in roughly equal amounts to living biomass (1.4 t C ha^?1 year^?1) and the non-living C pool (1.5 t C ha^?1 year^?1). Our estimate of biometric-based NEP was 25 % lower than the eddy covariance-based NEP in this pine forest, due partly to the underestimation of NPP and difficulty of estimation of soil and root respiration in the pine forest on lava flows that have large heterogeneity of soil depth. However, our results indicate that the mature pine forest acted as a significant carbon sink even when established on lava flow with low nutrient content in immature soils, and that sequestration strength, both in biomass and in soil organic matter, is large.     

Carbon and nitrogen storage in an age-sequence of <Emphasis Type="Italic">Pinus densiflora</Emphasis> stands in Korea

The carbon (C) and nitrogen (N) storage capabilities of Pinus densiflora in six different stand ages (10, 27, 30, 32, 44, and 71 years old) were investigated in Korea. Thirty sample trees were destructively harvested and 12 were excavated. Samples from the above and belowground tree components, coarse woody debris (CWD), forest floor, and mineral soil (0–30 cm) were collected. Tree biomass was highest in the 71-year-old stand (202.8 t ha⁻¹) and lowest in the 10-year-old stand (18.4 t ha⁻¹). C and N storage in the mineral soil was higher in the 71-year-old stand than in the other stands, mainly due to higher soil C and N concentrations. Consequently, the total ecosystem C and N storage (tree+forest floor+CWD+soil) was positively correlated with stand age: increasing from a minimum in the 10 year old stand (18.8 t C ha⁻¹ and 1.3 t N ha⁻¹) to a maximum in the 71-year-old stand (201.4 t C ha⁻¹ and 8.5 t N ha⁻¹). The total ecosystem C storage showed a similar sigmoidal pattern to that of tree C storage as a function of the age-sequence, while N storage in the CWD, forest floor and mineral soil showed no significant temporal trends. Our results provide important insights that will increase our understanding of C and N storage in P. densiflora stands and our ability to predict changes according to stand age in the region.     

Gaps and Soil C Dynamics in Old Growth Northern Hardwood–Hemlock Forests

Old growth forest soils are large C reservoirs, but the impacts of tree-fall gaps on soil C in these forests are not well understood. The effects of forest gaps on soil C dynamics in old growth northern hardwood–hemlock forests in the upper Great Lakes region, USA, were assessed from measurements of litter and soil C stocks, surface C efflux, and soil microbial indices over two consecutive growing seasons. Forest floor C was significantly less in gaps (19.0 Mg C ha⁻¹) compared to gap-edges (39.5 Mg C ha⁻¹) and the closed forest (38.0 Mg C ha⁻¹). Labile soil C (coarse particulate organic matter, cPOM) was significantly less in gaps and edges (11.1 and 11.2 Mg C ha⁻¹) compared to forest plots (15.3 Mg C ha⁻¹). In situ surface C efflux was significantly greater in gaps (12.0 Mg C ha⁻¹ y⁻¹) compared to edges and the closed forest (9.2 and 8.9 Mg C ha⁻¹ y⁻¹). Microbial biomass N (MBN) was significantly greater in edges (0.14 Mg N ha⁻¹) than in the contiguous forest (0.09 Mg N ha⁻¹). The metabolic quotient (qCO₂) was significantly greater in the forest (0.0031 mg CO₂ h⁻¹ g⁻¹/mg MBC g⁻¹) relative to gaps or edges (0.0014 mg CO₂ h⁻¹ g⁻¹/mg MBC g⁻¹). A case is made for gaps as alleviators of old growth forest soil C saturation. Relative to the undisturbed closed forest, gaps have significantly less labile C, significantly greater in situ surface C efflux, and significantly lower decreased qCO₂ values.     

Soil carbon storage, litterfall and CO2 efflux in fertilized and unfertilized larch (Larix leptolepis) plantations

This study evaluated the effects of forest fertilization on the forest carbon (C) dynamics in a 36-year-old larch (Larix leptolepis) plantation in Korea. Above- and below-ground C storage, litterfall, root decomposition and soil CO₂ efflux rates after fertilization were measured for 2 years. Fertilizers were applied to the forest floor at rates of 112 kg N ha⁻¹ year⁻¹, 75 kg P ha⁻¹ year⁻¹ and 37 kg K ha⁻¹ year⁻¹ for 2 years (May 2002, 2003). There was no significant difference in the above-ground C storage between fertilized (41.20 Mg C ha⁻¹) and unfertilized (42.25 Mg C ha⁻¹) plots, and the C increment was similar between the fertilized (1.65 Mg C ha⁻¹ year⁻¹) and unfertilized (1.52 Mg C ha⁻¹ year⁻¹) plots. There was no significant difference in the soil C storage between the fertilized and unfertilized plots at each soil depth (0–15, 15–30 and 30–50 cm). The organic C inputs due to litterfall ranged from 1.57 Mg C ha⁻¹ year⁻¹ for fertilized to 1.68 Mg C ha⁻¹ year⁻¹ for unfertilized plots. There was no significant difference in the needle litter decomposition rates between the fertilized and unfertilized plots, while the decomposition of roots with 1–2 mm diameters increased significantly with the fertilization relative to the unfertilized plots. The mean annual soil CO₂ efflux rates for the 2 years were similar between the fertilized (0.38 g CO₂ m⁻² h⁻¹) and unfertilized (0.40 g CO₂ m⁻² h⁻¹) plots, which corresponded with the similar fluctuation in the organic carbon (litterfall, needle and root decomposition) and soil environmental parameters (soil temperature and soil water content). These results indicate that little effect on the C dynamics of the larch plantation could be attributed to the 2-year short-term fertilization trials and/or the soil fertility in the mature coniferous plantation used in this study.     

Effects of logging on carbon dynamics of a jack pine forest in Saskatchewan,Canada

We calculated carbon budgets for a chronosequence of harvested jack pine (Pinus banksiana Lamb.) stands (0‐, 5‐, 10‐, and～29‐year‐old) and a～79‐year‐old stand that originated after wildfire. We measured total ecosystem C content (TEC), above‐, and belowground net primary productivity (NPP) for each stand. All values are reported in order for the 0‐, 5‐, 10‐, 29‐, and 79‐year‐old stands, respectively, for May 1999 through April 2000. Total annual NPP (NPP_T) for the stands (Mg C ha^?1 yr^?1±1 SD) was 0.9±0.3, 1.3±0.1, 2.7±0.6, 3.5±0.3, and 1.7±0.4. We correlated periodic soil surface CO₂ fluxes (R_S) with soil temperature to model annual R_S for the stands (Mg C ha^?1 yr^?1±1 SD) as 4.4±0.1, 2.4±0.0, 3.3±0.1, 5.7±0.3, and 3.2±0.2. We estimated net ecosystem productivity (NEP) as NPP_T minus R_H (where R_H was calculated using a Monte Carlo approach as coarse woody debris respiration plus 30–70% of total annual R_S). Excluding C losses during wood processing, NEP (Mg C ha^?1 yr^?1±1 SD) for the stands was estimated to be ?1.9±0.7, ?0.4±0.6, 0.4±0.9, 0.4±1.0, and ?0.2±0.7 (negative values indicate net sources to the atmosphere.) We also calculated NEP values from the changes in TEC among stands. Only the 0‐year‐old stand showed significantly different NEP between the two methods, suggesting a possible mismatch for the chronosequence. The spatial and methodological uncertainties allow us to say little for certain except that the stand becomes a source of C to the atmosphere following logging.     

Nitrogen Oxide Fluxes and Nitrogen Cycling during Postagricultural Succession and Forest Fertilization in the Humid Tropics

The effects of changes in tropical land use on soil emissions of nitrous oxide (N₂O) and nitric oxide (NO) are not well understood. We examined emissions of N₂O and NO and their relationships to land use and forest composition, litterfall, soil nitrogen (N) pools and turnover, soil moisture, and patterns of carbon (C) cycling in a lower montane, subtropical wet region of Puerto Rico. Fluxes of N₂O and NO were measured monthly for over 1 year in old (more than 60 years old) pastures, early- and mid-successional forests previously in pasture, and late-successional forests not known to have been in pasture within the tabonuco (Dacryodes excelsa) forest zone. Additional, though less frequent, measures were also made in an experimentally fertilized tabonuco forest. N₂O fluxes exceeded NO fluxes at all sites, reflecting the consistently wet environment. The fertilized forest had the highest N oxide emissions (22.0 kg N · ha⁻¹· y⁻¹). Among the unfertilized sites, the expected pattern of increasing emissions with stand age did not occur in all cases. The mid-successional forest most dominated by leguminous trees had the highest emissions (9.0 kg N · ha⁻¹· y⁻¹), whereas the mid-successional forest lacking legumes had the lowest emissions (0.09 kg N · ha⁻¹· y⁻¹). N oxide fluxes from late-successional forests were higher than fluxes from pastures. Annual N oxide fluxes correlated positively to leaf litter N, net nitrification, potential nitrification, soil nitrate, and net N mineralization and negatively to leaf litter C:N ratio. Soil ammonium was not related to N oxide emissions. Forests with lower fluxes of N oxides had higher rates of C mineralization than sites with higher N oxide emissions. We conclude that (a) N oxide fluxes were substantial where the availability of inorganic N exceeded the requirements of competing biota; (b) species composition resulting from historical land use or varying successional dynamics played an important role in determining N availability; and (c) the established ecosystem models that predict N oxide loss from positive relationships with soil ammonium may need to be modified. Received 22 February 2000; accepted 6 September 2000.     

Carbon stock and cycling in a bambooPhyllostachys bambusoides stand

Gross production and carbon cycling in aPhyllostachys bambusoides stand in Kyoto Prefecture, central Japan, were determined, and then a compartment model showing the carbon stock and cycling within the ecosystem was developed. Aboveground carbon stock was 52.3 tC ha⁻¹, increasing at a rate of 3.6 tC ha⁻¹ year⁻¹. Belowground carbon stock was 20.8 tC ha⁻¹ in the root system and 92.0 tC ha⁻¹ in the soil. Aboveground net production was 11.2 tC ha⁻¹ year⁻¹. Belowground net production was crudely estimated at 4.5 tC ha⁻¹ year⁻¹. The gross production was estimated at 41.8 tC ha⁻¹ year⁻¹ by summing the amount of outflow to the environment and the increment in biomass. Leaves consumed 13.7 tC ha⁻¹ year⁻¹ by respiration; the rest (41.8−13.7=28.1 tC ha⁻¹ year⁻¹) was surplus production of the leaves and flowed into the other compartments. The amounts of construction and maintenance respiration of the aboveground compartments were 3.4 and 18.5 tC ha⁻¹ year⁻¹, respectively. The annual amount of soil respiration was 11.2 tC ha⁻¹ year⁻¹. Soil respiration levels of 4.3 and 3.1 tC ha⁻¹ year⁻¹ were estimated for the flow of root respiration and root detritus. The proportion of net to gross production was 37%, which fell within the range of young and mature forests. A shorter life span of culms, compared to tree trunks, resulted in smaller biomass accumulation ratio (biomass/net production) in the ecosystem, of 4.66.     

Production,Respiration, and Overall Carbon Balance in an Old-growth <Emphasis Type="Italic">Pseudotsuga</Emphasis>-<Emphasis Type="Italic">Tsuga</Emphasis> Forest Ecosystem

Ground-based measurements of stores, growth, mortality, litterfall, respiration, and decomposition were conducted in an old-growth forest at Wind River Experimental Forest, Washington, USA. These measurements were used to estimate gross primary production (GPP) and net primary production (NPP); autotrophic respiration (R_a) and heterotrophic (R_h) respiration; and net ecosystem production (NEP). Monte Carlo methods were used to calculate uncertainty (expressed as ± 2 standard deviations of 200–400 calculations). Live carbon (C) stores were 39,800 g C m^–2 (34,800–44,800 g C m^–2). The store of C in detritus and mineral soil was 22,092 g C m^–2 (20,600–23,600 g C m^–2), and the total C stores were 61,899 g C m^–2 (56,600–67,700 g C m^–2). Total NPP was 597 g C m^–2 y^–1 (453 to 741 g C m^–2 y^–1). R_a was 1309 g C m^–2 y^–1 (845–1773 g C m^–2 y^–1), indicating a GPP of 1906 g C m^–2 y^–1 (1444–2368 g C m^–2 y^–1). R_h, including the respiration of heart rots in tree boles, was 577 g C m^–2 y^–1 (479–675 g C m^–2 y^–1). Long-term NEP was estimated to be +20 g C m^–2 y^–1 (–116 to +156 g C m^–2 y^–1), indicating this stand might be a small sink. These estimates contrast with the larger sink estimated at the same site using eddy-flux methods. Several hypotheses to explain this discrepancy were explored, including (a) undetected biomass increases, (b) underestimates of NPP, (c) unmeasured losses, and (d) a temporal mismatch between the two sets of measurements. The last hypothesis appears the most likely.     

Total Belowground Carbon Allocation in a Fast-growing Eucalyptus Plantation Estimated Using a Carbon Balance Approach

Trees allocate a large portion of gross primary production belowground for the production and maintenance of roots and mycorrhizae. The difficulty of directly measuring total belowground carbon allocation (TBCA) has limited our understanding of belowground carbon (C) cycling and the factors that control this important flux. We measured TBCA over 4 years using a conservation of mass, C balance approach in replicate stands of fast growing Eucalyptus saligna Smith with different nutrition management and tree density treatments. We measured TBCA as surface carbon dioxide (CO₂) efflux (“soil” respiration) minus C inputs from aboveground litter plus the change in C stored in roots, litter, and soil. We evaluated this C balance approach to measuring TBCA by examining (a) the variance in TBCA across replicate plots; (b) cumulative error associated with summing components to arrive at our estimates of TBCA; (c) potential sources of error in the techniques and assumptions; (d) the magnitude of changes in C stored in soil, litter, and roots compared to TBCA; and (e) the sensitivity of our measures of TBCA to differences in nutrient availability, tree density, and forest age. The C balance method gave precise estimates of TBCA and reflected differences in belowground allocation expected with manipulations of fertility and tree density. Across treatments, TBCA averaged 1.88 kg C m⁻² y⁻¹ and was 18% higher in plots planted with 10⁴ trees/ha compared to plots planted with 1111 trees/ha. TBCA was 12% lower (but not significantly so) in fertilized plots. For all treatments, TBCA declined linearly with stand age. The coefficient of variation (CV) for TBCA for replicate plots averaged 17%. Averaged across treatments and years, annual changes in C stored in soil, the litter layer, and coarse roots (−0.01, 0.06, and 0.21 kg C m⁻² y⁻¹, respectively) were small compared with surface CO₂ efflux (2.03 kg C m⁻² y⁻¹), aboveground litterfall (0.42 kg C m⁻² y⁻¹), and our estimated TBCA (1.88 kg C m⁻² y⁻¹). Based on studies from similar sites, estimates of losses of C through leaching, erosion, or storage of C in deep soil were less than 1% of annual TBCA. Received 6 March 2001; accepted 7 January 2002.     

Different patterns of ecosystem carbon accumulation between a young and an old-growth subtropical forest in Southern China

Using long-term (22 years) measurements from a young and an old-growth subtropical forest in southern China, we found that both forests accumulated carbon from 1982 to 2004, with the mean carbon accumulation rate at 227 ± 59 g C m⁻² year⁻¹ for young forest and 115 ± 89 g C m⁻² year⁻¹ for the old-growth forest. Allocation of the accumulated carbon was quite different between these two forests: the young forest accumulated a significant amount of carbon in plant live biomass, whereas the old-growth forest accumulated a significant amount of carbon in the soil. From 1982 to 2004, net primary productivity (NPP) increased for the young forest, and did not change significantly for the old-growth forest. The increase in NPP of the young forest resulted from recruitment of some dominant tree species characteristic of the subtropical mature forest in the region and an increase in tree density; decline of NPP of the old-growth forest was caused by increased mortality of the dominant trees.     

Species Structure, Dry Matter Dynamics and Carbon Flux of a Dry Tropical Forest in India

Species composition, plant biomass and net primary productivitywere studied on three sites of a dry tropical forest The forestwas characterized by small structure with 38–10.4 m2 ha^–1tree and 3 1–7 8 m² ha^–1 shrub basal cover Speciesdiversity was highest for the mid-slope site while the concentrationof dominance was greatest for the hill-top stand The beta diversitywas 3 1 Total standing crop of vegetation averaged 66 98 t ha^–1with 46 70 t ha^–1 in the tree layer, 13.97 t ha^–1in the shrub layer, 0.35 t ha^–1 in the herb layer, 2 83t ha^–1 in the litter layer and 3 13 t ha^–1 in fineroots Of the total annual litterfall (4 88–6.71 t ha^–1),69% was accounted for by leaves and 31% by non-leaf matter Netprimary production (NPP) ranged between 11 3 and 19 2 t ha^–1year^–1, to which the contributions of trees, shrubs andherbs averaged 72, 22 and 6%, respectively Contribution of rootsto NPP was substantial and ranged from 2 9 to 5 3 t ha^–1year^–1 A total of 83% of vegetation carbon was storedin the above-ground plant parts while the above-ground NPP wasresponsible for 72% of the total carbon input into the systemThe contribution of foliage, herbaceous vegetation and fineroots to carbon turnover was disproportionately larger comparedto their share in the total standing crop Carbon budgeting indicatedthat the forest was an accumulating system, over at least theshort term Dry tropical forest, biomass, litterfall, net primary production, carbon budget, carbon flux     

Soil Nutrients Limit Fine Litter Production and Tree Growth in Mature Lowland Forest of Southwestern Borneo

Efforts to improve models of terrestrial productivity and to understand the function of tropical forests in global carbon cycles require a mechanistic understanding of spatial variation in aboveground net primary productivity (ANPP) across tropical landscapes. To help derive such an understanding for Borneo, we monitored aboveground fine litterfall, woody biomass increment and ANPP (their sum) in mature forest over 29 months across a soil nutrient gradient in southwestern Kalimantan. In 30 (0.07 ha) plots stratified throughout the watershed (∼340 ha, 8–190 m a.s.l.), we measured productivity and tested its relationship with 27 soil parameters. ANPP across the study area was among the highest reported for mature lowland tropical forests. Aboveground fine litterfall ranged from 5.1 to 11.0 Mg ha⁻¹ year⁻¹ and averaged 7.7 ± 0.4 (mean ± 95 C.I.). Woody biomass increment ranged from 5.8 to 23.6 Mg ha⁻¹ year⁻¹ and averaged 12.0 ± 2.0. Growth of large trees (≥60 cm dbh) contributed 38–82% of plot-wide biomass increment and explained 92% of variation among plots. ANPP, the sum of these parameters, ranged from 11.1 to 32.3 Mg ha⁻¹ year⁻¹ and averaged 19.7 ± 2.2. ANPP was weakly related to fine litterfall (r ² = 0.176), but strongly related to growth of large trees at least 60 cm dbh (r ² = 0.848). Adjusted ANPP after accounting for apparent “mature forest bias” in our sampling method was 17.5 ± 1.2 Mg ha⁻¹ year⁻¹.Relating productivity measures to soil parameters showed that spatial patterning in productivity was significantly related to soil nutrients, especially phosphorus (P). Fine litterfall increased strongly with extractable P (r ² = 0.646), but reached an asymptote at moderate P levels, whereas biomass increment (r ² = 0.473) and ANPP (r ² = 0.603) increased linearly across the gradient. Biomass increment of large trees was more frequently and strongly related to nutrients than small trees, suggesting size dependency of tree growth on nutrients. Multiple linear regression confirmed the leading importance of soil P, and identified Ca as a potential co-limiting factor. Our findings strongly suggest that (1) soil nutrients, especially P, limit aboveground productivity in lowland Bornean forests, and (2) these forests play an important, but changing role in carbon cycles, as canopy tree logging alters these terrestrial carbon sinks. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Importance of disturbance history on net primary productivity in the world's most productive forests and implications for the global carbon cycle

Analysis of growth and biomass turnover in natural forests of Eucalyptus regnans, the world's tallest angiosperm, reveals it is also the world's most productive forest type, with fire disturbance an important mediator of net primary productivity (NPP). A comprehensive empirical database was used to calculate the averaged temporal pattern of NPP from regeneration to 250 years age. NPP peaks at 23.1 ± 3.8 (95% interquantile range) Mg C ha^?1 year^?1 at age 14 years, and declines gradually to about 9.2 ± 0.8 Mg C ha^?1 year^?1 at 130 years, with an average NPP over 250 years of 11.4 ± 1.1 Mg C ha^?1 year^?1, a value similar to the most productive temperate and tropical forests around the world. We then applied the age‐class distribution of E. regnans resulting from relatively recent historical fires to estimate current NPP for the forest estate. Values of NPP were 40% higher (13 Mg C ha^?1 year^?1) than if forests were assumed to be at maturity (9.2 Mg C ha^?1 year^?1). The empirically derived NPP time series for the E. regnans estate was then compared against predictions from 21 global circulation models, showing that none of them had the capacity to simulate a post‐disturbance peak in NPP, as found in E. regnans. The potential importance of disturbance impacts on NPP was further tested by applying a similar approach to the temperate forests of conterminous United States and of China. Allowing for the effects of disturbance, NPP summed across both regions was on average 11% (or 194 Tg C/year) greater than if all forests were assumed to be in a mature state. The results illustrate the importance of accounting for past disturbance history and growth stage when estimating forest primary productivity, with implications for carbon balance modelling at local to global scales.