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.     

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.     

Simulating root responses to grazing of a Mongolian grassland ecosystem

A new Sim-CYCLE grazing model has been obtained by combining a grazing model (Seligman et al. 1992, Ecol. Model. 60: 45–61) with the Sim-CYCLE model (Ito and Oikawa 2002, Ecol. Model. 151: 143–176). The new model has been validated against a set of field data obtained at Kherlen Bayaan-Ulaan (KBU) grassland. On the basis of the model, the root responses to grazing of KBU grassland have been studied under different conditions of stocking rates and precipitation. Model results indicate that both below-ground biomass (BB) and below-ground net primary production (BNPP) generally decrease with increasing stocking rate. However, if stocking rate is not higher than 0.7 sheep ha⁻¹, a sustainable state of the grassland ecosystem can be achieved after about 100 years, which suggests that the maximum sustainable stocking rate at KBU should be 0.7 sheep ha⁻¹. At the sustainable state, the maximum BB in a year is about 11 Mg DM ha⁻¹ under non-grazing condition, 5 Mg DM ha⁻¹ under 0.4 sheep ha⁻¹ stocking rate, and 4 Mg DM ha⁻¹ under 0.7 sheep ha⁻¹ stocking rate; the BNPP is 1.3 Mg DM ha⁻¹ year⁻¹ under non-grazing condition, and 0.6 Mg DM ha⁻¹ year⁻¹ under 0.4 sheep ha⁻¹ stocking rate, and 0.4 Mg DM ha⁻¹ year⁻¹ under 0.7 sheep ha⁻¹ stocking rate. Ratio of non-assimilation organ to assimilation organ (C/F) increases with increasing stocking rate. The C/F ratio is 10.99 under non-grazing conditions, and 12.11 under 0.7 sheep ha⁻¹ stocking rate. Root turnover rate decreases with increasing stocking rate. The rate is 12% each year under non-grazing conditions, and 11% each year under 0.7 sheep ha⁻¹ stocking rate. In addition, the effect of grazing on the grassland ecosystem under different scenarios of precipitation is also analyzed. Both BB and BNPP increase with increased precipitation, and vice versa. When precipitation is set to be 10% higher than the averaged from 1993 to 2002, the maximum sustainable stocking rate is 0.8 sheep ha⁻¹, and when the precipitation is set to be 15% lower than the averaged, the maximum sustainable stocking rate is 0.6 sheep ha⁻¹.     

Carbon, nitrogen and phosphorus in volcanic soils following afforestation with native birch (Betula pubescens) and introduced larch (Larix sibirica) in Iceland

Afforestation has become an important tool for soil protection and land reclamation in Iceland. Nevertheless, the harsh climate and degraded soils are growth-limiting for trees, and little is know about changes in soil nutrients in maturing forests planted on the volcanic soils. In the present chronosequence study, changes in C, N and total P in soil (0–10 and 10–20 cm depth) and C and N in foliar tissue were investigated in stands of native Downy birch (Betula pubescens Enrh.) and the in Iceland introduced Siberian larch (Larix sibirica Ledeb.). The forest stands were between 14 and 97 years old and were established on heath land that had been treeless for centuries. Soils were Andosols derived from basaltic material and rhyolitic volcanic ash. A significant effect of tree species was only found for the N content in foliar tissue. Foliar N concentrations were significantly higher and foliar C/N ratios significantly lower in larch needles than in birch leaves. There was no effect of stand age. Changes in soil C and the soil nutrient status with time after afforestation were little significant. Soil C concentrations in 0–10 cm depth in forest stands older than 30 years were significantly higher than in heath land and forest stands younger than 30 years. This was attributed to a slow accumulation of organic matter. Soil N concentrations and soil P_tot were not affected by stand age. Nutrient pools in the two soil layers were calculated for an average weight of soil material (400 Mg soil ha⁻¹ in 0–10 cm depth and 600 Mg soil ha⁻¹ in 10–20 cm depth, respectively). Soil nutrient pools did not change significantly with time. Soil C pools were in average 23.6 Mg ha⁻¹ in the upper soil layer and 16.9 Mg ha⁻¹ in the lower soil layer. The highest annual increase in soil C under forest compared to heath land was 0.23 Mg C ha⁻¹ year⁻¹ in 0–10 cm depth calculated for the 53-year-old larch stand. Soil N pools were in average 1.0 Mg N ha⁻¹ in both soil layers and did not decrease with time despite a low N deposition and the uptake and accumulation of N in biomass of the growing trees. Soil P_tot pools were in average 220 and 320 kg P ha⁻¹ in the upper and lower soil layer, respectively. It was assumed that mycorrhizal fungi present in the stands had an influence on the availability of N and P to the trees. Responsible Editor: Hans Lambers.     

The Imprint of Land-use History: Patterns of Carbon and Nitrogen in Downed Woody Debris at the Harvard Forest

Few data sets have characterized carbon (C) and nitrogen (N) pools in woody debris at sites where other aspects of C and N cycling are studied and histories of land use and disturbance are well documented. We quantified pools of mass, C, and N in fine and coarse woody debris (CWD) in two contrasting stands: a 73-year-old red pine plantation on abandoned agricultural land and a naturally regenerated deciduous forest that has experienced several disturbances in the past 150 years. Masses of downed woody debris amounted to 40.0 Mg ha⁻¹ in the coniferous stand and 26.9 Mg ha⁻¹ in the deciduous forest (20.4 and 13.8 Mg C ha⁻¹, respectively). Concentrations of N were higher and C:N ratios were lower in the deciduous forest compared to the coniferous. Pools of N amounted to 146 kg N ha⁻¹ in the coniferous stand and 155 kg N ha⁻¹ in the deciduous forest; both are larger than previously published pools of N in woody debris of temperate forests. Woody detritus buried in O horizons was minimal in these forests, contrary to previous findings in forests of New England. Differences in the patterns of mass, C, and N in size and decay classes of woody debris were related to stand histories. In the naturally regenerated deciduous forest, detritus was distributed across all size categories, and most CWD mass and N was present in the most advanced decay stages. In the coniferous plantation, nearly all of the CWD mass was present in the smallest size class (less than 25 cm diameter), and a recognizable cohort of decayed stems was evident from the stem-exclusion phase of this even-aged stand. These results indicate that heterogeneities in site histories should be explicitly included when biogeochemical process models are used to scale C and N stocks in woody debris to landscapes and regions. Received 27 April 2001; accepted 4 January 2002.     

Woody debris contribution to the carbon budget of selectively logged and maturing mid-latitude forests

Woody debris (WD) is an important component of forest C budgets, both as a C reservoir and source of CO₂ to the atmosphere. We used an infrared gas analyzer and closed dynamic chamber to measure CO₂ efflux from downed coarse WD (CWD; diameter≥7.5 cm) and fine WD (FWD; 7.5 cm>diameter≥2 cm) to assess respiration in a selectively logged forest and a maturing forest (control site) in the northeastern USA. We developed two linear regression models to predict WD respiration: one based on WD temperature, moisture, and size (R ²=0.57), and the other on decay class and air temperature (R ²=0.32). WD respiration (0.28±0.09 Mg C ha⁻¹ year⁻¹) contributed only ≈2% of total ecosystem respiration (12.3±0.7 Mg C ha⁻¹ year⁻¹, 1999–2003), but net C flux from CWD accounted for up to 30% of net ecosystem exchange in the maturing forest. C flux from CWD on the logged site increased modestly, from 0.61±0.29 Mg C ha⁻¹ year⁻¹ prior to logging to 0.77±0.23 Mg C ha⁻¹ year⁻¹ after logging, reflecting increased CWD stocks. FWD biomass and associated respiration flux were ≈7 times and ≈5 times greater, respectively, in the logged site than the control site. The net C flux associated with CWD, including inputs and respiratory outputs, was 0.35±0.19 Mg C ha⁻¹ year⁻¹ (net C sink) in the control site and −0.30±0.30 Mg C ha⁻¹ year⁻¹ (net C source) in the logged site. We infer that accumulation of WD may represent a small net C sink in maturing northern hardwood forests. Disturbance, such as selective logging, can enlarge the WD pool, increasing the net C flux from the WD pool to the atmosphere and potentially causing it to become a net C source.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Land-use conversion effects on CO2 emissions: from agricultural to hybrid poplar plantation

Land-use changes such as deforestation have been considered one of the main contributors to increased greenhouse gas emissions, while verifiable C sequestration through afforestation projects is eligible to receive C credits under the Kyoto Protocol. We studied the short-term effects on CO₂ emissions of converting agricultural land-use (planted to barley) to a hybrid poplar (Populus deltoids × Populus × petrowskyana var. Walker) plantation in the Parkland region in northern Alberta, where large areas are being planted to hybrid poplars. CO₂ emissions were measured using a static gas chamber method. No differences were found in soil temperature, volumetric moisture content, or soil respiration rates between the barley and Walker plots. The mean soil respiration rate in 2005 was 1.83 ± 0.19 (mean ± 1 SE) and 1.89 ± 0.13 μmol CO₂ m⁻² s⁻¹ in the barley and Walker plots, respectively. However, biomass production was higher in the barley plots, indicating that the agricultural land-use system had a greater ability to fix atmospheric CO₂. The C balance in the land-use systems were estimated to be a small net gain (before considering straw and grain removal through harvesting) of 0.03 ± 0.187 Mg C ha⁻¹ year⁻¹ in the barley plots and a net loss of 3.35 ± 0.080 Mg C ha⁻¹ year⁻¹ from the Walker poplar plots. Over the long-term, we expect the hybrid poplar plantation to become a net C sink as the trees grow bigger and net primary productivity increases.     

Cotton–cowpea intercropping and its N2 fixation capacity improves yield of a subsequent maize crop under Zimbabwean rain-fed conditions

Intercropping cotton (Gossypium hirsutum L.) and cowpea (Vigna unguiculata (L.) Walp) is one of the ways to improve food security and soil fertility whilst generating cash income of the rural poor. A study was carried out to find out the effect of cotton–cowpea intercropping on cowpea N₂-fixation capacity, nitrogen balance and yield of a subsequent maize crop. Results showed that cowpea suppressed cotton yields but the reduction in yield was compensated for by cowpea grain yield. Cowpea grain yield was significantly different across treatments and the yields were as follows: sole cowpea (1.6 Mg ha⁻¹), 1:1 intercrop (1.1 Mg ha⁻¹), and 2:1 intercrop (0.7 Mg ha⁻¹). Cotton lint yield was also significantly different across treatments and was sole cotton (2.5 Mg ha⁻¹), 1:1 intercrop (0.9 Mg ha⁻¹) and 2:1 intercrop (1.5 Mg ha⁻¹). Intercropping cotton and cowpea increased the productivity with land equivalence ratios (LER) of 1.4 and 1.3 for 1:1 and 2:1 intercrop treatments, respectively. There was an increase in percentage of N fixation (%Ndfa) by cowpea in intercrops as compared to sole crops though the absolute amount fixed (Ndfa) was lower due to reduced plant population. Sole cowpea had %Ndfa of 73%, 1:1 intercrop had 85% and 2:1 intercrop had 77% while Ndfa was 138 kg ha⁻¹ for sole cowpea, 128 kg ha⁻¹ for 1:1 intercrop and 68 kg ha⁻¹ for 2:1 intercrop and these were significantly different. Sole cowpea and the intercrops all showed positive N balances of 92 kg ha⁻¹ for sole cowpea and 1:1 intercrop, and 48 kg ha⁻¹ for 2:1 intercrop. Cowpea fixed N transferred to the companion cotton crop was very low with 1:1 intercrop recording 3.5 kg N ha⁻¹ and 2:1 intercrop recording 0.5 kg N ha⁻¹. Crop residues from intercrops and sole cowpea increased maize yields more than residues from sole cotton. Maize grain yield was, after sole cotton (1.4 Mg ha⁻¹), sole cowpea (4.6 Mg ha⁻¹), 1:1 intercrops (4.4 Mg ha⁻¹) and 2:1 intercrops (3.9 Mg ha⁻¹) and these were significantly different from each other. The LER, crop yields, %N fixation and, N balance and residual fertility showed that cotton–cowpea intercropping could be a potentially productive system that can easily fit into the current smallholder farming systems under rain-fed conditions. The fertilizer equivalency values show that substantial benefits do accrue and effort should be directed at maximizing the dry matter yield of the legume in the intercrop system while maintaining or improving the economic yield of the companion cash crop.     

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.     

Net anthropogenic phosphorus inputs: spatial and temporal variability in the Chesapeake Bay region

We estimated net anthropogenic phosphorus inputs (NAPI) in the Chesapeake Bay region. NAPI is an index of phosphorus pollution potential. NAPI was estimated by quantifying all phosphorus inputs and outputs for each county. Inputs include fertilizer applications and non-food phosphorus uses, while trade of food and feed can be an input or an output. The average of 1987, 1992, 1997, and 2002 NAPI for individual counties ranged from 0.02 to 78.46 kg P ha⁻¹ year⁻¹. The overall area-weighted average NAPI for 266 counties in the region was 4.52 kg P ha⁻¹ year⁻¹, indicating a positive net phosphorus input that can accumulate in the landscape or can pollute the water. Large positive NAPI values were associated with agricultural and developed land cover. County area-weighted NAPI increased from 4.43 to 4.94 kg P ha⁻¹ year⁻¹ between 1987 and 1997 but decreased slightly to 4.86 kg P ha⁻¹ year⁻¹ by 2002. Human population density, livestock unit density, and percent row crop land combined to explain 83% of the variability in NAPI among counties. Around 10% of total NAPI entering the Chesapeake Bay watershed is discharged into Chesapeake Bay. The developed land component of NAPI had a strong direct correlation with measured phosphorus discharges from major rivers draining to the Bay (R ² = 0.81), however, the correlation with the simple percentage of developed land was equally strong. Our results help identify the sources of P in the landscape and evaluate the utility of NAPI as a predictor of water quality.     

Nutrient transfer by leaf litterfall during a sugar maple decline episode at Lake Clair watershed,Québec,Canada

In a declining sugar maple (SM) stand, we tested the hypothesis that an increasing relative abundance of American beech (AB) and yellow birch (YB) would improve litter quality by providing a higher proportion of litterfall richer in base cations and lower in acidity. From 1989 to 2006, SM leaf fall diminished from 59% (1,718 kg ha⁻¹ year⁻¹) to 36% (915 kg ha⁻¹ year⁻¹) of the total leaf fall biomass. Overall, the increase in AB and YB litterfall compensated for the SM decrease, resulting in constant annual leaf litterfall fluxes (2,803 kg ha⁻¹ year⁻¹) over the period studied. However, because the leaf litter for AB and YB had Ca and Mg concentrations 2–3 times higher than did SM, Ca and Mg concentrations and fluxes in leaf litterfall significantly increased between 1989 and 2006. Leaf litterfall of AB and YB also has a higher base/acid ratio than SM. Consequently, changes in forest composition following SM decline led to a clear improvement in litterfall quality in terms of base cations content and fluxes and acid–base properties.     

The change of soil carbon stocks and fine root dynamics after land use change from a native pasture to a pine plantation

A published meta-analysis of worldwide data showed soil carbon decreasing following land use change from pasture to conifer plantation. A paired site (a native pasture with Themeda triandra dominant, and an adjacent Pinus radiata plantation planted onto the pasture 16 years ago) was set up as a case study to assess the soil carbon reduction and the possible reason for the reduction under pine, including the change in fine root (diameter <2 mm) dynamics (production and mortality). Soil analysis confirmed that soil carbon and nitrogen stocks to 100 cm under the plantation were significantly less than under the pasture by 20 and 15%, respectively. A 36% greater mass of fine root was found in the soil under the pasture than under the plantation and the length of fine root was about nine times greater in the pasture. Much less fine root length was produced and roots died more slowly under the plantation than under the pasture based on observations of fine root dynamics in minirhizotrons. The annual inputs of fine root litter to the top 100 cm soil, estimated from soil coring and minirhizotron observations, were 6.3 Mg dry matter ha⁻¹ year⁻¹ (containing 2.7 Mg C and 38.9 kg N) under the plantation, and 9.7 Mg ha⁻¹ year⁻¹ (containing 3.6 Mg C and 81.4 kg N) under the pasture. The reduced amount of carbon, following afforestation of the pasture, in each depth-layer of the soil profile correlated with the lower length of dead fine roots in the layer under the plantation compared with the pasture. This correlation was consistent with the hypothesis that the soil carbon reduction after land use change from pasture to conifer plantation might be related to change of fine root dynamics, at least in part.     

Measurements and Modeling of Carbon and Nitrogen Cycling in Agroecosystems of Southern Wisconsin: Potential for SOC Sequestration during the Next 50 Years

Landmanagement practices such as no-tillage agriculture and tallgrass prairie restoration have been proposed as a possible means to sequester atmospheric carbon, helping to refurbish soil fertility and replenish organic matter lost as a result of previous agricultural management practices. However, the relationship between land-use changes and ecosystem structure and functioning is not yet understood. We studied soil and vegetation properties over a 4-year period (1995–98), and assembled measurements of microbial biomass, soil organic carbon (SOC) and nitrogen (N), N-mineralization, soil surface carbon dioxide (CO₂) flux, and leached C and N in managed (maize; Zea mays L.) and natural (prairie) ecosystems near the University of Wisconsin Agricultural Research Station at Arlington. Field data show that different management practices (tillage and fertilization) and ecosystem type (prairie vs maize) have a profound influence on biogeochemistry and water budgets between sites. These measurements were used in conjunction with a dynamic terrestrial ecosystem model, called IBIS (the Integrated Biosphere Simulator), to examine the long-term effects of land-use changes on biogeochemical cycling. Field data and modeling suggest that agricultural land management near Arlington between 1860 and 1950 caused SOC to be depleted by as much as 63% (native SOC approximately 25.1 kg C m⁻²). Reductions in N-mineralization and microbial biomass were also observed. Although IBIS simulations depict SOC recovery in no-tillage maize since the 1950s and also in the Arlington prairie since its restoration was initiated in 1976, field data suggest otherwise for the prairie. This restoration appears to have done little to increase SOC over the past 24 years. Measurements show that this prairie contained between 28% and 42% less SOC (in the top 1 m) than the no-tillage maize plots and 40%–47% less than simulated potential SOC for the site in 1999. Because IBIS simulates competition between C3 and C4 grass species, we hypothesized that current restored prairies, which include many forbs not characterized by the model, could be less capable of sequestering C than agricultural land planted entirely in monocultural grass in this region. Model output and field measurements show a potential 0.4 kg C m⁻² y⁻¹ difference in prairie net primary production (NPP). This study indicates that high-productivity C4 grasslands (NPP = 0.63 kg C m⁻² y⁻¹) and high-yield maize agroecosystems (10 Mg ha⁻¹) have the potential to sequester C at a rate of 74.5 g C m⁻² y⁻¹ and 86.3 g C m⁻² y⁻¹, respectively, during the next 50 years across southern Wisconsin. Received 28 December 1999; accepted 11 December 2000.     

Altitudinal changes in carbon storage of temperate forests on Mt Changbai, Northeast China

A number of studies have investigated regional and continental scale patterns of carbon (C) stocks in forest ecosystems; however, the altitudinal changes in C storage in different components (vegetation, detritus, and soil) of forest ecosystems remain poorly understood. In this study, we measured C stocks of vegetation, detritus, and soil of 22 forest plots along an altitudinal gradient of 700–2,000 m to quantify altitudinal changes in carbon storage of major forest ecosystems (Pinus koraiensis and broadleaf mixed forest, 700–1,100 m; Picea and Abies forest, 1,100–1,800 m; and Betula ermanii forest, 1,800–2,000 m) on Mt Changbai, Northeast China. Total ecosystem C density (carbon stock per hectare) averaged 237 t C ha⁻¹ (ranging from 112 to 338 t C ha⁻¹) across all the forest stands, of which 153 t C ha⁻¹ (52–245 t C ha⁻¹) was stored in vegetation biomass, 14 t C ha⁻¹ (2.2–48 t C ha⁻¹) in forest detritus (including standing dead trees, fallen trees, and floor material), and 70 t C ha⁻¹ (35–113 t C ha⁻¹) in soil organic matter (1-m depth). Among all the forest types, the lowest vegetation and total C density but the highest soil organic carbon (SOC) density occurred in Betula ermanii forest, whereas the highest detritus C density was observed in Picea and Abies forest. The C density of the three ecosystem components showed distinct altitudinal patterns: with increasing altitude, vegetation C density decreased significantly, detritus C density first increased and then decreased, and SOC density exhibited increasing but insignificant trends. The allocation of total ecosystem C to each component exhibited similar but more significant trends along the altitudinal gradient. Our results suggest that carbon storage and partitioning among different components in temperate forests on Mt Changbai vary greatly with forest type and altitude.     

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.     

Decomposition and Fragmentation of Coarse Woody Debris: Re-visiting a Boreal Black Spruce Chronosequence

We re-visited a seven-stand boreal chronosequence west of Thompson, Manitoba, Canada, in which coarse woody debris (CWD) and its instantaneous decomposition were measured in 2000. New CWD measurements were performed in 2007, and tree inventories updated to provide mortality and snag failure data. These data were used to model CWD changes, compare methods of estimating decomposition, and infer possible fragmentation rates. Measured CWD was between 9.7 (in both the 77- and 43-year-old stands) and 80.4 (in the 18-year-old stand) Mg ha⁻¹ in 2007. Spatial variability was high; at most stands CWD levels had not changed significantly from 2000 to 2007. Tree mortality was a significant flux only in older stands, whereas snag fall rate varied by an order of magnitude, from 2.9% y⁻¹ (0.2 Mg ha⁻¹ y⁻¹) in the 9-year-old stand to 9.8% y⁻¹ (2.3 Mg ha⁻¹ y⁻¹) in the 12-year-old stand. A one-pool model based on these inputs underestimated actual 2000–2007 CWD decomposition in the younger stands, suggesting that fragmentation could be an important part of the carbon flux exiting the CWD pool. We compared three independent measures of annual decomposition (k): direct measurements of CWD respiration, rates based on the 7-year re-sampling effort described here, and rates inferred from the chronosequence design itself. Mean k values arrived at via these techniques were 0.06 ± 0.03, 0.05 ± 0.04, and 0.05 ± 0.05 y⁻¹, respectively. The four-pool model suggested that the transition rate between decay classes was 0.14–0.19 y⁻¹; the model was most sensitive to initial CWD values. Although the computed k values implied a problem with chronosequence site selection for at least one site, the overall CWD trend was consistent with a larger number of sites surveyed in the region.     

Impact of long-term nitrogen addition on carbon stocks in trees and soils in northern Europe

The aim of this study was to quantify the effects of fertiliser N on C stocks in trees (stems, stumps, branches, needles, and coarse roots) and soils (organic layer +0–10 cm mineral soil) by analysing data from 15 long-term (14–30 years) experiments in Picea abies and Pinus sylvestris stands in Sweden and Finland. Low application rates (30–50 kg N ha⁻¹ year⁻¹) were always more efficient per unit of N than high application rates (50–200 kg N ha⁻¹ year⁻¹). Addition of a cumulative amount of N of 600–1800 kg N ha⁻¹ resulted in a mean increase in tree and soil C stock of 25 and 11 kg (C sequestered) kg⁻¹ (N added) (“N-use efficiency”), respectively. The corresponding estimates for NPK addition were 38 and 11 kg (C) kg⁻¹ (N). N-use efficiency for C sequestration in trees strongly depended on soil N status and increased from close to zero at C/N 25 in the humus layer up to 40 kg (C) kg⁻¹ (N) at C/N 35 and decreased again to about 20 kg (C) kg⁻¹ (N) at C/N 50 when N only was added. In contrast, addition of NPK resulted in high (40–50 kg (C) kg⁻¹ (N)) N-use efficiency also at N-rich (C/N 25) sites. The great difference in N-use efficiency between addition of NPK and N at N-rich sites reflects a limitation of P and K for tree growth at these sites. N-use efficiency for soil organic carbon (SOC) sequestration was, on average, 3–4 times lower than for tree C sequestration. However, SOC sequestration was about twice as high at P. abies as at P. sylvestris sites and averaged 13 and 7 kg (C) kg⁻¹ (N), respectively. The strong relation between N-use efficiency and humus C/N ratio was used to evaluate the impact of N deposition on C sequestration. The data imply that the 10 kg N ha⁻¹ year⁻¹ higher deposition in southern Sweden than in northern Sweden for a whole century should have resulted in 2.0 ± 1.0 (95% confidence interval) kg m⁻² more tree C and 1.3 ± 0.5 kg m⁻² more SOC at P. abies sites in the south than in the north for a 100-year period. These estimates are consistent with differences between south and north in tree C and SOC found by other studies, and 70–80% of the difference in SOC can be explained by different N deposition.     

Coarse woody debris mass and nutrients in forest ecosystems of Korea

Coarse woody debris (CWD) is an essential component of forests. However, quantification of both the mass and nutrient content of CWD within a given environment tends to be a fairly labor-intensive proposition that requires long-term studies to be conducted for viable data to be obtained. As a result, various aspects of CWD in forest ecosystems remain somewhat poorly understood. In this review, we have compiled all available estimates of CWD mass and nutrients from both coniferous and deciduous forests in Korea. The CWD mass data varied substantially by forest type, age, location, and sampling time, ranging from 1.5 to 24.5 Mg ha⁻¹, and for the amount (kg ha⁻¹) of nutrients in the CWD, ranging from 3.5 to 23.6 for nitrogen (N), 0.8 to 4.7 for phosphorus (P), 3.9 to 13.3 for potassium (K), 25.9 to 30.9 for calcium (Ca), 1.4 to 4.2 for magnesium (Mg), and 0.1 to 0.6 for sodium (Na). The mass of CWD transferred from live trees to the forest floor ranged between 0.1 and 4.9 Mg ha⁻¹ year⁻¹, and these values were roughly equivalent to 26–42% of the annual litterfall inputs (2.5–10.8 Mg ha⁻¹ year⁻¹) for mixed Quercus spp. forests within the relevant region. Annual nutrients inputs (kg ha⁻¹ year⁻¹) through CWD decomposition were 0.7–1.6 for N, 0.04–0.3 for P, 0.3–1.0 for K, 1.7–3.1 for Ca, and 0.1–0.3 for Mg. Consequently, these results revealed that the ecological value of CWD for C and nutrient cycling was relatively insignificant. However, only a limited number of studies have been conducted on CWD in different coniferous or mixed deciduous forests in the region. As a direct result of this paucity of data, further long-term studies on CWD mass and nutrients in a variety of forest types are required in order to be able to evaluate accurately the ecological value of CWD on biodiversity and physical properties in Korean forest ecosystems.     

Litterfall dynamics in carbonate and deltaic mangrove ecosystems in the Gulf of Mexico

From 1996 to 2002, we measured litterfall, standing litter crop, and litter turnover rates in scrub, basin, fringe and riverine forests in two contrasting mangrove ecosystems: a carbonate-dominated system in the Southeastern Everglades and a terrigenous-dominated system in Laguna de Terminos (LT), Mexico. We hypothesized that litter dynamics is driven by latitude, geomorphology, hydrology, soil fertility and soil salinity stress. There were significant temporal patterns in LT with litterfall rates higher during the rainy season (2.4 g m⁻² day⁻¹) than during the dry season (1.8 g m⁻² day⁻¹). Total annual litterfall was significantly higher in the riverine forest (12.8 Mg ha⁻² year⁻¹) than in the fringe and basin forests (9.7 and 5.2 Mg ha⁻² year⁻¹, respectively). In Southeastern Everglades, total annual litterfall was also significantly higher during the rainy season than during the dry season. Spatially, the scrub forest had the lowest annual litterfall (2.5 Mg ha⁻² year⁻¹), while the fringe and basin had the highest (9.1 and 6.5 Mg ha⁻² year⁻¹, respectively). In LT, annual standing litter crop was 3.3 Mg ha⁻¹ in the fringe and 2.2 Mg ha⁻¹ in the basin. Litter turnover rates were significantly higher in the fringe mangrove forest (4.1 year⁻¹) relative to the basin forests (2.2 year⁻¹). At Southeastern Everglades there were significant differences in annual standing litter crop: 1.9, 3.3 and 4.5 Mg ha⁻¹ at scrub, basin and fringe mangrove sites, respectively. Furthermore, turnover rates were similar at both basin and fringe mangrove types (2.1 and 2.0 year⁻¹, respectively) but significantly higher than scrub mangrove forest (1.3 year⁻¹). These findings suggest that litter export is important in regulating litter turnover rates in frequently flooded riverine and fringe forests, while in infrequently flooded basin forests, in situ litter decomposition controls litter turnover rates.