The Importance of Termites to the CH<Subscript>4</Subscript> Balance of a Tropical Savanna Woodland of Northern Australia

Termites produce methane (CH₄) as a by-product of microbial metabolism of food in their hindguts, and are one of the most uncertain components of the regional and global CH₄ exchange estimates. This study was conducted at Howard Springs near Darwin, and presents the first estimate of CH₄ emissions from termites based on replicated in situ seasonal flux measurements in Australian savannas. Using measured fluxes of CH₄ between termite mounds and the atmosphere, and between soil and the atmosphere across seasons we determined net CH₄ flux within a tropical savanna woodland of northern Australia. By accounting for both mound-building and subterranean termite colony types, and estimating the contribution from tree-dwelling colonies it was calculated that termites were a CH₄ source of +0.24 kg CH₄-C ha⁻¹ y⁻¹ and soils were a CH₄ sink of −1.14 kg CH₄-C ha⁻¹ y⁻¹. Termites offset 21% of CH₄ consumed by soil resulting in net sink strength of −0.90 kg CH₄-C ha⁻¹ y⁻¹ for these savannas. For Microcerotermes nervosus (Hill), the most abundant mound-building termite species at this site, mound basal area explained 48% of the variation in mound CH₄ flux. CH₄ emissions from termites offset 0.1% of the net biome productivity (NBP) and CH₄ consumption by soil adds 0.5% to the NBP of these tropical savannas at Howard Springs.     

Andropogon gayanus (Gamba Grass) Invasion Increases Fire-mediated Nitrogen Losses in the Tropical Savannas of Northern Australia

Invasive alien grasses can substantially alter fuel loads and fire regimes which could have significant consequences for fire-mediated nutrient losses. The effects of the alien grass Andropogon gayanus Kunth. (Gamba grass) on fire-mediated nutrient losses was evaluated in Australia’s tropical savannas. Losses of macronutrients during fire were determined by comparing the nutrient pools contained in the fine fuel before fire and in the ash after fire. Pre-fire grass nutrient pools were significantly higher in A. gayanus plots than in native grass plots for all nutrients measured (N, P, K, S, Ca, and Mg). Nutrient losses were substantially higher in A. gayanus plots, with 113% higher losses for N, 80% for P, 56% for K, 63 for S, 355% for Ca, and 345% for Mg. However, only losses of N and Mg varied significantly between grass types. A simplified savanna ecosystem nutrient budget estimated that A. gayanus fires led to the net N loss of 20 kg ha⁻¹ y⁻¹. This is a conservative estimate because total fuel loads were relatively low (7.85 t ha⁻¹) for A. gayanus invaded plots leading to a relatively moderate intensity fire (6,408 kW m⁻¹). Higher A. gayanus fuel loads and fire intensities could potentially lead to losses of up to 61.5 kg N ha⁻¹ from the grass fuel. Over the long term, this is likely to lead to depletion of soil nutrients, particularly N, in the already low-fertility tropical savanna soils.     

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-Atmosphere Exchange of N<Subscript>2</Subscript>O and NO in Near-Natural Savanna and Agricultural Land in Burkina Faso (W. Africa)

In a combined field and laboratory study in the southwest of Burkina Faso, we quantified soil-atmosphere N₂O and NO exchange. N₂O emissions were measured during two field campaigns throughout the growing seasons 2005 and 2006 at five different experimental sites, that is, a natural savanna site and four agricultural sites planted with sorghum (n = 2), cotton and peanut. The agricultural fields were not irrigated and not fertilized. Although N₂O exchange mostly fluctuated between −2 and 8 μg N₂O–N m⁻² h⁻¹, peak N₂O emissions of 10–35 μg N₂O–N m⁻² h⁻¹ during the second half of June 2005, and up to 150 μg N₂O–N m⁻² h⁻¹ at the onset of the rainy season 2006, were observed at the native savanna site, whereas the effect of the first rain event on N₂O emissions at the crop sites was low or even not detectable. Additionally, a fertilizer experiment was conducted at a sorghum field that was divided into three plots receiving different amounts of N fertilizer (plot A: 140 kg N ha⁻¹; plot B: 52.5 kg N ha⁻¹; plot C: control). During the first 3 weeks after fertilization, only a minor increase in N₂O emissions at the two fertilized plots was detected. After 24 days, however, N₂O emission rates increased exponentially at plot A up to a mean of 80 μg N₂O–N m⁻² h⁻¹, whereas daily mean values at plot B reached only 19 μg N₂O–N m⁻² h⁻¹, whereas N₂O flux rates at plot C remained unchanged. The calculated annual N₂O emission of the nature reserve site amounted to 0.52 kg N₂O–N ha⁻¹ a⁻¹ in 2005 and to 0.67 kg N₂O–N ha⁻¹ a⁻¹ in 2006, whereas the calculated average annual N₂O release of the crop sites was only 0.19 kg N₂O–N ha⁻¹ a⁻¹ and 0.20 kg N₂O–N ha⁻¹ a⁻¹ in 2005 and 2006, respectively. In a laboratory study, potential N₂O and NO formation under different soil moisture regimes were determined. Single wetting of dry soil to medium soil water content with subsequent drying caused the highest increase in N₂O and NO emissions with maximum fluxes occurring 1 day after wetting. The stimulating effect lasted for 3–4 days. A weaker stimulation of N₂O and NO fluxes was detected during daily wetting of soil to medium water content, whereas no significant stimulating effect of single or daily wetting to high soil water content (>67% WHC_max) was observed. This study demonstrates that the impact of land-use change in West African savanna on N trace gas emissions is smaller—with the caveat that there could have been potentially higher N₂O and NO emissions during the initial conversion—than the effect of timing and distribution of rainfall and of the likely increase in nitrogen fertilization in the future.     

Stocks and dynamics of carbon in trees across a rainfall gradient in a tropical savanna

In this study, systematic variation in tree morphology across a rainfall gradient in Australia's tropical savanna biome and its implications for carbon stocks and dynamics were quantified. The aim was to support efforts to manage fire regimes to increase vegetative carbon stocks as a greenhouse gas mitigation strategy. The height of trees for a given trunk diameter declines with decreasing rainfall from 2000 to 300 mm and increasing dry season length across the Australian savanna biome. It is likely that increasing dry season length is the main driver of this decline rather declining rainfall per se. By taking account of the response of total basal area to rainfall and soil type, stand structure, and tree height and diameter relationships, the carbon stocks in live trees were estimated to decline from about 34 t ha^?1 in the wetter savannas to 6 t ha^?1 in the drier savannas. These values are broadly consistent with field‐based estimates. Because of the declining ratio of height to trunk diameter, trees of a given diameter in drier regions will be more likely to be killed by fires of a given intensity than trees in wetter regions. Thus single fires of given intensity are likely to have a greater proportionate impact on live tree carbon stock in drier savannas, but a much greater absolute impact in wetter savannas due to the greater total carbon stock. Projected decreases in early wet season rainfall under climate change scenarios, despite projections of little change in total precipitation in northern Australia, may lead to decreased carbon stock in live trees through two mechanisms: a reduction in total basal area and decreases in tree height for given trunk diameters.     

Nitrogen deposition in tropical forests from savanna and deforestation fires

We used satellite‐derived estimates of global fire emissions and a chemical transport model to estimate atmospheric nitrogen (N) fluxes from savanna and deforestation fires in tropical ecosystems. N emissions and reactive N deposition led to a net transport of N equatorward, from savannas and areas undergoing deforestation to tropical forests. Deposition of fire‐emitted N in savannas was only 26% of emissions – indicating a net export from this biome. On average, net N loss from fires (the sum of emissions and deposition) was equivalent to approximately 22% of biological N fixation (BNF) in savannas (4.0 kg N ha^?1 yr^?1) and 38% of BNF in ecosystems at the deforestation frontier (9.3 kg N ha^?1 yr^?1). Net N gains from fires occurred in interior tropical forests at a rate equivalent to 3% of their BNF (0.8 kg N ha^?1 yr^?1). This percentage was highest for African tropical forests in the Congo Basin (15%; 3.4 kg N ha^?1 yr^?1) owing to equatorward transport from frequently burning savannas north and south of the basin. These results provide evidence for cross‐biome atmospheric fluxes of N that may help to sustain productivity in some tropical forest ecosystems on millennial timescales. Anthropogenic fires associated with slash and burn agriculture and deforestation in the southern part of the Amazon Basin and across Southeast Asia have substantially increased N deposition in these regions in recent decades and may contribute to increased rates of carbon accumulation in secondary forests and other N‐limited ecosystems.     

Variable carbon losses from recurrent fires in drained tropical peatlands

Tropical peatland fires play a significant role in the context of global warming through emissions of substantial amounts of greenhouse gases. However, the state of knowledge on carbon loss from these fires is still poorly developed with few studies reporting the associated mass of peat consumed. Furthermore, spatial and temporal variations in burn depth have not been previously quantified. This study presents the first spatially explicit investigation of fire‐driven tropical peat loss and its variability. An extensive airborne Light Detection and Ranging data set was used to develop a prefire peat surface modelling methodology, enabling the spatially differentiated quantification of burned area depth over the entire burned area. We observe a strong interdependence between burned area depth, fire frequency and distance to drainage canals. For the first time, we show that relative burned area depth decreases over the first four fire events and is constant thereafter. Based on our results, we revise existing peat and carbon loss estimates for recurrent fires in drained tropical peatlands. We suggest values for the dry mass of peat fuel consumed that are 206 t ha⁻¹ for initial fires, reducing to 115 t ha⁻¹ for second, 69 t ha⁻¹ for third and 23 t ha⁻¹ for successive fires, which are 58–7% of the current IPCC Tier 1 default value for all fires. In our study area, this results in carbon losses of 114, 64, 38 and 13 t C ha⁻¹ for first to fourth fires, respectively. Furthermore, we show that with increasing proximity to drainage canals both burned area depth and the probability of recurrent fires increase and present equations explaining burned area depth as a function of distance to drainage canal. This improved knowledge enables a more accurate approach to emissions accounting and will support IPCC Tier 2 reporting of fire emissions.     

Frequent fires reduce tree growth in northern Australian savannas: implications for tree demography and carbon sequestration

Tropical savannas are typically highly productive yet fire‐prone ecosystems, and it has been suggested that reducing fire frequency in savannas could substantially increase the size of the global carbon sink. However, the long‐term demographic consequences of modifying fire regimes in savannas are difficult to predict, with the effects of fire on many parameters, such as tree growth rates, poorly understood. Over 10 years, we examined the effects of fire frequency on the growth rates (annual increment of diameter at breast height) of 3075 tagged trees, at 137 locations throughout the mesic savannas of Kakadu, Nitmiluk and Litchfield National Parks, in northern Australia. Frequent fires substantially reduced tree growth rates, with the magnitude of the effect markedly increasing with fire severity. The highest observed frequencies of mild, moderate and severe fires (1.0, 0.8 and 0.4 fires yr^?1, respectively) reduced tree growth by 24%, 40% and 66% respectively, relative to unburnt areas. These reductions in tree growth imply reductions in the net primary productivity of trees by between 0.19 t C ha^?1 yr^?1, in the case of mild fires, and 0.51 t C ha^?1 yr^?1, in the case of severe fires. Such reductions are relatively large, given that net biome productivity (carbon sequestration potential) of these savannas is estimated to be just 1–2 t C ha^?1 yr^?1. Our results suggest that current models of savanna tree demography, that do not account for a relationship between severe fire frequency and tree growth rate, are likely to underestimate the long‐term negative effects of frequent severe fires on tree populations. Additionally, the negative impact of frequent severe fires on carbon sequestration rates may have been underestimated; reducing fire frequencies in savannas may increase carbon sequestration to a greater extent than previously thought.     

Changes in soil carbon flux and carbon stock over a rotation of poplar plantations in northwest China

Forest soil is a major component of terrestrial ecosystems for carbon sequestration and plays an important role in the global carbon cycle. Soil carbon flux and soil carbon pools were investigated in a poplar plantation chronosequence over a rotation in northwest China. Based on continuous field observation in 2007, the results showed that mean soil CO₂ efflux rate was 5.54, 4.81, and 3.93 μmol CO₂ m⁻² s⁻¹ for stands of 2-, 8-, and 15-year-old, respectively, during the growing season. Significant differences in soil respiration of three age classes were mainly because soil temperature, carbon allocation, and fine root growth changed greatly with stand age. Multiple regression analysis suggested that soil temperature and fine root biomass in the upper layer could explain 78–85% of the variation in soil respiration. Mineral soil C stock at 0–40 cm depth was 55.77, 55.09, and 58.14 t ha⁻¹ in the 2-, 8-, and 15-year-old stands, respectively. The average rate of soil C sequestration was 0.13 t ha⁻¹ year⁻¹ following afforestation on former crop lands. Although the plantations had similar management practices and soil types since their establishment, many biotic and abiotic factors such as root biomass and turnover rate, soil condition of the plantations had undergone marked changes at different development stages, which could result in the remarkable differences in soil carbon flux and storage over a rotation. Our results highlight the importance of the development stage within a rotation of poplar plantation in assessment of soil carbon budget.     

Nitrous Oxide Emission from Grazed Grassland Under Different Management Systems

Nitrous oxide (N₂O) emissions from grazed grasslands are estimated to be approximately 28% of global anthropogenic N₂O emissions. Estimating the N₂O flux from grassland soils is difficult because of its episodic nature. This study aimed to quantify the N₂O emissions, the annual N₂O flux and the emission factor (EF), and also to investigate the influence of environmental and soil variables controlling N₂O emissions from grazed grassland. Nitrous oxide emissions were measured using static chambers at eight different grasslands in the South of Ireland from September 2007 to August 2009. The instantaneous N₂O flux values ranged from -186 to 885.6 μg N₂O-N m⁻² h⁻¹ and the annual sum ranged from 2 ± 3.51 to 12.55 ± 2.83 kg N₂O-N ha⁻¹ y⁻¹ for managed sites. The emission factor ranged from 1.3 to 3.4%. The overall EF of 1.81% is about 69% higher than the Intergovernmental Panel on Climate Change (IPCC) default EF value of 1.25% which is currently used by the Irish Environmental Protection Agency (EPA) to estimate N₂O emission in Ireland. At an N applied of approximately 300 kg ha⁻¹ y⁻¹, the N₂O emissions are approximately 5.0 kg N₂O-N ha⁻¹ y⁻¹, whereas the N₂O emissions double to approximately 10 kg N ha⁻¹ for an N applied of 400 kg N ha⁻¹ y⁻¹. The sites with higher fluxes were associated with intensive N-input and frequent cattle grazing. The N₂O flux at 17°C was five times greater than that at 5°C. Similarly, the N₂O emissions increased with increasing water filled pore space (WFPS) with maximum N₂O emissions occurring at 60–80% WFPS. We conclude that N application below 300 kg ha⁻¹ y⁻¹ and restricted grazing on seasonally wet soils will reduce N₂O emissions.     

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.     

Effects of Fire on Seedling Diversity and Plant Reproduction (Sexual vs. Vegetative) in Neotropical Savannas Differing in Tree Density

Little is known about the effects of fire on the structure and species composition of Neotropical savanna seedling communities. Such effects are critical for predicting long‐term changes in plant distribution patterns in these ecosystems. We quantified richness and density of seedlings within 144 plots of 1 m² located along a topographic gradient in long‐unburned (fire protected since 1983) and recently burned (September 2005) savannas in Brazil. These savannas differ in tree density and canopy cover. Sites along the gradient, however, did not differ in species composition prior to the fire. In recently burned savannas we also evaluated the contribution of vegetative reproduction relative to sexual reproduction by quantifying richness and density of root suckers. Finally, we tested seed tolerance to pulses of high temperatures—similar to those occurring during fires on the soil surface and below—of five dominant savanna tree species. Seedlings were more abundant and diverse in unburned than in burned savannas. Seedling species composition differed among unburned and burned savannas probably reflecting early differences in root: shoot biomass allocation patterns. In recently burned savannas, root suckers were more abundant and diverse than seedlings. Relatively long exposures (>10 min) of temperatures of 90 °C reduced seed germination in all studied species suggesting a negative effect of fire on germination of seeds located at or aboveground level. Because vegetative reproduction contributes more than sexual reproduction in burned environments, frequent fires are likely to cause major shifts in species composition of Neotropical savanna plant communities, favoring clonally produced recruits along tree density/topographic gradients.     

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.     

Fluxes of greenhouse gases from Andosols under coffee in monoculture or shaded by <Emphasis Type="Italic">Inga densiflora</Emphasis> in Costa Rica

The objective of this study was to evaluate the effect of N fertilization and the presence of N₂ fixing leguminous trees on soil fluxes of greenhouse gases. For a one year period, we measured soil fluxes of nitrous oxide (N₂O), carbon dioxide (CO₂) and methane (CH₄), related soil parameters (temperature, water-filled pore space, mineral nitrogen content, N mineralization potential) and litterfall in two highly fertilized (250 kg N ha⁻¹ year⁻¹) coffee cultivation: a monoculture (CM) and a culture shaded by the N₂ fixing legume species Inga densiflora (CIn). Nitrogen fertilizer addition significantly influenced N₂O emissions with 84% of the annual N₂O emitted during the post fertilization periods, and temporarily increased soil respiration and decreased CH₄ uptakes. The higher annual N₂O emissions from the shaded plantation (5.8 ± 0.3 kg N ha⁻¹ year⁻¹) when compared to that from the monoculture (4.3 ± 0.1 kg N ha⁻¹ year⁻¹) was related to the higher N input through litterfall (246 ± 16 kg N ha⁻¹ year⁻¹) and higher potential soil N mineralization rate (3.7 ± 0.2 mg N kg⁻¹ d.w. d⁻¹) in the shaded cultivation when compared to the monoculture (153 ± 6.8 kg N ha⁻¹ year⁻¹ and 2.2 ± 0.2 mg N kg⁻¹ d.w. d⁻¹). This confirms that the presence of N₂ fixing shade trees can increase N₂O emissions. Annual CO₂ and CH₄ fluxes of both systems were similar (8.4 ± 2.6 and 7.5 ± 2.3 t C-CO₂ ha⁻¹ year⁻¹, −1.1 ± 1.5 and 3.3 ± 1.1 kg C-CH₄ ha⁻¹ year⁻¹, respectively in the CIn and CM plantations) but, unexpectedly increased during the dry season.     

BIODIVERSITY RESEARCH: Turning up the heat: the impacts of Andropogon gayanus (gamba grass) invasion on fire behaviour in northern Australian savannas

Aim This study aimed to quantify changes in fire severity resulting from the invasion of Australia’s tropical savannas by the African grass Andropogon gayanus Kunth. (gamba grass). Location Mesic savannas of the Northern Territory, Australia. Method Byram’s fire‐line intensity (I_f), fuel load and architecture, and two post‐fire indicators of fire intensity – scorch height (SH) and char height (CH) of woody vegetation – were determined for fires in native grass savanna and A. gayanus invaded savanna. Leaf scorch is the height at which the fire’s radiant heat browns leaf tissue, and leaf char is the height that radiant heat blackens or consumes leaf tissue and provides an indirect measure of flame height. These data, and 5 years of similar data collected from the Kapalga Fire Project in Kakadu National Park, were used to develop empirical relationships between I_f and the post‐fire indices of fire intensity. Results A relationship between A. gayanus I_f and SH could not be developed because complete canopy scorch occurred in most A. gayanus fires, even at low I_f. In contrast, A. gayanus I_f was strongly correlated with CH. This empirical relationship was substantially different from that for native grass fires. For a given I_f, there was a significantly greater CH in invaded sites. This increase in radiant heat is attributable to the increased biomass (mean 3.6 t ha^?1 in native grasses compared to 11.6 t ha^?1 in A. gayanus) and height (approximately 0.5 m in native grasses compared to 4 m in A. gayanus) of the standing fine fuel. Main conclusion Andropogon gayanus invasion resulted in substantial changes in fire behaviour. This has important regional implications owing to the current (10,000–15,000 km²) and predicted (380,000 km²) area of invasion and the negative consequences for the native savanna biota that has evolved with frequent but relatively low‐intensity fire.     

Long-Term Nitrogen Storage and Soil Nitrogen Availability in Post-Fire Lodgepole Pine Ecosystems

Long-term, landscape patterns in inorganic nitrogen (N) availability and N stocks following infrequent, stand-replacing fire are unknown but are important for interpreting the effect of disturbances on ecosystem function. Here, we present results from a replicated chronosequence study in the Greater Yellowstone Ecosystem (Wyoming, USA) directed at measuring inorganic N availability (ion-exchange resin bags) and ecosystem N pools among 77 lodgepole pine stands that varied in age and density. Inorganic N availability ranged from 0.07 to 3.20 μN bag⁻¹ d⁻¹ and nitrate (NO₃⁻) was, on average, 65% of total resin-sorbed N. Total ecosystem N stocks (live + detrital + soil) averaged 109.9 ± 3.0 g N m⁻² (range = 63.7–185.8 g N m⁻²). Live N was 14%, detrital N was 29%, and soil N was 57% of total stocks. Soil NO₃⁻, total ecosystem N, live N, and detrital N generally increased with stand age, but soil N stocks decreased. Models (AIC_c) to predict soil N availability and N stocks included soil P, soil Ca, bulk density, and pH in addition to age (adj R ² ranged from 0.18 to 0.53) and density was included only for live N stocks. Patterns of N stocks and N availability with density were strongest for young stands (<20 years) regenerating from extensive fire in 1988; for example, litterfall N stocks increased with density (adj R ² = 0.86, P < 0.001) but inorganic N availability declined (adj R ² = 0.47, P < 0.003). Across the complex Yellowstone landscape, we conclude that N stocks and N availability are best predicted by a combination of local soil characteristics in addition to factors that vary at landscape scales (stand density and age). Overall, total ecosystem N stocks were recovered quickly following stand-replacing fire, suggesting that moderate increases in fire frequency will not affect long-term landscape N storage in Greater Yellowstone. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Author contributions   EAHS, MGT, and MGR conceived the study; DMK performed field research; EAHS and DMK oversaw laboratory analyses and analyzed data; EAHS wrote the paper.     

Methane Budget of a Black Forest Spruce Ecosystem Considering Soil Pattern

Study included seven soils, an adjacent spring and brook and was conducted to estimate CH₄ source and sink strengths of forest soils along a wetness gradient, i.e. their exchange with atmosphere (direct emission), and hydrosphere (indirect emission). Soils are represented by anaerobic Histosol, oxic Cambisols, Histosol with degraded peatlayers and Gleysols having intermediate redox state. They could be separated into three emission groups: CH₄ emitting (248–318 kg C ha⁻¹ a⁻¹), CH₄ uptake (−0.1 to −5 kg C ha⁻¹ a⁻¹), and soils on the edge of CH₄ uptake and release (−0.2–20 kg C ha⁻¹ a⁻¹). Although soils with CH₄ uptake were dominant (75%), the soil specific CH₄ budget identified the study field (6.53 ha) as CH₄ source (40.9 kg C ha⁻¹ a⁻¹). Not only CH₄ emissions, but also dissolved CH₄ in soil solution varied regularly with soil type. Individual soil solutions contained 0.008–151 μmol CH₄ l⁻¹. CH₄ vanished to negligible loads, when dissolved CH₄ passed an oxidative downslope soil zone, but promoted CH₄ uptake was measured at this soil. In turn, CH₄ was discharged to the atmosphere, when the soil solution left the pedosphere across an anaerobic soil zone. These measured indirect emissions were low (34 g C a⁻¹), but the values of individual soil solution indicate possible higher discharges (3.9 kg a⁻¹) at a different soil pattern. The results suggest that CH₄ uptake rates of temperate forests are overestimated.     

Landscape-Scale Simulation of Heterogeneous Fire Effects on Pyrogenic Carbon Emissions,Tree Mortality,and Net Ecosystem Production

Fire influences carbon dynamics from local to global scales, but many uncertainties remain regarding the remote detection and simulation of heterogeneous fire effects. This study integrates Landsat-based remote sensing and Biome-BGC process modeling to simulate the effects of high-, moderate-, and low-severity fire on pyrogenic emissions, tree mortality, and net ecosystem production. The simulation area (244,600 ha) encompasses four fires that burned approximately 50,000 ha in 2002–2003 across the Metolius Watershed, Oregon, USA, as well as in situ measurements of postfire carbon pools and fluxes that we use for model evaluation. Simulated total pyrogenic emissions were 0.732 Tg C (2.4% of equivalent statewide anthropogenic carbon emissions over the same 2-year period). The simulated total carbon transfer due to tree mortality was fourfold higher than pyrogenic carbon emissions, but dead wood decomposition will occur over decades. Immediately postfire, burned areas were a simulated carbon source (net C exchange: −0.076 Tg C y⁻¹; mean ± SD: −142 ± 121 g C m⁻² y⁻¹). As expected, high-severity, stand-replacement fire had disproportionate carbon impacts. The per-unit area effects of moderate-severity fire were substantial, however, and the extent of low-severity fire merits its inclusion in landscape-scale analyses. These results demonstrate the potential to reduce uncertainties in landscape to regional carbon budgets by leveraging Landsat-based fire products that account for both stand-replacement and partial disturbance.     

Tree Patches Show Greater N Losses but Maintain Higher Soil N Availability than Grassland Patches in a Frequently Burned Oak Savanna

Long-term prescribed fires have increased woody canopy openness and reduced nitrogen (N) cycling (that is, net N mineralization) in an oak savanna in Minnesota, USA. It is unclear how fire-induced shifts from oak-dominated to C4 grass-dominated vegetation contribute to this decline in N cycling compared to direct effects of increasing fire frequency promoting greater N losses. We determined (1) the magnitude of decline in net N mineralization in oak versus grass-dominated patches with increasing fire frequency and (2) if differences in net N mineralization between oak and grass patches in frequently burned oak savanna (burned 8 out of 10 years on average during the last 40 years) could be attributed to differences in N losses through volatilization and leaching or to plant traits affecting decomposition and mineralization. In situ net N mineralization declined with increasing fire frequency overall, but this decline was less in oak- than in grass-dominated patches, with oak-dominated patches having more than two times higher net N mineralization than grass-dominated patches. Greater net N mineralization in oak-dominated patches occurred despite greater N losses through volatilization and leaching (on average 1.8 and 1.4 g m⁻² y⁻¹ for oak- and grass-dominated patches, respectively), likely because of higher plant litter N concentration in the oak-dominated patches. As total soil N pools in the first 15 cm did not differ between oak- and grass-dominated patches (on average 83 g N m⁻²), N inputs from atmospheric deposition and uptake from deep soil layers may offset higher N losses. Our results further show that net N mineralization rates decline within 5 years after tree death and subsequent colonization by C4 grasses to levels observed in grass-dominated patches. Although long-term prescribed fires often directly reduce N stocks and cycling because of increased N losses, this study has shown that fire-induced shifts in vegetation composition can strongly contribute to the declines in N cycling in systems that are frequently disturbed by fires with potential feedbacks to plant productivity.     

Greenhouse gas emissions and carbon sequestration potential in restored wetlands of the Canadian prairie pothole region

North American prairie pothole wetlands are known to be important carbon stores. As a result there is interest in using wetland restoration and conservation programs to mitigate the effects of increasing greenhouse gas concentration in the atmosphere. However, the same conditions which cause these systems to accumulate organic carbon also produce the conditions under which methanogenesis can occur. As a result prairie pothole wetlands are potential hotspots for methane emissions. We examined change in soil organic carbon density as well as emissions of methane and nitrous oxide in newly restored, long-term restored, and reference wetlands across the Canadian prairies to determine the net GHG mitigation potential associated with wetland restoration. Our results indicate that methane emissions from seasonal, semi-permanent, and permanent prairie pothole wetlands are quite high while nitrous oxide emissions from these sites are fairly low. Increases in soil organic carbon between newly restored and long-term restored wetlands supports the conclusion that restored wetlands sequester organic carbon. Assuming a sequestration duration of 33 years and a return to historical SOC densities we estimate a mean annual sequestration rate for restored wetlands of 2.7 Mg C ha⁻¹year⁻¹ or 9.9 Mg CO₂ eq. ha⁻¹ year⁻¹. Even after accounting for increased CH₄ emissions associated with restoration our research indicates that wetland restoration would sequester approximately 3.25 Mg CO₂ eq. ha⁻¹year⁻¹. This research indicates that widescale restoration of seasonal, semi-permanent, and permanent wetlands in the Canadian prairies could help mitigate GHG emissions in the near term until a more viable long-term solution to increasing atmospheric concentrations of GHGs can be found.