Carbon dioxide balance of a tropical peat swamp forest in Kalimantan, Indonesia

Tropical peatlands, which coexist with swamp forests, have accumulated vast amounts of carbon as soil organic matter. Since the 1970s, however, deforestation and drainage have progressed on an enormous scale. In addition, El Niño and Southern Oscillation (ENSO) drought and large‐scale fires, which grow larger under the drought condition, are accelerating peatland devastation. That devastation enhances decomposition of soil organic matter and increases the carbon release to the atmosphere as CO₂. This phenomenon suggests that tropical peatlands have already become a large CO₂ source, but related quantitative information is limited. Therefore, we evaluated the CO₂ balance of a tropical peat swamp forest in Central Kalimantan, Indonesia, using 3 years of CO₂ fluxes measured using the eddy covariance technique from 2002 through 2004. The forest was disturbed by drainage; consequently, groundwater level (GL) was reduced. The net ecosystem CO₂ production (NEP) measurements showed seasonal variation, which was slightly positive or almost zero in the early dry season, and most‐negative late in the dry season or early the rainy season. This seasonality is attributable to the seasonal pattern of climate, tree phenology and fires. Slightly positive NEP resulted from smaller ecosystem respiration (RE) and larger gross primary production (GPP) under conditions of high photosynthetic photon flux density (PPFD) and large leaf area index (LAI). The most‐negative NEP resulted from smaller GPP and larger RE. The smaller GPP was related to high vapor pressure deficit (VPD), small LAI and low PPFD because of smoke from fires. The larger RE was related to low GL. Annual NEP values were estimated respectively as −602, −382 and −313 g C m⁻² yr⁻¹ for 2002, 2003 and 2004. These negative NEP values show that the tropical peat swamp forest, disturbed by drainage, functioned as a CO₂ source. That source intensity was highest in 2002, an ENSO year, mainly because of low PPFD caused by dense smoke emitted from large fires.     

CO2 emissions from an undrained tropical peatland: Interacting influences of temperature,shading and water table depth

Emission of CO₂ from tropical peatlands is an important component of the global carbon budget. Over days to months, these fluxes are largely controlled by water table depth. However, the diurnal cycle is less well understood, in part, because most measurements have been collected daily at midday. We used an automated chamber system to make hourly measurements of peat surface CO₂ emissions from chambers root‐cut to 30 cm. We then used these data to disentangle the relationship between temperature, water table and heterotrophic respiration (R_het). We made two central observations. First, we found strong diurnal cycles in CO₂ flux and near‐surface peat temperature (<10 cm depth), both peaking at midday. The magnitude of diurnal oscillations was strongly influenced by shading and water table depth, highlighting the limitations of relying on daytime measurements and/or a single correction factor to remove daytime bias in flux measurements. Second, we found mean daily R_het had a strong linear relationship to the depth of the water table, and under flooded conditions, R_het was small and constant. We used this relationship between R_het and water table depth to estimate carbon export from both R_het and dissolved organic carbon over the course of a year based on water table records. R_het dominates annual carbon export, demonstrating the potential for peatland drainage to increase regional CO₂ emissions. Finally, we discuss an apparent incompatibility between hourly and daily average observations of CO₂ flux, water table and temperature: water table and daily average flux data suggest that CO₂ is produced across the entire unsaturated peat profile, whereas temperature and hourly flux data appear to suggest that CO₂ fluxes are controlled by very near surface peat. We explore how temperature‐, moisture‐ and gas transport‐related mechanisms could cause mean CO₂ emissions to increase linearly with water table depth and also have a large diurnal cycle.     

Effects of disturbances on the carbon balance of tropical peat swamp forests

Tropical peatlands have accumulated huge soil carbon over millennia. However, the carbon pool is presently disturbed on a large scale by land development and management, and consequently has become vulnerable. Peat degradation occurs most rapidly and massively in Indonesia, because of fires, drainage, and deforestation of swamp forests coexisting with tropical peat. Peat burning releases carbon dioxide (CO₂) intensively but occasionally, whereas drainage increases CO₂ emission steadily through the acceleration of aerobic peat decomposition. Therefore, tropical peatlands present the threat of switching from a carbon sink to a carbon source to the atmosphere. However, the ecosystem‐scale carbon exchange is still not known in tropical peatlands. A long‐term field experiment in Central Kalimantan, Indonesia showed that tropical peat ecosystems, including a relatively intact peat swamp forest with little drainage (UF), a drained swamp forest (DF), and a drained burnt swamp forest (DB), functioned as net carbon sources. Mean annual net ecosystem CO₂ exchange (NEE) (± a standard deviation) for 4 years from July 2004 to July 2008 was 174 ± 203, 328 ± 204 and 499 ± 72 gC m^?2 yr^?1, respectively, for the UF, DF, and DB sites. The carbon emissions increased according to disturbance degrees. We found that the carbon balance of each ecosystem was chiefly controlled by groundwater level (GWL). The NEE showed a linear relationship with GWL on an annual basis. The relationships suggest that annual CO₂ emissions increase by 79–238 gC m^?2 every 0.1 m of GWL lowering probably because of the enhancement of oxidative peat decomposition. In addition, CO₂ uptake by vegetation photosynthesis was reduced by shading due to dense smoke from peat fires ignited accidentally or for agricultural practices. Our results may indicate that tropical peatland ecosystems are no longer a carbon sink under the pressure of human activities.     

Evapotranspiration of tropical peat swamp forests

In Southeast Asia, peatland is widely distributed and has accumulated a massive amount of soil carbon, coexisting with peat swamp forest (PSF). The peatland, however, has been rapidly degraded by deforestation, fires, and drainage for the last two decades. Such disturbances change hydrological conditions, typically groundwater level (GWL), and accelerate oxidative peat decomposition. Evapotranspiration (ET) is a major determinant of GWL, whereas information on the ET of PSF is limited. Therefore, we measured ET using the eddy covariance technique for 4–6 years between 2002 and 2009, including El Niño and La Niña events, at three sites in Central Kalimantan, Indonesia. The sites were different in disturbance degree: a PSF with little drainage (UF), a heavily drained PSF (DF), and a drained burnt ex‐PSF (DB); GWL was significantly lowered at DF, especially in the dry season. The ET showed a clear seasonal variation with a peak in the mid‐dry season and a large decrease in the late dry season, mainly following seasonal variation in net radiation (R_n). The R_n drastically decreased with dense smoke from peat fires in the late dry season. Annual ET forced to close energy balance for 4 years was 1636 ± 53, 1553 ± 117, and 1374 ± 75 mm yr^?1 (mean ± 1 standard deviation), respectively, at UF, DF, and DB. The undrained PSF (UF) had high and rather stable annual ET, independently of El Niño and La Niña events, in comparison with other tropical rainforests. The minimum monthly‐mean GWL explained 80% of interannual variation in ET for the forest sites (UF and DF); the positive relationship between ET and GWL indicates that drainage by a canal decreased ET at DF through lowering GWL. In addition, ET was decreased by 16% at DB in comparison with UF chiefly because of vegetation loss through fires.     

Carbon balance in the tundra, boreal forest and humid tropical forest during climate change: scaling up from leaf physiology and soil carbon dynamics

Carbon exchange by the terrestrial biosphere is thought to have changed since pre-industrial times in response to increasing concentrations of atmospheric CO₂ and variations (anomalies) in inter-annual air temperatures. However, the magnitude of this response, particularly that of various ecosystem types (biomes), is uncertain. Terrestrial carbon models can be used to estimate the direction and size of the terrestrial responses expected, providing that these models have a reasonable theoretical base. We formulated a general model of ecosystem carbon fluxes by linking a process-based canopy photosynthesis model to the Rothamsted soil carbon model for biomes that are not significantly affected by water limitation. The difference between net primary production (NPP) and heterotrophic soil respiration (R_h) represents net ecosystem production (NEP). The model includes (i) multiple compartments for carbon storage in vegetation and soil organic matter, (ii) the effects of seasonal changes in environmental parameters on annual NEP, and (iii) the effects of inter-annual temperature variations on annual NEP. Past, present and projected changes in atmospheric CO₂ concentration and surface air temperature (at different latitudes) were analysed for their effects on annual NEP in tundra, boreal forest and humid tropical forest biomes. In all three biomes, annual NEP was predicted to increase with CO₂ concentration but to decrease with warming. As CO₂ concentrations and temperatures rise, the positive carbon gains through increased NPP are often outweighed by losses through increased R_h, particularly at high latitudes where global warming has been (and is expected to be) most severe. We calculated that, several times during the past 140 years, both the tundra and boreal forest biomes have switched between being carbon sources (annual NEP negative) and being carbon sinks (annual NEP positive). Most recently, significant warming at high latitudes during 1988 and 1990 caused the tundra and boreal forests to be net carbon sources. Humid tropical forests generally have been a carbon sink since 1960. These modelled responses of the various biomes are in agreement with other estimates from either field measurements or geochemical models. Under projected CO₂ and temperature increases, the tundra and boreal forests will emit increasingly more carbon to the atmosphere while the humid tropical forest will continue to store carbon. Our analyses also indicate that the relative increase in the seasonal amplitude of the accumulated NEP within a year is about 0–14% year^?1 for boreal forests and 0–23% year^?1 in the tundra between 1960 and 1990.     

Climatic controls on the carbon and water balances of a boreal aspen forest, 1994–2003

The carbon and water budgets of boreal and temperate broadleaf forests are sensitive to interannual climatic variability and are likely to respond to climate change. This study analyses 9 years of eddy‐covariance data from the Boreal Ecosystem Research and Monitoring Sites (BERMS) Southern Old Aspen site in central Saskatchewan, Canada and characterizes the primary climatic controls on evapotranspiration, net ecosystem production (F_NEP), gross ecosystem photosynthesis (P) and ecosystem respiration (R). The study period was dominated by two climatic extremes: extreme warm and cool springs, which produced marked contrasts in the canopy duration, and a severe, 3‐year drought. Annual F_NEP varied among years from 55 to 367 g C m⁻² (mean 172, SD 94). Interannual variability in F_NEP was controlled primarily by factors that affected the R/P ratio, which varied between 0.74 and 0.96 (mean 0.87, SD 0.06). Canopy duration enhanced P and F_NEP with no apparent effect on R. The fraction of annual photosynthetically active radiation (PAR) that was absorbed by the canopy foliage varied from 38% in late leaf‐emergence years to 51% in early leaf‐emergence years. Photosynthetic light‐use efficiency (mean 0.0275, SD 0.026 mol C mol⁻¹ photons) was relatively constant during nondrought years but declined with drought intensity to a minimum of 0.0228 mol C mol⁻¹ photons during the most severe drought year. The impact of drought on F_NEP varied with drought intensity. Years of mild‐to‐moderate drought suppressed R while having little effect on P, so that F_NEP was enhanced. Years of severe drought suppressed both R and P, causing either little change or a subtle reduction in F_NEP. The analysis produced new insights into the dominance of canopy duration as the most important biophysical control on F_NEP. The results suggested a simple conceptual model for annual F_NEP in boreal deciduous forests. When water is not limiting, annual P is controlled by canopy duration via its influence on absorbed PAR at constant light‐use efficiency. Water stress suppresses P, by reducing light‐use efficiency, and R, by limiting growth and/or suppressing microbial respiration. The high photosynthetic light‐use efficiency showed this site to be a highly productive boreal deciduous forest, with properties similar to many temperate deciduous forests.