Tuomas Laurila

Net carbon dioxide losses of northern ecosystems in response to autumn warming

The carbon balance of terrestrial ecosystems is particularly sensitive to climatic changes in autumn and spring, with spring and autumn temperatures over northern latitudes having risen by about 1.1 °C and 0.8 °C, respectively, over the past two decades. A simultaneous greening trend has also been observed, characterized by a longer growing season and greater photosynthetic activity. These observations have led to speculation that spring and autumn warming could enhance carbon sequestration and extend the period of net carbon uptake in the future. Here we analyse interannual variations in atmospheric carbon dioxide concentration data and ecosystem carbon dioxide fluxes. We find that atmospheric records from the past 20 years show a trend towards an earlier autumn-to-winter carbon dioxide build-up, suggesting a shorter net carbon uptake period. This trend cannot be explained by changes in atmospheric transport alone and, together with the ecosystem flux data, suggest increasing carbon losses in autumn. We use a process-based terrestrial biosphere model and satellite vegetation greenness index observations to investigate further the observed seasonal response of northern ecosystems to autumnal warming. We find that both photosynthesis and respiration increase during autumn warming, but the increase in respiration is greater. In contrast, warming increases photosynthesis more than respiration in spring. Our simulations and observations indicate that northern terrestrial ecosystems may currently lose carbon dioxide in response to autumn warming, with a sensitivity of about 0.2 PgC °C-1, offsetting 90% of the increased carbon dioxide uptake during spring. If future autumn warming occurs at a faster rate than in spring, the ability of northern ecosystems to sequester carbon may be diminished earlier than previously suggested.

Land management and land-cover change have impacts of similar magnitude on surface temperature

The direct effects of land-cover change on surface climate are increasingly well understood, but fewer studies have investigated the consequences of the trend towards more intensive land management practices. Now, research investigating the biophysical effects of temperate land-management changes reveals a net warming effect of similar magnitude to that driven by changing land cover.

Soil and total ecosystem respiration in agricultural fields: effect of soil and crop type

A study was made of the effect of soil and crop type on the soil and total ecosystem respiration rates in agricultural soils in southern Finland. The main interest was to compare the soil respiration rates in peat and two different mineral soils growing barley, grass and potato. Respiration measurements were conducted during the growing season with (1) a closed-dynamic ecosystem respiration chamber, in which combined plant and soil respiration was measured and (2) a closed-dynamic soil respiration chamber which measured only the soil and root-derived respiration. A semi-empirical model including separate functions for the soil and plant respiration components was used for the total ecosystem respiration (TER), and the resulting soil respiration parameters for different soil and crop types were compared. Both methods showed that the soil respiration in the peat soil was 2–3 times as high as that in the mineral soils, varying from 0.11 to 0.36 mg (CO2) m−2 s−1 in the peat soil and from 0.02 to 0.17 mg (CO2) m−2 s−1 in the mineral soils. The difference between the soil types was mainly attributed to the soil organic C content, which in the uppermost 20 cm of the peat soil was 24 kg m−2, being about 4 times as high as that in the mineral soils. Depending on the measurement method, the soil respiration in the sandy soil was slightly higher than or similar to that in the clay soil. In each soil type, the soil respiration was highest on the grass plots. Higher soil respiration parameter values (Rs0, describing the soil respiration at a soil temperature of 10 °C, and obtained by modelling) were found on the barley than on the potato plots. The difference was explained by the different cultivation history of the plots, as the potato plots had lain fallow during the preceding summer. The total ecosystem respiration followed the seasonal evolution in the leaf area and measured photosynthetic flux rates. The 2–3-fold peat soil respiration term as compared to mineral soil indicates that the cultivated peat soil ecosystem is a strong net CO2 source.

Impact of the sulphur dioxide sources in the Kola Peninsula on air quality in northernmost Europe

Abstract An account is presented of sulphur pollution in northernmost continental Europe, based on analysis of recent observations and on dispersion model calculations. To complement the routine daily observations made at background stations, SO 2 has also been measured using an hourly registering monitor at Janiskoski in the Russian Kola Peninsula. Sulphur dioxide emissions from the Kola Peninsula, totalling 600 Gg (10 3 tonnes) yr −1 , have a dominant impact on SO 2 concentrations and S deposition over large areas, producing an environmental load exceeding the thresholds for potential detrimental effects. The major part of these emissions is produced by two non-ferrous smelters. Due to these sources, SO 2 concentrations vary considerably within the study area, from the low “remote background” values to pollution episodes with −100 μ g(SO 2 )m −3 as an hourly average, depending on wind direction. High concentrations also occur in summer, providing a great potential for dry deposition and suggesting that exposure is a highly episodic process. On an annual basis, dry deposition predominates over wet deposition. Precipitation acidity is related mainly to sulphate, and neutralization by alkaline cations is exceptionally low in the north. According to model calculations, a potentially critical deposition of 0.3 g(S) m −2 yr −1 is exceeded over an area of 150,000 km 2 , 32,000 km 2 of which are in Finland and 19,000 km 2 in Norway. Within this area the contribution of smelter emissions to sulphur deposition ranges from 40% to almost 100%.

CO2 exchange of a sedge fen in southern Finland—the impact of a drought period

Eddy covariance (EC) measurements of net ecosystem CO2 exchange (NEE) were conducted on a boreal sedge fen in southern Finland (61°50’N, 24.12’E) during a 1.5-yr period covering two summers in 2004.2005. The EC data were complemented by chamber measurements, which enabled the partition of the daytime NEE into respiration and photosynthesis. A special emphasis was put on the hydrometeorological responses of CO2 exchange during a drought period in July 2005. A mean CO2 efflux of 0.009 mg CO2 m-2 s-1 was observed during mid-winter (January.February), while the night-time respiration during the two Julys averaged 0.09 mgCO2 m-2 s-1. During both years the mean midday uptake in late July was about -0.16 mgCO2 m-2 s-1. An annual CO2 balance of -188 g CO2 m-2 was observed in 2005.Aslightly higher net sink of -219 gCO2 m-2 was estimated for 2004. The drought period experienced in July 2005 caused a clear depression in the daily NEE values. From the combined analysis of EC and chamber measurements it was concluded that this was mainly due to increased respiration, but evidence was also found of suppressed photosynthesis due to a high VPD.

Spatial variation in plant community functions regulates carbon gas dynamics in a boreal fen ecosystem

The aim of this study was to asses how the variability in carbon gas exchange at the plant community scale affected the C gas exchange estimates at the ecosystem scale in a fen that was homogeneous in a micrometeorological sense, that is, had an even surface topography and plant cover. CO2 and CH4 exchange was measured at the plant community scale with chambers and at the ecosystem scale with the eddy covariance (EC) technique. Community-scale measurements were upscaled to the ecosystem scale by weighting the community-specific estimates by the area of the community. All communities were net CO2 sinks and CH4 sources during the growing season, but net ecosystem production (NEP) and CH4 emissions ranged from 21 to 190 g CO2-C m-2 and from 4.3 to 13 g CH4-C m-2, respectively, between the communities. The seasonal estimates of NEP and CH4, upscaled to the 200 m radius from the EC tower, were 82 and 7.9 g CH4-C m-2, which agreed well with the EC measurements. As the communities differed markedly in their C gas dynamics, their proportions controlled the ecosystem scale estimates. Successful upscaling required detailed knowledge on the proportions and leaf area of the communities.

Annual CO2 exchange of a peat field growing spring barley or perennial forage grass

[1] We report on net ecosystem CO 2 exchange (NEE) measurements conducted with the eddy covariance method over agricultural peat soil in the 2-year period between October 2000 and October 2002. In 2001, spring barley and undersown grass were sown on the site. After the barley harvest, the perennial forage grass was left to grow, so that in 2002 the field was growing grass. A higher maximum net CO 2 uptake was observed for barley than for grass during the height of the summer, peaking at about -1.0 and -0.75 mg CO 2 m s -1 , respectively. The maximum nighttime total ecosystem respiration was measured in July and was similar for both crops, about 0.35 mg CO 2 m -2 s -1 . During the growing season the field acted as a daily CO 2 sink for only 40 days in barley versus 84 days in grass. In the winter the average carbon dioxide efflux varied from 15.6 to 16.5 μg CO 2 m -2 s -1 . The annual NEE of the agricultural peat soil growing barley and grass was 771 ± 104 and 290 ± 91 g CO 2 m -2 , respectively. The longer net CO 2 uptake period was the main reason for the lower annual NEE for grass; however, owing to the higher amount of grass biomass produced the net ecosystem production (NEP), calculated as the sum of NEE and removed biomass, was slightly larger for grass (452 g C m -2 ) than for barley (336 g C m -2 ). These results show that the organic peat is still undergoing rapid decomposition after more than 100 years of cultivation activity. In addition, switching from an annual to a perennial crop did not turn the field into a CO 2 sink, at least during a 1-year period.

On the spatial distribution and seasonal variation of lower-troposphere ozone over Europe

Surface ozone data from 25 Europeanlow-altitude sites and mountain sites located between79°N and 28°N were studied. The analysiscovered the time period March 1989–February 1993.Average summer and winter O3 concentrations inthe boundary layer over the continent gave rise togradients that were strongest in the north-west tosouth-east direction and west-east direction, respectively. WintertimeO3 ranged from 19 to 27 ppbover the continent, compared to about 32 ppb at thewestern border, while for summer the continentalO3 values ranged between 39 and 56 ppb and theoceanic mixing ratios were around 37 ppb. In the lowerfree troposphere average wintertime O3 mixingratios were around 38 ppb, with only an 8 ppbdifference between 28°N and 79°N. For summerthe average O3 levels decreased from about 55 ppbover Central Europe to 32 ppb at 79°N. Inaddition, O3 and Ox(= O3 + NO2)in polluted and clean air were compared. Theamplitudes of the seasonal ozone variations increasedin the north-west to south-east direction, while thetime of the annual maximum was shifted from spring (atthe northerly sites) to late summer (at sites inAustria and Hungary), which reflected the contributionof photochemical ozone production in the lower partsof the troposphere.

Seasonal CO2 balances of a subarctic mire

Micrometeorological measurements of CO2 and energy fluxes were carried out in a peatland ecosystem in northern Finland (69°08′N, 27°17′E) during a measurement period from April to the end of October 1997. The summer of 1997 was exceptionally warm and dry as compared to the climatological normal period of 1961–1990, and the effects of the high temperature and lowered water table were clearly seen in the CO2 fluxes. The highest individual downward flux densities of about −0.25 mg(CO2) m−2 s−1 took place at the end of July, while the highest respiration rates of 0.15 mg(CO2) m−2 s−1 were observed later in August. During the first days of measurements in April the median of respiration flux densities through the snow cover was about 0.006 mg(CO2) m−2 s−1. In correspondence to the CO2 fluxes the strongest sink terms in the daily net ecosystem exchange (NEE) balances of about −6 g(CO2) m−2 d−1 were observed in July. The highest positive balances of about 4 g(CO2) m−2 d−1 were observed in early June and in August. The daily balances in April were about 0.6 g(CO2) m−2 d−1. The net balances for the sink period (June 15 to August 26, 1997) and for the 6-month measurement period were −188 g m−2 and −30 g m−2, respectively. The wintertime CO2 balance was estimated by modeling the NEE using the NEE values from the first measurement week in April. The wintertime balances obtained yielded estimates for annual balances in the range of 62 to 72 g m−2 yr−1.

The ambient concentrations of biogenic hydrocarbons at a northern European, boreal site.

Abstract Concentrations of monoterpenes, 1,8-cineol and light hydrocarbons were measured in Potsonvaara, Ilomantsi, Eastern Finland during two growing seasons in 1997 and 1998. The measuring site was located on the top of a hill, outside a mixed forest. The monthly average summer concentrations of isoprene were 0.3–1.7 ppbC and monoterpenes and 1,8-cineol together 1.6–3.2 ppbC. Isoprene and α-pinene were the most abundant compounds throughout the growing season, but β-pinene, Δ3-carene, camphene, 1,8-cineol, sabinene and limonene were found as well. Isoprene and sabinene concentrations started to increase later than the concentrations of other compounds, and were better correlated with each other than with other compounds. Diurnal variations of monoterpenes show a minimum in the daytime and a maximum at night, except sabinene at midsummer, that has maximum concentrations during the day. The field data support the idea that the effective temperature sum can be used to predict the initiation of emissions of isoprene and also terpene emissions from Betula pendula.