Anders Lindroth

Respiration as the main determinant of carbon balance in European forests

Carbon exchange between the terrestrial biosphere and the atmosphere is one of the key processes that need to be assessed in the context of the Kyoto Protocol. Several studies suggest that the terrestrial biosphere is gaining carbon, but these estimates are obtained primarily by indirect methods, and the factors that control terrestrial carbon exchange, its magnitude and primary locations, are under debate. Here we present data of net ecosystem carbon exchange, collected between 1996 and 1998 from 15 European forests, which confirm that many European forest ecosystems act as carbon sinks. The annual carbon balances range from an uptake of 6.6 tonnes of carbon per hectare per year to a release of nearly 1 t C ha -1 yr-1, with a large variability between forests. The data show a significant increase of carbon uptake with decreasing latitude, whereas the gross primary production seems to be largely independent of latitude. Our observations indicate that, in general, ecosystem respiration determines net ecosystem carbon exchange. Also, for an accurate assessment of the carbon balance in a particular forest ecosystem, remote sensing of the normalized difference vegetation index or estimates based on forest inventories may not be sufficient.

Terrestrial Gross Carbon Dioxide Uptake: Global Distribution and Covariation with Climate

Carbon Cycle and Climate Change As climate change accelerates, it is important to know the likely impact of climate change on the carbon cycle (see the Perspective by Reich). Gross primary production (GPP) is a measure of the amount of CO2 removed from the atmosphere every year to fuel photosynthesis. Beer et al. (p. 834, published online 5 July) used a combination of observation and calculation to estimate that the total GPP by terrestrial plants is around 122 billion tons per year; in comparison, burning fossil fuels emits about 7 billion tons annually. Thirty-two percent of this uptake occurs in tropical forests, and precipitation controls carbon uptake in more than 40% of vegetated land. The temperature sensitivity (Q10) of ecosystem respiratory processes is a key determinant of the interaction between climate and the carbon cycle. Mahecha et al. (p. 838, published online 5 July) now show that the Q10 of ecosystem respiration is invariant with respect to mean annual temperature, independent of the analyzed ecosystem type, with a global mean value for Q10 of 1.6. This level of temperature sensitivity suggests a less-pronounced climate sensitivity of the carbon cycle than assumed by recent climate models. A combination of data and models provides an estimate of how much photosynthesis by all the world’s plants occurs each year. Terrestrial gross primary production (GPP) is the largest global CO2 flux driving several ecosystem functions. We provide an observation-based estimate of this flux at 123 ± 8 petagrams of carbon per year (Pg C year−1) using eddy covariance flux data and various diagnostic models. Tropical forests and savannahs account for 60%. GPP over 40% of the vegetated land is associated with precipitation. State-of-the-art process-oriented biosphere models used for climate predictions exhibit a large between-model variation of GPP’s latitudinal patterns and show higher spatial correlations between GPP and precipitation, suggesting the existence of missing processes or feedback mechanisms which attenuate the vegetation response to climate. Our estimates of spatially distributed GPP and its covariation with climate can help improve coupled climate–carbon cycle process models.

The human footprint in the carbon cycle of temperate and boreal forests

Temperate and boreal forests in the Northern Hemisphere cover an area of about 2 × 107 square kilometres and act as a substantial carbon sink (0.6–0.7 petagrams of carbon per year). Although forest expansion following agricultural abandonment is certainly responsible for an important fraction of this carbon sink activity, the additional effects on the carbon balance of established forests of increased atmospheric carbon dioxide, increasing temperatures, changes in management practices and nitrogen deposition are difficult to disentangle, despite an extensive network of measurement stations. The relevance of this measurement effort has also been questioned, because spot measurements fail to take into account the role of disturbances, either natural (fire, pests, windstorms) or anthropogenic (forest harvesting). Here we show that the temporal dynamics following stand-replacing disturbances do indeed account for a very large fraction of the overall variability in forest carbon sequestration. After the confounding effects of disturbance have been factored out, however, forest net carbon sequestration is found to be overwhelmingly driven by nitrogen deposition, largely the result of anthropogenic activities. The effect is always positive over the range of nitrogen deposition covered by currently available data sets, casting doubts on the risk of widespread ecosystem nitrogen saturation under natural conditions. The results demonstrate that mankind is ultimately controlling the carbon balance of temperate and boreal forests, either directly (through forest management) or indirectly (through nitrogen deposition).

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.

Global patterns of land‐atmosphere fluxes of carbon dioxide, latent heat, and sensible heat derived from eddy covariance, satellite, and meteorological observations

We upscaled FLUXNET observations of carbon dioxide, water, and energy fluxes to the global scale using the machine learning technique, model tree ensembles (MTE). We trained MTE to predict site-level gross primary productivity (GPP), terrestrial ecosystem respiration (TER), net ecosystem exchange (NEE), latent energy (LE), and sensible heat (H) based on remote sensing indices, climate and meteorological data, and information on land use. We applied the trained MTEs to generate global flux fields at a 0.5 degrees x 0.5 degrees spatial resolution and a monthly temporal resolution from 1982 to 2008. Cross-validation analyses revealed good performance of MTE in predicting among-site flux variability with modeling efficiencies (MEf) between 0.64 and 0.84, except for NEE (MEf = 0.32). Performance was also good for predicting seasonal patterns (MEf between 0.84 and 0.89, except for NEE (0.64)). By comparison, predictions of monthly anomalies were not as strong (MEf between 0.29 and 0.52). Improved accounting of disturbance and lagged environmental effects, along with improved characterization of errors in the training data set, would contribute most to further reducing uncertainties. Our global estimates of LE (158 +/- 7 J x 10(18) yr(-1)), H (164 +/- 15 J x 10(18) yr(-1)), and GPP (119 +/- 6 Pg C yr(-1)) were similar to independent estimates. Our global TER estimate (96 +/- 6 Pg C yr(-1)) was likely underestimated by 5-10%. Hot spot regions of interannual variability in carbon fluxes occurred in semiarid to semihumid regions and were controlled by moisture supply. Overall, GPP was more important to interannual variability in NEE than TER. Our empirically derived fluxes may be used for calibration and evaluation of land surface process models and for exploratory and diagnostic assessments of the biosphere.

Energy exchange and water budget partitioning in a boreal minerogenic mire

This study investigated patterns and controls of the seasonal and inter-annual variations in energy fluxes (i.e., sensible heat, H, and latent heat, lambda E) and partitioning of the water budget (i.e., precipitation, P; evapotranspiration, ET; discharge, Q; and soil water storage, Delta S) over five years (2001-2005) in a boreal oligotrophic fen in northern Sweden based on continuous eddy covariance, water table level (WTL), and weir measurements. For the growing season (May 1 to September 31), the 5 year averages (+/- standard deviation) of the midday (10:00 to 14:00 h) Bowen ratio (beta, i.e., H/lambda E) was 0.86 +/- 0.08. Seasonal and inter-annual variability of beta was mainly driven by lambda E which itself was strongly controlled by both weather (i.e., vapor pressure deficit, D, and net radiation, R-n) and physiological parameters (i.e., surface resistance). During the growing season, surface resistance largely exceeded aerodynamic resistance, which together with low mean values of the actual ET to potential ET ratio (0.55 +/- 0.05) and Priestley-Taylor alpha (0.89) suggests significant physiological constrains on ET in this well-watered fen. Among the water budget components, the inter-annual variability of ET was lower (199 to 298 mm) compared to Q (225 to 752 mm), with each accounting on average for 34 and 65% of the ecosystem water loss, respectively. The fraction of P expended into ET was negatively correlated to P and positively to R-n. Although a decrease in WTL caused a reduction of the surface conductance, the overall effect of WTL on ET was limited. Non-growing season (October 1 to April 30) fluxes of H, lambda E, and Q were significant representing on average -67%, 13%, and 61%, respectively, of their growing season sums (negative sign indicates opposite flux direction between the two seasons). Overall, our findings suggest that plant functional type composition, P and R-n dynamics (i.e., amount and timing) were the major controls on the partitioning of the mire energy and water budgets. This has important implications for the regional climate as well as for ecosystem development, nutrient, and carbon dynamics. Citation: Peichl, M., J. Sagerfors, A. Lindroth, I. Buffam, A. Grelle, L. Klemedtsson, H. Laudon, and M. B. Nilsson (2013), Energy exchange and water budget partitioning in a boreal minerogenic mire, J. Geophys. Res. Biogeosci., 118, 1-13, doi:10.1029/2012JG002073.

Energy Partitioning Between Latent And Sensible Heat Flux During The Warm Season At Fluxnet Sites

The warm season (mid-June through late August) partitioning between sensible (H) and latent (LE) heat flux, or the Bowen ratio (beta=H/LE), was investigated at 27 sites over 66 site years within the international network of eddy covariance sites (FLUXNET). Variability in beta across ecosystems and climates was analyzed by quantifying general climatic and surface characteristics that control flux partitioning. The climatic control on beta was quantified using the climatological resistance (R-i), which is proportional to the ratio of vapor pressure deficit (difference between saturation vapor pressure and atmospheric vapor pressure) to net radiation (large values of R-i decrease beta). The control of flux partitioning by the vegetation and underlying surface was quantified by computing the surface resistance to water vapor transport (R-c, with large values tending to increase beta). There was a considerable range in flux partitioning characteristics (R-c, R-i and beta) among sites, but it was possible to define some general differences between vegetation types and climates. Deciduous forest sites and the agricultural site had the lowest values of R-c and beta (0.25-0.50). Coniferous forests typically had a larger R-c and higher beta (typically between 0.50 and 1.00 but also much larger). However, there was notable variability in R-c and R-i between coniferous site years, most notably differences between oceanic and continental climates and sites with a distinct dry summer season (Mediterranean climate). Sites with Mediterranean climates generally had the highest net radiation, R-c, R-i, and beta. There was considerable variability in beta between grassland site years, primarily the result of interannual differences in soil water content and R-c

Past, Present, and Future Controls on Levels of Persistent Organic Pollutants in the Global Environment

Understanding the legacy of persistent organic pollutants requires studying the transition from primary to secondary source control.

CO2 exchange at the floor of a boreal forest

Net CO2 exchange at the forest floor in a mixed boreal spruce and pine forest in central Sweden was studied during 1996. Forest floor CO 2 efflux was measured continuously by means of a ventilated open soil chamber, covering a surface area of 0.6 m 2 . The chamber was transparent and thus measured soil respiration by night, and soil respiration reduced by photosynthetic uptake by forest floor vegetation by day. Maximum nocturnal efflux rates were 0.2‐0.7 and daytime rates were 0.05‐0.2 mg m 2 s 1 . Measured efflux rates were higher than reported in other studies, but have earlier been found to agree with nocturnal CO2 exchange of the forest ecosystem measured by eddy-covariance technique. Soil temperature at 5 cm explained 49% of the variation in nocturnal soil respiration, while moss and air temperature explained 29 and 17% of the variation, respectively. For the relationship with soil temperature at 5 cm, base respiration rate and effective Q10, derived from data over the seasonal course, were 0.04 mg m 2 s 1 and 4.75, respectively. Corresponding figures for the relationship with air temperature were 0.11 mg m 2 s 1 and 1.89. Over the 6 months (May‐October) covered by measurements, August had the largest CO2 efflux, 0.89 kg m 2 and May the smallest efflux, 0.20 kg m 2 . During daytime photosynthetic uptake by forest floor vegetation reduced potential soil efflux through respiration by ca. 20%. On an annual basis total forest floor respiration was estimated to be 4.5 kg CO2 m 2 and gross photosynthesis to be 0.7 kg CO2 m 2 , resulting in a net efflux of 3.8 kg CO2 m 2 . ©2000 Elsevier Science B.V. All rights reserved.

Transpiration response to soil moisture in pine and spruce trees in Sweden

Variation in transpiration and conductance between individual trees of Scots pine and Norway spruce was investigated in a mixed 50-year-old stand in central Sweden. Daily transpiration rates were measured by the tissue heat balance method on five trees of each species during a dry, warm growing season. Daytime averages of sapflow, climatic variables and soil water content were used to fit an empirical model of tree conductance for each tree. Conductance per unit needle area was about twice as high in pine as in spruce, while equal-sized trees transpired similarly in both species. Conductance generally decreased more steeply with increasing vapour pressure deficit and increased faster with increasing light in pine than in spruce, although one individual spruce behaved more like the pines. Inclusion of a linear or exponential function for air temperature improved the model for pine, but of the spruces, only one tree showed a clear temperature dependency. The response to decreasing soil water content varied widely; the spruces tended to be more sensitive to drought than the pines. When the drought was at its worst, no sapflow could be detected in some of the trees. On average, the reduction in transpiration began when ca. 80% of the extractable water in the rooting zone had been depleted.