Marian Pavelka

Dependence of the Q10 values on the depth of the soil temperature measuring point

The parameter Q10 is commonly used to express the relationship between soil CO2 efflux and soil temperature. One advantage of this parameter is its application in a model expression of respiration losses of different ecosystems. Correct specification of Q10 in these models is indispensable. Soil surface CO2 efflux and soil temperature at different depths were measured in a 21-year-old Norway spruce stand and a mountain grassland site located at the Experimental Ecological Study Site Bily Kriz, Beskydy Mts. (NE Czech Republic), using automated gasometric systems. A time-delay and goodness-of-fit between soil CO2 efflux and soil temperature at different measuring depths were determined. Wide ranges of values for the time-delay of CO2 efflux in response to temperature, Q10 and the determination coefficient (R2) between CO2 efflux and temperature were obtained at the both sites. The values of Q10 and the CO2 time-delay increased with depth, while the R2 of the CO2-temperature relationship significantly decreased. Soil temperature records obtained close to the soil surface showed the highest values of R2 and the lowest value of the time-delay at both sites. Measurement of soil temperature at very shallow soil layer, preferably at the soil surface, is highly recommended to determine useable values of Q10. We present a new procedure to normalize Q10 values for soil temperatures measured at different depths that would facilitate comparison of different sites.

A new mass conservation approach to the study of CO2 advection in an alpine forest

A new method is proposed for the computation of CO2 Net Ecosystem Exchange (NEE) and its components in a forest ecosystem. Advective flux is estimated by taking into account the air mass conservation principle. For this purpose, wind and dry air density values on the surface of the control volume are first corrected and then the advective flux is estimated on the surface of the control volume. Turbulent flux is also computed along the surface of the control volume while storage flux is computed inside the volume. Additional characteristics of this method are that incompressibility of the mean flow is not assumed a priori, and that vertical and horizontal advective fluxes are not treated separately, but their sum is estimated directly. The methodology is applied to experimental data collected with a three-dimensional scheme at the alpine site of Renon during the Advex project (July 2005). The advection flux was found to be prevailing positive at night and negative during the day, as was found in previous studies on advection for the same site, but showed a lower scatter in half-hour calculated values. We tested the effect of its summation on turbulent and storage fluxes to produce half-hourly values of NEE. Nighttime NEE values were used in functional relations with soil temperature, daytime values with PPFD. The effect of addition of the advection component was an increase in the values of parameters indicating ecosystem respiration, quantum yield, and photosynthetic capacity. The coefficient of correlation between NEE and environmental drivers increased. (Less)

Glass Domes with Adjustable Windows: A Novel Technique for Exposing Juvenile Forest Stands to Elevated CO2 Concentration

We present a new technological approach for in situ investigation of long-term impacts of elevated CO2 concentration (EC) on juvenile forests characterised by an intensive community level and canopy closure phase. Construction of the glass domes is based on the properties of earlier tested open-top chambers (OTCs). An air climatisation device together with an adjustable window system, that forms the shell cover of the domes, is able to keep the required [CO2] in both time and spatial scales with the relatively small consumption of supplied CO2. This is achieved by half-closing the windows on the windward side. We evidenced good coupling of treated trees to the atmosphere, including mutual interactions among trees. The semi-open design of the domes moderates the problems of strong wind, humidity, and temperature gradients associated with OTCs. The frequency distributions of the environmental variations within the domes indicate that: air temperature is maintained within the ambient range ±1.0 °C for ca. 80 % of the time, and changes in the relative air humidity vary from −15 to 0 % for ca. 82 % of the time. The most important chamber effect is associated with the penetration of solar irradiance, which is reduced by 26 % compared to the open condition outside the domes. The dimensions of the domes are 10×10 m in length and 7 m high in the central part. The experiment was done in three identical stands of twelve-year-old Norway spruce trees. The 56 trees are planted at two different spacings to estimate the impacts of stand spatial structure in relation to EC.

Global parameterization and validation of a two-leaf light use efficiency model for predicting gross primary production across FLUXNET sites

Light use efficiency (LUE) models are widely used to simulate gross primary production (GPP). However, the treatment of the plant canopy as a big leaf by these models can introduce large uncertainties in simulated GPP. Recently, a two-leaf light use efficiency (TL-LUE) model was developed to simulate GPP separately for sunlit and shaded leaves and has been shown to outperform the big-leaf MOD17 model at six FLUX sites in China. In this study we investigated the performance of the TL-LUE model for a wider range of biomes. For this we optimized the parameters and tested the TL-LUE model using data from 98 FLUXNET sites which are distributed across the globe. The results showed that the TL-LUE model performed in general better than the MOD17 model in simulating 8 day GPP. Optimized maximum light use efficiency of shaded leaves (epsilon(msh)) was 2.63 to 4.59 times that of sunlit leaves (epsilon(msu)). Generally, the relationships of epsilon(msh) and epsilon(msu) with epsilon(max) were well described by linear equations, indicating the existence of general patterns across biomes. GPP simulated by the TL-LUE model was much less sensitive to biases in the photosynthetically active radiation (PAR) input than the MOD17 model. The results of this study suggest that the proposed TL-LUE model has the potential for simulating regional and global GPP of terrestrial ecosystems, and it is more robust with regard to usual biases in input data than existing approaches which neglect the bimodal within-canopy distribution of PAR.

Carbon exchange between ecosystems and atmosphere in the Czech Republic is affected by climate factors.

By comparing five ecosystem types in the Czech Republic over several years, we recorded the highest carbon sequestration potential in an evergreen Norway spruce forest (100%) and an agroecosystem (65%), followed by European beech forest (25%) and a wetland ecosystem (20%). Because of a massive ecosystem respiration, the final carbon gain of the grassland was negative. Climate was shown to be an important factor of carbon uptake by ecosystems: by varying the growing season length (a 22-d longer season in 2005 than in 2007 increased carbon sink by 13%) or by the effect of short- term synoptic situations (e.g. summer hot and dry days reduced net carbon storage by 58% relative to hot and wet days). Carbon uptake is strongly affected by the ontogeny and a production strategy which is demonstrated by the comparison of seasonal course of carbon uptake between coniferous (Norway spruce) and deciduous (European beech) stands.

Extreme warm temperatures alter forest phenology and productivity in Europe

Recent climate warming has shifted the timing of spring and autumn vegetation phenological events in the temperate and boreal forest ecosystems of Europe. In many areas spring phenological events start earlier and autumn events switch between earlier and later onset. Consequently, the length of growing season in mid and high latitudes of European forest is extended. However, the lagged effects (i.e. the impact of a warm spring or autumn on the subsequent phenological events) on vegetation phenology and productivity are less explored. In this study, we have (1) characterised extreme warm spring and extreme warm autumn events in Europe during 2003-2011, and (2) investigated if direct impact on forest phenology and productivity due to a specific warm event translated to a lagged effect in subsequent phenological events. We found that warmer events in spring occurred extensively in high latitude Europe producing a significant earlier onset of greening (OG) in broadleaf deciduous forest (BLDF) and mixed forest (MF). However, this earlier OG did not show any significant lagged effects on autumnal senescence. Needleleaf evergreen forest (NLEF), BLDF and MF showed a significantly delayed end of senescence (EOS) as a result of extreme warm autumn events; and in the following years spring phenological events, OG started significantly earlier. Extreme warm spring events directly led to significant (p=0.0189) increases in the productivity of BLDF. In order to have a complete understanding of ecosystems response to warm temperature during key phenological events, particularly autumn events, the lagged effect on the next growing season should be considered.

Environmental factors influencing the relationship between stem CO 2 efflux and sap flow

Key messageBeside temperature, soil moisture was found as the most important environmental factor influencing the relationship between stem CO2 efflux and sap flow.AbstractStem CO2 efflux is an important component of the forest carbon balance. Even after several studies on this issue, there is still uncertainty about the influence of the sap flux on stem CO2 efflux. This study analyses stem CO2 efflux and sap flow measured on Norway spruce [Picea abies (L.) Karst] trees and environmental factors influencing this relationship during the growing seasons of 2010 and 2011. Stem CO2 efflux measurements were performed using an automatic dynamic closed gasometrical system, whilst sap flow measurements were carried out by applying a sap flow method heat pulse velocity. Stem CO2 efflux was positively correlated with stem temperature; sap flow was positively correlated with incident global radiation. During optimal soil moisture conditions, stem CO2 efflux and sap flow were positively correlated while during dry conditions, stem CO2 efflux and sap flow were not positively correlated. Almost all significant correlations between stem CO2 efflux and sap flow were not controlled by any investigated environmental factor.

Branch CO2 efflux in vertical profile of Norway spruce tree

Branch CO2 efflux of Norway spruce tree [Picea abies (L.) Karst.] was measured in ten branches at five different whorls during the growing season 2004 (from June till October) in campaigns of 3–4 times per month at the Beskydy Mts., the Czech Republic. Branch CO2 efflux was measured using a portable infrared gas analyzer (LI-6250, LI-COR, Inc., USA), operating as a closed system. Branch woody-tissue temperature was measured continuously in 10-min intervals for each sample position during the whole experiment period. On the basis of relation between CO2 efflux rate and woody-tissue temperature, a value of Q10 and of normalized CO2 efflux rate (E10–CO2 efflux rate at 10°C) was calculated for each sampled position. Estimated Q10 values ranged from 2.12 to 2.89, and E10 ranged from 0.41 to 1.19 μmolCO2m−2s−1. Differences in branch CO2 efflux were found between orientations, east-side branches presented higher efflux rate than west-side branches. The highest branch CO2 efflux rate values were measured in August and the lowest in October, which corresponds with woody-tissue temperature and growth processes during these periods. Branch CO2 efflux was significantly and positively correlated with branch position within canopy and woody-tissue temperature. Branches from the upper whorls showed higher CO2 efflux activity and seasonal dynamics than branches from the lower whorls.

Winter respiratory C losses provide explanatory power for net ecosystem productivity

M. Haeni, R. Zweifel, W. Eugster, A. Gessler, S. Zielis, C. Bernhofer, A. 7 Carrara, T. Grünwald, K. Havránková, B. Heinesch, M. Herbst, A. Ibrom, A. 8 Knohl, F. Lagergren, B.E. Law, M. Marek, G. Matteucci, JH. McCaughey, S. 9 Minerbi, L. Montagnani, E. Moors, J. Olejnik, M. Pavelka, K. Pilegaard, G. 10 Pita, A. Rodrigues, M. J. Sanz Sánchez, M.-J. Schelhaas, M. Urbaniak, R. 11 Valentini, A. Varlagin, T. Vesala, C. Vincke, J. Wu, and N. Buchmann 12

Forest Ecosystem as a Source of Co2 During Growing Season: Relation to Weather Conditions

Abstract Net ecosystem production reflects the potential of the ecosystem to sequestrate atmospheric CO2. Daily net ecosystem production of a mountain Norway spruce forest of the temperate zone (Czech Republic) was determined using the eddy covariance method. Growing season days when the ecosystem was a CO2 source were examined with respect to current weather conditions. During the 2005, 2006, and 2007 growing seasons, there were 44, 65, and 39 days, respectively, when the forest was a net CO2 source. The current weather conditions associated with CO2 release during the growing seasons were: cool and overcast conditions at the beginning or end of the growing seasons characterized by a 3-year mean net ecosystem production of -7.2 kg C ha-1 day-1; overcast or/and rainy days (-23.1 kg C ha-1 day-1); partly cloudy and hot days (-11.8 kg C ha-1 day-1); and overcast and hot days (-13.5 kg C ha-1 day-1). CO2 release was the highest during the overcast or/and rainy conditions (84%, average from all years), which had the greatest impact during the major production periods. As forests are important CO2 sinks and more frequent weather extremes are expected due to climate change, it is important to predict future forest carbon balances to study the influence of heightened variability in climatic variables.