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A growing body of scientific evidence indicates that we have entered the Anthropocene Epoch. Many assert that society has exceeded sustainable ecological planetary boundaries and that altered biogeophysical processes are no longer reversible to natural rates of ecosystem functioning. To properly and successfully address societal needs for the future, more holistic and complex methods need to be applied at various spatial and temporal scales. The increasingly interconnected nature of human and natural environments—from individuals to large megacities and entire continents and from cells through ecosystems to the biosphere as a whole (e.g., as seen in the carbon cycle)—demand new and often interdisciplinary and international approaches to address emerging global challenges. With that perspective in mind, the Czech Republic’s National Climate Program was established in 1991 with the aim to understand the impact of global environmental change on society. The National Climate Program was updated in 2017 to formulate a new Climate Protection Policy. Here, we outline the multifaceted problems that climate change poses for the Czech Republic, as well as a new scientific infrastructure and approaches directed to better understanding the effects of climate change on our ecosystems, water resources, urban environment, agriculture, human health, and general economy.

One of the greatest threats posed by ongoing climate change may be regarded the drought caused by changes in precipitation distribution. The aim of presented study was to characterize reactions to dry conditions and conditions without drought stress on gross primary production (GPP) and net ecosystem production (NEP) of spruce and beech forests, as these two species dominate within the European continent. Daily courses of GPP and NEP of these two species were evaluated in relation to an expected decrease in CO2 uptake during dry days. The occurrence of CO2 uptake hysteresis in daily production was also investigated. Our study was performed at Bilý Křiž (spruce) and Stitna (beech) mountain forest sites during 2010–2012 period. We applied eddy covariance technique for the estimation of carbon fluxes, vapor pressure deficit and precipitation characteristics together with the SoilClim model for the determination of drought conditions, and the inverse of the Penman–Monteith equation to compute canopy conductance. Significant differences were found in response to reduced water supply for both species. Spruce reacts by closing its stomata before noon and maintaining a reduced photosynthetic activity for the rest of the day, while beech keeps its stomata open as long as possible and slightly reduces photosynthetic activity evenly throughout the entire day. In the spruce forest, we found substantial hysteresis in the light response curve of GPP. In the beech forest, the shape of this curve was different: evening values exceeded morning values.

A long-standing problem in micrometeorology is that at most eddy covariance sites around the world, the sum of the sensible and latent heat flux measurements is less than the available energy, resulting in the so-called energy balance closure problem. This study utilised the national network of eddy covariance towers in the Czech Republic to examine the degree of energy balance closure at sites covering a wide variety of vegetation types and terrain complexities. The degree of energy balance closure at each site varied depending on the method used to calculate the closure fraction. When the closure was computed using linear regressions of half-hourly sums of turbulent heat fluxes against half-hourly available energy values, closure ranged from 0.68 (beech forest) to 0.81 (spruce forest). However, when closure was computed using the bulk energy balance ratio method, values ranged from 0.61 to 0.73. Highest closure occurred in moderately unstable atmospheric conditions, while closure also increased with increases in the correlation coefficients for vertical wind velocity and water vapour, and vertical wind velocity and sonic temperature. Lowest closure was found at a beech forest in the Carpathian Mountains, where evidence suggested that the complex topography to the south of the eddy covariance tower was influencing the airflow and resulting in poor energy balance closure results. Energy balance closure was also particularly low at a rapeseed cropland, and this was attributed to the low frequency of moderately unstable to strongly unstable conditions at the site.

Although most studies find temperature, soil moisture and water table to be important environmental factors that affect peatland carbon dynamics, the role of vegetation communities has been investigated less. Therefore, this study investigates whether peatland ecosystems produce heterogeneous CO2 fluxes due to differences in vegetation community. In addition, the study also examines which major environmental factors influence this vegetation. To achieve the aims of this study, four sites with different vegetation communities were established in a semi-natural peatland ecosystem in Poland. CO2 flux measurements were carried out using a closed dynamic chamber system. Measurement campaigns were carried out from April until December 2008, every 2–3 weeks. Measured ecosystem respiration (Reco) and net ecosystem exchange (NEE) rates showed daily and seasonal variation at all investigated sites. Reco presented a strong dependence on soil temperature at the 5 cm depth, while NEE showed a strong dependence on solar radiation. The mean temperature sensitivity (Q10) for the four sites ranged between 3.17 and 8.3. The highest NEE and Reco values were obtained at the site represented by Caricetum elatae and the lowest NEE and Reco at the site represented by Calamagrostietum neglectae.

Accurate predictions of net ecosystem productivity (NEPc) of forest ecosystems are essential for climate change decisions and requirements in the context of national forest growth and greenhouse gas inventories. However, drivers and underlying mechanisms determining NEPc (e.g., climate and nutrients) are not entirely understood yet, particularly when considering the influence of past periods. Here we explored the explanatory power of the compensation day (cDOY)-defined as the day of year when winter net carbon losses are compensated by spring assimilation-for NEPc in 26 forests in Europe, North America, and Australia, using different NEPc integration methods. We found cDOY to be a particularly powerful predictor for NEPc of temperate evergreen needleleaf forests (R2=0.58) and deciduous broadleaf forests (R2=0.68). In general, the latest cDOY correlated with the lowest NEPc. The explanatory power of cDOY depended on the integration method for NEPc, forest type, and whether the site had a distinct winter net respiratory carbon loss or not. The integration methods starting in autumn led to better predictions of NEPc from cDOY then the classical calendar method starting 1 January. Limited explanatory power of cDOY for NEPc was found for warmer sites with no distinct winter respiratory loss period. Our findings highlight the importance of the influence of winter processes and the delayed responses of previous seasons' climatic conditions on current year's NEPc. Such carry-over effects may contain information from climatic conditions, carbon storage levels, and hydraulic traits of several years back in time. (Less)

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.

Summary 1. Cloud cover affects carbon exchange between biota and the atmosphere. Recent studies have demonstrated that an increase in the diffuse radiation fraction enhances the photosynthetic efficiency of canopies. Although the exact mechanism behind this effect is not clear, a more even distribution of light among leaves across the vertical profile of the canopy is considered to be the most important cause of this difference. 2. To test this hypothesis, the net ecosystem production (NEP) of a Norway spruce forest (30-year-old) was measured under cloudy and sunny skies by the eddy covariance method. In parallel, measurements of the diurnal courses of gas exchange and chlorophyll fluorescence parameters were made in the upper sun (5th whorl; 1-year-old needles), middle (8th and 10th whorl; 1- and 2-year-old needles) and lower shade (15th whorl; >2-year-old needles) shoots. 3. The higher diffuse radiation fraction during cloudy days resulted in significantly higher ecosystem carbon uptake than at corresponding incident photosynthetic photon flux density on sunny days. Our shoot-level data show that shoots from deep within the canopy contribute substantially to the overall carbon balance during cloudy days. But, although shade-adapted shoots had a markedly positive carbon balance over a 24-h period on cloudy days, their performance was impaired on sunny days contributing only a marginal or even negative carbon balance from the middle and shaded parts of the canopy. The uppermost sun shoots contributed 78% of the total carbon assimilated during a sunny day, but only 43% during a cloudy day. 4. In addition, afternoon depression of canopy NEP and CO2 assimilation rates of the uppermost shoots (5th and 8th whorl) occurred in response to irradiance on sunny days, characterized by significant decreases in CO2 uptake and apparent quantum yield; however, this depression did not occur under cloudy conditions. Stomatal and non-stomatal regulations of carbon assimilation in the afternoon are discussed.

Temporal variation of leaf area index (LAI) in two young Norway spruce stands with different densities was monitored during eight consecutive growing seasons (1998–2005). We focused on: (1) LAI dynamics and above-ground mass production of both spruce stands and their comparison, (2) leaf area duration (LADU), crop production index (CPI) and leaf area efficiency (LAE) evaluation, and (3) thinning impact on the above-mentioned parameters. Also, we tried to deduce the most effective LAI value for the Norway spruce forest investigated. The LAI values of both spruce stands showed a typical seasonal course. To describe the LAI dynamics of the stand, we recommend taking LAI measurements within short time intervals at the time of budding and needle expansion growth (i.e., in early spring) and close to the LAI peak, when the twig growth has been completed. The reason was that after reaching the seasonal maximum, no significant differences between subsequently obtained values were found in the following 2 months. Therefore, we recommend this period for the estimation of seasonally representative LAI values, enabling the comparison of various spruce stands. The maximum hemi-surface LAI value reached 12.4. Based on our results, the most effective LAI values for maximum above-ground biomass production were within the range of 10–11. We found an LAI over these values to be less effective for additional production of above-ground biomass. In forest practice, thinning intensity is mostly described by percentage of stocking reduction. We want to show that not only thinning intensity, but also the type of thinning is important information. The type of thinning significantly affected the stand above-ground biomass increment, canopy openness, stand LAI and LAI efficiency. The stimulating effect of high-type thinning was observed; the LAE as well as the CPI increased. Low-type thinning had no such effects on LAE increments compared to the high-type thinning with similar intensity.

Anemometer and CO2 concentration data from temporary campaigns performed atsix CARBOEUROFLUX forest sites were used to estimate the importance of non-turbulentfluxes in nighttime conditions. While storage was observed to be significant only duringperiods of both low turbulence and low advection, the advective fluxes strongly influence thenocturnal CO 2 balance, with the exception of almost flat and highly homogeneous sites. Onthe basis of the main factors determining the onset of advective fluxes, the ‘advection velocity’,which takes net radiation and local topography into account, was introduced as a criterion tocharacterise the conditions of storage enrichment/depletion. Comparative analyses of the sixsites showed several common features of the advective fluxes but also some substantial dif-ferences. In particular, all sites where advection occurs show the onset of a boundary layercharacterised by a downslope flow, negative vertical velocities and negative vertical CO2concentration gradients during nighttime. As a consequence, vertical advection was observedto be positive at all sites, which corresponds to a removal of CO2 from the ecosystem. Themain differences between sites are the distance from the ridge, which influences the boundary-layer depth, and the sign of the mean horizontal CO2 concentration gradients, which isprobably determined by the source/sink distribution. As a consequence, both positive andnegative horizontal advective fluxes (corresponding respectively to CO 2 removal from theecosystem and to CO 2 supply to the ecosystem) were observed. Conclusive results on the importance of non-turbulent components in the mass balance require, however, further experimental investigations at sites with different topographies, slopes, different land covers, which would allow a more comprehensive analysis of the processes underlying the occurrence of advective fluxes. The quantification of these processes would help to better quantify nocturnal CO2 exchange rates.

International audience; Measuring turbulent fluxes with the eddy covariance method has become a widely accepted and powerful tool for the determination of long term data sets for the exchange of momentum, sensible and latent heat, and trace gases such as CO2 between the atmosphere and the underlying surface. Several flux networks developed continuous measurements above complex terrain, e.g. AmeriFlux and EUROFLUX, with a strong focus on the net exchange of CO2 between the atmosphere and the underlying surface. Under many conditions basic assumptions for the eddy covariance method in its simplified form, such as stationarity of the flow, homogeneity of the surface and fully developed turbulence of the flow field, are not fulfilled. To deal with non-ideal conditions which are common at many FLUXNET sites, quality tests have been developed to check if these basic theoretical assumptions are valid. In the framework of the CARBOEUROFLUX project, we combined quality tests described by Foken and Wichura (1996) with the analytical footprint model of Schmid (1997). The aim was to identify suitable wind sectors and meteorological conditions for flux measurements. These tools were used on data of 18 participating sites. Quality tests were applied on the fluxes of momentum, sensible and latent heat, and on the CO2-flux, respectively. The influence of the topography on the vertical wind component was also checked. At many sites the land use around the flux towers is not homogeneous or the fetch may not be large enough. So the relative contribution of the land use type intended to be measured was also investigated. Thus the developed tool allows comparative investigations of the measured turbulent fluxes at different sites if using the same technique and algorithms for the determination of the fluxes as well as analyses of potential problems caused by influences of the surrounding land use patterns.

Radiation use efficiency values estimation based on the biomass increment (one approach) and on NPP from eddy covariance (two approaches) estimation of NPP brings the values of 0.13, 0.40, and 0.47 g (C) MJ −1 , respectively. The productivity of terrestrial ecosystems is primarily reliant on the absorption of solar radiation energy and its conversion into biomass. Monteith (1977) first introduced the concept of radiation use efficiency (RUE), which expresses the effectiveness of a plant stand to use solar radiation for the formation of new biomass and to maintain existing biomass. The presented paper uses a long-term, decadal, time series of biomass data, which is based on forest inventory data and an allometric relation, and on the application of eddy covariance (EC) estimation of Net Primary Production (NPP). These approaches provide different values of light use efficiency (LUE). LUE is based on direct carbon exchange estimation, LUE i , which denotes instantaneous efficiency based on the relationship between the daily sum of incident global radiation (GR i ) and NPP and LUES, calculated as the ratio between the sum of NPP and the sum of GR i per growing season. RUE is based on direct yearly biomass increment expressed in carbon units (carbon = 0.5 × biomass) divided by the sum of GR i per year. The obtained values amount to 0.13, 0.40, and 0.47 g(C) MJ−1 for RUE, LUES, and LUE i , respectively. The higher value of LUE i reflects a direct relation with the efficiency of photosynthetic carbon pumping. In contrast, the RUE value, based on biomass inventories, is the result of woody mass formation that is caused by several mutually related physiological processes and “wastages” of radiation utilization.