Michal V. Marek

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.

Thermal optimality of net ecosystem exchange of carbon dioxide and underlying mechanisms.

• It is well established that individual organisms can acclimate and adapt to temperature to optimize their functioning. However, thermal optimization of ecosystems, as an assemblage of organisms, has not been examined at broad spatial and temporal scales. • Here, we compiled data from 169 globally distributed sites of eddy covariance and quantified the temperature response functions of net ecosystem exchange (NEE), an ecosystem-level property, to determine whether NEE shows thermal optimality and to explore the underlying mechanisms. • We found that the temperature response of NEE followed a peak curve, with the optimum temperature (corresponding to the maximum magnitude of NEE) being positively correlated with annual mean temperature over years and across sites. Shifts of the optimum temperature of NEE were mostly a result of temperature acclimation of gross primary productivity (upward shift of optimum temperature) rather than changes in the temperature sensitivity of ecosystem respiration. • Ecosystem-level thermal optimality is a newly revealed ecosystem property, presumably reflecting associated evolutionary adaptation of organisms within ecosystems, and has the potential to significantly regulate ecosystem-climate change feedbacks. The thermal optimality of NEE has implications for understanding fundamental properties of ecosystems in changing environments and benchmarking global models.

Stand microclimate and physiological activity of tree leaves in an oak-hornbeam forest

SummaryIn an uneven-aged, multi-species oak-hornbeam forest at Báb, SW Slovakia (former IBP Forest Research Site), a series of micrometeorological and ecophysiological measurements started in 1985. The aims of the work are to improve understanding of physiological processes (photosynthesis, respiration, and transpiration) of adult trees and stand microclimate, to collect data for simulation of the canopy (stand) photosynthesis and for ecological synthesis of the functioning of the forest ecosystem. In this paper, photosynthetically active radiation (PAR), air temperature (AT) and relative humidity (RH), wind speed (WS), and CO2 concentration ([CO2]) in and above the forest are characterized for the fully leaved season, using diurnal courses, vertical profiles and isodiagrams (isopleths). Approximately 50% of incident PAR was absorbed by the upper 4–5 m layer of leaves and only approximately 5% or less penetrated to the forest floor. Vertical gradients of AT and RH were generally low, but large differences in diurnal ranges of AT and RH were observed between vertical levels. The upper leaf canopy greatly reduced WS, and at a height of about 14 m above the ground it was close to zero. The highest diurnal [CO2] maximum and variations occurred at 1 m above the ground, and the lowest above the forest. In “good” light conditions in the forest, the entire leaf canopy (overstorey and understorey canopy) is a large sink of CO2. At night the forest stand is a source of CO2, the largest internal source being the soil and forest floor.

Test of Accuracy of LAI Estimation by LAI-2000 under Artificially Changed Leaf to Wood Area Proportions

The accuracy of LAI-2000 Plant Canopy Analyzer for leaf (LAI) and plant (PAI) area indexes measurements was tested in 20-year-old Norway spruce stand using the reduction of canopy biomass. Needle and branch areas were reduced progressively upward every one meter. Values of effective leaf area index (LAIe), as an uncorrected product of LAI-2000, were compared with directly estimated LAI and PAI values after each reduction step. LAI-2000 underestimates PAI and LAI values according to LAI-2000 rings readings, and varied proportions between leaf and wood areas. The values of LAIc have been increased with decreasing of the view angle of the relevant LAI-2000 rings. Therefore, the underestimation of LAI becomes smaller when the readings near the horizon are masked. More accurate results, for projected LAI (LAIp) calculation, are produced by LAI-2000 when some dense grids of measurement points and the most vertical ring readings (0 –13 °) are used. Correction factor 1.6 is possible to use for unreduced canopy hemi-surface LAI estimation, when the last rings (i.e. 5th and 4th rings, 47 –74 °) are excluded. Correction factor of 1.25 can be used to compute LAIp if the angle readings under 43 °are also masked.

Mechanistic differences in utilization of absorbed excitation energy within photosynthetic apparatus of Norway spruce induced by the vertical distribution of photosynthetically active radiation through the tree crown

Abstract An attempt to reveal the causal relation between gradient of photosynthetically active radiation (PhAR) through Norway spruce crown and differentiation of photosynthetic apparatus under field conditions is presented. The exposure of exposed (E) needles to higher photosystem (PS) II excitation pressure than for shaded (S) ones led to diminution of light harvesting complexes (LHCs), increased photosynthetic capacity (PNmax, by 70%) and increased capacity of nonradiative dissipation (NPQ, by 30%). Further analyses of chlorophyll a (Chl a) fluorescence revealed several differences between S and E needles concerning NPQ: (a) for E needles the NPQ provides more efficient protection against overreduction of PS II reaction centres; (b) the steeper dependence of F0 quenching on NPQ (by 53%) in E needles than in S needles supports the view that nonradiative dissipation localized in LHCs was enhanced for E needles; (c) for E needles induction of NPQ was accompanied by a significant decrease in energy transfer from Chl b to Chl a (by 20% and/or 15%) as estimated from 77 K excitation spectra of Chl a fluorescence measured from PS II and PS I, respectively. This implied that down-regulation of light harvesting function operates in PS I as well as in PS II. The physiological significance of these results regarding the adaptation of Norway spruce photosynthetic apparatus to the gradient of PhAR through the tree crown is discussed.

Soil (N) modulates soil C cycling in CO2-fumigated tree stands: a meta-analysis

Under elevated atmospheric CO(2) concentrations, soil carbon (C) inputs are typically enhanced, suggesting larger soil C sequestration potential. However, soil C losses also increase and progressive nitrogen (N) limitation to plant growth may reduce the CO(2) effect on soil C inputs with time. We compiled a data set from 131 manipulation experiments, and used meta-analysis to test the hypotheses that: (1) elevated atmospheric CO(2) stimulates soil C inputs more than C losses, resulting in increasing soil C stocks; and (2) that these responses are modulated by N. Our results confirm that elevated CO(2) induces a C allocation shift towards below-ground biomass compartments. However, the increased soil C inputs were offset by increased heterotrophic respiration (Rh), such that soil C content was not affected by elevated CO(2). Soil N concentration strongly interacted with CO(2) fumigation: the effect of elevated CO(2) on fine root biomass and -production and on microbial activity increased with increasing soil N concentration, while the effect on soil C content decreased with increasing soil N concentration. These results suggest that both plant growth and microbial activity responses to elevated CO(2) are modulated by N availability, and that it is essential to account for soil N concentration in C cycling analyses.

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.

Effect of season, needle age and elevated CO2 concentration on photosynthesis and Rubisco acclimation in Picea abies.

While downward photosynthetic acclimation in response to elevated CO(2) (EC) is frequently accompanied by reduction in Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase), the exact mechanism behind this decrease and its dynamics are not well understood. We comprehensively studied Rubisco adjustment to EC in coniferous Picea abies using an electrophoretic (protein content), spectrophotometric (initial (RA(initial)) and total (RA(total)) inxa0vitro Rubisco activities), and gas-exchange (maximum carboxylation activity inxa0vivo (V(Cmax))) techniques. With respect to differing carbon sink strength and nitrogen remobilization, we hypothesized greater acclimation of photosynthesis in one-year-old as compared to current-year needles and at the end than at the beginning of the vegetation season. EC treatment led to a decrease in V(Cmax) values in current-year needles, but the ribulose-1,5-bisphosphate (RuBP)-limited rate of photosynthesis (J(max)) remained unaffected. Indeed, both V(Cmax) and J(max) were reduced by the EC treatment in one-year-old needles. The extent of photosynthetic acclimation in EC plants did not increase, however, during the vegetation season. EC decreased the activation state of Rubisco (RA(initial)/RA(total)) by 16% and 5% in current-year and one-year-old needles, respectively (averaged over the growing season). While during spring (short-term effect) EC treatment did not influence the Rubisco content per unit leaf area and decreased its specific activity (activity per unit Rubisco mass) in both needle age classes studied, exposure to EC during the entire vegetation season tended to reduce the Rubisco content while increasing its specific activity. Irrespective of CO(2) treatment and needle age, a hyperbolic-decay relationship was observed between Rubisco-specific activity and its content.

Seasonal changes of photosynthetic assimilation of Norway spruce under the impact of enhanced UV-B radiation

Abstract The cloned saplings of Norway spruce (7 years old) were exposed to enhanced UV-B irradiation (+25%) continuously over three growing seasons. An analysis of CO 2 assimilation and pigment composition was performed in the beginning (June) and the end (September) of the third growing season to evaluate the influence of long-term elevated UV-B irradiation. The UV-B was administered at levels that were, at any moment, at 125% of the ambient UV-B radiation. The results of the long-term experiment supports an idea on cumulative exposure UV-B effect. Four months under the influence of enhanced UV-B radiation was responsible for the depression of A N over the whole interval of investigated PPFD/ C i . A significant decrease in A Nmax. (up to 9%) and α (up to 22%), and increase in Γ i (up to 57%) and of R D (up to 19%) was found. The long-term effect of enhanced UV-B radiation caused a significant decrease in A Nsat (up to 60%) and τ (up to 8%), and a significant increase in Γ C (up to 27%), compared to controls. The chlorophyll a + b content was significantly decreased in the UV-B exposed variant. This change was mainly caused by a decrease in the chlorophyll a content (up to 20%). Thus, the negative effect of the long-term 21% enhancement UV-B radiation on photosynthetic gas exchange and photosynthetic pigment is concluded. Investigations of the gas-exchange parameters showed that the main limitation of assimilation can be identified at the level of RuBP regeneration related to electron transport. This finding is supported by the analysis of photosynthetic pigment content that indicates a simultaneous decrease of the chlorophyll a content and the antenna size. Thus, the potential increased sensitivity to the photoinhibition of photosynthesis resulting from the long-term influence of enhanced UV-B radiation is suggested.

Long-term exposure of Norway spruce to elevated CO2 concentration induces changes in photosystem II mimicking an adaptation to increased irradiance

Summary Fifteen-year-old Norway spruces ( Picea abies [L.] Karst.) were grown in open top chambers (OTC) at ambient (A) and elevated (i.e. ambient + 350 μmol(CO 2 ) mol −1 ) concentrations of CO 2 (E) for four growing seasons (1992–1995). During this time period several examples of the depression of photosynthetic activities were observed for E needles. In order to better characterize the nature of this depression the gas exchange and fluorescence parameters were analyzed on current year needles during the last season (July 1995). The photon flux density response curves of CO 2 uptake (P N ) revealed a significandy reduced stimulation of P N for E needles as compared with short-term exposure to doubled CO 2 . Moreover, the sudden exposure of E shoots to 350μmol(CO 2 ) mol −1 at saturating irradiance revealed a depression of both P Nmax (by 20%) and quantum yield of PS II (by 32%) compared with A shoots measured at 350μmol(CO 2 ) mol −1 . The data supporting the diminished light harvesting system of photosystem II (PS II) in E shoots compared with A shoots were obtained from pigment analysis, low temperature fluorescence spectra and Chl a fluorescence induction kinetics. The relative proportion of inactive reaction centres of PS II determined from F pl of the fluorescence induction was 20% higher for E needles. These changes found for E needles mimicked an adaptation of PS II to increased irradiance compared with A needles. As the irradiance exposure was the same for the examined needles from both E and A spruces we suggest that these changes reported for E needles resulted from the feed-back limitation of photochemical reactions due to suppressed electron transport through the plastoquinone pool.