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Atmospheric carbon dioxide (CO 2 ) has increased substantially since the industrial revolution began, and physiological responses to elevated atmospheric CO 2 concentrations reportedly alter the biometry and wood structure of trees. Additionally, soil nutrient availability may play an important role in regulating these responses. Therefore, in this study, we grew 288 two-year-old saplings of sessile oak ( Quercus petraea (Matt.) Liebl.) in lamellar glass domes for three years to evaluate the effects of CO 2 concentrations and nutrient supply on above- and belowground biomass, wood density, and wood structure. Elevated CO 2 increased above- and belowground biomass by 44.3% and 46.9%, respectively. However, under elevated CO 2 treatment, sapling wood density was markedly lower (approximately 1.7%), and notably wider growth rings—and larger, more efficient conduits leading to increased hydraulic conductance—were observed. Moreover, despite the vessels being larger in saplings under elevated CO 2 , the vessels were significantly fewer ( p = 0.023). No direct effects of nutrient supply were observed on biomass growth, wood density, or wood structure, except for a notable decrease in specific leaf area. These results suggest that, although fewer and larger conduits may render the xylem more vulnerable to embolism formation under drought conditions, the high growth rate in sessile oak saplings under elevated CO 2 is supported by an efficient vascular system and may increase biomass production in this tree species. Nevertheless, the decreased mechanical strength, indicated by low density and xylem vulnerability to drought, may lead to earlier mortality, offsetting the positive effects of elevated CO 2 levels in the future.

Quantitative wood anatomy is critical for establishing climate reconstruction proxies, understanding tree hydraulics, and quantifying carbon allocation. Its accuracy depends upon the image acquisition methods, which allows for the identification of the number and dimensions of vessels, fibres, and tracheids within a tree ring. Angiosperm wood is analysed with a variety of different image acquisition methods, including surface pictures, wood anatomical micro-sections, or X-ray computed micro-tomography. Despite known advantages and disadvantages, the quantitative impact of method selection on wood anatomical parameters is not well understood. In this study, we present a systematic uncertainty analysis of the impact of the image acquisition method on commonly used anatomical parameters. We analysed four wood samples, representing a range of wood porosity, using surface pictures, micro-CT scans, and wood anatomical micro-sections. Inter-annual patterns were analysed and compared between methods from the five most frequently used parameters, namely mean lumen area ( ), vessel density ( ), number of vessels ( ), mean hydraulic diameter ( ), and relative conductive area ( ). A novel sectorial approach was applied on the wood samples to obtain intra-annual profiles of the lumen area ( ), specific theoretical hydraulic conductivity ( ), and wood density ( ). Our quantitative vessel mapping revealed that values obtained for hydraulic wood anatomical parameters are comparable across different methods, supporting the use of easily applicable surface picture methods for ring-porous and specific diffuse-porous tree species. While intra-annual variability is well captured by the different methods across species, wood density ( ) is overestimated due to the lack of fibre lumen area detection. Our study highlights the potential and limitations of different image acquisition methods for extracting wood anatomical parameters. Moreover, we present a standardized workflow for assessing radial tree ring profiles. These findings encourage the compilation of all studies using wood anatomical parameters and further research to refine these methods, ultimately enhancing the accuracy, replication, and spatial representation of wood anatomical studies.

In Central Europe, the most important pioneer species are silver birch and European aspen. Changes in disturbance regime and an economic interest for this species have led to studies on this species and stands. Two naturally regenerated dense stands of birch (Betula pendula Roth – silver birch monoculture) and aspen (Populus tremula L. – European aspen monoculture) were selected from a Querceto – Fagetum mesotrophicum site to observe responses under the same conditions in Central Europe. Both stands regenerated after the allochthonous Norway spruce stands dieback at the site in 1999. Within a 10 m × 25 m transect established in both stands, the diameter at breast height (DBH) of all the trees was measured between 2015 and 2020. In addition, the height and position were recorded for all trees, and sample trees of both species were felled for biomass measurement. A higher volume production of aspen at the beginning (107.48/96.80 m3) and at the end of the experiment (178.32/143.08 m3) was accompanied with a lower above-ground wood biomass (WAB). The WAB of birch increased from 81.9 t·ha–1 to 103.3 t·ha–1 and aspen allocated 79.5 t·ha–1 to 94.8 t·ha–1 of biomass. The current annual increment of biomass for these stands was 4.3 t·ha–1 and 3.1 t·ha–1 in the age range of 17 to 22 years. The culmination of the volume increment has not yet occurred in any of the stands, but the mean annual increment of wood biomass has already been reached for both stands. Furthermore, the aspen stand tended to be more dynamic in terms of biomass allocation and mortality. Also, the lower self-tolerance of aspen confirmed our hypothesis: the two native pioneer species differ in their social behaviour within monospecific stands.

Tree stems represent a long-lived biomass compartment for atmospheric carbon sequestration. While terrestrial biosphere models predict rising carbon sequestration in forests, direct observations of tree growth are inconclusive due to varying standardization procedures of tree-ring series and complex factors influencing stem growth such as moisture and nutrient deficits and anthropogenic carbon and nitrogen fertilization. The mismatch between tree-ring-based observations, repeated inventories at permanent plots, and predictions of biospheric models represents a significant knowledge gap limiting forecasting of future forest growth. Using the novel approach free of tree-ring standardization trials and focusing on even-aged trees sampled from uneven-aged forest stands, we present a robust comparison of tree stem diameter changes in temperate forests between 1990 and 2015 along environmental gradients in Central Europe. The stem sizes of four out of five species showed significant enlargement while also partly increasing stem growth limitation due to moisture availability. The largest increase in stem diameter was recorded for late succession species on fertile sites. By contrast, the stem size of early-succession species on dry and nutrient-poor sites remained unaltered. Stems of mature trees in present-day forests are, on average, 8% thicker than their counterparts in 1990 consistent with trends predicted by terrestrial biosphere models. We demonstrated that, despite increasing drought limitation, temperate tree species enlarged their stems. Viewed in conjunction with older permanent plot data, Central-European temperate forests exhibited almost half century of continuous stem enlargement, potentially impacting forest functioning in terms of size-sensitive characteristics such as susceptibility to drought and disturbances.

Droughts, amplified by climate change, pose a significant threat to the success of both artificially and naturally regenerated forests. Understanding how these changes affect the initial stages of saplings development is crucial for forest establishment, particularly for ecologically and economically important species like Norway spruce and sessile oak in Central Europe. This study investigated the impact of crown reduction (CR) by 50% of crown length on saplings of each species. Automatic dendrometers were installed on 24 saplings per species to precisely monitor growth and water-related stem changes. The main objective was to investigate the potential ameliorative effect of CR on water-stressed saplings during their initial development. Our study hypothesized that CR, by decreasing leaf area and consequently water use, would improve water availability and facilitate sapling growth. The results indicate that CR may enhance soil water availability thereby supporting the growth of water-stressed Norway spruce saplings but not those of sessile oak. The tree water deficit – an indicator of tree water status – significantly improves in Norway spruce saplings subjected to CR ( < 0.05). Conversely, this treatment resulted in the depletion of stem water status in sessile oak saplings. The species-specific growth phenology revealed that CR led to an increase in the number of growing days for Norway spruce compared to sessile oak saplings. In summary, CR may be considered a beneficial method for alleviating stress in Norway spruce saplings, especially during drought. In addition, further testing in field conditions is necessary to confirm these results.

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 CO 2 uptake during dry days. The occurrence of CO 2 uptake hysteresis in daily production was also investigated. Our study was performed at Bílý Kříž (spruce) and Štítná (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.

Fast and precise leaf area index (LAI) estimation of a forest stand is frequently needed for a wide range of ecological studies. In the presented study, we compared side-by-side two instruments for performing LAI estimation (i.e. LaiPen LP 100 as a “newly developed device” and LAI-2200 PCA as the “world standard”), both based on indirect optical methods for performing LAI estimation in pure Norway spruce (Picea abies (Linnaeus) H. Karsten) stands under different thinning treatments. LAI values estimated by LaiPen LP 100 were approximate 5.8% lower compared to those measured by LAI-2200 PCA when averaging all collected data regardless of the thinning type. Nevertheless, when we considered the differences among LAI values at each measurement point within a regular grid, LaiPen LP 100 overestimated LAI values compared to those from LAI-2200 PCA on average by 1.4%. Therefore, both instruments are comparable. Similar LAI values between thinning from above (A) and thinning from below (B) approaches were indirectly detected by both instruments. The highest values of canopy production index and leaf area efficiency were observed within the stand thinned from above (plot A).

Key messageEffect of drought during 2017 and 2018 resulted in radial stem increment reduction to 78% and 61%, respectively, of the levels occurring in normal year 2016 in Central Europe.Norway spruce (Picea abies (L.) Karst.) is currently the most threatened commercial tree species in Central Europe. This is due to increased drought stress from advancing climate change as well as the species’ distribution outside its natural range. Tree water status and water movement through a tree are key parameters influencing tree growth and vitality. This study is focused on the growth and stress reaction of spruce to climatic conditions, analysing stem diameter variation along an elevation gradient (381–995 m a.s.l.) in the Czech Republic. Tree water deficit based on the zero-growth concept (TWD), calculated from high-frequency dendrometer records and the temporal dynamics of radial growth, was studied for 3 years (2016–2018). Two of these 3 years were affected by severe drought during the growing season. Contrary to our expectations, the observed TWD showed no clear linear decline with rising elevation. The most severe tree desiccation was observed in experimental sites at middle elevations of about 600 m a.s.l. Here, we show that both the timing and level of tree water deficit had an impact on annual stem radial increment (SRIannual). Severe drought had a substantial negative impact on SRIannual of Norway spruce in both 2017 and 2018. Drought conditions in 2017 and 2018 resulted in reduction of SRIannual relative to measurements for the wetter year in 2016 to 78% and 61%, respectively. We report the evidence that the current climatic conditions in the Central European region are not suitable for growing Norway spruce at lower and middle elevations and that forest management needs to react immediately to this situation.

Atmospheric carbon dioxide (CO.sub.2) has increased substantially since the industrial revolution began, and physiological responses to elevated atmospheric CO.sub.2 concentrations reportedly alter the biometry and wood structure of trees. Additionally, soil nutrient availability may play an important role in regulating these responses. Therefore, in this study, we grew 288 two-year-old saplings of sessile oak (Quercus petraea (Matt.) Liebl.) in lamellar glass domes for three years to evaluate the effects of CO.sub.2 concentrations and nutrient supply on above- and belowground biomass, wood density, and wood structure. Elevated CO.sub.2 increased above- and belowground biomass by 44.3% and 46.9%, respectively. However, under elevated CO.sub.2 treatment, sapling wood density was markedly lower (approximately 1.7%), and notably wider growth rings-and larger, more efficient conduits leading to increased hydraulic conductance-were observed. Moreover, despite the vessels being larger in saplings under elevated CO.sub.2, the vessels were significantly fewer (p = 0.023). No direct effects of nutrient supply were observed on biomass growth, wood density, or wood structure, except for a notable decrease in specific leaf area. These results suggest that, although fewer and larger conduits may render the xylem more vulnerable to embolism formation under drought conditions, the high growth rate in sessile oak saplings under elevated CO.sub.2 is supported by an efficient vascular system and may increase biomass production in this tree species. Nevertheless, the decreased mechanical strength, indicated by low density and xylem vulnerability to drought, may lead to earlier mortality, offsetting the positive effects of elevated CO.sub.2 levels in the future.

Atmospheric carbon dioxide (CO.sub.2) has increased substantially since the industrial revolution began, and physiological responses to elevated atmospheric CO.sub.2 concentrations reportedly alter the biometry and wood structure of trees. Additionally, soil nutrient availability may play an important role in regulating these responses. Therefore, in this study, we grew 288 two-year-old saplings of sessile oak (Quercus petraea (Matt.) Liebl.) in lamellar glass domes for three years to evaluate the effects of CO.sub.2 concentrations and nutrient supply on above- and belowground biomass, wood density, and wood structure. Elevated CO.sub.2 increased above- and belowground biomass by 44.3% and 46.9%, respectively. However, under elevated CO.sub.2 treatment, sapling wood density was markedly lower (approximately 1.7%), and notably wider growth rings-and larger, more efficient conduits leading to increased hydraulic conductance-were observed. Moreover, despite the vessels being larger in saplings under elevated CO.sub.2, the vessels were significantly fewer (p = 0.023). No direct effects of nutrient supply were observed on biomass growth, wood density, or wood structure, except for a notable decrease in specific leaf area. These results suggest that, although fewer and larger conduits may render the xylem more vulnerable to embolism formation under drought conditions, the high growth rate in sessile oak saplings under elevated CO.sub.2 is supported by an efficient vascular system and may increase biomass production in this tree species. Nevertheless, the decreased mechanical strength, indicated by low density and xylem vulnerability to drought, may lead to earlier mortality, offsetting the positive effects of elevated CO.sub.2 levels in the future.