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Heatwaves exert disproportionately strong and sometimes irreversible impacts on forest ecosystems. These impacts remain poorly understood at the tree and species level and across large spatial scales. Here, we investigate the effects of the record-breaking 2018 European heatwave on tree growth and tree water status using a collection of high-temporal resolution dendrometer data from 21 species across 53 sites. Relative to the two preceding years, annual stem growth was not consistently reduced by the 2018 heatwave but stems experienced twice the temporary shrinkage due to depletion of water reserves. Conifer species were less capable of rehydrating overnight than broadleaves across gradients of soil and atmospheric drought, suggesting less resilience toward transient stress. In particular, Norway spruce and Scots pine experienced extensive stem dehydration. Our high-resolution dendrometer network was suitable to disentangle the effects of a severe heatwave on tree growth and desiccation at large-spatial scales in situ, and provided insights on which species may be more vulnerable to climate extremes., This work utilised the network of dendrometer observations established by the COST Action network STReESS (grant FP1106). We acknowledge the involved networks TreeNet, Swiss Long-term Forest Ecosystem Research Programme LWF, French National Network for Long-term FOrest ECOsystem Monitoring RENECOFOR, the German Long Term Ecosystem Research Network LTER-D, the Italian Long Term Ecosystem Research Network ILTER, the Integrated Carbon Observation System ICOS and Tree-Watch.net. R.L.S. acknowledges funding from the Special Research Fund (BOF) of Ghent University for Postdoctoral Fellowships and the Spanish Ministry of Science, Innovation, and Universities (Juan de la Cierva Programme, grant IJC2018-036123-I). R.L.P., R.Z., P.F., and G.v.A. acknowledge funding from the Federal Office for the Environment FOEN (00.0365.PZ I 0427-0562, 09.0064.PJ/R301-0223; project treenet.info) and the Swiss National Science Foundation SNF (20FI21_148992, 20FI_173691, P2BSP3_184475; project LOTFOR 150205 and Grant 20FI20_173691; project ICOS-CH). J.M.V. and M.M. acknowledge funding from the Spanish MINECO via competitive grants CGL2013-46808-R and CGL2017-89149-C2-1-R. R.P. acknowledges funding from the grant RTI2018-095297-J-I00 (Spain) and by a Humboldt Research Fellowship for Experienced Researchers (Germany).

Despite growing interest in predicting plant phenological shifts, advanced spring phenology by global climate change remains debated. Evidence documenting either small or large advancement of spring phenology to rising temperature over the spatio-temporal scales implies a potential existence of a thermal threshold in the responses of forests to global warming. We collected a unique data set of xylem cell-wall-thickening onset dates in 20 coniferous species covering a broad mean annual temperature (MAT) gradient (−3.05 to 22.9°C) across the Northern Hemisphere (latitudes 23°–66° N). Along the MAT gradient, we identified a threshold temperature (using segmented regression) of 4.9 ± 1.1°C, above which the response of xylem phenology to rising temperatures significantly decline. This threshold separates the Northern Hemisphere conifers into cold and warm thermal niches, with MAT and spring forcing being the primary drivers for the onset dates (estimated by linear and Bayesian mixed-effect models), respectively. The identified thermal threshold should be integrated into the Earth-System-Models for a better understanding of spring phenology in response to global warming and an improved prediction of global climate-carbon feedbacks.

Global warming and its influence on the environment has become a popular and widespread issue. Nowadays, an analogy of a high latitude ecosystem during the greenhouse type of climate does not exist. The Cretaceous polar ecosystem gave us a unique possibility of understanding these extreme ecosystems and the specific adaptations of organisms to these conditions. Such types of ecosystem are crucial for a better understanding of possible future climate changes. This thesis focusses on the adaptation strategies of land plants during the Late Cretaceous in the Arctic peninsula. The land plants responded to these specific solar conditionals in two different ways: evergreen or deciduous. The fossil material for this study comes from Brandy Bay and Crame Col, James Ross Island, Antarctic. The material was collected in a continuous sequence from Kotic point to Santa Marta Formation (Cenomanian - Companian). A detailed and systematic analysis was performed on five out of fifty-five samples that well represented the studied region and age: Agathoxylon kellerense, Agathoxylon antarcticus, Araucarioxylon chapmanae, Podocarpoxylon multiparenchymatosum and Phoroxylon sp. Based on the detailed study of Agathoxylon kellerense (sample number AN34) wood anatomy and growth rings structure, adaptation strategies were...

VENCLOVÁ, S. (2018): Kritické zhodnocení popsaných zkřemenělých dřev z třeboňské pánve (jižní Čechy) a zpracování nových nálezů [Re-evaluation of the published silicified woods from the Třeboň Basin (South Bohemia) and systematical description of new finds. Mgr. Thesis, in Czech.] - 92 pp., Faculty of Science, Charles University, Prague, Czech Republic. Abstract: In this study sillicified woods were observed from three localities from Třeboň basin. Woods were investigated by thin sections. It was observed one species belonging to Gymnosperms, for now it is determined to taxodioid type conifer. The rest of woods belong to Angiosperms, high probably belonging to genus Paraphyllanthoxylon. By PCA analysis was investigated mutual similarity of the woods within and with related species. Investigated woods can be considered as new species of genus Paraphyllanthoxylon. Stratigraphycal position was not determined. It was discused the affiliation of sillicified woods with Lipnice series. Key words: South Bohemian Basins, Lipnice formation, Paraphyllanthoxylon, sillicified woods

RESUMÉ Tato práce se zabývá vlivem teplotních poměrů na intra-anuální růst smrku ztepitélo (Picea Abies (L.) Karsten) na alpínské hranici lesa v Krkonoších ve vegetačních sezónách 2011 a 2012. Monitoring probíhal na dvou lokalitách na jižním svahu Luční hory. První lokalita se nacházela v nadmořské výšce 1310 m n. m., přibližně na horní hranici zapojeného lesa, druhá pak v nadmořské výšce 1450 m n. m., ve stromových skupinkách nad horní hranicí lesa. Mezi lokalitami byl pozorován poměrně vysoký vertikální teplotní gradient jak u teplot vzduchu (1,2 - 1,7 řC), tak u teplot půdy v hloubce 10 cm (0,8 - 1,2 řC). Teplotní rozdíl mezi lokalitami se pak projevil v nižším pozorovaném přírůstu na výše položené lokalitě. Pozorované počátky kambiální aktivity se pohybovaly od 26. 4. do 21. 5. Začátek kambiální aktivity je ovlivněn množstvím sněhové pokrývky akumulované na lokalitách v průběhu zimy a s tím souvisejícím datem jejího odtátí, teplotami půdy a teplotami v přízemní vrstvě atmosféry. Délka kambiální aktivity se pohybovala mezi 55 - 77 dny, přičemž její délka byla nižší na výše položené lokalitě. Diferenciace buněk nastala bezprostředně po počátku kambiální aktivity a trvala od 111 do 132 dní. V porovnání sledovaných růstových sezón vyšla lépe růstová sezóna 2012, ve které byl průměrný přírůst stromů vyšší na...

The thesis presents systematical study and taphonomical evaluation of fusite wood specimens collected in the middle bench of the main lignite seam in the Bilina mine. In the introduction part, there are a general characteristic of the Most Basin and its geological setting with respect to the main lignite seam. Consequently, the charcoal is described in detail. Generally, the charcoaled wood, which was produced during wildfires in the Tertiary forests, shows well preserved anatomical details. The final part resumes systematics and taphonomy of the charcoal specimens collected by the author.