Carpathian forest ecosystems play a major role in providing ecosystem services and ecological stability in Europe. Increasing drought frequency and intensity is a reality in plains and hills regions, becoming a threat also for mountain forest ecosystems. Forests are among the most vulnerable terrestrial ecosystems to these extremes. The impacts of climate change vary from the reduction of productivity and loss of overall ecosystem biodiversity to even the mortality of trees. Our study assessed the growth and drought resilience of three main forest species from the Carpathians: Picea abies, Abies alba, and Fagus sylvatica.
Based on an extensive tree ring data network consisting of over 6000 trees from 158 plots for P. abies, 64 plots for A. alba, and 65 plots for F. sylvatica, distributed along an elevation gradient, we (i) evaluate the basal area increment variability and (ii) quantify the resilience to the most extreme drought years from last 100 years. To assess the tree's capacity to react to water deficit, we used the resilience indices: resistance, recovery, and recovery period.
Our results show an evident growth decline of P. abies from elevation below 1400 m, with a similar trend in the case of A. alba from elevation below 800 m. A. alba and F. sylvatica show an increasing trend of basal area increment in plots from medium and high elevations. F. sylvatica and A. alba have a higher resistance and resilience to water deficit at low elevations than P. abies. Generally, the recovery period from drought events increases with elevation from all species. 

How to cite: Popa, A., Balabasciuc, M., and Popa, I.: Trees resilience to drought in forest ecosystems from Eastern Carpathians, Romania, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1712, https://doi.org/10.5194/egusphere-egu25-1712, 2025.

Extreme climate events pose a potential threat to the forest structure stability, triggering tree growth release (GR) and possible shifts in tree social status. This study used a dendrochronological approach to investigate changes in tree competition in a 100-year-old Japanese cedar (Cryptomeria japonica) plantation at the JinGangZuan site, managed by the Experimental Forest of National Taiwan University. Tree-ring data was used to reconstruct historical GR events and explored the relationship between GR and the maximum wind speed. In addition, we evaluated tree social status shifts by comparing each sample tree’s diameter at breast height (DBH) with the stand’s quadratic mean DBH from forest inventory data.

Our findings identified a major GR event around 1970 and revealed a significant positive correlation (r = 0.73, P = 0.01) between GR and maximum wind speed. Most high wind events occurred during typhoon periods. Despite these disturbances, the overall social structure remained largely unchanged among the 113 sampled trees, suggesting that GR does not necessarily drive significant status shifts. Specifically, 79.6% of trees retained their original social status (“big trees remain big and small trees remain small”), while 20.4% exhibited some degree of status change. Most codominant-to-dominant status changes occurred in early growth stages, while status change in later stages appear to be randomly distributed. We suspected disturbances tend to remove weaker trees, complicating direct comparisons among individuals.

Notably, typhoon strength has increased over the past four decades and is expected to intensify further under global warming. Although this study suggests minimal impacts of disturbances on forest structure at the JinGangZuan site, proactive adaptation measures are advisable. Tree-ring analysis offers valuable insights into stand development following extreme events, providing critical guidance for refining forest management strategies in an era of changing environmental conditions.

How to cite: Tsai, C.-C., Wang, Y.-F., and Cheng, S.-T.: Examining Forest Structure and Growth Dynamics Using Dendrochronology, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15521, https://doi.org/10.5194/egusphere-egu25-15521, 2025.

In numerous ecological systems, forthcoming critical transitions can be identified using a variety of methods for deriving early warning indicators. Several methods focus on characteristics of time-series related to system behaviour or properties, including changes in time-series variability. One such method is conditional heteroskedasticity (CH). CH defines a time series as having a non-constant variability, that is also dependent on the variability at previous time-steps, where increases in variability indicate that the system under study is nearing a critical transition. Here, we apply this concept to time series of radial growth, measured as tree-ring widths: a general autoregressive conditional heteroskedasticity (GARCH) model is used to produce a CH time-series from detrended tree-ring data. By analysing the variability trends within this time series, conclusions can be made relating to the system’s proximity to transition. Whilst this form of analysis is not a novel concept in the field of ecology, such a thorough examination of the models’ ability to detect change in the variability of tree-ring data is yet to be carried out. We propose the application of a dual-model approach, using both GARCH and VS-Lite models, with an aim of determining the efficacy of such a strategy to detect not only changes in tree-growth stability, but more specifically changes induced by climate stressors. This approach has the potential to forecast impending critical transitions in tree-growth behaviour, possible fluctuations in the rate of mortality, and quantify the influence of climate on growth stability at both the tree and site-level.   

How to cite: Egan, S. and Zang, C.: Using GARCH Models to Detect Forthcoming Transitions in Tree Growth, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8655, https://doi.org/10.5194/egusphere-egu25-8655, 2025.

The ability of forests to continue providing important ecosystem services and mitigating climate change depends on their ability to adapt to global change pressures, such as more frequent climate extremes (specifically drought and heatwaves) and changes in atmospheric pollutants, such as reactive nitrogen compounds. On the one hand, nitrogen deposition could stimulate tree growth in a CO2 richer word, but on the other hand increasing atmospheric nitrogen input, above the critical load, could result in forest dieback, through soil acidification and nutrient imbalances but also by making trees more vulnerable to climate extremes. How do these global change components interact and affect forest carbon, water and nitrogen cycling? What are tree ecolophysiological mechanisms involved? Are those mechanisms synchronized (in terms of magnitude and temporal trends) at tree and ecosystem scales? Does nitrogen deposition affect tree and forest responses to climate extremes? 
In order to answer these fundamental questions, we considered 12 forests along a climate and nitrogen deposition gradient (from 3 to 42 kg ha-1 yr-1) in Europe, including four of the most widespread tree species in European forests: Fagus sylvatica, Quercus spp., Picea abies, Pinus sylvestris. Forests sites were selected within established networks, namely ICOS and eLTER (for the ecosystem scale measurements of carbon and water fluxes with eddy covariance technique and other ecological parameters) and ICP Forests (for atmospheric nitrogen deposition). We will present preliminary results on the combinination of existing data on ecosystem fluxes with dendroecological data (growth and stable carbon, oxygen and nitrogen isotope ratios) to explore multidecadal changes in forest water-use efficiency and elucidate tree physiological mechanisms underpinning those responses. Moreover, in specific years characterized by climate extremes, an intra-annual isotope approach will be considered to evaluate possible divergences among tree species in the physiological signal and between tree and ecosystem responses, but also to elucidate the contribution of nitrogen deposition in affecting responses to climate extremes. 
Collaborators at the ICOS and ICP Forests sites selected for the study are greatly acknowledged

How to cite: Matteucci, G., Montedoro, M., Rossi, M., Mazzenga, F., and Guerrieri, R.: Effects of nitrogen deposition and climate extremes on European forests: combining stable isotopes in tree rings and ecosystem fluxes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12400, https://doi.org/10.5194/egusphere-egu25-12400, 2025.

Understanding changes in the physiological responses of trees to disturbances, and establishing proxies to reconstruct past events, is of high importance in a changing world. Recent studies have demonstrated the potential of δ²H in tree-ring cellulose as a proxy for physiological changes in carbon utilization, reflecting shifts between the use of current assimilates and stored C sources. These findings might explain the considerable annual variations in the strength of the δ¹⁸O and δ²H (O-H) relationship despite the shared hydrological pathway, underlining the complex interaction of hydrological and physiological processes. One of the situations where there is a clear disruption of carbon assimilation and tree functioning is defoliation events. Thus, tree-ring isotopes can be utilized to test the physiological signal recorded in tree rings by quantifying changes in δ¹³C, δ²H and δ¹⁸O values, and the decoupling of the O-H relationship. Here, we investigated the isotopic fingerprint of abiotic and biotic defoliation events in tree-ring cellulose, including (i) late-spring frost on European beech near its upper elevational limit in the Swiss Jura(ii) pine processionary moth outbreaks in northern Italy, and (iii) cockchafer moth outbreaks on archaeological oak material from Central European lowlands. Across all defoliation types, a common fingerprint was identified with significantly enriched δ²H, depleted δ¹⁸O, resulting in the decoupled (negative) O-H relationship, and non-affected δ¹³C values.  As defoliation causes reduced fresh carbon assimilation, the remobilization of stored non-structural carbohydrates (NSC) is likely the fundamental process for plant growth, metabolism, and canopy re-flushing. NSC differ in their isotopic ratio compared to fresh photosynthates, by exhibiting ²H-enrichment and ¹⁸O-depletion, explaining the negative O-H relationship in tree-ring cellulose. Since defoliation has been shown to induce allocation shifts by prioritizing NSC storage over radial growth, foliage loss also leads to substantial secondary growth reductions which was observed across all defoliation types. The generally non-significant changes in δ¹³C between outbreak and non-outbreak years indicate minor impacts on leaf stomatal conductance. In conclusion, this common isotopic fingerprint provides valuable insight into past defoliation events and their reconstruction, which is particularly relevant in the context of rapid environmental change.

How to cite: Vitali, V., Hofmann, C. F., Obojes, N., Martínez-Sancho, E., Holloway-Phillips, M., Reim, J. S., Argelich Ninot, M., Muigg, B., Tegel, W., Camarero Marinez, J. J., Saurer, M., and Vitasse, Y.: Tracking the isotopic fingerprint of defoliation in tree rings, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5180, https://doi.org/10.5194/egusphere-egu25-5180, 2025.

Tree-ring blue intensity (BI), a proxy measuring blue light reflection from tree-ring cores, has emerged as a promising tool for climate reconstruction, yet its application in East Asia remains limited. Here we evaluate the dendroclimatological potential of tree-ring blue intensity using Korean pine (Pinus koraiensis Siebold & Zucc.) samples from Mt. Seorak in the Korean Peninsula. We developed chronologies using tree-ring width (TRW) and three BI parameters (earlywood BI, maximum latewood BI, and delta BI), with delta BI (DBI) exhibiting robust correlations with summer (July-August) mean surface air temperature (SAT). Despite weak series intercorrelation in the BI chronologies, the DBI measurements preserved consistent temperature signals from the early 20th century to present, in contrast to the inconsitent climate response in the TRW data. The DBI chronology further demonstrated potential for reconstructing large-scale atmospheric patterns, including the circumglobal teleconnection pattern and subtropical jet stream. Our findings establish BI methodology as a valuable tool for enhancing climate reconstructions in subtropical East Asia, particularly in regions where traditional TRW measurements prove inadequate for capturing summer temperature signals.

How to cite: Kim, M.-S., Fuentes, M., Linderholm, H., Lidman, F., Koh, Y., Choi, C., Woo, S.-H., and Jeong, J.-H.: The First Tree-Ring Blue Intensity Measurements from the Korean Peninsula: Exploring Summer Temperature Signals, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6466, https://doi.org/10.5194/egusphere-egu25-6466, 2025.

    Dynamic fluctuations in sea surface temperature within the North Atlantic Ocean (NA-SST) exert a pivotal influence on global climate change. Yet, the specific impacts of these variations on the temperature shifts within the Qinling-Bashan Mountains (QBM) of China, remain to be fully elucidated. Furthermore, a debate surrounding the manifestation of the Little Ice Age (LIA) within the QBM is ongoing, primarily due to the scarcity of reliable, long-term, and high-resolution temperature records. To bridge these knowledge gaps, in this paper, we present a ~250-year temperature reconstruction (Tmax_11-7) developed based on a tree-ring-width chronology from the QBM. The new reconstruction not only aligns with a local winter temperature reconstruction based on historical document evidence, but also reveals robust regional and hemispheric temperature signals. Intriguingly, the study shows that the warming trend observed in the QBM since the Industrial Era is less pronounced than that witnessed across China and the Northern Hemisphere. The Tmax_11-7 reconstruction provides a glimpse into the final stages of the LIA, highlighting a relatively colder 19th century followed by a warmer 20th century. Notably, the warming trend post-1970s remains within the bounds of the 250-year temperature framework. The research further uncovers that NA-SST has predominantly governed the QBM's temperature fluctuations over the past centuries, as evidenced by the tight positive correlation between Atlantic multidecadal Oscillation (AMO)/Atlantic multidecadal variability (AMV) and Tmax_11-7. However, this dominant influence of NA-SST was temporarily subdued between 1900 and 1930 CE due to the interference of Pacific Decadal Oscillation (PDO). This finding serves as a reminder that future temperature projections for the QBM must account not only for the primary role of NA-SST but also the modulating effects of the PDO. The study is essential for shaping effective environmental management and adaptation strategies in the region.

How to cite: Cai, Q. and Liu, Y.: The influence of North Atlantic sea surface temperature fluctuations on the climate of the Qinling-Bashan Mountains over the past 250 years, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8087, https://doi.org/10.5194/egusphere-egu25-8087, 2025.

    Climate change has played a pivotal role in shaping Chinese history, especially during the Ming Dynasty. Previous studies have focused primarily on the Chongzhen megadrought, which is widely considered as the primary climatic perturbation behind the demise of the Ming Dynasty. However, relatively little is known about other severe drought events and their potential impact on the dynasty collapse. Additionally, the characteristics of an exceptional climatic anomaly termed the “Late Ming Weak Monsoon Period” are unclear. In this study, we reconstructed the historical variations of Palmer Drought Severity Index for July–September based on tree-ring stable oxygen isotopes (δ¹⁸O) from 1556 CE to 2015 CE in southwest Loess Plateau. Our study reveals a significant weakening of the Asian summer monsoon between 1561 CE and 1661 CE, consistent with the Late Ming Weak Monsoon Period, and unravels its structural characteristics in details. Our reconstruction also captures a distinct humidification trend over northwest China since the early 2000s and suggests that this current humidification trend will persist in this region for the next few years. Notably, in addition to the well-known Chongzhen megadrought, our study records the severe Wanli megdrought (1585–1590 CE) during the late Ming Dynasty, an event that rarely featured in earlier studies, exhibiting comparable duration and severity to the Chongzhen megadrought. Further analysis indicates that Wanli megadrought may have served as an early trigger for the collapse of the Ming Dynasty. Furthermore, our analysis implicates the El Niño–Southern Oscillation as a contributing factor in both the Wanli and Chongzhen megadroughts, and thus to the ultimate collapse of the Ming Dynasty by affecting the Asian summer monsoon intensity.

How to cite: Ren, M. and Liu, Y.: The collapse of the Ming Dynasty actually began with the Wanli megadrought: insights from a hydroclimate reconstruction based on tree-ring δ18O over the past 460 years, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8716, https://doi.org/10.5194/egusphere-egu25-8716, 2025.

Peatland trees are valuable archives of paleoclimatic information; however, gaps persist in understanding the relationships between tree growth, peatland hydrology, and hydroclimate variables. While previous research in peatlands has mainly focused on tree-ring widths (TRW), yielding inconclusive results, the potential of stable carbon (δ¹³C) and oxygen (δ¹⁸O) isotopes in tree rings remains unexplored. In this study, we develop TRW, δ¹³C, and δ¹⁸O chronologies of Scots pine trees located in a Swedish peatland and a reference site on bedrock with a mineral soil layer. We assess their responses to hydroclimate conditions and evaluate their potential for reconstructing hydroclimate variations. Our findings show significant differences in mean TRW and δ¹³C values between the peatland and reference sites. Moreover, while all three proxies exhibit uniform year-to-year variations across sites, we observe discrepancies in long-term trends, particularly in δ¹³C. Although the climate sensitivity of TRW is weak and non-homogenous, the δ¹³C and δ¹⁸O peatland and reference chronologies contain robust and consistent signals, with a maximum sensitivity to water table, precipitation, and vapor pressure deficit (VPD) variations during summer. Both δ¹³C and δ¹⁸O chronologies show stable relationships with three key hydroclimate variables over time. In conclusion, while TRWs from living peatland pines at our sites have limited potential to record high-frequency hydroclimate information, δ¹³C and δ¹⁸O chronologies can serve as excellent proxies for the reconstruction of past hydroclimate changes.

How to cite: Janecka, K., Treydte, K., Piccinelli, S., Francon, L., Argelich Ninot, M., Edvardsson, J., Corona, C., Lehsten, V., and Stoffel, M.: Peatland trees contain strong and temporally stable hydroclimate information in tree-ring δ13C and δ18O records, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13082, https://doi.org/10.5194/egusphere-egu25-13082, 2025.