Climate warming predispose sessile oak forests to drought-induced tree mortality regardless of management legacies

Climate warming-related drought could become a major driver of large-scale forest dieback. However, little is known about how past management legacies modulate the climate-growth responses during recent dieback episodes in central European oak forests. Here, we examine the role played by past management –unmanaged old-growth vs. managed forests– in recent tree mortality events occurring in Quercus petraea (Matt.) Liebl. stands across large areas of western Romania. We analyze how stand structure (tree size, competition) and climatic factors (drought, temperature and precipitation) drive tree radial growth patterns in neighboring standing dead and living trees. We analyzed basal area increments (BAI) trends, past management legacies and climate- and drought-growth relationships during the 20th century to distinguish the roles and interactions on recent warming-induced dieback. We observed that temperature rises and changes in atmospheric water demand during growing season let to increasing drought stress during the late 20th century affecting both managed and unmanaged forests. Dead trees from old-growth and managed forests showed lower growth than living trees after dieback onset. In both forests, dead and living trees displayed divergent growth patterns after dry 1980s, indicating that dieback was triggered by severe extreme conditions. Dead trees from managed stands experienced significant stronger growth reductions after 1980s though they experienced less tree-to-tree competition than dead trees in old-growth forest. High stand density negatively drove growth and enhanced climate sensitivity in old-growth stands. Competition acted synergistically with climate warming and drought causing tree mortality regardless of the management legacies in of Q. petraea forests. Our retrospective assessment of growth rates in relation with climate and structure changes offers valuable information for further forest conservation and management decisions of Q. petraea forests. These findings highlight the importance of past uses legacies driving recent forest dieback in temperate oak forests, making them more vulnerable under forecasted climate-warming related droughts in central Europe.Nemoral Forests under Climate Extremes (NEMKLIM Project, grant number 3517861300), German Federal Agency for Nature Conservation (Bundesamt für Naturschutz,BfN), German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety, Germany, project PN 19070506, Romanian National Authority from Scientific Research and Innovation, Romanian Ministry of Education and Research, CNCS-UEFISCDI, project number PN-III-P4-ID-PCE-2020- 2696, within PNCDI III. projects VUL-BOS project (UPO-1263216 and PinCaR (UHU-1266324), FEDER Funds, Andalusia Regional Government, Consejería de Economía, Conocimiento, Empresas y Universidad 2014-2020), project LESENS (RTI2018-096884-B-C33), Spanish Ministry of Science, Innovation and Universities.This work was supported by NEMKLIM project: Nemoral Forests under Climate Extremes (NEMKLIM Project, grant number 3517861300), financed by the German Federal Agency for Nature Conservation (Bundesamt für Naturschutz,BfN) and the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety, Germany and by project PN 19070506 financed by Romanian National Authority from Scientific Research and Innovation. I.C. Petritan was supported by a grant of the Romanian Ministry of Education and Research, CNCS-UEFISCDI, project number PN-III-P4-ID-PCE-2020- 2696, within PNCDI III. R. S ´anchez-Salguero was supported by VUL-BOS project (UPO-1263216, FEDER Funds, Andalusia Regional Government, Consejería de Economía, Conocimiento, Empresas y Universidad 2014-2020) and LESENS (RTI2018-096884-B-C33) project from the Spanish Ministry of Science, Innovation and Universities. A. Hevia was supported by PinCaR project (UHU-1266324, FEDER Funds, Andalusia Regional Government, Consejería de Economía, Conocimiento, Empresas y Universidad 2014-2020). We are also grateful to Lucian Toiu, Nicu Tudose, George Sarbu and Gheorghe Stefan for help in collecting field data. We appreciate the permission and logistic support given by staff of the Barzava and Codrii Beiusului si Sfanta Maria Forest Districts, we are grateful especially to eng. Gheorghe Marc and eng. Jeno Ferko. The authors declare no conflicts of interest

Structural dynamics of deciduous mixed stands in the Hyrcanian forests, northern Iran

Forest structure as an effect of forest dynamics can be used to characterize biophysical processes, biodiversity and ecosystem functions. This study examines the structural components of old-growth forest stands located in the Caspian forests, north of Iran. We measured forest-related attributes in five plots located in the Kheyrud Forest, Mazandaran Province, Iran in 2004 and 2014, respectively. The mean stand density was of 338.6 tree ha-1, the mean volume was of 389.2 m3 ha-1, and the volume of deadwood accounted for 34.1 m3 ha-1. The mean stocking volume per hectare was for beech: 3.95; hornbeam: 7.00; oak: 1.39; maple: 0.60 and other species: 2.55 m3 ha-1. Meanwhile, the mean volume of the felled trees, other cuttings, snags and logs were 31.3, 1.1, 12.6, and 21.5 m3 ha-1, respectively. The mean abundance of gap size on small (<200 m2), intermediate (200-500 m2), and large (>500 m2) classes were calculated as 53.5, 37.9, and 14.1%, respectively. No significant difference in size gap distribution was detected over 10 years, neither any gap size class. The coarse woody debris (CWD) percentage from total deadwood volume varied from 67% to 93%, whereas the fine woody debris (FWD) amount was estimated ranged between 7 and 33%. No significant differences between 2004 and 2014 was found for CWD and FWD. Comparison of spatial pattern results of tree species in 2004 and 2014 implies that there were no significant changes in the mingling index, uniform angle index, or diameter differentiation at species and stand levels, except oak and maple species that showed a significant change in diameter growing. In other words, despite the harvesting of the trees, spatial pattern indices have not changed significantly. These results indicate the performance of ecological forestry programs by foresters in the Hyrcanian Forest stands and their increasing consideration to the ecological principles of the forest. The results are useful in the sense that they characterize the stand structure components which are fundamental to performing silvicultural treatments based on the emulation of natural forest structural dynamics

A comparative analysis of foliar chemical composition and leaf construction costs of beech (Fagus sylvatica L.), sycamore maple (Acer pseudoplatanus L.) and ash (Fraxinus excelsior L.) saplings along a light gradient

• Construction cost (g glucose g−1), chemical composition and morphology of leaves of beech (Fagus sylvatica L.) and two co-occurring valuable broadleaved species (sycamore maple – Acer pseudoplatanus L. – and ash – Fraxinus excelsior L.) were investigated along a horizontal light gradient (3–60% of above canopy radiation) and from top to bottom within the crowns in a fairly even-aged mixed-species thicket established by natural regeneration beneath a patchy shelterwood canopy. • Construction cost and carbon concentration increased with irradiance in ash and sycamore maple and were independent of irradiance in beech. Leaf traits expressed on an area basis, like construction cost, nitrogen content and leaf mass (LMA) increased significantly with irradiance in all three species and decreased from top to bottom within crowns. • The shade tolerant beech invested more glucose to produce a unit foliar biomass, but less to build a unit foliar area due to lower LMA. Thereby beech was able to display a greater total leaf area, what at least in parts counterbalanced the lower values of Na as compared to ash and sycamore maple

Soil fauna biodiversity responses to simulated anthropogenic disturbances in European oak forests (HoliSoils H-2020 project)

Only AbstractSoil biodiversity is essential for forest ecosystem functioning as they play key roles in litter decomposition or nutrient recycling. However, perturbations due to anthropogenic (thinning/clear-cut/slash removal) sources can lead to strong impacts on soil biodiversity. In a context of climate-smart forestry, our knowledge about soil biodiversity responses to forest perturbations need to be improved. In the framework of HoliSoils project, we stablished three study sites in oak-dominated forests located in Spain (Quercus faginea), France (Quercus pubescens) and Romania (Quercus robur) to study the impact of tree removal intensity on soil biodiversity. The first results recorded in Spain pointed out a negative effect of both tree and slash removals whatever the soil biota group considered. In addition, the intensity of these effects increased with organism size. Next steps will include a comparison of response patterns between the three study sites and a survey of these responses over longer times

The impact of insect herbivory on biogeochemical cycling in broadleaved forests varies with temperature

Herbivorous insects alter biogeochemical cycling within forests, but the magnitude of these impacts, their global variation, and drivers of this variation remain poorly understood. To address this knowledge gap and help improve biogeochemical models, we established a global network of 74 plots within 40 mature, undisturbed broadleaved forests. We analyzed freshly senesced and green leaves for carbon, nitrogen, phosphorus and silica concentrations, foliar production and herbivory, and stand-level nutrient fluxes. We show more nutrient release by insect herbivores at non-outbreak levels in tropical forests than temperate and boreal forests, that these fluxes increase strongly with mean annual temperature, and that they exceed atmospheric deposition inputs in some localities. Thus, background levels of insect herbivory are sufficiently large to both alter ecosystem element cycling and influence terrestrial carbon cycling. Further, climate can affect interactions between natural populations of plants and herbivores with important consequences for global biogeochemical cycles across broadleaved forests

A single-tree approach to determine climate-growth patterns of European beech and their seasonality in the species southern distribution area

Dry and warm climate conditions in southern Europe represent clear limits for European beech (Fagus sylvatica) growth near the species southern distribution limit, but it is unclear how aridification and changes in seasonal precipitation regimes will affect these forests at the individual level. We explored climate-growth relationships and the seasonality of peak climate signals in European beech using daily climate data and a large collection of tree-ring width series from southern and southeastern Europe through Generalised Linear Mixed Models (GLMMs). In most cases we found a positive and significant influence of precipitation on tree growth, and a significant negative effect of maximum temperature. Predictions from the GLMMs revealed a positive impact of precipitation during an 88 day window from spring to early summer (mid-April to mid-July), for an average tree across our network. This critical growing time window ranged from 75 days in warmer and drier conditions, and extended up to 100 days in areas with mild temperatures and moderate summer precipitation. Maximum temperatures negatively affected trees for an average of 27 day window in summer (June-July). This period was reduced to <10 days in locations with wetter and colder summers, rising up to 45 days in sites with drier and warmer summers. The positive effect of precipitation on growth was stronger and commenced earlier in larger trees. Similarly, the negative effects of maximum temperatures were more pronounced for larger trees. The use of daily climate data and a tree-centred approach allowed for capturing critical temporal dynamics in climate-growth relationships that are often overlooked by conventional methods. These insights significantly enhance our understanding of climatic factors influencing individual beech growth at the edge of its distribution range and their seasonal variations

Jet stream position explains regional anomalies in European beech forest productivity and tree growth.

The mechanistic pathways connecting ocean-atmosphere variability and terrestrial productivity are well-established theoretically, but remain challenging to quantify empirically. Such quantification will greatly improve the assessment and prediction of changes in terrestrial carbon sequestration in response to dynamically induced climatic extremes. The jet stream latitude (JSL) over the North Atlantic-European domain provides a synthetic and robust physical framework that integrates climate variability not accounted for by atmospheric circulation patterns alone. Surface climate impacts of north-south summer JSL displacements are not uniform across Europe, but rather create a northwestern-southeastern dipole in forest productivity and radial-growth anomalies. Summer JSL variability over the eastern North Atlantic-European domain (5-40E) exerts the strongest impact on European beech, inducing anomalies of up to 30% in modelled gross primary productivity and 50% in radial tree growth. The net effects of JSL movements on terrestrial carbon fluxes depend on forest density, carbon stocks, and productivity imbalances across biogeographic regions

Identifying drivers of non-stationary climate-growth relationships of European beech.

The future performance of the widely abundant European beech (Fagus sylvatica L.) across its ecological amplitude is uncertain. Although beech is considered drought-sensitive and thus negatively affected by drought events, scientific evidence indicating increasing drought vulnerability under climate change on a cross-regional scale remains elusive. While evaluating changes in climate sensitivity of secondary growth offers a promising avenue, studies from productive, closed-canopy forests suffer from knowledge gaps, especially regarding the natural variability of climate sensitivity and how it relates to radial growth as an indicator of tree vitality. Since beech is sensitive to drought, we in this study use a drought index as a climate variable to account for the combined effects of temperature and water availability and explore how the drought sensitivity of secondary growth varies temporally in dependence on growth variability, growth trends, and climatic water availability across the species' ecological amplitude. Our results show that drought sensitivity is highly variable and non-stationary, though consistently higher at dry sites compared to moist sites. Increasing drought sensitivity can largely be explained by increasing climatic aridity, especially as it is exacerbated by climate change and trees' rank progression within forest communities, as (co-)dominant trees are more sensitive to extra-canopy climatic conditions than trees embedded in understories. However, during the driest periods of the 20th century, growth showed clear signs of being decoupled from climate. This may indicate fundamental changes in system behavior and be early-warning signals of decreasing drought tolerance. The multiple significant interaction terms in our model elucidate the complexity of European beech's drought sensitivity, which needs to be taken into consideration when assessing this species' response to climate change

No Future Growth Enhancement Expected at the Northern Edge for European Beech due to Continued Water Limitation.

With ongoing global warming, increasing water deficits promote physiological stress on forest ecosystems with negative impacts on tree growth, vitality, and survival. How individual tree species will react to increased drought stress is therefore a key research question to address for carbon accounting and the development of climate change mitigation strategies. Recent tree-ring studies have shown that trees at higher latitudes will benefit from warmer temperatures, yet this is likely highly species-dependent and less well-known for more temperate tree species. Using a unique pan-European tree-ring network of 26,430 European beech (Fagus sylvatica L.) trees from 2118 sites, we applied a linear mixed-effects modeling framework to (i) explain variation in climate-dependent growth and (ii) project growth for the near future (2021-2050) across the entire distribution of beech. We modeled the spatial pattern of radial growth responses to annually varying climate as a function of mean climate conditions (mean annual temperature, mean annual climatic water balance, and continentality). Over the calibration period (1952-2011), the model yielded high regional explanatory power (R2 = 0.38-0.72). Considering a moderate climate change scenario (CMIP6 SSP2-4.5), beech growth is projected to decrease in the future across most of its distribution range. In particular, projected growth decreases by 12%-18% (interquartile range) in northwestern Central Europe and by 11%-21% in the Mediterranean region. In contrast, climate-driven growth increases are limited to around 13% of the current occurrence, where the historical mean annual temperature was below ~6°C. More specifically, the model predicts a 3%-24% growth increase in the high-elevation clusters of the Alps and Carpathian Arc. Notably, we find little potential for future growth increases (-10 to +2%) at the poleward leading edge in southern Scandinavia. Because in this region beech growth is found to be primarily water-limited, a northward shift in its distributional range will be constrained by water availability

Response of planted beech (Fagus sylvatica L.) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) saplings to herbaceous and small shrubs control on clearcuts

The reaction of young beech (Fagus sylvatica L.) and Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) saplings on competition of two types of vegetation-(1) gramineous with mainly Agrostis capillaries, Calama-grostis epigejos, Deschampsia flexuosa, and (2) small shrubs with mainly Rubus fruticosus and R. idaeus-on clear cuts on two sites was studied for 2 years. Half the sample saplings were released from competing vegetation by repeated herbicide applications. This treatment significantly raised the diameter increment in both species at the site with higher competition intensity, and more strongly after the removal of small shrubs than after the removal of grasses. Sapling length increment was not significantly affected. After being released from small shrubs, saplings of both species developed a smaller specific fine root length (cm g(-1) fine root biomass) than unreleased saplings during the second year which was characterized by low rainfall. Root nitrogen concentration significantly increased after weed control in both vegetation types. Sapling foliar content of main nutritional elements was negatively related to dry mass and total chemical content of surrounding ground vegetation. Based on these results, a release from ground vegetation could be a useful tool to improve growth of planted beech and Douglas-fir saplings on sites with well-developed small shrubs competition (mainly by Rubus fruticosus and R. idaeus), or under fairly dry conditions.Deutsche Forschungsgemeinschaft (DFG) [Lu 452/9-1