Growth and resilience responses of Scots pine to extreme droughts across Europe depend on pre‐drought growth conditions

Global climate change is expected to further raise the frequency and severity of extreme events, such as droughts. The effects of extreme droughts on trees are difficult to disentangle given the inherent complexity of drought events (frequency, severity, duration, and timing during the growing season). Besides, drought effects might be modulated by trees’ phenotypic variability, which is, in turn, affected by long‐term local selective pressures and management legacies. Here, we investigated the magnitude and the temporal changes of tree‐level resilience (i.e., resistance, recovery, and resilience) to extreme droughts. Moreover, we assessed the tree‐, site‐, and drought‐related factors and their interactions driving the tree‐level resilience to extreme droughts. We used a tree‐ring network of the widely distributed Scots pine (Pinus sylvestris ) along a 2800 km latitudinal gradient from southern Spain to northern Germany. We found that the resilience to extreme drought decreased in mid‐elevation and low productivity sites from 1980‐1999 to 2000‐2011 likely due to more frequent and severe droughts in the later period. Our study showed that the impact of drought on tree‐level resilience was not dependent on its latitudinal location, but rather on the type of sites trees were growing at and on their growth performances (i.e., magnitude and variability of growth) during the pre‐drought period. We found significant interactive effects between drought duration and tree growth prior to drought, suggesting that Scots pine trees with higher magnitude and variability of growth in the long term are more vulnerable to long and severe droughts. Moreover, our results indicate that Scots pine trees that experienced more frequent droughts over the long‐term were less resistant to extreme droughts. We therefore conclude that the physiological resilience to extreme droughts might be constrained by their growth prior to drought, and that more frequent and longer drought periods may overstrain their potential for acclimation.Marie Skłodowska-Curie Individual Fellowship (PROJECT ID: 749051-REFOREST), Postdoctoral grant (IJCI-2015-25845, FEDER funds), RTI2018-096884-B-C31, RTI2018-096884-B-C33 projects (Ministry of Ministry of Science, Innovation and Universities, Spain), VULBOS project (UPO-1263216, FEDER Funds, Andalusia Regional Government, Consejería de Economía, Conocimiento, Empresas y Universidad 2014-2020), PinCaR project (UHU-1266324, FEDER Funds, Andalusia Regional Government, Consejería de Economía, Conocimiento, Empresas y Universidad 2014-2020), Bavarian Ministry of Science, Bavarian Climate Research Network (bayklif). project DENDROKLIMA by the German Waldklimafond (FKZ 28W-C-4-077-01), ST327 Bavarian State Ministry for Food, Agriculture, and Forestry. Landesforst MecklenburgVorpommern, Landeskompetenzzentrum Forst Eberswalde and Nordwestdeutsche Forstliche Versuchsanstalt

Precision and accuracy of tree-ring-based death dates of mountain pines in the Swiss National Park

Key message : Mountain pines in the Swiss National Park show evidence of partial cambial mortality, which affects the precision of tree-ring-based death dates, followed by lagged crown mortality. Abstract: The time of tree death is commonly reconstructed by dating the outermost ring of tree-ring series. However, due to the occurrence of partial cambial mortality, the date of the outermost tree ring may vary between different locations on the tree stem. Furthermore, a tree may continue to live following the formation of the most recent tree ring. In this study, we quantified precision and accuracy of tree-ring-based death dates from 229 dead mountain pines (Pinus montana) from a 28km2 study area in the Swiss National Park. For almost two-thirds of the trees, a maximum difference of just 0-4years between the dates of cambial mortality from three increment cores was observed, however, for a few trees the difference reached 30-65years. Higher maximum differences between the dates of cambial mortality are expected for trees on steep slopes, for old trees or for trees that died a long time ago. For 84% of dead mountain pines, which were sampled in a permanent sample plot with 2-year remeasurement intervals, the difference between the date of observed crown mortality and the death date determined from three cores was 0-5years. Sampling two or just one core per tree decreases the accuracy of tree-ring-based death dates. Based on the findings of our study, we recommend a prior assessment of the precision and accuracy of tree-ring-based death dates for any dendroecological study dealing with the reconstruction of tree mortalit

Pine and mistletoes: how to live with a leak in the water flow and storage system?

The mistletoe, Viscum album, living on Scots pine (Pinus sylvestris) has been reported barely to regulate its transpiration and thus heavily to affect the gas exchange of its host. The extent of this mistletoe effect and its underlying mechanism has, so far, only been partially analysed. In this study, pine branches with different mistletoe infestation levels were investigated by sap flow gauges and analysed with a modelling approach to identify the mistletoe-induced stomatal regulation of pine and its consequences for the water and carbon balances of the tree. It was found that Viscum album barely regulates its stomata and that pines consequently compensate for the additional water loss of mistletoes by closing their own stomata. Despite the reduced stomatal aperture of the needles, the total water loss of branches with mistletoes increased. Furthermore, the increasingly closed stomata reduced carbon assimilation for the pine. Such a negative effect of the mistletoes on pine's stomatal conductance and carbon gain was particularly strong during dry periods. Our study therefore suggests that mistletoe-induced stomatal closure is a successful mechanism against dying from hydraulic failure in the short term but increases the risk of carbon starvation in the long term. With the current conditions in Valais, Switzerland, a tree with more than about 10-20% of its total leaf area attributable to mistletoes is at the threshold of keeping a positive carbon balance. The currently increasing mistletoe abundance, due to increasing mean annual temperatures, is therefore accelerating the ongoing pine decline in many dry inner-Alpine valley

Is forest mushroom productivity driven by tree growth? Results from a thinning experiment

• Most of the edible forest mushrooms are mycorrhizal and depend on carbohydrates produced by the associated trees. Fruiting patterns of these fungi are not yet fully understood since climatic factors alone do not completely explain mushroom occurrence. • The objective of this study was to retrospectively find out if changing tree growth following an increment thinning has influenced the diversity patterns and productivity of associated forest mushrooms in the fungus reserve La Chanéaz, Switzerland. • The results reveal a clear temporal relationship between the thinning, the growth reaction of trees and the reaction of the fungal community, especially for the ectomycorrhizal species. The tree-ring width of the formerly suppressed beech trees and the fruit body number increased after thinning, leading to a significantly positive correlation between fruit body numbers and tree-ring width. • Fruit body production was influenced by previous annual tree growth, the best accordance was found between fruit body production and the tree-ring width two years previously. • The results support the hypothesis that ectomycorrhizal fruit body production must be linked with the growth of the associated host trees. Moreover, the findings indicate the importance of including mycorrhizal fungi as important players when discussing a tree as a carbon source or sin

Drought as an Inciting Mortality Factor in Scots Pine Stands of the Valais, Switzerland

During the 20th century, high mortality rates of Scots pine (Pinus silvestris L.) have been observed over large areas in the Rhône valley (Valais, Switzerland) and in other dry valleys of the European Alps. In this study, we evaluated drought as a possible inciting factor of Scots pine decline in the Valais. Averaged tree-ring widths, standardized tree-ring series, and estimated annual mortality risks were related to a drought index. Correlations between drought indices and standardized tree-ring series from 11 sites showed a moderate association. Several drought years and drought periods could be detected since 1864 that coincided with decreased growth. Although single, extreme drought years had generally a short-term, reversible effect on tree growth, multi-year drought initiated prolonged growth decreases that increased a tree's long-term risk of death. Tree death occurred generally several years or even decades after the drought. In conclusion, drought has a limiting effect on tree growth and acts as a bottleneck event in triggering Scots pine decline in the Valai

Land-use and climate change effects in forest compositional trajectories in a dry Central-Alpine valley

• Increased mortality of Scots pine (Pinus sylvestris L.) and spreading of deciduous trees are observed in the Swiss Rhone valley. Previous research identified climate change effects as main drivers of this trend. On the local scale, we hypothesize that legacies from past anthropogenic disturbances are superimposed on climate effects. • We reconstructed land-use history and quantified changes in tree species composition from 1930 to 1994 on 9468 ha of forested land. The aim was to analyze the contribution of anthropogenic disturbances to the observed changes and to disentangle human impact from climate change effects. • At altitudes below 1 200 m a.s.l. we found a shift from pine (−11.4%) to deciduous trees (+11%) with significantly lower increase of deciduous trees in stands formerly used for grazing and/or litter collecting. Conversely, pine decrease was not correlated with former anthropogenic disturbances. We interpret pine mortality as an effect of increased drought stress due to climate change while spread of deciduous trees is driven by land-use change. Grazing and litter collecting hindered deciduous tree regeneration and it was not until their abandonment a few decades ago that forest composition started to change. At higher elevations the shift from Norway spruce (Picea abies; −8.5%) to European larch (Larix decidua; +8.2%) corresponds to silvicultural management schemes, aimed at promoting larch recruitment. • Our study illustrates the importance of disentangling climate from land-use change effects for understanding shifts in forest composition. The findings are relevant for other regions in the European Alps where forests undergo comparable environmental change

Below Average Midsummer to Early Autumn Precipitation Evolved Into the Main Driver of Sudden Scots Pine Vitality Decline in the Swiss Rhône Valley

The vitality of Scots pine (Pinus sylvestris L.) is declining since the 1990s in many European regions. This was mostly attributed to the occurrence of hotter droughts, other climatic changes and secondary biotic stressors. However, it is still not well understood which specific atmospheric trends and extremes caused the observed spatio-temporal dieback patterns. In the Swiss Rhône valley, we identified negative precipitation anomalies between midsummer and early autumn as the main driver of sudden vitality decline and dieback events. Whereas climate change from 1981 to 2018 did not lead to a reduced water input within this time of the year, the potential evapotranspiration strongly increased in spring and summer. This prolonged and intensified the period of low soil moisture between midsummer and autumn, making Scots pines critically dependent on substantial precipitation events which temporarily reduce the increased water stress. Thus, local climate characteristics (namely midsummer to early autumn precipitation minima) are decisive for the spatial occurrence of vitality decline events, as the lowest minima outline the most affected regions within the Swiss Rhône valley. Mortality events will most likely spread to larger areas and accelerate the decline of Scots pines at lower elevations, whereas higher altitudes may remain suitable Scots pine habitats. The results from our regional study are relevant on larger geographic scales because the same processes seem to play a key role in other European regions increasingly affected by Scots pine dieback events

Drought alters timing, quantity, and quality of wood formation in Scots pine

Drought has been frequently discussed as a trigger for forest decline. Today, large-scale Scots pine decline is observed in many dry inner-Alpine valleys, with drought discussed as the main causative factor. This study aimed to analyse the impact of drought on wood formation and wood structure. To study tree growth under contrasting water supply, an irrigation experiment was installed in a mature Scots pine (Pinus sylvestris L.) forest at a xeric site in a dry inner-Alpine valley. Inter- and intra-annual radial increments as well as intra-annual variations in wood structure of pine trees were studied. It was found that non-irrigated trees had a noticeably shorter period of wood formation and showed a significantly lower increment. The water conduction cells were significantly enlarged and had significantly thinner cell walls compared with irrigated trees. It is concluded that pine trees under drought stress build a more effective water-conducting system (larger tracheids) at the cost of a probably higher vulnerability to cavitation (larger tracheids with thinner cell walls) but without losing their capability to recover. The significant shortening of the growth period in control trees indicated that the period where wood formation actually takes place can be much shorter under drought than the ‘potential' period, meaning the phenological growth perio

Growth response of Scots pine with different crown transparency status to drought release

• Context : One short-term adjustment of trees to drought is the reduction of photosynthetic tissues via leaf shedding. But in conifers, it usually takes several years to fully restore needle mass and assimilation capacity. • Aims : This study aims to evaluate whether leaf shedding sustainably damages conifers or if these trees still have the ability to recover from drought with respect to their foliage and wood formation. • Methods : An irrigation experiment was established in a mature dry forest to test the growth reactions of Scots pine (Pinus sylvestris L.) differing in crown transparency (low, medium, high) to a drought release by irrigation in comparison with equivalent control trees growing under naturally dry conditions on the same site. • Results : Drought and high crown transparency had a combined negative effect on radial tree growth: Control trees with medium to high crown transparencies showed a substantially shorter growth period and a long-lasting growth depression in response to the severe summer drought in 2003. However, all trees benefited from irrigation, irrespective of their crown status, and immediately increased growth in response to irrigation. • Conclusion : The progressed drought-induced defoliation seemed to be a weakening factor for trees suffering from drought, but this can be reversed if the water supply is improve

The upward shift in altitude of pine mistletoe ( Viscum album ssp. austriacum ) in Switzerland—the result of climate warming?

Pine mistletoe (Viscum album ssp. austriacum) is common in natural Scots pine (Pinus sylvestris L.) forests in the alpine Rhone Valley, Switzerland. This semi-parasite, which is regarded as an indicator species for temperature, increases the drought stress on trees and may contribute to the observed pine decline in the region. We recorded mistletoes on representative plots of the Swiss National Forest Inventory ranging from 450 to 1,550m a.s.l. We found mistletoe on 37% of the trees and on 56% of all plots. Trees infested with mistletoe had a significantly higher mortality rate than non-infested trees. We compared the current mistletoe occurrence with records from a survey in 1910. The current upper limit, 1,250m, is roughly 200m above the limit of 1,000-1,100m found in the earlier survey 100 years ago. Applying a spatial model to meteorological data we obtained monthly mean temperatures for all sites. In a logistic regression mean winter temperature, pine proportion and geographic exposition significantly explained mistletoe occurrence. Using mean monthly January and July temperatures for 1961-1990, we calculated Skre's plant respiration equivalent (RE) and regressed it against elevation to obtain the RE value at the current mistletoe elevation limit. We used this RE value and temperature from 1870-1899 in the regression and found the past elevation limit to be at 1,060m, agreeing with the 1910 survey. For the predicted temperature rise by 2030, the limit for mistletoe would increase above 1,600m altitud