Tree rings as an early warning against catastrophic landslides: Assessing the potential of dendrochronology for determining slope stability

We investigated three slopes (in southern Poland, the Carpathian Mts, and the Sudeten Mts) subject to catastrophic, sudden landslides. To reconstruct past landslide activity, we analysed the eccentricity of tree rings in the stems of Norway spruce (Picea abies) using a per cent eccentricity index method. We obtained data on year-by-year changes in eccentricity patterns of single specimens, as well as data on landslide events dated from the whole population of trees sampled on each slope (13–30 spruce trees). These data supplied indirect information on the temporal variability of landslide activity on the three slopes revealing that all three slopes were subject to frequent landslide activity (recurrence intervals 2.0–2.7 years) well before catastrophic events that occurred in 1997 and 2010. The study also showed that 3–5 years before a catastrophic event the sample trees started to record increasing ground instability demonstrated as an uninterrupted, sudden increase in the eccentricity of single trees. Our results suggest that the application of dendrochronological methods can reveal slopes at an increased risk of catastrophic landsliding well in advance. The methods we applied show great promise for forecasting catastrophic landslides and assessing landslide hazard, slope stability and the effectiveness of engineering works undertaken to stabilise landslides.     

Growth pattern of Picea rubens prior to canopy recruitment

A majority (72%) of Picea rubens Sarg. (red spruce) trees in an old-growth spruce-fir forest in the Great Smoky Mountains underwent episodes of radial growth suppression and release before they reached the forest canopy. Prior to canopy recruitment, trees experienced an average of 1.43 and a maximum of 7 suppression periods with an average ring width of 0.257 mm. Duration of suppression periods ranged from 4 to 79 years with an average of 19.05 years, which was significantly shorter than the average duration of release periods (29.00 years). Mean ring width in a suppression period was negatively correlated with duration of the suppression period. The opposite was true for release periods. The severity of suppression had no significant effect on mean ring width in subsequent release periods. Greater suppression was observed in the recent growth pattern of current non-canopy trees than in the historical growth pattern reconstructed from current canopy trees. Growth releases prior to canopy recruitment, triggered by small gaps or neighboring gaps, displayed a continuous pattern over the last two centuries and had consistent frequency in recent decades. By contrast, canopy recruitment resulting from large-gap disturbances was discontinuous over the last two centuries, and less frequent after 1850.     

Growth trends and climate responses of Norway spruce along elevational gradients in East-Central Europe

Key message

Decadal growth variability of Norway spruce increases with elevation. Recent temperature sensitivity and growth enhancement are limited to trees growing in the zone adjacent to timberline.

Abstract

Growth trends and climate responses of forest trees along elevational gradients are not fully understood. A deeper insight is, however, fundamental for predicting ecosystem functioning and productivity under future climate change. Supplementary to the effects of elevation and regional provenance on tree growth are sample depth, uneven representation of sample age and varying site conditions. Furthermore, there is only a limited number of studies addressing growth changes along elevational gradients, while at the same time applying tree-ring standardization methods that are sensitive to trend preservation. Here, we introduce 12 novel tree-ring width chronologies of Norway spruce (Picea abies[L.] Karst.) from four elevational belts encompassing montane forests and the local timberline in three regions in East-Central Europe between 15° and 19°E. Each chronology is characterized by sufficient sample replication and a comparable age structure between 1906 and 2010. Tree growth near timberline revealed substantial medium-frequency variability and sharply increasing ring widths since the 1980s. Medium-frequency growth variability of lower elevation trees was, however, relatively small, and growth rates over the last decade were either stable or even decreased. During the last four decades, Norway spruce from higher elevations exhibited a reduced response to autumn temperatures preceding ring formation. In contrast, trees from the lower-montane zone increased their sensitivity to drought during the same time. Our results emphasize not only different but also instable growth trends and climate responses of forest trees along altitudinal gradients, which should be considered in future forest management strategies.

     

Divergent growth responses of healthy and declining spruce trees to climatic stress: A case study from the Western Carpathians

Long-lived tree species optimize resource allocation processes, and there are likely trade-offs between the growth and response to environmental stressful conditions. The study aims to investigate the growth of healthy and declining Norway spruce trees using dendrochronological analysis. We have selected a natural high mountain spruce population and established 27 sampling plots in Poľana Mts. We measured 270 trees, from which we visually evaluated 133 individuals. Moreover, 51 trees were sampled for increment cores. Our study reveals thatspecific extreme combination of winter, spring and summer temperatures and precipitation sums within the particular year 1974 induced long-term growth divergence of neighboring spruce trees of comparable age and size and differentiated between healthy and declining (highly defoliated) trees. This period accounts for a decrease in size of relativized increments by 30.2% in declining trees compared to visually healthy individuals. Trees with a low ability to recover from climatic stress compensate the response to environmental conditions by lower growth rates. In high mountain temperate forests, the long-term growth decline of spruce trees last about 40 years. The single-tree selection silvicultural system should be preferred in high mountain conditions, where frequent small-scale, low-intensity disturbances drive stand dynamics. Trees showing low growth resilience to revealed combination of weather conditions should be preferentially removed from stands , especially within the transformation of even-aged spruce monocultures using selective cuttings.     

Does ice crystal formation in buds explain growth disturbances in boron-deficient Norway spruce?

Loss of apical dominance in boron-deficient trees has been suggested to be due to frost damage of terminal buds and leaders. Excessive nitrogen (N) supply can exacerbate boron (B) deficiency by the dilution-effect. N may also have direct effects on winter hardiness. We studied frost hardening of buds of Norway spruce (Picea abies L. Karst.) in healthy-looking trees and in trees with growth disturbances. The effect of B and N on frost hardiness was studied in a factorial fertilisation experiment during cold acclimation. Frost hardiness was determined by differential temperature analysis (DTA) and scoring of visual damage. In a DTA profile of apical buds with a piece of stem, low-temperature exotherm (LTE) predicted bud injury, while two of the observed high-temperature exotherms and two of the observed intermediate-temperature exotherms were non injurious. Appearance of LTE followed changes in air temperature. The risk of frost damage was not affected by fertilisation treatments or previously observed growth disturbances. However, when the bud structure was deformed by severe B deficiency, the supercooling ability disappeared. Such buds are probably killed by freezing in nature and therefore, frost damage may play a secondary role in the development of growth disturbances.     

Needle analyses and graphic vector analyses of Norway spruce and Scots pine stands

 The aims of this study were to test the nutritional condition of young stands of Norway spruce and Scots pine in southernmost Norway, where atmospheric inputs of anthropogenic nitrogen and strong acids, as well as deposition of magnesium by sea spray, are relatively high. There has been general concern about forest health in this region, specifically about nutrient deficiencies and nitrogen saturation caused by the atmospheric pollution. To expand our knowledge about these subjects, needle analyses and graphic vector analyses, as well as tree vigour index and height growth response to fertiliser application, were used as diagnostic tools. The overall conclusion is that phosphorous limitation is probably more frequent than expected in the coastal zone of southern Norway, especially in spruce on shallow soils. It is hypothesised that phosphorous limitation might be a more general problem or will arise as one in a 30 – 60 km coastal zone stretching from southernmost Sweden along the coast to the north and then westwards along the south coast of Norway. Nitrogen saturation in southernmost Norway might already occur on some sites with shallow soils. Needle analyses and graphic vector analyses can yield erroneous results if stress factors other than nutrient supply, e. g. severe drought, determine the growth rate of trees. In addition, the graphic vector analyses require that nutrients applied to the test plots are available to roots early in the growing season, and that needle nutrient concentrations in late autumn reflect those when the needles were formed. Received: 30 May 1995/Accepted: 27 December 1995     

Longterm assessment of spruce vitality in the Fichtelgebirge (West Germany) under ongoing acid depostion

Historical land-use and especially acid precipitation seem to have altered the natural habitat conditions for Norway spruce (Picea abies (L.) Karst.) in Central Europe.Many forest decline studies of the last ten years evaluate the continual anthropogenic stress situation for Norway spruce. Especially in regions with high acidic deposition, regeneration of spruce stands seems to be impossible without soil melioration. In a first estimate the assumed H⁺-deposition and additionally the propensity of soils to acidification in spruce stands is characterized. After studying several soil parameters, exposition and climatic factors were processed by means of a Geographical Information System in order to draw a balance of human impact on habitat conditions of Norway spruce.Both the spatial distribution of spruce vitality under natural conditions (without human impact) in the research area Fichtelgebirge (northeast Bavaria) and the prospective vitality of spruce under actual human impact is shown. From this comparison it is concluded that without air pollution abatement and without amelioration of the heavily acidified sites spruce regeneration can not be expected even in the long run. The actual forest damage and some restoration programs give evidence of this assumption.     

Macroscopic indicators for the retrospective assessment of Norway spruce crown response to stress in the Krkonoše Mountains

The synergistic effect of a number of natural and anthropogenic stress factors is probably responsible for forest decline in Central Europe. The macroscopic indicators of Norway spruce crown status, as well as branch structure transformation, were proposed to reconstruct the retrospective stress response of a tree to stress conditions over a 30-year period. Based on crown defoliation and the proportion of secondary shoots, five categories of tree response to stress factors were introduced: resistant trees (crown defoliation ≤35%, secondary shoots ≤65%); resilient trees (crown defoliation ≤35%, secondary shoots >65%); susceptible moderately transformed trees (crown defoliation >35%, secondary shoots ≤65%); susceptible strongly transformed trees (crown defoliation >35%, 65% < secondary shoots ≤90%); and exhausted trees to the environment (crown defoliation >35%, secondary shoots >90%). These stress response categories are discussed assuming a sink/source relationship on carbohydrate allocation. The relationship between stress response categories and current regenerative processes in the crown, assessed by macroscopic markers on bud development and shoot growth, is outlined. The research has been carried out mainly in three permanent research plots of old autochthonous Norway spruce forest stands in the Krkonoše Mts. (Czech Republic) under different historical stress loads.     

Abrupt growth changes in Norway spruce and Yezo spruce near an industrial district in Hokkaido, Japan

 Increments in the radii of Norway spruce (Picea abies Karst.) and Yezo spruce (Picea jezoensis Carr.) trees that revealed symptoms of a decline in growth were analyzed by dendrochronological methods in an attempt to correlate past reductions in growth with their main causes. The trees were growing at different sites near the industrial district of Tomakomai, Hokkaido. A skeleton plot method was used to construct a series of pointer years that revealed the number of trees with a clear reduction in growth or recovery from such a reduction. An analysis of “abrupt growth changes” demonstrated that at least two periods of growth reduction were common to a large number of Norway spruce trees. The reduction events were related to the records of industrial activity near the forest and meteorological data. The growth reduction in the 1970s coincided with the start of operation of certain local factories, and its extent was related to the distance from the industrial region. By contrast, a reduction in growth in 1984 was detected at all the Norway spruce sites and the extent was approximately the same at all sites. This phenomenon was related to extreme drought conditions. Growth of Yezo spruce trees was less sensitive to industrial activity and to drought than that of Norway spruce. Thus, differences in response to air pollution and drought were observed between the two species. Received: 20 February 1996 / Accepted: 29 April 1996     

亚高山云冷杉混交林树木生长释放与干扰分析

对云南碧塔海亚高山云冷杉林内４个样地冠层树木的生长压制和释放的历史,用树木年轮分析方法进行了重建,然后根据生长释放频率推测林冠干扰强度（每１０年冠层树木的死亡百分率）。４个林分（１个中龄林,３个成过熟林）生长释放的平均百分率为４８％～９２％。中龄林内,平均生长释放频率为７１％／１０ａ,成过熟林则为７４％～９５％／１０ａ,在油麦吊云杉〔Ｐｉｃｅａｂｒａｃｈｙｔｙｌａｖａｒ．ｃｏｍｐｌａｎａｔａ（Ｍａｓｔ．）ＣｈｅｎｇｅｘＲｅｈｄ．〕占优势的林分和大果红杉（Ｌａｒｉｘｐｏｔａｎｉｎｉｖａｒ．ｍａｃｒｏｃａｒｐａＬａｗ）油麦吊云杉混交林分内,估测的林冠干扰强度分别为４８％／１０ａ和５９％／１０ａ。     

Structure,spatio-temporal dynamics and disturbance regime of the mixed beech–silver fir–Norway spruce old-growth forest of Biogradska Gora (Montenegro)

The structure and the spatio-temporal dynamics of the mixed beech–silver fir–Norway spruce old-growth forest of Biogradska Gora (Montenegro) have been analysed at different spatial scales: at the landscape scale, using a high-resolution SPOT5 satellite image and at the stand level with an intensive field survey. This remote-sensing approach has been used to obtain a land cover map in order to define the main vegetation types and to detect the large canopy gaps (>150 m²). The structural characteristics have been delineated in a 50-ha study area in which a regular 120-m grid was superimposed over a 1:10,000 raster map and 30 sampling points have been obtained. The forest is characterized by a high volume of living trees (1029.6 m³ ha^? 1) and coarse woody debris (420.4 m³ ha^? 1) and by small-scale disturbances (individual trees to small groups) with a low incidence of intermediate disturbances (18 forest canopy gaps>150 m² over 1230 ha). The two approaches have proved useful to delineate the spatio-temporal dynamics. The Biogradska Gora forest dynamics are dominated by very small-scale processes, which are partially autogenic and partially caused by allogenic factors. The influence of large-scale or intermediate disturbances has shown to be negligible.     

Spatial Patterns in Hyphal Growth and Substrate Exploitation within Norway Spruce Stems Colonized by the Pathogenic White-Rot Fungus Heterobasidion parviporum

In Norway spruce, a fungistatic reaction zone with a high pH and enrichment of phenolics is formed in the sapwood facing heartwood colonized by the white-rot fungus Heterobasidion parviporum. Fungal penetration of the reaction zone eventually results in expansion of this xylem defense. To obtain information about mechanisms operating upon heartwood and reaction zone colonization by the pathogen, hyphal growth and wood degradation were investigated using real-time PCR, microscopy, and comparative wood density analysis of naturally colonized trees with extensive stem decay. The hyphae associated with delignified wood at stump level were devoid of any extracellular matrix, whereas incipient decay at the top of decay columns was characterized by a carbohydrate-rich hyphal sheath attaching hyphae to tracheid walls. The amount of pathogen DNA peaked in aniline wood, a narrow darkened tissue at the colony border apparently representing a compromised region of the reaction zone. Vigorous production of pathogen conidiophores occurred in this region. Colonization of aniline wood was characterized by hyphal growth within polyphenolic lumen deposits in tracheids and rays, and the hyphae were fully encased in a carbohydrate-rich extracellular matrix. Together, these data indicate that the interaction of the fungus with the reaction zone involves a local concentration of fungal biomass that forms an efficient translocation channel for nutrients. Finally, the enhanced production of the hyphal sheath may be instrumental in lateral expansion of the decay column beyond the reaction zone boundary.To grow to great heights, trees continually replace their water- and nutrient-conducting elements. Older elements, such as the heartwood that is formed in many trees, gradually become nonconductive. In contrast to the living sapwood, heartwood lacks active defense mechanisms against microbes. However, lignin, the polymer coating cell wall polysaccharides, is highly resistant to microbial degradation. In fact, white-rot fungi, besides having evolved the ability to tolerate or detoxify the secondary metabolites accumulating in heartwood, are the only organisms capable of efficiently degrading lignin. Following establishment in the heartwood of living trees, the colonies of pathogenic white-rot fungi expand and eventually also threaten the conductive sapwood.The white-rot fungus Heterobasidion annosum sensu lato, composed of three species with overlapping geographic distributions and host ranges in Europe (23), is the most important pathogen of Norway spruce (Picea abies L. Karst) in boreal forests. Primary infection of Norway spruce stands by H. annosum sensu lato takes place through fresh thinning stumps or wounds on roots and at the base of the stem. Basidiospores landing on these entrance points give rise to mycelia which colonize the root systems, and eventually the fungus spreads into the stem heartwood. At sites infested with Heterobasidion parviporum, a species primarily restricted to Norway spruce, roots of saplings can become infected by the fungus after around 10 years of growth (25). Stem colonization usually initiates only after the heartwood has started to develop, which in Norway spruce takes place in trees 25 to 40 years old (17). Due to relatively rapid axial spread within heartwood, the decay column caused by H. annosum sensu lato often is up to 10 m high in the stems of mature Norway spruce trees.In response to sapwood challenge by an expanding heartwood-based colony of H. annosum sensu lato, Norway spruce forms a so-called reaction zone (RZ) in the border area between healthy sapwood and colonized heartwood. This xylem defense is characterized by high pH due to increased carbonate content and enrichment of phenolic compounds, particularly lignans, some of which have shown antifungal properties in bioassays (14, 30, 31). Although several wood decay fungi are able to eventually penetrate the RZ regions formed in trees, the strategies employed by fungi to breach these unique defense barriers are poorly understood (24). The purpose of this study was to obtain information about the mechanisms operating in heartwood colonization and expansion of the decay column via penetration of the RZ. To do this, we examined spatial growth of H. parviporum and the associated substrate exploitation patterns within naturally colonized mature stems of Norway spruce.     

Structure and stand development in three subalpine Norway spruce (Picea abies (L.) Karst.) stands in Paneveggio (Trento, Italy)

1. Three permanent plots (100×0 m) were established in the subalpine Norway spruce (Picea abies (L.) Karst.) forest of Paneveggio in the spring of 1993, to begin a long-term forest ecosystem research project. The main purpose of these plots was to provide information about subalpine Norway spruce stand dynamics and to provide suggestions for close-to-nature silviculture. 2. The three stands were selected to represent the most common forest structures in the Paneveggio forest. The first stand is close to forestry roads, has a relatively regular and continuous canopy, and thinning and cutting operations only ended in the 1980s; the second stand is far from forest roads and has developed without anthropogenic influence for several decades; the third one is located at the present upper limit of the pure spruce forest and, apparently, was heavily used in the past as a pasture. 3. The first step in the investigation was to describe the structure and to study the history of the three stands using both written evidence from manage- ment plans and biological archives from tree rings. 4. The stands in plots 1 and 2 began to establish after a disturbance that removed part of the previous stands according to dendroecological studies, which are partially supported by written evidence. The remaining parts of these stands were eliminated by two major disturbances that occurred during the following decades. Written records about the use of the forest lead us to assume that the initial disturbances that occurred in the two stands were logging activities as a part of a group shelterwood system. The stand in plot 2 has developed without significant human interference for about half a century as confirmed by the presence of many dead trees. The stand in plot 3 consists of old trees that were part of an open stand and a secondary population that established after cessation of grazing. 5. The study has confirmed that dendroecological techniques can be used to identify occurrence and intensity of previous disturbance in forests stands, although at Paneveggio it is difficult to distinguish between natural and anthropogenic disturbances in the tree ring record. The presence of human activity necessitates investigation of multiple lines of evidence. 6. Paneveggio's forest management plans were useful in the interpretation of the data obtained through dendroecological analysis, although events did not always correspond because data from the management plans (yearly thinning, felling, wind-throw damage) never gave stand-level details, but applied to areas of several hectares. Despite these limitations, the information included in the management plans is of crucial importance in studying stand history and only by using all these sources of information is it possible to delineate the most important features of the history and disturbance that affected the origin and subsequent growth of the forest stands.     

The spatio-temporal pattern of historical disturbances of an evergreen broadleaved forest in East China: a dendroecological analysis

Evergreen broadleaved forests (EBLF), the zonal forest ecosystem of the subtropical zone in east China, have been degraded from recent anthropogenic disturbance. Understanding the role of past disturbances in EBLFs would be helpful to the restoration of degraded EBLFs. We used dendroecological techniques to reconstruct the disturbance history of a secondary EBLF dominated by Schima superba and Castanopsis carlesii in Tiantong National Forest Park (29°48′N, 121°47′E), Zhejiang Province, East China. The disturbances were inferred from tree-ring growth release and long-term establishment patterns obtained from 91 overstory trees. The initial growth rates of these trees were compared to trees originating in the understory to evaluate the intensity of past disturbances. The spatial distribution patterns of disturbances were portrayed with tree mapping. The results revealed that there were five disturbances, averaging one disturbance per decade over the past half century. The first disturbance event was probably most intense given that most canopy trees established at that time and displayed high initial growth rates. The timing of the second disturbance event coincided with the documented selective logging. The last three disturbances, having lower tree growth responses and a clumped spatial distribution of gap creation, were probably the result of recurring typhoons. The first two disturbances led to tree regeneration and secondary succession, represented mainly by long-lived deciduous trees in the forest. The subsequent disturbances facilitated the stand development process, creating a complex three-dimensional structure from a pre-existing single-age cohort. This study suggests that EBLFs affected by large disturbances can recover in a few decades and the frequent gap disturbances probably facilitate its process in the early successional stages.     

Large old trees increase growth under shifting climatic constraints: Aligning tree longevity and individual growth dynamics in primary mountain spruce forests

In a world of accelerating changes in environmental conditions driving tree growth, tradeoffs between tree growth rate and longevity could curtail the abundance of large old trees (LOTs), with potentially dire consequences for biodiversity and carbon storage. However, the influence of tree-level tradeoffs on forest structure at landscape scales will also depend on disturbances, which shape tree size and age distribution, and on whether LOTs can benefit from improved growing conditions due to climate warming. We analyzed temporal and spatial variation in radial growth patterns from ~5000 Norway spruce (Picea abies [L.] H. Karst) live and dead trees from the Western Carpathian primary spruce forest stands. We applied mixed-linear modeling to quantify the importance of LOT growth histories and stand dynamics (i.e., competition and disturbance factors) on lifespan. Finally, we assessed regional synchronization in radial growth variability over the 20th century, and modeled the effects of stand dynamics and climate on LOTs recent growth trends. Tree age varied considerably among forest stands, implying an important role of disturbance as an age constraint. Slow juvenile growth and longer period of suppressed growth prolonged tree lifespan, while increasing disturbance severity and shorter time since last disturbance decreased it. The highest age was not achieved only by trees with continuous slow growth, but those with slow juvenile growth followed by subsequent growth releases. Growth trend analysis demonstrated an increase in absolute growth rates in response to climate warming, with late summer temperatures driving the recent growth trend. Contrary to our expectation that LOTs would eventually exhibit declining growth rates, the oldest LOTs (>400 years) continuously increase growth throughout their lives, indicating a high phenotypic plasticity of LOTs for increasing biomass, and a strong carbon sink role of primary spruce forests under rising temperatures, intensifying droughts, and increasing bark beetle outbreaks.     

Association genetics identifies a specifically regulated Norway spruce laccase gene,PaLAC5, linked to Heterobasidion parviporum resistance

It is important to improve the understanding of the interactions between the trees and pathogens and integrate this knowledge about disease resistance into tree breeding programs. The conifer Norway spruce (Picea abies) is an important species for the forest industry in Europe. Its major pathogen is Heterobasidion parviporum, causing stem and root rot. In this study, we identified 11 Norway spruce QTLs (Quantitative trait loci) that correlate with variation in resistance to H. parviporum in a population of 466 trees by association genetics. Individual QTLs explained between 2.1 and 5.2% of the phenotypic variance. The expression of candidate genes associated with the QTLs was analysed in silico and in response to H. parviporum hypothesizing that (a) candidate genes linked to control of fungal sapwood growth are more commonly expressed in sapwood, and; (b) candidate genes associated with induced defences are respond to H. parviporum inoculation. The Norway spruce laccase PaLAC5 associated with control of lesion length development is likely to be involved in the induced defences. Expression analyses showed that PaLAC5 responds specifically and strongly in close proximity to the H. parviporum inoculation. Thus, PaLAC5 may be associated with the lignosuberized boundary zone formation in bark adjacent to the inoculation site.     

Seasonality matters—The effects of past and projected seasonal climate change on the growth of native and exotic conifer species in Central Europe

Norway spruce is one of the economically most important tree species in Central European forestry. However, its high susceptibility to droughts poses a strong challenge to its cultivation under future conditions with likely more frequent and prolonged droughts and shifts in the seasonal climate. To compensate for expected losses of forest areas suitable for the cultivation of spruce, more drought-tolerant species are required. Silver fir and Douglas fir are two potential candidates, which promise lower drought susceptibility and equal or even higher yield when compared to Norway spruce.Using the Black Forest as a regional case study, we assessed the effects of seasonal climate change, including drought stress, on tree-ring width formation of these three economically relevant conifer species over the last 60 years. In addition, we projected potential species-specific growth changes under different climate change scenarios until 2100.Our results suggest that both silver fir and Douglas fir will possibly experience growth increases in a warmer future climate, as predicted under the 4.5 and 8.5 Representative Concentration Pathway (RCP) climate change scenarios, whereas growth of spruce is expected to decline. Moreover, drought susceptibility in silver fir and Douglas fir is lower than in spruce, as shown for past drought events, and their ability to benefit from milder winters and springs could play a major role in their capacity to compensate for drier summers in the near to mid-term future. This study highlights the need to advance our understanding of the processes that drive drought resistance and resilience in tree species to guide management strategies in the face of climate change.     

Size-dependent responses to summer drought in Scots pine, Norway spruce and common oak

In this study, we provide a detailed analysis of tree growth and water status in relation to climate of three major species of forest trees in lower regions of Bavaria, Southern Germany: Scots pine (Pinus sylvestris), Norway spruce (Picea abies) and common oak (Quercus robur). Tree-ring chronologies and latewood δ¹³C were used to derive measures for drought reaction across trees of different dimensions: growth reduction associated with drought years, long-term growth/climate relations and stomatal control on photosynthesis. For Scots pine, growth/climate relations indicated a stronger limitation of radial growth by high summer temperatures and low summer precipitation in smaller trees in contrast to larger trees. This is corroborated by a stronger stomatal control on photosynthesis for smaller pine trees under average conditions. In dry years, however, larger pine trees exhibited stronger growth reductions. For Norway spruce, a significantly stronger correlation of tree-ring width with summer temperatures and summer precipitation was found for larger trees. Additionally, for Norway spruce there is evidence for a change in competition mode from size-asymmetric competition under conditions with sufficient soil water supply to a more size-symmetric competition under dry conditions. Smaller oak trees showed a weaker stomatal control on photosynthesis under both dry and average conditions, which is also reflected by a significantly faster recovery of tree-ring growth after extreme drought events in smaller oak trees. The observed patterns are discussed in the context of the limitation-caused matter partitioning hypothesis and possible species-specific ontogenetic modifications.     

Decomposition and element concentrations of Norway spruce needle litter with differing B, N, or P status

Adequate boron (B) nutrition may decrease concentrations of phenolic compounds and enhance structural integrity and lignification in plants, compared with suboptimal B. This could affect decomposition in areas where B deficiencies are common. The mass loss and changes in element concentrations in Norway spruce needle litter were studied with combinations of litter from high-B and low-B trees, incubated for 29 months, in either B fertilised or control plots without B addition. The litter originated from the same Norway spruce field experiments. Additionally, the field experiments included long-term N and P treatments. Initially, lowest lignin concentrations were found in Norway spruce litter from the treatment P and particularly in the combination B?+?P, and highest in the B?+?N fertilised plots. The mass loss of Norway spruce litter was not affected by the treatments. However, B_litter increased Cu accumulation. The litter from the B?+?P fertilised plots accumulated considerably more Al, Ca, S and Zn than the other treatments, whereas B together with N reduced the remaining amounts of these elements. Reduced nutrient release from litter may have far-reaching consequences on nutrient cycles in forests.     

Silver fir and Douglas fir are more tolerant to extreme droughts than Norway spruce in south‐western Germany

Improving our understanding of the potential of forest adaptation is an urgent task in the light of predicted climate change. Long‐term alternatives for susceptible yet economically important tree species such as Norway spruce (Picea abies) are required, if the frequency and intensity of summer droughts will continue to increase. Although Silver fir (Abies alba) and Douglas fir (Pseudotsuga menziesii) have both been described as drought‐tolerant species, our understanding of their growth responses to drought extremes is still limited. Here, we use a dendroecological approach to assess the resistance, resilience, and recovery of these important central Europe to conifer species the exceptional droughts in 1976 and 2003. A total of 270 trees per species were sampled in 18 managed mixed‐species stands along an altitudinal gradient (400–1200 m a.s.l.) at the western slopes of the southern and central Black Forest in southwest Germany. While radial growth in all species responded similarly to the 1976 drought, Norway spruce was least resistant and resilient to the 2003 summer drought. Silver fir showed the overall highest resistance to drought, similarly to Douglas fir, which exhibited the widest growth rings. Silver fir trees from lower elevations were more drought prone than trees at higher elevations. Douglas fir and Norway spruce, however, revealed lower drought resilience at higher altitudes. Although the 1976 and 2003 drought extremes were quite different, Douglas fir maintained consistently the highest radial growth. Although our study did not examine population‐level responses, it clearly indicates that Silver fir and Douglas fir are generally more resistant and resilient to previous drought extremes and are therefore suitable alternatives to Norway spruce; Silver fir more so at higher altitudes. Cultivating these species instead of Norway spruce will contribute to maintaining a high level of productivity across many Central European mountain forests under future climate change.