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Das Stammdickenwachstum von Buchen und Tannen in Baden-Württemberg unter dem Einfluss von Witterung und Klimaveränderungen : Langfristige Trends und kurzfristige Reaktionen auf Trockenheit
Abstract
Das Wachstum von Bäumen wird maßgeblich von der Wasserverfügbarkeit
beeinflusst. Da für Baden-Württemberg steigende Temperaturen, abnehmende
Sommerniederschläge und eine Zunahme der Dauer von Trockenperioden prognostiziert werden, ist Kenntnis über die Reaktion von Bäumen auf diesen Klimawandel unabdingbar für eine zukunftsfähige Forstwirtschaft. In Baden-Württemberg ist die Buche die wichtigste Laubbaumart und kommt flächendeckend vor. Die Tanne ist nach der Fichte die häufigste Nadelbaumart mit Vorkommen vorrangig in den Wuchsgebieten Schwarzwald und Südwestdeutsches Alpenvorland. Studien deuten darauf hin, dass Buchen in Baden-Württemberg in den letzten Jahrzehnten vor allem in tieferen Lagen bereits abnehmende Zuwächse aufweisen – mutmaßlich aufgrund zunehmender Aridität. Auch für die Tanne wird vermutet, dass diese zwar positiv auf zunehmende Temperaturen, nicht jedoch auf zunehmende Trockenheit reagiert. Untersuchungen aus anderen Regionen Europas kommen jedoch zu teils abweichenden Ergebnissen, sodass sich noch kein klares Bild ergibt.
Im Rahmen der vorliegenden Arbeit soll daher für Buchen und Tannen in Baden-Württemberg untersucht werden, (1) wie die Witterung generell das Radialwachstum beeinflusst, (2) welche langfristigen Trends, vor allem seit den 1980er Jahren, auftreten und (3) welche Reaktion das Radialwachstum auf Trockenperioden zeigt und wie diese durch verschiedene Eigenschaften der Trockenheit beeinflusst wird. Dazu wurden von (vor)herrschenden Bäumen auf Wald-Dauerbeobachtungsflächen der Landesanstalt für Umwelt Baden-Württemberg Bohrkerne gewonnen und die gemessenen Jahrringserien hinsichtlich der inter-annuellen Schwankungen der Jahrringbreiten und langfristiger Veränderungen des Zuwachsniveaus ausgewertet. Um die Reaktion auf Trockenheit zu untersuchen wurden zunächst basierend auf dem Standardised-Evapotranspiration-Precipitation-Index Trockenjahre ermittelt, für die dann Trockenstressindizes berechnet wurden. Da diese eine hohe Variabilität zwischen den Trockenjahren aufweisen, wurden für die Zuwachsreaktion im Trockenjahr (Resistenz) und die Veränderung des Zuwachses nach dem Trockenjahr im Vergleich zum Zuwachs im Trockenjahr (Erholung) Modelle aufgestellt, in die die verschiedenen Eigenschaften der Trockenperiode, die etwa deren Dauer, Intensität und zeitliches Auftreten quantifizieren, sowie verschiedene Eigenschaften des Baums im Trockenjahr als unabhängige Variablen eingingen.
Die Auswertung der Buchen zeigt, dass (1) das Radialwachstum von Buchen von der Wasserverfügbarkeit limitiert wird, wobei im Norden Baden-Württembergs vor allem die Niederschläge der vorangegangenen Herbst- und Wintermonate relevant sind und im Südosten Baden-Württembergs die Niederschläge des Frühsommers. So konnten auch (2) auf fast allen Untersuchungsflächen abnehmende Grundflächenzuwächse seit den 1980er bzw. 1990er Jahren festgestellt werden, doch wird neben zunehmender Trockenheit auch ein Einfluss der in diesem Zeitraum ebenfalls zunehmenden Bestandesgrundfläche nicht ausgeschlossen. Dennoch zeigen die Buchen (3) in Zeiten abnehmender Zuwächse im Vergleich zu Zeiträumen stabilen Zuwachsniveaus eine höhere Resistenz und Erholung, und schnell wachsende Buchen eine geringere Resistenz und Erholung als langsam wachsende Bäume, sodass hier ein Trade-Off zwischen oberirdischer Produktivität und Trockenheitstoleranz vermutet wird. Jedoch ist die Resistenz der Buchen auch dann höher, wenn die Trockenperiode in einen Zeitraum mittelfristig besserer Wasserversorgung fällt, was darauf hindeutet, dass vor allem eine zunehmende Frequenz von Trockenjahren negative Auswirkungen auf die Buche hat. Unerwartet ist der bedeutende Einfluss der Fruchtbildung auf die Resistenz der Buche – diese nimmt mit zunehmender Mastintensität im Trockenjahr substantiell ab. Dieses Ergebnis zeigt deutlich, dass eine fehlende Berücksichtigung der Fruktifikation zu Fehlinterpretationen der Trockenheitstoleranz der Buche führen kann.
Die Auswertung der Tannen zeigt, dass (1) auf das Radialwachstum die Witterung nur in geringem Maße einen Einfluss ausübt und entgegen der Erwartungen hier ebenfalls vor allem der Niederschlag eine wichtige Rolle spielt und nicht die Temperatur. Die Tannen weisen (2) langfristig auffällige Zuwachstrends auf, die weitgehend mit denen anderer Untersuchungen in Europa vergleichbar sind, vor allem hinsichtlich einer Zuwachsdespression mit sehr schmalen Jahrringen in den 1970er Jahren, auf die ein beispielloser Anstieg der jährlichen Zuwächse folgt, der in den 1990er Jahren bzw. im ersten Jahrzehnt des 21. Jahrhunderts kulminiert. Aufgrund der hohen Heterogenität der Verläufe innerhalb der einzelnen Untersuchungsflächen wird jedoch davon ausgegangen, dass nicht Klimaveränderungen allein, sondern auch hier Veränderungen der Bestandesstruktur einen maßgeblichen Einfluss haben. Auch die Tannen (3) reagieren mit einem geringeren Zuwachseinbruch auf Trockenheit, wenn die mittelfristige Wasserversorgung gut ist. Für die Erholung ist vor allem das zeitliche Auftreten der stärksten Trockenheit relevant, vor allem in Kombination mit dem anschließenden Wiederanstieg der Wasserverfügbarkeit. Auch bei den Tannen weisen schnell wachsende Bäume eine geringere Resistenz und Erholung auf als
langsam wachsende.
Insgesamt geben die Ergebnisse der vorliegenden Arbeit einen umfassenden Überblick über das Radialwachstum von Buchen und Tannen in Baden-Württemberg und den Einfluss der Witterung auf verschiedenen Ebenen und liefert daher einen wichtigen Beitrag zum Verständnis des Stammdickenwachstums dieser beiden wichtigen Baumarten unter den bereits zu beobachtenden und zukünftigen Klimaveränderungen. Bezüglich der Buchen ist vor allem die großräumige Übereinstimmung der inter-annuellen Schwankungen der Jahrringbreite trotz teils stark abweichender langfristiger Trends erstaunlich. Hier liefert die vorliegende Arbeit einen Hinweis darauf, dass abgesehen von der Höhenlage auch die geografische Lage eine Rolle für die Entwicklung des Radialwachstums spielt – möglicherweise aufgrund unterschiedlicher intra-annueller Niederschlagsverteilungen und gerade vor dem Hintergrund deren Veränderungen im Zuge des Klimawandels.
Bezüglich der Reaktion des Radialwachstums von Buchen und Tannen in Baden-Württemberg auf Trockenheit zeigt die vorliegende Arbeit die erste umfassende Untersuchung zur Variabilität der Zuwachsreaktion zwischen Trockenjahren und zu den zugrundeliegenden Einflussfaktoren. Dabei wird deutlich, dass weder die Trockenperioden verschiedener Jahre miteinander vergleichbar sind noch die Zuwachsreaktionen der Bäume, da sowohl die Intensität und das zeitliche Auftreten innerhalb der Vegetationsperiode als auch die Eigenschaften der Bäume und vor allem das zeitgleiche Auftreten von starker Fruktifikation diese beeinflussen. Wenn also die Toleranz gegenüber Trockenheit zwischen verschiedenen Jahren, Beständen und Baumarten verglichen werden soll, ist es notwendig, sämtliche Einflussfaktoren zu berücksichtigen.
... As mast years with intense reproductive effort might reduce the proportion of net primary production available for vegetative growth of trees, fructification is discussed as potential confounding factor influencing the results of the analysis of resilience components (Seifert and Müller-Starck, 2009;Hacket-Pain et al., 2017;Hirsch, 2019). For a better interpretation of the Lloret-indices and as complement to the inferential statistics described above, we used data of the forest monitoring program of the German province of Baden-Württemberg (Meining et al., 2020), which records the annual fructification intensity of individual forest trees in an 8 x 8 km sampling grid using an ordinal scale from 0 (absent) to 3 (abundant). ...
... A possible explanation of the decreased drought resistance of all species in 2018 might be the high coning intensity in the same year of all investigated species, which likely had caused substantial reallocations of carbohydrates due to the sink strength of cone production while simultaneously decreasing the proportion of net primary production available for vegetative growth of trees (van der Hacket-Pain et al., 2017;Hirsch, 2019). In contrast, Seifert and Müller-Starck (2009) did not detect trade-offs between cone production and vegetative growth of Norway spruce during and after mast years. ...
The conifer tree species Norway spruce (Picea abies), silver fir (Abies alba) and Douglas fir (Pseudotsuga menziesii) are important elements in tree species composition and forest management of Central European forests, but their potential to thrive under anticipated climatic changes is still debated controversially. This study contributes a multivariate analysis of resilience components based on increment cores sampled at breast height of Norway spruce, silver fir and Douglas fir trees growing along elevational gradients in Southwestern Germany. We aimed to gain novel insights into the species-specific and elevational response of tree growth and wood density variables during the extreme drought events of the years 2003 and 2018. Our results for Norway spruce corroborate projections of its ongoing decline during climate change as the reductions of wood density and biomass production indicated high drought sensitivity at all elevations. Moreover, resilience indices of mean tree-ring density, maximum latewood density, tree-ring width and biomass production were even lower after the drought of 2018 compared to the previous drought of 2003. Silver fir, a potential substitute tree species for Norway spruce, showed unexpected results with resistance and resilience indices being significantly lower in 2018 compared to 2003 indicating that silver fir might be more vulnerable to drought than previously expected, especially at low elevations. In contrast, the superior growth rates and higher levels of drought tolerance of Douglas fir were especially pronounced during the drought of 2018 and visible across the entire elevational gradient, even though high coning intensity was present for all investigated tree species as a possible confounding factor to exacerbate the drought stress effects in the study region.
In this paper, we discuss an extension to two popular approaches to modeling complex structures in ecological data: the generalized additive model (GAM) and the hierarchical model (HGLM). The hierarchical GAM (HGAM), allows modeling of nonlinear functional relationships between covariates and outcomes where the shape of the function itself varies between different grouping levels. We describe the theoretical connection between HGAMs, HGLMs, and GAMs, explain how to model different assumptions about the degree of intergroup variability in functional response, and show how HGAMs can be readily fitted using existing GAM software, the mgcv package in R. We also discuss computational and statistical issues with fitting these models, and demonstrate how to fit HGAMs on example data. All code and data used to generate this paper are available at: github.com/eric-pedersen/mixed-effect-gams.
Crown traits and competition attributes have an important effect on tree radial increment. Relationships among these elements are modeled using the distributions of the crown characteristics in a given calendar year, but these patterns can differ over time. The suitability of the patterns during recovery and normal growth was investigated using silver fir, Abies alba Mill., in old-growth forests. Generalized additive models (GAMs) for silver fir in the older (OG, trees aged 136–300 years) and younger (YG, trees aged 45–135 years) generations were developed. To test the validity of these GAMs, field data sets representing silver fir recovery and normal growth were used. For silver fir in OG, crown transparency had the largest effect on tree growth, explaining more than 25% of the variance. For silver fir in YG, relative crown length had the largest effect on tree growth, explaining more than 15% of the variance. The absolute relative prediction errors, AREmin and AREmax, were less than 0.03 and 1.50 mm, respectively. The developed GAMs are suitable during recovery and normal growth, but the GAMs were fitted to a relatively small area, neglecting climatic gradients and different disturbance types. This type of investigation should be continued on a larger scale.
Foresters from many countries are seeking for tree species or provenances able to cope with expected climate change. While it becomes clear that some temperate tree species will increasingly suffer from climate warming, the fate of the ecologically and economically important silver fir (Abies alba Mill.) remains uncertain and debated because the ecological requirements of this species, as well as its resilience to drought, are still unclear. On the one hand, paleoecological studies reveal that this species was widely distributed under much warmer climate, suggesting a high potential to cope with ongoing and future climate warming. On the other hand, species distribution models generally predict a strong decline of its climatic suitability in the future. This paper aims to clarify the potential of this species to thrive in central and western Europe under predicted climate warming by reviewing the knowledge gained from different fields. Based on insight from different fields, we argue that silver fir has a great potential to thrive under warmer conditions in western and central Europe provided sufficient rainfall, as forecasted by climate models for most regions by 2100. For instance, dendroecological studies demonstrate that silver fir is more resistant and resilient to drought compared to co-occurring species such as Norway spruce, European beech and larch. The most prominent obstacle for increasing the proportion of fir in mixed forests nowadays is ungulate browsing that often prevents its upgrowth.
A novel methodological framework is presented for climate-sensitive modeling of annual radial stem increment using year-ring width time series. The approach is based on a generalized additive model with penalized regression splines together with a distributed time lag model taking into account smooth nonlinear effects of a series of monthly temperature and precipitation values, as well as their interactions. Climate effects are also assumed to vary smoothly with time lag. The model framework enables both the detrending of the individual time series and the regression modeling to be performed simultaneously in a single model step. The approach is applied to year-ring width time series of Norway spruce (Picea abies (L.) H. Karst.) trees in Tyrol, Austria. The marginal response curves show that tree growth is mainly promoted by high temperatures in late spring and early summer and by precipitation in fall and winter. Summer drought does not have a negative influence on the current year’s radial increment; however, when it is associated with high temperatures, it lowers the increment in the subsequent growth period. Higher winter precipitation in conjunction with lower temperatures has a positive effect. A significant non-climate related long-term growth trend is demonstrated, probably reflecting NOx and SO2 emission trends in Austria.
Background
Climate change is affecting forest growth and vitality globally, driving shifts in tree species distributions, which could result in major changes of important ecosystem services. One of the main forest functions in the region of the Black Forest is timber production. A large proportion of that timber is obtained from Norway spruce (Picea abies) monocultures. However, it has been shown that spruce is highly drought-sensitive. Its growth potential under future climatic conditions, which are characterized by an increasing frequency and intensity of summer droughts, has been questioned. In the long term, it will be inevitable to replace large proportions of current spruce forests with other tree species. Here I assessed the suitability of two potential substitute species, which are economically interesting and presumably more drought-tolerant, than spruce: Silver fir (Abies alba) and Douglas-fir (Pseudotsuga menziesii). Since mixed-species forests are assumed to increase ecological stability of forests in relation to stress and disturbances and thus to reduce future risks, the drought-tolerance of these three conifer species was assessed in mixed stands.
Aim
The aim of this thesis was to provide an assessment of the growth potential these three species under future climatic conditions in the Black Forest. A direct comparison of their drought responses and an understanding of their interactions in mixed-species stands is still lacking. The responses to past extreme drought events were measured along altitudinal gradients through a dendrochronological analysis. Climate correlation analyses were used to quantify the potential effect of shifts in seasonality of precipitation and temperature on radial growth of these species for the past 60 years and under future climate scenarios. I also quantified the net effects of mixing these three species on radial growth in “normal” and in drought years.
Research questions
1) Are Silver Fir and Douglas-fir more resistant and resilient to extreme drought events than Norway spruce across altitudinal gradients in the Black Forest?
2) How will these tree species react to future shifts in seasonality of precipitation and temperatures connected to climate change?
3) Does the mixing of the tree species influence their growth and resistance and resilience to drought?
Study site and sampling design
A total of 270 trees per species were sampled in 18 managed and mature mixed-species stands at the western slopes of the southern, central and northern Black Forest in southwest Germany.
Under predicted climate change, native silver fir (Abies alba) and European beech (Fagus sylvatica) are the most likely replacement species for the Norway spruce (Picea abies) monocultures planted across large parts of continental Europe. Our current understanding of the adaptation potential of fir-beech mixed forests to climate change is limited because long-term responses of the two species to environmental changes have not yet been comprehensively quantified. We compiled and analysed tree-ring width (TRW) series from 2855 dominant, co-dominant, sub-dominant and suppressed fir and beech trees sampled in 17 managed and unmanaged mixed beech-fir forest sites across Continental Europe, including Bosnia and Herzegovina, Germany, Italy, Romania and Slovakia. Dendroecological techniques that combine various detrending methods were used to investigate variation in radial growth of co-occurring fir and beech trees. Coincidental with peak SO2 emissions, the growth of silver fir declined between 1950 and 1980 at most sites, whereas beech growth increased during this period. Correspondent to a significant warming trend from 1990–2010, average beech growth declined, but silver fir growth increased. Long-term growth patterns and growth-climate sensitivity of fir and beech trees did not significantly differ between managed and unmanaged forests. Multi-decadal changes in the growth rate of all vertical tree classes were similar. In contrast to previous indications of limited drought susceptibility of beech mixed stands, this study suggests that the mixture of tree species in forest stands does not necessarily prevent growth depressions induced by long-term environmental change. Our results further imply that forest management does not necessarily alter their sensitivity to environmental changes.
The reach of different tree species’ crowns and the velocity of gap closure during the occupation of canopy gaps resulting from mortality and thinning during stand development determine species-specific competition and productivity within forest stands. However, classical dendrometric methods are rather inaccurate or even incapable of time- and cost-effectively measuring 3D tree structure, crown dynamics and space occupation non-destructively. Therefore, we applied terrestrial laser scanning (TLS) in order to measure the structural dynamics at tree and stand level from gap cutting in 2006 until 2012 in pure and mixed stands of Norway spruce (Picea abies [L.] Karst.) and European beech (Fagus sylvatica L.). In conclusion, our results suggest that Norway spruce invests newly available above-ground resources primarily into DBH as well as biomass growth and indicate a stronger resilience against loss of crown mass induced by mechanical damage. European beech showed a vastly different reaction, investing gains from additional above-ground resources primarily into faster occupation of canopy space. Whether our sample trees were located in pure or mixed groups around the gaps had no significant impact on their behavior during the years after gap cutting.
Phenotypic variability within forest species populations is considered of special relevance for local adaptation under new environments, albeit it has been analyzed to a lesser extent than inter-population phenotypic variability. A common garden study was carried out to assess phenotypic variability in response to water stress in half-sibling families from a marginal population of Fagus sylvatica L. at its south-western range edge distribution in Europe. Two irrigation regimes were applied, well-watered (WW) seedlings and those submitted to weekly cycles of drying-rewatering of growth media. Seedling growth and their leaf functional traits were recorded during the last cycle of water stress. Most of the phenotypic changes were explained by phenotypic plasticity in response to water stress, but there was also a significant effect of family in the expression of some of the studied traits. The relationship of carbon isotope fractioning with gas exchange traits across families under WW conditions did not follow the same pattern as the phenotypic trends. The leaf net photosynthesis across families was modified by the nitrogen content on a leaf mass basis that was in turn correlated positively with leaf nitrogen isotope fractionation. The results point to an important role of leaf nitrogen in determining the intrinsic water-use efficiency (WUE) across families. Variation in WUE was ruled mainly by control of stomatal conductance to water vapor under water stress, but by leaf net photosynthesis under wet conditions. Relatively high inter-family phenotypic variability in growth and functional traits were observed. Within-population phenotypic variability, and the plasticity of some of the studied traits, is of fundamental importance to cope with the harsher environments beech will have to endure in the future at different points in its distribution range.
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.
Key messageRecent growth changes (1980–2007) in Western European forests strongly vary across tree species, and range from +42% in mountain contexts to −17% in Mediterranean contexts. These changes reveal recent climate warming footprint and are structured by species' temperature (−) and precipitation (+) growing conditions. ContextUnprecedented climate warming impacts forests extensively, questioning the respective roles of climatic habitats and tree species in forest growth responses. National forest inventories ensure a repeated and spatially systematic monitoring of forests and form a unique contributing data source. AimsA primary aim of this paper was to estimate recent growth changes in eight major European tree species, in natural contexts ranging from mountain to Mediterranean. A second aim was to explore their association with species’ climatic habitat and contemporary climate change. Methods
Using >315,000 tree increments measured in >25,000 NFI plots, temporal changes in stand basal area increment (BAI) were modelled. Indicators of climate normals and of recent climatic change were correlated to species BAI changes. ResultsBAI changes spanned from −17 to +42% over 1980–2007 across species. BAI strongly increased for mountain species, showed moderate/no increase for generalist and temperate lowland species and declined for Mediterranean species. BAI changes were greater in colder/wetter contexts than in warmer/drier ones where declines were observed. This suggested a role for climate warming, further found more intense in colder contexts and strongly correlated with species BAI changes. Conclusion
The predominant role of climate warming and species climatic habitat in recent growth changes is highlighted in Western Europe. Concern is raised for Mediterranean species, showing growth decreases in a warmer climate with stable precipitation.
A central issue in plant evolutionary ecology is to understand how several coordinated suites of traits (i.e. traits syndrome) may be jointly selected within a single species. This study aims to describe patterns of variation and co‐variation of functional traits in a water‐stressed tree population and test their relationships with performance traits.
Within a Mediterranean population of Fagus sylvatica experiencing recurrent summer droughts, we investigated the phenotypic variation of leaf unfolding phenology, leaf area (LA), leaf mass per area (LMA), leaf water content (LWC), water use efficiency (WUE) estimated by carbon isotopic discrimination (d13C), twig Huber‐value (HV: the stem cross‐section divided by the leaf area distal to the stem), wood density (WDens), and leaf nitrogen content (Nmass).
First, a principal component analysis revealed that two main axes structured the phenotypic variability: the first axis opposed leaf unfolding earliness and LWC to LMA and WUE; the second axis opposed LA to HV. These two axes can be interpreted as the opposition of two strategies (water economy versus water uptake) at two distinct scales (leaf for the first axis and branches for the second axis). Second, we found that LMA, LA, leaf unfolding and LWC responded differently to competition intensity, while WUE, WDens and HV did not correlate with competition. Third, we found that all studied functional traits were related to growth and/or reproductive performance traits and that these relationships were frequently non‐linear, showing strong interactions between traits.
By highlighting phenotypic clustering of functional traits involved in response to water stress and by evidencing antagonistic selection favouring intermediate trait values as well as trait combinations, our study brought new insights on how natural selection operates on plant functional traits in a stressful environment.
Interannual variation in radial growth is influenced by a range of physiological processes, including variation in annual reproductive effort, although the importance of reproductive allocation has rarely been quantified. In this study, we use long stand-level records of annual seed production, radial growth (tree ring width) and meteorological conditions to analyse the relative importance of summer drought and reproductive effort in controlling the growth of Fagus sylvatica L., a typical masting species. We show that both summer drought and reproductive effort (masting) influenced growth. Importantly, the effects of summer drought and masting were interactive, with the greatest reductions in growth found in years when high reproductive effort (i.e. mast years) coincided with summer drought. Conversely, mast years that coincided with non-drought summers were associated with little reduction in radial growth, as were drought years that did not coincide with mast years. The results show that the strength of an inferred trade-off between growth and reproduction in this species (the cost of reproduction) is dependent on environmental stress, with a stronger trade-off in years with more stressful growing conditions. These results have widespread implications for understanding interannual variability in growth, and observed relationships between growth and climate.
European beech (Fagus sylvatica L.) is one of the economically most important broadleaved tree species in Central Europe. However, beech shows high drought sensitivity and calls for profound research to test its ability to cope with limited water resources. Here, we investigated the drought tolerance of beech to the 2003 drought as influenced by Kraft class, aspect and thinning intensity. Annual basal area increment data of 126 sample trees from southwest Germany were used to assess the variability of drought tolerance indices, by comparing three social classes (predominant, dominant and co-dominant), two contrasting sites [a dry southwest (SW) aspect and a moist northeast (NE) aspect], and three treatments [control, strong thinning (stand basal area 15 m2 ha−1) and very strong thinning (stand basal area 10 m2 ha−1)] in mature beech stands. Our results show that the co-dominant and dominant trees had lower growth recovery and lower growth resilience after the drought, compared to the predominant trees. The differences between aspects pointed to a growth–drought tolerance trade-off, in which trees on the SW aspect displayed lower growth rates but higher resilience indices than trees on the moist NE aspect. Furthermore, our results suggest that the resistance to and resilience after the 2003 drought significantly increased for the thinned trees. Our results provide novel insights into the linkage between the forest stand management and drought tolerance of beech under contrasting sites. We conclude that thinning can partially alleviate effects of severe drought on European beech forests in southwest Germany and can be applied as an adaptive measure to increase the mitigation potential of beech stands.
Key message
In the center of the species’ distribution range in Germany, European beech stands show continued climate-change-related growth decline since the 1980s at low elevations, but growth increase at high elevations.
Abstract
Contradicting reports exist about the climate-change sensitivity of European beech (Fagus sylvatica), showing either a sensitive response of radial growth to dry and hot summer episodes and long-term growth decline with recent warming, or apparent insensitivity of growth and a remarkable potential for post-stress recovery. With a dendroecological study along an altitudinal transect in the center of the species’ distribution range in south-western Germany, we analyzed the climate response of radial growth with variation in altitude (110–1230 m) and associated temperature (10.6–3.5 °C MAT) and precipitation change (755–1788 mm year⁻¹ MAP). Climate sensitivity analysis showed that annual stem increment was strongly limited at low elevations (110–300 m) by low precipitation in April/May, but by low summer temperatures at 1230 m. At intermediate elevation (640 m), indications of both moisture and temperature limitation were found. The differences in climate sensitivity were linked to contrasting long-term growth trends. At 110–300 m, radial growth has continually decreased since about the 1980s, while it has increased at 1230 m. Our results from the four stands suggest for the study region that the abiotic control of beech radial growth switches from moisture to temperature limitation at a threshold situated between 160 and 235 mm of precipitation in April/May (which corresponds to 200 and 313 mm of precipitation in June–August), in accordance with dendroecological results from other Central European lowland regions. This indicates that, with further warming and drying of the climate, beech may suffer in lowland and lower montane regions of Central Europe from reduced vitality and productivity, whereas it may profit from warming in montane to upper montane elevation. We conclude that climate-change-related growth decline is more widespread in the center of the species’ distribution range than previously thought, which is highly relevant for forestry planning.
Key message
The study found an increased investment into stem growth (compared to root growth) if trees were surrounded by a complementary species. This response is consistent with known patterns about root–stem allometry under favorable conditions (humidity and stand density).
Abstract
The study investigated partitioning of resources between roots and stems in mono-species and mixed-species stands of Douglas-fir and European beech at four different sites. We combined tree ring analyses of stems and coarse roots to scrutinize root–stem allometry with a focus on how it is influenced by species mixture and humidity. The results show that allometry in mixed stands changed in favor of stem growth for both species. The greatest relative allocation into stem growth was observed for individual trees which were completely surrounded by trees of the other species. The data indicate that a decrease of stand density, which was used as a proxy for tree competition, has the same effect on allocation. To analyze the influence of humidity, we used a long- and short-term index. Based on these, we can show that allocation changes with general site conditions and annual humidity variations. We found that on both time scales, both species increase resource investment into stem growth if conditions are more humid. Under harsher conditions, allocation shifts into root growth. The findings contribute to understanding the overyielding in mixed stands. Mixing Douglas-fir and European beech leads to the same allocation patterns as an improvement of site conditions. We suggest that for both species, mixture is equivalent to growing on a better site.
Temperate forests are expected to be particularly vulnerable to drought and soil drying because they are not adapted to such conditions and perform best in mesic environments. Here we ask (i) how sensitively four common temperate tree species (Fagus sylvatica, Picea abies, Acer pseudoplatanus and Fraxinus excelsior) respond in their water relations to summer soil drying and seek to determine (ii) if species-specific responses to summer soil drying are related to the onset of declining water status across the four species. Throughout 2012 and 2013 we determined tree water deficit (TWD) as a proxy for tree water status from recorded stem radius changes and monitored sap flow rates with sensors on 16 mature trees studied in the field at Lägeren, Switzerland. All tree species responded equally in their relative maximum TWD to the onset of declining soil moisture. This implies that the water supply of all tree species was affected by declining soil moisture and that none of the four species was able to fully maintain its water status, e.g., by access to alternative water sources in the soil. In contrast we found strong and highly species-specific responses of sap flow to declining soil moisture with the strongest decline in P. abies (92%), followed by F. sylvatica (53%) and A. pseudoplatanus (48%). F. excelsior did not significantly reduce sap flow. We hypothesize the species-specific responses in sap flow to declining soil moisture that occur despite a simultaneous increase in relative TWD in all species reflect how fast these species approach critical levels of their water status, which is most likely influenced by species-specific traits determining the hydraulic properties of the species tree.
Weather and its lagged effects have been associated with interannual variability and synchrony of fruit production for several tree species. Such relationships are used often in hypotheses relating interannual variability in fruit production with tree resources or favourable pollinating conditions and with synchrony in fruit production among sites through the Moran effect (the synchronisation of biological processes among populations driven by meteorological variability) or the local availability of pollen. Climatic teleconnections, such as the North Atlantic Oscillation (NAO), representing weather packages, however, have rarely been correlated with fruit production, despite often being better predictors of ecological processes than is local weather. The aim of this study was to test the utility of seasonal NAO indices for predicting interannual variability and synchrony in fruit production using data from 76 forests of Abies alba, Fagus sylvatica, Picea abies, Pseudotsuga menziesii, Quercus petraea, and Q. robur distributed across central Europe. Interannual variability in fruit production for all species was significantly correlated with seasonal NAO indices, which were more prominently important predictors than local meteorological variables. The relationships identified by these analyses indicated that proximal causes were mostly responsible for the interannual variability in fruit production, supporting the premise that local tree resources and favourable pollinating conditions are needed to produce large fruit crops. Synchrony in fruit production between forests was mainly associated with weather and geographical distance among sites. Also, fruit production for a given year was less variable among sites during warm and dry springs (negative spring NAO phases). Our results identify the Moran effect as the most likely mechanism for synchronisation of fruit production at large geographical scales and the possibility that pollen availability plays a role in synchronising fruit production at local scales. Our results highlight the influence of the NAO on the patterns of fruit production across western Europe.
Biogeographical and ecological theory suggests that species distributions should be driven to higher altitudes and latitudes as global temperatures rise. Such changes occur as growth improves at the poleward edge of a species distribution and declines at the range edge in the opposite or equatorial direction, mirrored by changes in the establishment of new individuals. A substantial body of evidence demonstrates that such processes are underway for a wide variety of species. Case studies from populations at the equatorial range edge of a variety of woody species have led us to understand that widespread growth decline and distributional shifts are underway. However, in apparent contrast, other studies report high productivity and reproduction in some range edge populations. We sought to assess temporal trends in the growth of the widespread European beech tree (Fagus sylvatica) across its latitudinal range. We explored the stability of populations to major drought events and the implications for predicted widespread growth decline at its equatorial range edge. In contrast to expectations, we found greatest sensitivity and low resistance to drought in the core of the species range, whilst dry range edge populations showed particularly high resistance to drought and little evidence of drought-linked growth decline. We hypothesize that this high range edge resistance to drought is driven primarily by local environmental factors that allow relict populations to persist despite regionally unfavourable climate. The persistence of such populations demonstrates that range-edge decline is not ubiquitous and is likely to be driven by declining population density at the landscape scale rather than sudden and widespread range retraction.
We studied the effect of local weather conditions on intra-annual wood formation dynamics and wood structure of European beech (Fagus sylvatica L.) from a temperate location in the Czech Republic in two consecutive years, 2010 and 2011, characterized by different amounts of precipitation. Microcores were taken at weekly intervals and transverse sections of cambial and xylem tissue were prepared for light microscopic observation. Air temperature and soil moisture content were measured daily at the research plot. Tree-ring formation patterns and vessel features showed different responses to climatic factors in the two years. In 2010, the onset of cambial cell production occurred almost 10 days later than in 2011, when a considerably reduced amount of rainfall was already observed in the winter and spring months, as shown in Standardized Precipitation Index (SPI) values. Lack of precipitation in 2011 caused premature cessation of cambial cell division and markedly narrower annual xylem increments. Vessel density and water conductive area were higher in 2011 than in 2010. Average vessel size in general did not change. In response to local weather conditions, beech controls its hydraulic conductivity mainly by changing the number of vessels and tree growth rate, followed by vessel size. The lower sensitivity of vessel diameter to hydrological alterations confirms previous studies by other authors.
Two drought stress indices were applied to managed as well as old-growth beech forests and gaps for the 2001 to 2013 period to aid in the development of an efficient tool for field water supply diagnosis. The relative extractable soil water (REW), which was calculated from the soil water content in the root zone, and the transpiration index (TI), calculated as the ratio between the actual and potential transpiration were used. Both indices were calculated on a daily basis using the water balance model BROOK90, which was fitted and tested using measured data on throughfall and soil water content. A sensitivity analysis apportioned to the input parameters of the drought stress indices was conducted to assess uncertainty. Both drought stress indices showed the greatest drought stress in the years 2009, 2003 and 2011, as also indicated by the Standardized Precipitation Evapotranspiration Index (SPEI) at the nearest meteorological station. However, drought stress intensity and duration differed between the indices and study sites. Greater water supply stress was shown in the forests than the gaps. Furthermore, the agreement among the indices was smaller for gaps compared with forests, which implies that careful index selection is needed when comparing water supply stresses in different stages of forest stand development. Due to the low amount of input data required and the parameters that can be measured with relative ease in the field, REW might be an efficient tool for field water supply diagnosis when analyzing the drought stresses of similar forest types and at unique stages of development. REW satisfactorily indicated drought stress in forests but to a lesser extent in gaps. TI demonstrated more consistent differences in drought stress between forests and gaps and therefore proved to be the appropriate index for a detailed analysis of drought stress variation between different stages of forest stand development. However, due to a greater number of required input data and more demanding parameters, TI appears to be a more complex tool than REW for field water supply diagnosis in forests.
Least-squares means are predictions from a linear model, or averages thereof. They are useful in the analysis of experimental data for summarizing the effects of factors, and for testing linear contrasts among predictions. The lsmeans package (Lenth 2016) provides a simple way of obtaining least-squares means and contrasts thereof. It supports many models fitted by R (R Core Team 2015) core packages (as well as a few key contributed ones) that fit linear or mixed models, and provides a simple way of extending it to cover more model classes.
Tree growth is strongly influenced, among other factors, by climate. Much knowledge regarding climate–growth relationships has been gained by studying tree rings. However, sufficient tree-ring data is rarely available if climate effects are required to be representative for large spatial scales, for example, in order to be used in scenario models to estimate forest development under climate change. Alternative data sources include large-scale forest inventories, although these usually provide lower temporal resolutions than tree rings. When working with temporally sparsely-resolved growth data, the question of how climate–growth relationships aggregate over time becomes relevant. To overcome this trade-off between spatial representativeness and temporal resolution, this study aims at optimally using the information contained in the annual resolution of tree rings to derive recommendations regarding the choice of climate variables for modeling tree growth based on forest inventories. We evaluated for Picea abies and Fagus sylvatica, which part of the year (spring, summer, vegetation period, whole year) and whether mean or extreme climatic conditions within inventory intervals should be taken into consideration. A three-step approach was used: (1) we used response functions to quantify the effect of monthly precipitation and temperature on annual basal area increments, (2) we temporally aggregated the annual basal area increments to hypothetical intervals of five and ten years, and correlated them with climate means and extremes – from different parts of the year – within the aggregated intervals, and (3) we fitted linear mixed-effects models to simultaneously quantify the effects of the climate variables, site characteristics and the years of the hypothetical inventories. The results did not generally differ between both species. Variables based on conditions during the whole year and partly during spring performed better than variables based on conditions during summer or the vegetation period. Defining the year as the period between October of the previous year and September of the current year allows possible lag effects of previous autumn and winter conditions to be taken into consideration. Mean climatic conditions reached or exceeded the correlations of the extremes and mostly performed similar to or better than the extremes in the models. Our results indicate that these relationships are insensitive to the often arbitrarily determined years, in which inventories take place. These findings can serve as basic recommendations for the choice of climate variables when modeling climate effects on multi-year growth of P. abies and F. sylvatica in the European lowlands.
Tree growth depends, among other factors, largely on the prevailing climatic conditions. Therefore, changes to tree growth patterns are to be expected under climate change. Here, we analyze the tree-ring growth response of three major European tree species to projected future climate across a climatic (mostly precipitation) gradient in northeastern Germany. We used monthly data for temperature, precipitation, and the standardized precipitation evapotranspiration index (SPEI) over multiple time scales (1, 3, 6, 12, and 24 months) to construct models of tree-ring growth for Scots pine (Pinus sylvestris L.) at three pure stands, and for common beech (Fagus sylvatica L.) and pedunculate oak (Quercus robur L.) at three mature mixed stands. The regression models were derived using a two-step approach based on partial least squares regression (PLSR) to extract potentially well explaining variables followed by ordinary least squares regression (OLSR) to consolidate the models to the least number of variables while retaining high explanatory power. The stability of the models was tested through a comprehensive calibration-verification scheme. All models were successfully verified with R²s ranging from 0.21 for the western pine stand to 0.62 for the beech stand in the east. For growth prediction, climate data forecasted until 2100 by the regional climate model WETTREG2010 based on the A1B Intergovernmental Panel on Climate Change (IPCC) emission scenario was used. For beech and oak, growth rates will likely decrease until the end of the 21st century. For pine, modeled growth trends vary and range from a slight growth increase to a weak decrease in growth rates. The climatic gradient across the study area will possibly affect the future growth of oak with larger growth reductions towards the drier east. For beech, site-specific adaptations seem to override the influence of the climatic gradient. We conclude that Scots pine has great potential to remain resilient to projected climate change without any greater impairment, whereas common beech and pedunculate oak will likely face lesser growth under the expected warmer and dryer climate conditions. The results call for an adaptation of forest management to mitigate the negative effects of climate change for beech and oak.
Understanding drought sensitivity of tree species and its intra-specific variation is required to estimate the effects of climate change on forest productivity, carbon sequestration and tree mortality as well as to develop adaptive forest management measures. Here, we studied the variation of drought reaction of six European Abies species and ten provenances of Abies alba planted in the drought prone eastern Austria. Tree-ring and X-ray densitometry data were used to generate early-and latewood measures for ring width and wood density. Moreover, the drought reaction of species and provenances within six distinct drought events between 1970 and 2011, as identified by the standardized precipitation index, was determined by four drought response measures. The mean reaction of species and provenances to drought events was strongly affected by the seasonal occurrence of the drought: a short, strong drought at the beginning of the growing season resulted in growth reductions up to 50%, while droughts at the end of the growing season did not affect annual increment. Wood properties and drought response measures showed significant variation among Abies species as well as among A. alba provenances. Whereas A. alba provenances explained significant parts in the variation of ring width measures, the Abies species explained significant parts in the variation of wood density parameters. A consistent pattern in drought response across the six drought events was observed only at the inter-specific level, where A. nordmanniana showed the highest resistance and A. cephalonica showed the best recovery after drought. In contrast, differences in drought reaction among provenances were only found for the milder drought events in 1986, 1990, 1993 and 2000 and the ranking of provenances varied at each drought event. This indicates that genetic variation in drought response within A. alba is more limited than among Abies species. Low correlations between wood density parameters and drought response measures suggest that wood density is a poor predictor of drought sensitivity in Abies spec.
Key message
Beech trees were able to cope with the drought of 2003. Harmful water shortage has been avoided by an effective stomatal closure while use of carbon storage pools may have prevented carbon starvation and growth reduction.
Abstract
We applied hydrodynamic modeling together with a tree ring stable isotope approach to identify the physiological responses of beech trees to changing environmental conditions. The drought conditions of the extreme hot and dry summer in 2003 were hypothesized to significantly influence the radial growth of European beech mainly triggered by the stomatal response towards water scarcity leading, in turn, to a decline in carbon assimilation. The functional–structural single tree modeling approach applied, revealed in fact a strong limitation of water use and carbon gain during drought. However, tree ring width data did not show a clear drought response and no differentiation in radial growth during six subsequent years examined (2002–2007) has been observed. Using integrated results from mechanistic carbon–water balance simulations, tree ring carbon and oxygen isotope analysis and tree ring width measurements we postulate that the suggested drought-induced growth decline has been prevented by the remobilization of stored carbohydrates, an early onset in growth and the relatively late occurrence of the severe drought in 2003. Furthermore, we demonstrate that the stomatal response played a significant role in avoiding harmful water tension that would have caused xylem dysfunction. As a result of the combined investigation with physiological measurements (stable isotope approach) and hydrodynamic modeling of stomatal aperture, we could give insights into the physiological control of mature beech tree functioning under drought. We conclude that beech trees have been operating at their hydraulic limits and that the longer or repeated drought periods would have affected the growth considerably.
Link to full text: http://www.jstatsoft.org/v17/i01
Relative importance is a topic that has seen a lot of interest in recent years, particularly in applied work. The R package relaimpo implements six different metrics for assessing relative importance of regressors in the linear model, two of which are recommended - averaging over orderings of regressors and a newly proposed metric (Feldman 2005) called pmvd. Apart from delivering the metrics themselves, relaimpo also provides (exploratory) bootstrap confidence intervals. This paper offers a brief tutorial introduction to the package. The methods and relaimpo’s functionality are illustrated using the data set swiss that is generally available in R. The paper targets readers who have a basic understanding of multiple linear regression. For the background of more advanced aspects, references are provided.
European beech shows mast fruiting at intervals of 2-20 years with a recent increase in frequency. It is not precisely known which climatic or endogenous factors are the proximate causes of masting. We recorded fruit mass production in 11 beech stands across a climate gradient over 4 years, analyzed the influence of climatic, edaphic, and stand structural parameters on fructification, and quantified carbon (C) and nitrogen (N) allocation to leaf and fruit mass production. The solar radiation total in June and July of the year preceding a mast year (JJ−1) was the parameter most closely related to fruit mass production, whereas no influence was found for drought. Radiation induced flowering and subsequent fruit production in beech apparently through a threshold response when the long-term mean of June-July radiation was exceeded by more than 5%. Full masting was associated with a significantly smaller leaf size and stand leaf area in the mast year and it significantly lowered foliar N content in the mast and post-mast year. We conclude that radiation totals and the N status of the foliage jointly govern the temporal pattern of masting in beech, presumably by controlling the photosynthetic activity in early summer. Anthropogenic increases in N deposition and atmospheric [CO2] thus have the potential to increase masting frequency which can substantially alter forest productivity and forest biogeochemical cycles.
Temperate forests with shade-tolerant canopy tree species can develop vertical structures of varying complexity. Forests with Abies alba Mill. and Fagus sylvatica L. can be composed of one-, two-, and multi-storied patches and selection patches. A dominant view in forest ecology is that unmanaged forests tend to have greater structural heterogeneity than managed stands. Structural integrity, however, may differ among forest developmental stages. The main objective of this study was to compare the tree diameter complexity in managed and unmanaged patches during the early developmental stage.
Data were collected between 2016 and 2018 in the Świętokrzyskie Mountains in Central Europe. The investigated tree communities were dominated by A. alba and F. sylvatica. Sample plots representing the growing-up developmental stage were randomly selected; of these, 30 plots were in managed stands, and 30 plots were in unmanaged forests. The diameter at breast height (DBH) distribution patterns were determined using hierarchical cluster analysis (HCA), clustering indices, and finite mixture models.
Three main DBH distribution patterns were identified for the managed stands (K-A, K-B, and K-C). These patterns consisted of three or two sub-populations. The patterns represented structurally diversified patches composed of trees of all ages with multi-, three- or two-layered canopies and with intensive natural processes of regeneration. Two main DBH distribution patterns were identified for the unmanaged forests (S-A, and S-B). These patterns consisted of two clearly separated sub-populations. They are typical in patches with two-layered canopies, and the trees from the upper layer had a large share (40–60%). The distinguished DBH distribution patterns indicated there was greater tree size diversity in the managed stands than in the unmanaged forests. When comparing managed versus unmanaged patches, it is important to consider the developmental stage.
The microclimate in forest ecosystems can be altered by modifications of stand structure due to forest management or natural forest development. Current forest management practices in Central Europe and North America aim to promote structural heterogeneity and maintain forest canopy cover, which is known to be a major driver of forest microclimate. Here, we investigated the impacts of forest management and structural heterogeneity on the diurnal temperature range (DTR) in 128 managed forest stands in three climatically different locations (Swabian Alb, Hainich-Dün and Schorfheide-Chorin) in Central Europe. Increasing structural heterogeneity by promoting tree size diversity and differentiation increased vertical stratification and resulted in an impaired DTR during the vegetation period. Linear regression models with geographic location, elevation above sea level, canopy openness and measures of structural heterogeneity as explanatory variables explained 79.4–80.9% of variance in DTR. However, the overall effect of structural heterogeneity on DTR was small. Differences in DTR between plots of different main tree species could be attributed to differences in canopy openness and light transmission, whereas tree species diversity had no significant effect on DTR. Unmanaged forests were characterized by a significantly lower DTR than managed, even-aged forests. DTR in uneven-aged stands managed under single tree selection was comparable to unmanaged stands. Terrestrial laser scanning (TLS) derived measures of canopy openness and vertical structure allowed to explain 79.4% of variance in DTR considering geographic location and elevation, which can also be assessed by TLS with integrated GPS and an altimeter. We conclude that structural characteristics of forest stands other than canopy openness contribute marginally to variation in forest microclimate. However, the analyses of structure-microclimate analyses indicate that effects of stand structure on DTR might be more pronounced in regions with low precipitation during the vegetation period.
The photosynthetic responses to the combined effect of drought and heat stress were studied in leaves of 19-years-old European beech (Fagus sylvatica L.) trees originating from five provenances in Central Europe differing by altitude (55-1250 m), and grown in the same experimental plot. The measurements were conducted at the beginning of heat waves, under two different conditions: during a dry period (middle of July) and after recovery in wetter period, at the beginning of August. The decreases of stomatal conductance (gs ) and net photosynthesis rate (ACO2 ) during drought and heat wave stress were very similar in all provenances. However, we observed distinct response of PSII photochemistry on combined drought and high temperature stress, well associated with altitude of origin of the beech provenances. Measurements of pulse amplitude modulated (PAM) fluorescence identified maintenance of a high electron transport rate in beech provenances from high altitudes under drought and heat wave conditions, associated with some decrease of excitation pressure on PSII. This can be explained by enhanced capacity of alternative electron sinks to utilize the excess of electrons as a photoprotective mechanism. The analyses of fast chlorophyll fluorescence kinetics confirmed the differences in responses of PSII photochemistry between provenances originating from different altitudes. Compared with provenances at higher altitudes, we found more sensitive response (i.e. more limited electron transport at the PSII acceptor side and changes in the size of light harvesting complexes) to drought and heat stress, in those growing at low altitudes. Our results support the hypothesis that the provenances originating from higher altitudes possess enhanced phenotypic plasticity related to photoprotective responses, resulting from the long-term adaptation to marginal mountain conditions.
Global change affects the functioning of forest ecosystems and the services they provide, but little is known about the interactive effects of co-occurring global change drivers on important functions such as tree growth and vitality. In the present study we quantified the interactive (i.e. synergistic or antagonistic) effects of atmospheric nitrogen (N) deposition and climatic variables (temperature, precipitation) on tree growth (in terms of tree-ring width, TRW), taking forest ecosystems with European beech (Fagus sylvatica L.) as an example. We hypothesised that (i) N deposition and climatic variables can evoke non-additive responses of the radial increment of beech trees, and (ii) N loads have the potential to strengthen the trees' sensitivity to climate change. In young stands, we found a synergistic positive effect of N deposition and annual mean temperature on TRW, possibly linked to the alleviation of an N shortage in young stands. In mature stands, however, high N deposition significantly increased the trees' sensitivity to increasing annual mean temperatures (antagonistic effect on TRW), possibly due to increased fine root dieback, decreasing mycorrhizal colonization or shifts in biomass allocation patterns (aboveground vs. belowground). Accordingly, N deposition and climatic variables caused both synergistic and antagonistic effects on the radial increment of beech trees, depending on tree age and stand characteristics. Hence, the nature of interactions could mediate the long-term effects of global change drivers (including N deposition) on forest carbon sequestration. In conclusion, our findings illustrate that interaction processes between climatic variables and N deposition are complex and have the potential to impair growth and performance of European beech. This in turn emphasises the importance of multiple-factor studies to foster an integrated understanding and models aiming at improved projections of tree growth responses to co-occurring drivers of global change.
Understanding the effects of nitrogen deposition, ozone and climate on tree growth is important for planning sustainable forest management also in the future. The complex interplay of all these factors cannot be covered by experiments. Here we use observational data of mature forests for studying associations of various biotic and abiotic factors with tree growth. A 30 year time series on basal area increment of Fagus sylvatica L. and Picea abies Karst. in Switzerland was analyzed to evaluate the development in relation to a variety of predictors. Basal area increment of Fagus sylvatica has clearly decreased during the observation period. For Picea abies no trend was observed. N deposition of more than 26 (beech) or 20–22 kg N ha− 1 year− 1 (Norway spruce) was negatively related with basal area increment, in beech stronger than in Norway spruce. High N deposition loads and low foliar K concentrations in Fagus were correlated with increased drought sensitivity. High air temperatures in winter were negatively related with basal area increment in Norway spruce in general and in beech at high N:Mg ratio or high N deposition while on an average the relation was positive in beech. Fructification in beech was negatively related to basal area increment. The increase of fructification observed during the last decades contributed thus to the growth decrease. Ozone flux was significantly and negatively correlated with basal area increment both in beech and Norway spruce. The results show clear non-linear effects of N deposition on stem increment of European beech and Norway spruce as well as strong interactions with climate which have contributed to the growth decrease in beech and may get more important in future. The results not only give suggestions for ecological processes but also show the potential of an integral evaluation of observational data.
Increasing frequency and intensity of drought extremes associated with global change are a key challenge for forest ecosystems. Consequently, the quantification of drought effects on tree growth as a measure of vitality is of highest concern from the perspectives of both science and management. To date, a multitude of drought indices have been used to accompany or replace primary climatic variables in the analysis of drought-related growth responses. However, it remains unclear how individual drought metrics compare to each other in terms of their ability to capture drought signals in tree growth.
A useful approach to monitor tree response to climate change and environmental extremes is the recording of long-term time series of stem radial variations obtained with precision dendrometers. Here, we study the impact of environmental stress on seasonal growth dynamics and productivity of yellow birch (Betula alleghaniensis Britton) and sugar maple (Acer saccharum Marsh.) in the Great Lakes, St Lawrence forest region of Ontario. Specifically, we research the effects of a spring heat wave in 2010, and a summer drought in 2012 that occurred during the 2005–14 study period. We evaluated both growth phenology (onset, cessation, duration of radial growth, time of maximum daily growth rate) and productivity (monthly and seasonal average growth rates, maximum daily growth rate, tree-ring width) and tested for differences and interactions among species and years. Productivity of sugar maple was drastically compromised by a 3-day spring heat wave in 2010 as indicated by low growth rates, very early growth cessation and a lagged growth onset in the following year. Sugar maple also responded more sensitively than yellow birch to a prolonged drought period in July 2012, but final tree-ring width was not significantly reduced due to positive responses to above-average temperatures in the preceding spring. We conclude that sugar maple, a species that currently dominates northern hardwood forests, is vulnerable to heat wave disturbances during leaf expansion, which might occur more frequently under anticipated climate change.
The importance of European beech (Fagus sylvatica L.) for the Central European forest and wood sector demands profound research to examine the adaptive capacity of beech forests
to changing environmental conditions. Quantitative wood anatomy is a valuable tool for studying the relation between structural
and functional traits of trees, but due to the laborious methodology not many studies have thus far been performed on the
conductive tissue of broadleaf tree species with diffuse-porous wood structure. The aim of our research was to test the effects
of aspect and thinning on vessel anatomical features of European beech (vessel density, vessel size, total vessel area, vessel
groups and hydraulic conductivity). Our analysis of increment cores of trees sampled from a long-term experimental research
area on the Swabian Alb showed that (i) the variations in different vessel traits were mainly controlled by tree-ring width.
Additionally, we could observe that (ii) thinning contributed to a safer water transport by decreasing vessel size and that
(iii) the aspect modified these responses. Our results provide new insights into the plastic response of European beech wood
anatomy to warmer climatic conditions and demonstrated that thinning of the forest stands modified the water-conducting system
to become more resistant against hydraulic failure.
The first edition of this book has established itself as one of the leading references on generalized additive models (GAMs), and the only book on the topic to be introductory in nature with a wealth of practical examples and software implementation. It is self-contained, providing the necessary background in linear models, linear mixed models, and generalized linear models (GLMs), before presenting a balanced treatment of the theory and applications of GAMs and related models. The author bases his approach on a framework of penalized regression splines, and while firmly focused on the practical aspects of GAMs, discussions include fairly full explanations of the theory underlying the methods. Use of R software helps explain the theory and illustrates the practical application of the methodology. Each chapter contains an extensive set of exercises, with solutions in an appendix or in the book’s R data package gamair, to enable use as a course text or for self-study.
Climate warming will increase the drought exposure of many forests world‐wide. It is not well understood how trees adapt their hydraulic architecture to a long‐term decrease in water availability.
We examined 23 traits characterizing the hydraulic architecture and growth rate of branches and the dependent foliage of mature European beech ( Fagus sylvatica ) trees along a precipitation gradient (855–594 mm yr ⁻¹ ) on uniform soil. A main goal was to identify traits that are associated with xylem efficiency, safety and growth.
Our data demonstrate for the first time a linear increase in embolism resistance with climatic aridity (by 10%) across populations within a species. Simultaneously, vessel diameter declined by 7% and pit membrane thickness ( T m ) increased by 15%. Although specific conductivity did not change, leaf‐specific conductivity declined by 40% with decreasing precipitation. Of eight plant traits commonly associated with embolism resistance, only vessel density in combination with pathway redundancy and T m were related.
We did not confirm the widely assumed trade‐off between xylem safety and efficiency but obtained evidence in support of a positive relationship between hydraulic efficiency and growth. We conclude that the branch hydraulic system of beech has a distinct adaptive potential to respond to a precipitation reduction as a result of the environmental control of embolism resistance.
Some forest-related studies on possible effects of climate change conclude that growth potential of European beech (Fagus sylvatica L.) might be impaired by the predicted increase in future serious drought events during the growing season. Other recent research suggests that not only multi-year increment rates but also growth resistance and recovery of beech during respectively after dry years may differ between pure and mixed stands. Thus, we combined dendrochronological investigations and wood stable isotope measurements to further investigate the impact of neighborhood diversity on long-term performance, short-term drought response and soil water availability of European beech in three major geographic regions of Germany. During the last 4 decades, target trees whose competitive neighborhood consisted of co-occurring species exhibited a superior growth performance compared to beeches in pure stands of the same investigation area. This general pattern was also found in exceptional dry years. Although the summer droughts of 1976 and 2003 predominantly caused stronger relative growth declines if target trees were exposed to interspecific competition, with few exceptions they still formed wider annual rings than beeches growing in close-by monocultures. Within the same study region, recovery of standardized beech target tree radial growth was consistently slower in monospecific stands than in the neighborhood of other competitor species. These findings suggest an improved water availability of beech in mixtures what is in line with the results of the stable isotope analysis. Apparently the magnitude of competitive complementarity determines the growth response of target beech trees in mixtures. Our investigation strongly suggest that the sensitivity of European beech to environmental constrains depends on neighborhood identity. Therefore, the systematic formation of mixed stands tends to be an appropriate silvicultural measure to mitigate the effects of global warming and droughts on growth patterns of Fagus sylvatica. This article is protected by copyright. All rights reserved.
