This paper introduces a new approach-the Principal Component Gradient Analysis (PCGA)-to detect ecological gradients in time-series populations, i.e. several time-series originating from different ...individuals of a population. Detection of ecological gradients is of particular importance when dealing with time-series from heterogeneous populations which express differing trends. PCGA makes use of polar coordinates of loadings from the first two axes obtained by principal component analysis (PCA) to define groups of similar trends. Based on the mean inter-series correlation (rbar) the gain of increasing a common underlying signal by PCGA groups is quantified using Monte Carlo Simulations. In terms of validation PCGA is compared to three other existing approaches. Focusing on dendrochronological examples, PCGA is shown to correctly determine population gradients and in particular cases to be advantageous over other considered methods. Furthermore, PCGA groups in each example allowed for enhancing the strength of a common underlying signal and comparably well as hierarchical cluster analysis. Our results indicate that PCGA potentially allows for a better understanding of mechanisms causing time-series population gradients as well as objectively enhancing the performance of climate transfer functions in dendroclimatology. While our examples highlight the relevance of PCGA to the field of dendrochronology, we believe that also other disciplines working with data of comparable structure may benefit from PCGA.
Basal area increment (BAI) is increasingly used in tree-ring based studies as it provides a direct measure of wood production and thus allows for the interpretation of growth trends. BAIs from ...increment cores are generally calculated while assuming circular growth patterns. However, observation of stem discs shows that many ring shapes are characterized by some form of eccentricity, not only characterized by deviations from circular ring shapes but also by piths not being centrally located. This observation poses to the question with what accuracy BAIs are calculated from increment cores. To quantify the estimation bias in BAI, we have developed a method that mimics eccentric tree growth by simulation. Various aspects of eccentricity are incorporated to created ‘stem discs’ with realistic appearances. Since BAI time series for our simulated discs are known, we can evaluate the accuracy of BAI calculation methods from cores. The ‘coring’ is simulated by taking cores at the thickest and thinnest sides of the simulated discs, whereby the number of cores is varied from one to four. In our simulations, we choose two calculation methods, namely the traditional circular approach and one that is based on the assumption of elliptical growth shapes. We find that bias in calculated BAI values is highly influenced by the number of cores taken, with a dramatic decrease from one to four cores. Furthermore, trend patterns in BAI series might be misleading in case of highly eccentric growth patterns. Based on these findings, we discuss the consequences for the interpretation of existing literature, where BAI analyses are based on one or two cores (along with the assumption of circular ring shapes). Such consequences are, however, difficult to quantify since we have no eccentricity statistics of tree growth within a forest stand. Therefore, we do not know the randomness of eccentricity within a stand, and thus to what extent chronology building (i.e. averaging BAI estimates over multiple trees) may reduce estimation bias. To lower BAI bias, we recommend to base BAI calculations on as many cores as possible. For individual trees with high levels of eccentricity, taking four increment cores seems necessary to reasonably estimate their basal area increments.
The mechanistic pathways connecting ocean-atmosphere variability and terrestrial productivity are well-established theoretically, but remain challenging to quantify empirically. Such quantification ...will greatly improve the assessment and prediction of changes in terrestrial carbon sequestration in response to dynamically induced climatic extremes. The jet stream latitude (JSL) over the North Atlantic-European domain provides a synthetic and robust physical framework that integrates climate variability not accounted for by atmospheric circulation patterns alone. Surface climate impacts of north-south summer JSL displacements are not uniform across Europe, but rather create a northwestern-southeastern dipole in forest productivity and radial-growth anomalies. Summer JSL variability over the eastern North Atlantic-European domain (5-40E) exerts the strongest impact on European beech, inducing anomalies of up to 30% in modelled gross primary productivity and 50% in radial tree growth. The net effects of JSL movements on terrestrial carbon fluxes depend on forest density, carbon stocks, and productivity imbalances across biogeographic regions.
The climate sensitivity of radial growth in European beech (
Fagus
sylvatica
L.) was analyzed within a narrow valley in the Swabian Alb (southwestern Germany). We collected stem disks from three ...aspects (NE, NW and SW) of trees belonging to different social classes. Common climatic factors limiting growth across the valley were identified using a principal component analysis (PCA). Further, we performed hierarchical cluster analysis (HCA), redundancy analysis (RDA) and bootstrapped correlation analysis to reveal differences in chronologies and climate-growth relationships between aspect and social class. Climatic variables considered in our analyses were monthly and seasonal data on temperature and precipitation, as well as a self-calibrating Palmer drought severity index (sc-PDSI). We identified drought in the period June–August as the most prominent factor limiting growth across the valley. Dominant and co-dominant trees at the NW and SW aspects were found to be particularly drought sensitive, whereas intermediate trees were less susceptible to drought. Underlying causes of established climate–growth relationships are discussed in the context of drought susceptibility, tree-size modulation and tree physiological processes.
The knowledge about potential climate change impacts on forests is continuously expanding and some changes in growth, drought induced mortality and species distribution have been observed. However ...despite a significant body of research, a knowledge and communication gap exists between scientists and non-scientists as to how climate change impact scenarios can be interpreted and what they imply for European forests. It is still challenging to advise forest decision makers on how best to plan for climate change as many uncertainties and unknowns remain and it is difficult to communicate these to practitioners and other decision makers while retaining emphasis on the importance of planning for adaptation.
In this paper, recent developments in climate change observations and projections, observed and projected impacts on European forests and the associated uncertainties are reviewed and synthesised with a view to understanding the implications for forest management. Current impact assessments with simulation models contain several simplifications, which explain the discrepancy between results of many simulation studies and the rapidly increasing body of evidence about already observed changes in forest productivity and species distribution. In simulation models uncertainties tend to cascade onto one another; from estimating what future societies will be like and general circulation models (GCMs) at the global level, down to forest models and forest management at the local level.
Individual climate change impact studies should not be uncritically used for decision-making without reflection on possible shortcomings in system understanding, model accuracy and other assumptions made. It is important for decision makers in forest management to realise that they have to take long-lasting management decisions while uncertainty about climate change impacts are still large. We discuss how to communicate about uncertainty – which is imperative for decision making – without diluting the overall message. Considering the range of possible trends and uncertainties in adaptive forest management requires expert knowledge and enhanced efforts for providing science-based decision support.
•Uncertainty is inherent to climate change impact assessments.•Extreme events are only weakly represented in many assessments.•The range of possible impacts has so far been underestimated in most studies.•Some general trends are common to all climate projections.•Guidance is needed to interpret state-of-the-art knowledge and give helpful advice.
Bioclimate envelope models have been widely used to illustrate the discrepancy between current species distributions and their potential habitat under climate change. However, the realism and correct ...interpretation of such projections has been the subject of considerable discussion. Here, we investigate whether climate suitability predictions correlate to tree growth, measured in permanent inventory plots and inferred from tree‐ring records. We use the ensemble classifier RandomForest and species occurrence data from ~200,000 inventory plots to build species distribution models for four important European forestry species: Norway spruce, Scots pine, European beech, and pedunculate oak. We then correlate climate‐based habitat suitability with volume measurements from ~50‐year‐old stands, available from ~11,000 inventory plots. Secondly, habitat projections based on annual historical climate are compared with ring width from ~300 tree‐ring chronologies. Our working hypothesis is that habitat suitability projections from species distribution models should to some degree be associated with temporal or spatial variation in these growth records. We find that the habitat projections are uncorrelated with spatial growth records (inventory plot data), but they do predict interannual variation in tree‐ring width, with an average correlation of .22. Correlation coefficients for individual chronologies range from values as high as .82 or as low as −.31. We conclude that tree responses to projected climate change are highly site‐specific and that local suitability of a species for reforestation is difficult to predict. That said, projected increase or decrease in climatic suitability may be interpreted as an average expectation of increased or reduced growth over larger geographic scales.
Species distribution models for four major European tree species are validated against independent forest growth data. We conclude that species distribution models can predict temporal but not spatial variation in forest growth.
The future performance of the widely abundant European beech (Fagus sylvatica L.) across its ecological amplitude is uncertain. Although beech is considered drought-sensitive and thus negatively ...affected by drought events, scientific evidence indicating increasing drought vulnerability under climate change on a cross-regional scale remains elusive. While evaluating changes in climate sensitivity of secondary growth offers a promising avenue, studies from productive, closed-canopy forests suffer from knowledge gaps, especially regarding the natural variability of climate sensitivity and how it relates to radial growth as an indicator of tree vitality. Since beech is sensitive to drought, we in this study use a drought index as a climate variable to account for the combined effects of temperature and water availability and explore how the drought sensitivity of secondary growth varies temporally in dependence on growth variability, growth trends, and climatic water availability across the species' ecological amplitude.
Our results show that drought sensitivity is highly variable and non-stationary, though consistently higher at dry sites compared to moist sites. Increasing drought sensitivity can largely be explained by increasing climatic aridity, especially as it is exacerbated by climate change and trees' rank progression within forest communities, as (co-)dominant trees are more sensitive to extra-canopy climatic conditions than trees embedded in understories. However, during the driest periods of the 20th century, growth showed clear signs of being decoupled from climate. This may indicate fundamental changes in system behavior and be early-warning signals of decreasing drought tolerance. The multiple significant interaction terms in our model elucidate the complexity of European beech's drought sensitivity, which needs to be taken into consideration when assessing this species' response to climate change.
Growth variability, growth trends, and climatic water availability interact and significantly influence climate-growth relationships of growth in European beech. Display omitted
•Linear mixed-effects model used to derive individual and interactive effects.•Growth variability and trends are more influential during wet conditions.•Growth variables are decoupled and replaced by climate during dry conditions.•Climate sensitivity decouples at driest conditions, indicating early-warning signal.
The magnitude and frequency of soil frost events might increase in northern temperate regions in response to climate warming due to reduced insulation caused by declining snow cover. In temperate ...deciduous forests, increased soil frost severity can hamper tree growth and increase the mortality of fine roots, soil fauna and microorganisms, thus altering carbon and nutrient cycling. From single-site studies, however, it is unclear how the sensitivities of these responses change along continental gradients from regions with low to high snowfall. We conducted a gradient design snow cover and soil temperature manipulation experiment across a range of lowland beech forest sites to assess the site-specific sensitivity of tree growth and biogeochemical cycling to soil cooling. Even mild and inconsistent soil frost affected tree increment, germination, litter decomposition and the retention of added
15
N. However, the sensitivity of response (treatment effect size per degree of warming or cooling) was not related to prevailing winter climate and snow cover conditions. Our results support that it may be valid to scale these responses to simulated winter climate change up from local studies to regional scales. This upscaling, however, needs to account for the fact that cold regions with historically high snowfall may experience increasingly harsh soil frost conditions, whereas in warmer regions with historically low snowfall, soil frost may diminish. Thus, despite the uniform biotic sensitivity of response, there may be opposing directions of winter climate change effects on temperate forests along continental temperature gradients due to different trends of winter soil temperature.
Information about the resistance and adaptive potential of tree species and provenances is needed to select suitable planting material in times of rapidly changing climate conditions. In this study, ...we evaluate growth responses to climatic fluctuations and extreme events for 12 provenances of northern red oak (
L.) that were tested across three trial sites with distinct environmental conditions in Germany. Six provenances each were sourced from the natural distribution in North America and from introduced stands in Germany. We collected increment cores of 16 trees per provenance and site. Dendroecological methods were used to compare provenance performance and establish climate-growth relationships to identify the main growth limiting factors. To evaluate the provenance response to extreme drought and frost events, three site-specific drought years were selected according to the Standardized Precipitation Evapotranspiration Index (SPEI) and 2010 as a year with an extreme late frost event. Resistance indices for these years were calculated and assessed in relation to overall growth performance. We observed a high variation in growth and in the climate sensitivity between sites depending on the prevailing climatic conditions, as well as a high intra-specific variation. Overall, summer drought and low temperatures in the early growing season appear to constrain the growth of red oak. The resistance of provenances within sites and extreme years showed considerable rank changes and interaction effects. We did not find a trade-off between growth and resistance to late frost, namely, fast growing provenances had a high frost hardiness. Further, there was no evidence for a trade-off between growth and drought hardiness. Still, responses to drought or late frost differ between provenances, pointing to dissimilar adaptive strategies. Provenances from introduced (i.e. German) stands represent suitable seed sources, as they combine a higher growth and frost hardiness compared to their North American counterparts. Drought hardiness was slightly higher in the slow-growing provenances. The results provide a better understanding of the variable adaptive strategies between provenances and help to select suitable planting material for adaptive forest management.
Poplar species such as aspen (Populus tremula L.) play a very important role in the forest formation process not only in Eastern European regions. Unfortunately, such aspen stands are often severely ...affected by fungal diseases, causing mainly core rot. In this study, the indirect effects of thinning on the phytosanitary condition of aspen by promotion of tree growth were investigated. Two thinning methods, manual (thinning from below) and mechanical thinning (schematic), were applied to young stands dominated by Eurasian aspen to study their effects on tree growth and health. All trees were measured at breast height and diameter frequency distribution was determined twice, i.e. three and 24 years after the beginning of the experiment. In addition, during the second measurement, tree-ring samples were obtained from individual trees to evaluate growth and wood decay damage. Neither manual nor mechanical thinning of aspen significantly increased its growth at the stand level, but positive effects on individual trees were observed in plots where mechanical thinning was applied. The thicker the trees, the less decayed they were. The analysis suggests that thinning in general should not be used to increase stand production, but the positive effects of mechanical thinning on individual aspens can be recommended to promote the growth of individual vigorous trees.
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