Schweingruber’s cosmos of inspiration

Born 29^th February 1936 near Bern, Switzerland, Fritz Hans Schweingruber worked as a teacher until 1965, obtained a PhD in botany from the University of Basel in 1972 (where he also obtained a Professorship in 1976), and started his lifelong career at the Swiss Federal Research Institute WSL in Birmensdorf right afterwards. Fritz developed a dendrochronological network across much of the Northern Hemisphere, expanded wood (xylem) science beyond forests, implemented wood anatomical techniques into dendroecology and paleoclimatology, and enthusiastically trained thousands of students, of which hundreds remained actively involved in the still emerging field of tree-ring research. Though Fritz died 7^th January 2020 after an extraordinary academic career, his intellectual legacy will continue to inspire scholars around the world.     

Dendrochronological research of Scots pine (Pinus sylvestris L.) radial growth in vicinity of industrial pollution

The aim of this research is to investigate changes in the annual radial increment of Scots pine (Pinus sylvestris L.) in the vicinity of intensive (3–10 km) and moderate (11–20 km) industrial pollution during different growth periods (growth promotion, inhibition, and recovery). Low level of emission was beneficial for tree growth during the growth promotion period, and the annual radial increment in the zones of intensive and moderate pollution increased by approximately 15–25% and 10%, respectively. Severe loss was reported to forests during the growth inhibition period when nitrogen and sulphur dioxide emissions were 37–40 thousand (thou.) tons per year. About 40–45% tree radial increment loss was observed in the stands closest to the pollution source, and 15–20% loss was observed for the most distant stands. The stabilization of radial growth decrease and the beginning of recovery of damaged stands began in 1988–1992, when the annual amount of industrial emissions and environmental pollution were considerably reduced. The stabilization of radial growth and the initiation of recovery after pollution reduction were high for the most damaged stands. Their radial increment was stable and close to that of the control stands in 2000–2011. Stands with less damage growing further from the pollution source were recovered earlier, and their radial increment stabilized near the control increment in 1995–1999. The results of linear regression analysis demonstrated that the impact of pollution is different for stands growing at different distances from the plant, and the impact decreases with distance (R² = 0.78 and R² = 0.75, respectively; p < 0.05).     

A methodological framework to optimize models predicting critical dates of xylem phenology based on dendrometer data

Dendrometer measurements are a frequently used alternative to the laborious and time consuming microcoring to investigate intra-annual growth dynamics of trees. However, since dendrometer data not only comprise cambial growth, but also hydrological fluctuations, both signals need to be disentangled to derive critical dates of xylem phenology from dendrometer data. For this purpose, various approaches can be found in the literature, however a systematic comparison of the different options is still missing. In this study we present a methodological framework to evaluate the accuracy of different mathematical fittings to derive tree-ring phenology from dendrometer data and apply this approach to a data set comprised of three conifer species where high-resolution band dendrometer measurements and microcore sampling have been done in parallel. Based on our study we provide evidence that the most common approaches to derive onset and cessation of xylem cell enlargement from dendrometer data, i.e. applying absolute and relative thresholds to deterministic models fitted to dendrometer data, caused systematic deviations from the reference of xylogenesis observations, but could be improved by small adaptations. The most precise and unbiased predictions of xylem growth phenology were obtained by fitting the three-parameter Gompertz model to the dendrometer data and applying a relative threshold of 3.5% of annual increment to the model predictions for growth onset and an absolute threshold of 4.5 µm day^-1 based on the first model derivative for growth cessation. Our framework enables an improved usage of dendrometer data for the prediction of the onset and cessation of xylem cell enlargement, which are important ecological indicators to quantify the effects environmental changes on forest growth and the terrestrial carbon cycle.     

Comparing automatically generated and manually measured tree-ring transects of growth trends with Hawaiian sandalwood as an example species

Tree-ring measurements are a primary quantitative tool used in numerous scientific disciplines. Some species, however, exhibit morphological complexities leading to significant uncertainty in these measurements. Hawaiian Sandalwood (Santalum paniculatum) stems, for example, often develop asymmetric growth features that hinder tree-ring measurements. These features include faint-ring boundaries and wedging rings which disappear in portions of the cross-section. In this work we a use a novel two-dimensional transect methodology and our own open-source software, svg-dendro, to analyze particularly difficult cross-sections. Our method accomplishes this by first tracing all rings by hand and automatically generating a user-specified number of transects. On average, these traced measurements had more sensitivity to tree-ring variability without losing important equivalencies with the traditional binocular stereomicroscope technique (e.g., radii, skewness) as indicated by greater mean variance for ring number, mean tree-ring width, and standard deviation. All S. paniculatum samples had ring wedging, where certain sides of the stem had many locally absent tree rings but to different intensities. The new technique allows us to analyze the shift from complete rings with little to no wedging to rings with more wedging starting between the 19th and 40th ring, where deep stem lobes begin forming. The new method also reveals the difficulty in measuring these trees, as the wedging creates multiple lobes with different visible ring counts. This research suggests that this two-dimensional methodology would be best applied to non-circular trees with fewer incomplete rings, supporting the importance of species and population selection. Overall, we have developed an efficient and flexible means to measure otherwise unmeasurable growth features in tree samples through representing tree-ring boundaries as curves and developing software to sort and map transects.     

An attempt to dendrochronologically date house features at the King site (9FL5), a 16th century Late Mississippian town in northwestern Georgia,USA

The King site is a Late Mississippian (ca. 1400–1540 CE) aboriginal town located in northwestern Georgia along the Coosa River associated with the Coosa Chiefdom. The site was settled ca. 1530 but was occupied for perhaps only 50 years or so based on the lack of horizontal stratigraphy. The site was visited by members of either or both the Hernando de Soto expedition in 1540 and/or the Tristan de Luna expedition in 1560. In 1974, archaeologists discovered and removed 36 sections of subterranean charred pine posts from six house features. Our objectives were to determine if the tree rings on these posts could be dendrochronologically dated to verify the dates of site occupation and confirm the construction sequence of several houses determined originally via stratigraphic and archaeological evidence. We were able to graphically and statistically crossmatch 13 measurement series representing 10 posts from 5 of the 6 structures, yielding a 157-year floating chronology (average interseries correlation = 0.60). We were unable to absolutely crossdate this floating chronology with the only regional reference chronology long enough (back to 1378 CE) to reach the 16th century, an eastern red cedar chronology from eastern Tennessee. Archaeological evidence indicated Houses 8 and 23.4 were built later in the King site occupancy, confirmed by the tree-ring dates as both houses have the youngest tree rings of the five structures. House 14 had the oldest outermost tree rings but archaeological evidence suggests this house also was likely constructed late in the King site occupancy. We propose some posts were salvaged and reused from abandoned houses as the King site became rapidly depopulated in the last 10–20 years of site occupancy, thus explaining the age of posts used in House 14. We urge archaeologists working in the Southeastern U.S. to consider developing a more formal process for exhuming and preserving charcoalized wood remains from archaeological sites so that these samples can be evaluated using dendrochronological techniques.     

Inverse effects of recent warming on trees growing at the low and high altitudes of the Dabie Mountains,subtropical China

Understanding how tree rings of different species at different elevations respond to climate, and whether their relationship is stable over time is crucial not only for credible palaeoclimatic reconstructions, but also for better awareness of forest growth dynamics and therefore making scientific management decisions against a background of global warming. In this work, six tree-ring chronologies of Pinus taiwanensis at three sites from 800 to 1550 m above sea level (asl), and Pinus massoniana at three sites from 500 to 650 m asl were developed in the Dabie Mountains (DBS), subtropical China. In recent decades, only P. taiwanensis at 1550 m asl showed a positive growth-trend. Both P. taiwanensis at 800 m asl and three low-altitude P. massoniana at 500–650 m asl showed negative growth trends. Climate-growth relationship analyses revealed that 1) temperature was the dominant climatic factor that controlled tree-ring growth in DBS. It exerted most influence on trees growing at the lower and upper limit, than for trees growing at altitudes from 650 to 1450 m asl. Only precipitation in October positively influenced tree growth of P. massoniana at all three sites and P. taiwanensis at 800 m asl; 2) There was a shift in temperature’s impact on trees, that was from significantly negative relationship at the four lower altitudes, mainly for the current growing-season, to significantly positive relationship in previous February–July period at the highest altitude. The higher the altitude, the more significant the lag effect of temperature on trees; 3) The influence of temperature on tree growth at most altitudes were variable over time. The positive influence of temperature on trees at 1550 and 1450 m asl was comparatively stable during the early period of instrumental records. It strengthened evidently since the early 1990s, which is coincidental with the timing of the evident temperature increase in DBS. On the contrary, the negative impacts of temperature on tree growth at 800, 650 and 500 m asl had weakened since the early 1990s. Besides the increase of water use efficiency of trees, we speculated that the increasing influence of precipitation in May and July weakened the relationship between temperature and tree rings at low altitudes. This work points out that the upper and lower limits of forest in DBS offer the preferred locations for future sampling in climate reconstruction, but the stability of tree growth and climate over time should be considered. Moreover, forest management should give priority to altitude factors, in addition to tree species representation.     

Dendrochronology and dendroclimatology of yew in Poland

This study concerns dendrochronology and dendroclimatology of yew growing in Poland. The yew (Taxus baccata L.), a long-lived, slow-growing tree, is regarded as a species threatened with extinction. The eastern boundary of its range transects Poland. The analyses were performed on 34 yew populations which are protected as parts of nature reserves, as nature monuments, or which are planned to be protected. Samples were collected using Pressler borer from a total of 774 trees yielding 1307 profiles. Classical dating techniques (including cross-dating method) were subsequently used to reconstruct 34 local chronologies. Average tree-ring width was 0.84 mm and ranged from 0.27 to 1.47 mm. For most stands, the age of the yew trees is overestimated. The studied populations most frequently are 100–200 years old, and the oldest yew trees in Poland are growing in the KS population (age estimated at 500–600 years). Average tree-ring width was found to be strongly dependent on the chronology length/tree age. The 674 pointer years calculated for the local chronologies served as a basis for determining pointer years for the entire study area. Negative years include: 1862, 1865, 1917, 1927, 1940, 1947, 1956, 1963, 1969, 1976, 1979, 1993, 1996, and 2003. Positive years are: 1884, 1914, 1916, 1965, 1977, 1988, 1997, and 2007. Analysis of weather conditions in those years demonstrated a strong relationship between tree-ring width and air temperature in winter, pre-spring, and early spring. Higher-than-average temperatures during those seasons correlate positively with yew tree-ring width. Response function analysis performed for local and regional chronologies point to a dominant role of air temperature in December of the year preceding growth and in January, February, and March of the current year (linear relationships). June precipitation is an additional factor affecting tree-ring width in some areas of northern and northwestern Poland: the higher the precipitation, the wider the tree-rings. The results obtained, particularly those concerning growth-climate relationships and dendroclimatic regionalisation, can be used in the on-going programme of yew restitution in Poland.     

Moisture changes over the past 467 years in the central Hexi Corridor,northwestern China

The long-term characteristics of precipitation in arid and semi-arid areas are of great interest because these areas are very sensitive to climate change and human activities. The Hexi Corridor is an arid and semi-arid region in northwestern China that also is an important sector of the Silk Road Economic Belt; despite the region’s dependence on precipitation, annually resolved, long-term moisture records are still lacking for this region. Here, a standard tree-ring width chronology spanning 1484–2015 AD is developed for the Hexi Corridor using Qilian juniper (S. przewalskii Kom.). The chronology is used to reconstruct moisture changes in the region over the past 467 years. Correlation analyses indicate that the tree-ring width index has a significant positive correlation with the June SPEI (standardized precipitation evapotranspiration index) on a twelve-month time scale (r=0.73, n=65, p<0.001). We used this information to build a transfer function that explains 52.5% of the variance in the SPEI reconstructed for the period from 1549 CE to 2015 CE. Our study area experienced clear alternations between dry and wet periods. Especially long wet periods include 1600–1650 AD and 1762–1804 AD; long dry periods include 1670–1693 AD, 1917–1970 AD, and 1990–2015 AD. The 1920s was the most severe period of drought in the last 467 years. The results of wavelet analysis and running correlation analysis indicate that the atmospheric circulation system experienced a notable shift around the 1800s, after which point the role of the westerly system grew more pronounced.     

Comparison of growth-climate relationship of sabina przewalskii at different timberlines along a precipitation gradient in the northeast Qinghai-Xizang Plateau,China北大核心CSCD

Aims To test the hypothesis that water is the main limiting factor of tree growth at the arid alpine timberline, and to explore the effects of water on growth-climate relationships of Sabina przewalskii along a precipitation gradient in the northeast Qinghai-Xizang Plateau. Methods Three sides were selected to sample the alpine timberline along a precipitation gradient in the northeast Qinghai-Xizang Plateau: Halihatu National Forest Park in Wulan County (HL, annual precipitation 217 mm), Qushigang in Dulan County (QS, 281 mm) and Hebei Forest Farm in Tongde County (HB, 470 mm). The correlation and response analysis at seasonal and extreme climate year scales were used to examine the spatial variations of the growth-climate relationship of S. przewalskii at different timberlines. Important findings Our results do not support the hypothesis that water is the main limiting factor of tree growth at the arid alpine timberline. The effect of precipitation on the radial growth of S. przewalskii were consistent across all three sampling sites, while the effects of temperature were different across sites. At HL site (low precipitation), the winter and summer minimum temperature were the main limiting factor of S. przewalskii radial growth, and this relationship did not significantly change in different extreme climate years. At QS site (middle precipitation), the radial growth of S. przewalskii was mainly limited by the minimum temperature in spring and summer, but its effect was weaker than that at low precipitation site. At HB site (high precipitation), the spring temperature had a significant negative effect on tree growth, and the positive effect of spring precipitation on tree growth was significantly enhanced in comparison with those at low and middle precipitation sites, especially in extreme high temperature and drought years. Summer precipitation did not significantly affect tree growth at high precipitation site. Our results did not support the hypothesis that the radial growth of trees at alpine timberline in arid/humid area is mainly limited by water/temperature. However, precipitation at timberline will affect the relationship between tree growth and temperature at different seasons. With the warming and humidification of the northeastern Qinghai-Xizang Plateau, the climatic limiting factors of tree growth in different timberline areas may be complicated. © 2018 Editorial Office of Chinese Journal of Plant Ecology. All rights reserved.     

Variation of different tree-ring parameters in samples from each terminal shoot of a Norway spruce tree

Variability of wood parameters in a tree is sometimes a rather nebulous concept since variability is evident within single cells, from early to latewood, from pith to bark and from stem base to the top of a tree. So far, stem analyses have been done using a restricted number of parameters, mostly ring-width, and using a restricted number of samples in the longitudinal direction. This study analyses a number of parameters from a single tree. An 81-year-old spruce tree was felled and internodial discs were taken from each annual terminal shoot. All tree rings in each disc were measured and a whole-stem analysis was completed for the following parameters: ring-width, mean ring density, maximum density, percentage of latewood, type of transition from early to latewood, intra-annual density fluctuation, number of resin ducts per tree-ring and position of resin ducts within the tree-rings. All parameters showed calendar-year patterns, visible as lines parallel to the bark. The most clear calendar-year pattern was seen for the type of transition from early to latewood and for intra-annual density fluctuations. The strongest inter-series correlation between calendar rings was seen for ring-width. None of the parameters showed significant inter-series correlations for cambial rings. These results may help us to understand how cores or discs taken at breast height represent the entire tree.