Schweingruber’s cosmos of inspiration

Born 29 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 January 2020 after an extraordinary academic career, his intellectual legacy will continue to inspire scholars around the world. 1. Chronology development, anatomical assessment and impact study for a fulfilled life Fritz had a near-perfect photographic memory that allowed him to store huge amounts of data sustainably and access all kind of information rapidly. Of the sheer infinite number of trees, shrubs, dwarf shrubs and herbs he sampled and/or collected during the past five decades from a range of biogeographic habitats between 0 and 6150 m asl on six continents, he was able to tell short, though almost always inspiring stories. Together with his tremendous diligence, he catalysed his scientific memories and generated 39 fascinating books, of which many were published by Springer (e.g. Schweingruber et al., 2011). His opus magnum “Tree Rings and Environment. Dendroecology” (Schweingruber, 1996) is a standard for any tree-ring researcher, and his last book – an anatomical atlas of aquatic and wetland plant stems – was published just recently (Schweingruber et al., 2020). Fritz contributed to more than 200 journal articles, often as the senior author, and his numerous ring width, density and anatomy data will continue to play an important role in many studies. Right from the beginning of his academic career in the 1970s, it was essential for him to make all data freely available; this generosity, unfortunately, was not infectious to everybody and many scholars are still not convinced by the mutual benefits of open data access. Fritz’s liberal attitude, however, pathed his international success. In providing chronologies since 1970s, Fritz became the most prolific contributor to the International Tree-Ring Data Bank (ITRDB). As a role model for many of us, he is credited as principal investigator for 451 collections and co-investigator for another 75. Rigorously advocating open data accessibility and the unprejudiced exchange of intellectual property, Fritz was essential for the collegial sprit of our community. The immense paleoclimatic value of Schweingruber’s Northern Hemisphere tree-ring width and density networks is best reflected in three out of several highly-cited and hugely influential articles published in Science and Nature: Esper et al. (2002) and Briffa et al. (1998a) demonstrated the ability of well-replicated and carefully standardized tree-ring chronologies to preserve and capture low-frequency growth and temperature variability as well as the rapid response to volcanic activity during the past centuries, while probably being affected by reduced climate sensitivity since the 1980s (Briffa et al., 1998b). Another Nature article still describes the scientific benchmark for disentangling biotic and abiotic drivers of boreal forest productivity (Vaganov et al., 1999). Numerous ring width and maximum latewood density chronologies Fritz had developed since the early-1970s formed the backbone of various IPCC reports (https://www.ipcc.ch/), and his early work on species identification, wood provenance and cross-dating was crucial for the creation and maintenance of modern dendroarchaeology (Schweingruber, 1975). Fritz was a virtuoso on the microtome, a subtle craftsman, and an obsessive collector. He was a passionate explorer of epistemological concepts. Using transverse, tangential and radial sections of tens of https://doi.org/10.1016/j.dendro.2020.125680 Received 14 February 2020; Received in revised form 19 February 2020; Accepted 19 February 2020 ⁎ Corresponding author at: Department of Geography, University of Cambridge, Downing Place, CB2 3EN, UK. E-mail address: ulf.buentgen@geog.cam.ac.uk (U. Büntgen). Dendrochronologia 60 (2020) 125680 Available online 22 February 202


Chronology development, anatomical assessment and impact study for a fulfilled life
Fritz had a near-perfect photographic memory that allowed him to store huge amounts of data sustainably and access all kind of information rapidly. Of the sheer infinite number of trees, shrubs, dwarf shrubs and herbs he sampled and/or collected during the past five decades from a range of biogeographic habitats between 0 and 6150 m asl on six continents, he was able to tell short, though almost always inspiring stories. Together with his tremendous diligence, he catalysed his scientific memories and generated 39 fascinating books, of which many were published by Springer (e.g. Schweingruber et al., 2011). His opus magnum "Tree Rings and Environment. Dendroecology" (Schweingruber, 1996) is a standard for any tree-ring researcher, and his last bookan anatomical atlas of aquatic and wetland plant stemswas published just recently (Schweingruber et al., 2020). Fritz contributed to more than 200 journal articles, often as the senior author, and his numerous ring width, density and anatomy data will continue to play an important role in many studies.
Right from the beginning of his academic career in the 1970s, it was essential for him to make all data freely available; this generosity, unfortunately, was not infectious to everybody and many scholars are still not convinced by the mutual benefits of open data access. Fritz's liberal attitude, however, pathed his international success. In providing chronologies since 1970s, Fritz became the most prolific contributor to the International Tree-Ring Data Bank (ITRDB). As a role model for many of us, he is credited as principal investigator for 451 collections and co-investigator for another 75. Rigorously advocating open data accessibility and the unprejudiced exchange of intellectual property, Fritz was essential for the collegial sprit of our community. The immense paleoclimatic value of Schweingruber's Northern Hemisphere tree-ring width and density networks is best reflected in three out of several highly-cited and hugely influential articles published in Science and Nature: Esper et al. (2002) and Briffa et al. (1998a) demonstrated the ability of well-replicated and carefully standardized tree-ring chronologies to preserve and capture low-frequency growth and temperature variability as well as the rapid response to volcanic activity during the past centuries, while probably being affected by reduced climate sensitivity since the 1980s (Briffa et al., 1998b). Another Nature article still describes the scientific benchmark for disentangling biotic and abiotic drivers of boreal forest productivity (Vaganov et al., 1999). Numerous ring width and maximum latewood density chronologies Fritz had developed since the early-1970s formed the backbone of various IPCC reports (https://www.ipcc.ch/), and his early work on species identification, wood provenance and cross-dating was crucial for the creation and maintenance of modern dendroarchaeology (Schweingruber, 1975).
Fritz was a virtuoso on the microtome, a subtle craftsman, and an obsessive collector. He was a passionate explorer of epistemological concepts. Using transverse, tangential and radial sections of tens of https T thousands of plant samples from almost all biomes on six continents ( Fig. 1), Fritz revealed the large anatomical variability of stems Schweingruber and Börner, 2018) (Fig. 2), including bark and pith . His emerging interest in combining wood anatomy, plant physiology and dendroecology (Schweingruber et al., 2011) inspired a generation of young scholars worldwide to advance their scope beyond trees . The nuanced interpretation of anatomical features, of which many have been described for the first time by Fritz, however, was only possible due to the refinement of cutting and staining techniques (Gärtner and Schweingruber, 2013;Gärtner et al., 2014Gärtner et al., , 2015. Due to this global xylem database (Büntgen Fig. 1. Examples of Schweingruber's sampling sites: Temperature-constrained, northern most limit of plant growth in east Greenland around 73°N, aquatic limitation of terrestrial plant growth at 0 m asl in north-western France, drought-induced, arid limit of plant growth in the dessert of Libya, upper distribution limit of plant growth in Ladakh, western Himalaya (from left to right).  (Körner et al., 2019). In so doing, he provided inspiration for several projects and courses, and contributed substantially to the most recent challenge of the long-lasting treeline paradigm (Crivellaro and Büntgen, 2020). Two examples of his continuous enthusiasm for the investigation of changes in woody vegetation in arctic and alpine tundra ecosystems can be found in coastal east Greenland and Ladakh in the northwest Himalayas (Büntgen et al., 2015;Dolezal et al., 2016), where he conducted fieldwork at the northern and upper limits of vascular plant growth in 2011 and 2014, respectively (Fig. 3). Eloquently expanding wood (xylem) sciences beyond the Earth's forested regions Schweingruber and Berger, 2018), Fritz was the senior author of both studies. Fritz's tireless action was never constrained by socio-cultural or political factors. He was a prospective cosmopolitan interested in gaining and sharing knowledge. His influence on Russia's tree-ring community since the early-1990s, including the leading research centres in Krasnoyarsk and Yekaterinburg, resulted in numerous scientific expeditions, data and knowledge exchanges, summer schools and training programmes, research breakthroughs and publications, andmost importantlylifelong friendships.
Fritz was an incredible teacher, able to motivate people of all ages and backgrounds. In 1986, he invented the legendary "Dendroecological Fieldweek", and later on created the famous "International Course on Wood Anatomy & Tree-Ring Ecology" in the eastern Swiss Alps; both will continue in his tradition. Making knowledge accessible in an easy way was always his prime goal (Kaennel and Schweingruber, 1995). Due to many teaching experiences and research collaborations, Fritz had established a distinct value system, driven by diligence, pragmatism, directness and stamina, which enabled him to differentiate between constructive scientific criticism (Büntgen and Schweingruber, 2010) and personal animosity.
With much vision and spirit, Fritz had the ambition to introduce time to plant ecology. He therefore proposed a different type of dendroecology based on collecting as much samples as possible from different age classes and species across the landscape (including herbs and grasses), preparing anatomical thin sections, and ignoring cross-dating, thus breaking a taboo in dendrochronology. With this disciplinary expansion in mind, Fritz emphatically suggested to prioritize five research avenues at the interface of wood anatomy and dendroecology: i) determine individual plant ages and community age structures; ii) consider site-and species-specific differences in plant morphological characteristics and stem anatomical features; iii) expand stem anatomical and xylem chronological investigations from tundra shrubs to herbs; iv) estimate historical changes in biomass production and carbon allocation within and among plant species; and v) apply communitybased dendroecological studies to associate changes in age structure and recruitment intensity with environmental and climatic factors.
Fritz was highly creative and always searching for new frontiers and inspiration. During the recent past, he explored linkages between art and nature, for which he developed innovative models of realising (macroscopically) and explaining (ecologically) form and function from the cell to the globe. Gifted with the rare ability to see the unknown, he invented an exciting cosmos of novelty.
Fritz's lifetime achievement would have been impossible without his wife. His thoughts were sharp until the very end, and his last decisions were an obvious continuation of his oeuvre, respectfully demonstrating character strength, pragmatism, far-sightedness, and conjugal security. Those who knew Fritz will never forget him, and generations of scholars will be inspired by his legacy.

Declaration of Competing Interest
Authors declare no conflict of interest.

Acknowledgements
We are grateful to the worldwide tree-ring and wood anatomical community/ies for continuing the intellectual and ethical heritage of Professor Fritz Hans Schweingruber within and beyond disciplinary boundaries, across cultural and political borders, and hopefully also over many generations of inquisitive students. Paolo Cherubini kindly commented on an earlier version of this manuscript and handled the editorial process. Fig. 3. Example of Schweingruber's fieldwork performance: Fritz collecting dwarf shrubs near Scoresbysund around 70°N in coastal east Greenland, August 2011, which generated at least three important discoveries: Arctic dwarf shrubs i) often recruit in pulses, ii) can grow more than 200 years, and iii) tend to be less lignified.