Celebrating Thomas van der Hammen’s 100th anniversary: contributions of an explorer palynologist to neotropical paleoecology

Thomas van der Hammen (1924–2010) of the Netherlands passed the secondary school (HBS) exam in the summer of 1944, but universities were closed during the Second World War (WWII). In 1943 he spent some time in the natural history museum in Denekamp as assistant of the director, Mr. Bernink. He joined the Dutch Youth Organization for Nature Studies (NJN). In 1939 he published his first paper in the journal of the NJN. In 1946, at the age of 22, he started his study of geology at Leiden University. In the academic curriculum excursions in the field to European countries developed as a tradition. Thomas studied geology and botany at the geological institute of Leiden University. His PhD study focused on the history of vegetation, climate and landscape development of the Lateglacial of Twente, central Eastern Netherlands (Van der Hammen 1951). He continued on previous work by his supervisor Frans Florschütz (1887–1965) (Van der Vlerk and Florschütz 1953; see also Hooghiemstra and Richards 2022). During the previous ice-age (Saalian) Scandinavian ice had pushed moraines up to some 70 m above present-day sea level. Since 1939 Van der Vlerk and Florschütz had been working on the preparation of a book Nederland in het ijstijdvak (‘The Netherlands during the ice ages’). Although dated, it still is a joy to read this classical book about the Pleistocene history of the Netherlands (Van der Vlerk and Florschütz 1950). Professors Florschütz and Van der Vlerk (1892–1974) worked with an enthusiastic team of PhD students, amongst others Waldo Zagwijn (1928–2018) (Hooghiemstra and Hoek 2019) and Thomas van der Hammen (Hooghiemstra et al. 2010) (Figure 1).

In 1950, Thomas participated in the International Botanical Congress in Stockholm. Palynologists were well represented which was a great opportunity to meet many of the authorities who laid the foundations of palynology (Figure 2).

The laboratory of Johannes Iversen (1904–1971) was a central place to improve one’s skills. Thomas visited Iversen’s laboratory in 1950, a year before he published his PhD thesis in 1951. He continued his career at the Geological Survey of Colombia in Bogotá where he also started teaching palynology at different universities. His explorative studies of sediment archives from the Cretaceous to the Quaternary in Colombia and adjacent countries, stand as a model of how sediment archives became used to study the history of neotropical ecosystems.

Thomas was a warm and charismatic person. Always in a good mood and inquiring after the interests of students. He wrote poems, played the violin, made stone sculptures, was a skilful dancer, and loved to see ballet in the theatre. He might be characterised as a romanticus. Thomas showed much love and respect for nature. Thomas admired the life and work of Saint Francis of Assisi, an Italian mystic, poet and catholic friar who founded the religious order of the Franciscans. Ater retirement Thomas built a chapel on his land in honour of Saint Francis of Assisi.

Thomas was well introduced into the Dutch and Colombian academic communities. From 1974 to 1983 I got my palynological education at the bachelor’s and master’s levels from Thomas and the activities of the Dutch Palynological Society. After a postdoc period in Göttingen I returned in 1987 to Thomas’ group and a vivid collaboration started. On the occasion of his 100^th anniversary a symposium was organised at the Servicio Geológico Colombiano (formerly Ingeominas) in Bogotá. This paper is a tribute to the explorer scientist Thomas van der Hammen based on 35 years of cooperation.

Shortly after his thesis defence (1951) Thomas moved to his new employer, the Colombian Geological Survey (Ingeominas) in Bogotá. The oil industry was in need of palynological expertise. He started teaching courses and trained Colombian geologists in palynology and palaeobotany as a stratigraphical tool. In 1953 he married in Bogotá Anita Malo, and they had three children: Tomas, Clara and Cornelis. In 1954 he laid a foundation for a systematic nomenclature in palynology, an important step in the further development of pre-Quaternary studies. Thomas published Principios para la nomenclatura palinológica sistemática (Van der Hammen 1954, 1956) in the Boletin Geológico. Thomas was fascinated by the wealth of fossils found in Villa de Leiva. Padre Huertas was the curator of the site. In 1957 he gave Thomas Bertsch’ Lehrbuch der Pollenanalyse (Bertsch 1942), a key document of pollen analysis in the 1930s. Padre Huertas signed this book with ‘A Thomas, como hommenage de admiración gratitud. P. Gustavo Huertas, Bogota DE 1.957’. Enrique González-Guzman was among his promising students. Between 1960 and 1965 Enrique and Thomas published three important papers (Van der Hammen and González 1960, 1964, 1965) Unfortunately for Thomas, Enrique decided not to continue in science, and choose a career in the oil industry.

Educated as a ‘temperate climate palynologist’, Thomas was most fascinated by obtaining a first understanding of the history of tropical ecosystems. Climate change must have changed the elevational position of vegetation belts and he expected this mechanism to be instrumental in reconstructing past climate change. Above the upper forest line (UFL) at ca. 3200 m, the iconic stemrosettes of the genus Espeletia, locally called frailejones, captivate all visitors of the páramo. ‘Frailejon’ comes from ‘fraile’ (monk), a name given to members of a religious order. The páramo is often immersed in fog, the ascending clouds cause the tall stemrosettes to look like a person.

Thomas was inspired by the studies of the neotropical flora and vegetation of Alexander von Humboldt (1769–1859) (Von Humboldt and Bonpland 1807, 2009; Hoorn et al. 2019; Falk et al. 2022), the Colombian botanist José Celestino Mutis (1732–1808) (Villegas 1992) and José Cuatrecasas (1903–1996) (Cuatrecasas 1934, 1958, 1989; Bonifacino et al. 2009). Roberto Jaramillo-Mejia (1919–2006) introduced Thomas into the Colombian flora. Roberto received no formal academic training but began working for the Colombian National Herbarium in Bogóta, when he left school in 1948. Having no patience for paperwork, meetings or administration, he concentrated on collecting plants in the field and looking after the collections. Roberto Jaramillo was greatly admired and respected by all who worked in the herbarium, many of whom were taught by him in the art of making a good herbarium vouchers and in caring for the collections. His knowledge of the Colombian flora was unsurpassed.

In Europe pollen analysis flourished already in the 1930s (Bertsch 1942) but it was absent in the tropical regions. First pioneers in pollen analysis started in the tropics in the early 1950s, when the name ‘pollen analysis’ had already changed into ‘palynology’ (Hooghiemstra and Richards 2022). Eduard van Zinderen Bakker (1907–2002) set up a research group in Bloemfontein, South Africa, with studies on the environmental history in sub-Saharan Africa (Coetzee and Van Zinderen-Bakker 1988; Neumann and Scott 2018; Hooghiemstra and Richards 2022). Dan Livingstone (1927–2016) at Duke University focused research on lakes of east tropical Africa (Livingstone et al. 1959; Livingstone 1967, 1983, 2010; Livingstone and Van der Hammen 1978). He was the initiator of early plans for deep coring of the African lakes which helped to pave the way for later drilling in Lake Malawi and Lake Bosumtwi (Stager 2017). Paul Colinvaux (1930–2016) was among Livingstone’s graduate students. Willem van Zeist (1924–2016) performed pioneering studies of environmental change and the development of agriculture in the Middle East (Hooghiemstra and Birks 2018). Being among the first generation of pioneers, Thomas set up palynological research in South America. A decade later, in the early 1960s, more research groups settled in the tropics, such as in Pondicherry, India. In the 1970s palynological research groups with focus on the tropics flourished (Flenley 1979; Williams and Faure 1980).

Thomas contributed to the International Geophysical Year 1957–1958 by collecting a deep sediment core from the Bogotá basin (Hooghiemstra et al. 2022). He cored in 1958 at the grounds of the Geological Survey in Bogotá, located at a place where ancient lake Bogotá often had its shoreline. After a first core of 30 m (Van der Hammen and González 1960, 1963) he raised a 195 m deep sediment column in 1959 (Van der Hammen and González 1964). Pollen analysis was carried out by Enrique González. Results proved that the sedimentary basin included long records of the Pleistocene glacial interglacial cycles. Unfortunately, the sedimentary archive included many gaps reflecting the repetitive episodes with low lake-level stands. Therefore, the record was food for thought for new cores to be raised (Van der Hammen 1981).

His alma mater professor Frans Florschütz retired in December 1958 and left a vacancy in the Geological Institute. Thomas returned to Leiden University with many ideas to document the record of ice ages in the Northern Andes. This new and relevant field of interdisciplinary research on the cutting edge of biology and earth sciences was challenging in Dutch academia (Van Veen 2012). Studies on the Pleistocene history of tropical ecosystems were welcome at WOTRO, the branch of the Netherlands Organization for Scientific Research (NWO) with focus on research in the tropics. Thomas organised a scientific Colombian-Dutch network. Over decades he was successful obtaining grants from WOTRO for PhDs, postdocs, and technical assistance.

Thomas became immersed in international debates. For example, about the pluvial theory, suggesting that a glacial period in Europe coincided with pluvial period in Africa (Maarleveld and Van der Hammen 1959; Fairbridge 1964). This hypothesis was rejected by Eduard van Zinderen Bakker (Van Zinderen Bakker 1966). Many questions in Thomas’ PhD thesis (1951) were still unaddressed. Twente offered an excellent area to give students a multidisciplinary training including geology, climate history, vegetation history and human impact. Thomas and Lex Wijmstra (1932–2004) developed new research in the valley of the Dinkel River aiming to further improve the understanding of environmental, climate and vegetation change. Long profiles over tens of metres were made to be able to interpret stratigraphical changes in terms of past environmental change.

The year 1965 was characterised by significant change. Frans Florschütz, the founding father of palynology in the Netherlands, died. J.H. Germeraad (working at Shell and representing industry), Van der Hammen (working at the University of Leiden and representing academia) and Waldo Zagwijn (working at the Geological Survey and representing governmental organisations) advised the Royal Netherlands Geology and Mining Association (KNGMG) to organise a series of palynological meetings for all concerned. Because of the confidentiality of the results from industry they anticipated some restrictions. These meetings developed into the Dutch Palynological Society, a member of the IFPS. In 1965 Thomas was appointed at the University of Amsterdam to start a research group with focus on the (neo)tropics as part of the Hugo de Vries laboratory. Thomas accepted this invitation on the condition that his ‘right hand’ Lex Wijmstra could also make the move to the University of Amsterdam. Thomas and Lex started a field course in Twente. New profiles encountered in the field were explored and sampled ad hoc. The best students were selected for the main projects. This eight-day course has received much appreciation for 40 years (up to 2014). In 1971 Lex Wijmstra completed his PhD thesis about the Guyana coastal plain (Wijmstra 1971) and this step in his career allowed him to develop his own research lines. Handling and analysing pollen data by computer attracted his interests and he guided Thomas’ research group into the era of computers. In 1976, Thomas was appointed professor (1 day/week) at Free University in Amsterdam.

Geographers had explored past South American environmental conditions earlier. Thomas was much inspired by the publications of a few of them as their subjects related closely to the palaeoecological studies in Colombia. Carl Troll (1899–1975) at the University of München studied geography, climate and landscape ecology of mountainous countries (eg Troll 1968; Troll and Lauer 1978). Between 1926 and 1929 he was on a research journey through a number of Andean countries including Panama, Colombia, Ecuador, Peru, Bolivia and Chile. Wilhelm Lauer (1923–2007) at the University of Bonn attracted Thomas’ attention for his studies in the Northern Andes. A basic paper is Vom Wesen der Tropen. Klimaökologische Studien zum Inhalt und Abgrenzung eines irdischen Landschaftsgürtels (The essentials of the tropics. Climate-ecological studies on composition and biogeography of elevational landscape zones) (Lauer 1975). He published a thorough study of the Páramo de Papallacta in Ecuador (Lauer and Rafiqpoor 2000). Botanist Olav Hedberg (1923–2007) at the University of Uppsala had studied the altitudinal vegetation gradient in the East African mountains (eg Hedberg 1951, 1955, 1969; Brochmann et al. 2022). His transect study served as a primer for studies along the altitudinal gradient in the Northern Andes. Hedberg showed that vegetation studies along an altitudinal gradient are necessary for a proper interpretation of pollen records. Thomas created a position for neotropical vegetation studies in his research group and Antoine Cleef was appointed on that position (Hooghiemstra 2021).

Colleagues brought a visit to Thomas’ laboratory which housed unique pollen reference collections and a reprint collection which were regularly consulted. Alan Horowitz, known from a.o. The Quaternary of Israel (Horowitz 1979) visited Thomas’ pollen laboratory in 1969–1970. Alan Graham (1934–2021), known from a.o. Vegetation and vegetational history of northern Latin America (Graham 1973) spent some time in Amsterdam as a visiting research scientist.

Biologists speculated on mechanisms to explain the stunning high biodiversity in the Amazonian rainforest. Geologist Jürgen Haffer (1932–2010) travelled much in Amazonia for oil exploration. However, he also liked to observe birds. He noticed that different toucanet species had their own distribution area in Amazonia without overlap. Palaeoecologists learned that during the Last Glacial Maximum (LGM), the climate had been drier. Haffer made use of that understanding and hypothesised that the current Amazonian rainforest had fragmented into glacial refugia separated by savanna (Haffer 1969). The toucanet populations had been isolated for most (>50%) of glacial time and five different toucanet species developed. This elegant concept of a Pleistocene ‘species pump’, driven by the sequence of ice-ages, was well received by evolutionary biologists. Ghilian Prance was among the first reporting phytogeographic results in support of Haffers' theory (Prance 1973). The search for supporting evidence spread like a virus among biogeographers and palaeoecologists. Thomas’ first paper on this issue (Van der Hammen 1972) led to the PhD thesis on Manaus by Maria Lucia Absy (Absy 1979; Absy et al. 1991). She showed in pollen records from abandoned oxbow lakes a dynamic Holocene vegetation history. Long sedimentary archives covering a period of a full ice age cycle yet have not been found in Amazonia. At many places sediments are eroded few millennia after deposition. At a symposium in 1979 of the Associacion of Tropical Biology in Caracas, Prance invited many specialists and asked them to draw the areas with highest biodiversity of their specific group on a map of Amazonia (Prance 1982). The pre-assumption that for many different taxonomic groups (plants and animals), areas with a high biodiversity would overlap was hardly supported. The deadlock in finding evidence stimulated further research but also stimulated authors to use an increasingly firm word choice in their publications. Thomas summarised the understanding of fragmenting and merging of gene pools (distribution patterns) at high elevations (páramos; Van der Hammen 1974) and Amazonian rainforest in the light of changing dominance of dry periods (savanna; Van der Hammen 1982; Van der Hammen and Absy 1994). In the absence of evidence I was less convinced of the forest refugia hypothesis than Thomas was. I had a friendly relationship with Paul Colinvaux and I even could have been his postdoc if Paul’s research proposal had been granted in the early 1980s. In 1997 the Netherlands Organisation for Scientific Research (NWO) invited Paul Colinvaux to The Hague to give a lecture on his view on the ice-age Amazon. Paul presented the main lecture (Colinvaux 1997) and I was invited to be the co-referent (Hooghiemstra 1997). Afterwards Paul sent me a copy of the illustrated 1980 version of his book Why big fierce animals are rare with the inscription ‘For Henry Hooghiemstra with thanks for the Huygens and in the expectation that he and the author will go forward together backward to the Amazon past. Paul Colinvaux, Balboa, January 1998’. Later, in the preface of ‘Amazon pollen manual and atlas’ Paul expressed again his appreciation for this event in The Hague (Colinvaux et al. 1999, p. 15). At the turn of the century two palaeoecological papers discussed the pro and contra sides of Haffer’s hypothesis (Colinvaux et al. 2000; Van der Hammen and Hooghiemstra 2000). The disagreement between Thomas and Paul became deep. Paul was the better writer and succeeded in bringing his views to high ranked journals.

In the book ‘Amazonia: landscape and species evolution’ edited by Carina Hoorn and Frank Wesselingh it was shown that Amazonian biodiversity had developed between roughly 50 and 20 Ma, during the Oligocene and Miocene (Hoorn and Wesselingh 2010). The increase of biodiversity was a by-product of the upheaval of the Andes and had no relation with Pleistocene ice-age dynamics. Forty years of debate and research had triggered research to find support for this elegant hypothesis. Vicariance models and arguments what refugia could have separated in space and time had been intensively discussed. How after 1969 the refugia hypothesis had been supported, as well as falsified by others, was presented in a recent overview (Gomes da Rocha and Kaefer 2019). This paper focuses on ecological mechanisms which are relevant under all environmental conditions and in all hypotheses. However, framing evolutionary dynamics and biodiversity levels in geological time and in phylogenies of selected taxa received, in this paper, little attention. During the last decade understanding of the palaeoenvironmental settings in the Andes and the Amazon drainage basin increased substantially (Hoorn and Wesselingh 2010; Hoorn et al. 2022a; Hoorn et al. 2022b; Hoorn et al. 2023). This progress is in contrast with the poor evidence found for Amazonia during the LGM. Amazonia is the last big area in the world with little evidence how it was covered by vegetation during the last glacial. Palynologists have a task to find locations where the Amazon drainage system has not eroded sediments that accumulated over long periods of time.

Thomas was fascinated by the history of human occupation in Colombia. He worked with archaeologists Gonzalo Correal-Urrego and W.R. Hurt to investigate the early hunter-gatherers who lived on the high plain of Bogotá (Hurt et al. 1972; Hurt et al. 1976; Van der Hammen and Correal Urrego 1978). The Tequendama rock shelters have been inhabited from 11 kyr BP onwards continuing into the Herrera and Muisca periods (Hurt et al. 1976). Elisabeth Schreve-Brinkman worked with Thomas in the Al Abra rock shelters, located in the north of the high plain near Zipaquirá. In the several hundred rock shelters, first evidence of human settlement in the Americas was found dated at 12,400 yr BP (Schreve-Brinkman 1978; Van der Hammen 1978).

The northern Andes is a geologically, young mountain range (Hoorn and Wesselingh 2010) and reflects a tectonically active area. Local subsidence led to sedimentary basins in which sediments accumulated over long periods of time (Van der Hammen et al. 1973). Sediment sequences of hundreds of metres thick are rare (Hooghiemstra et al. 2022). In 1977 Thomas had organised another deep drilling in the Bogotá basin, located near the village of Funza at sufficient distance from the place where the ancient lake frequently had its border. A 357 m deep core Funza-1 was collected by Ingeominas (currently Servicio Geológico Colombiano). WOTRO supported a PhD project for the analysis of this long core (Hooghiemstra 1984). Developing a chronology for long sediment sequences is challenging and over the years different age models have been proposed. The sediment infill of the Bogotá basin got its final age model on the basis of the 586 m deep core Funza-2 (Torres et al. 2013). The pollen record reach 2.25 Ma back in time whereas the sediment record might touch the Pliocene-Quaternary boundary (2.58 Ma).

Thomas addressed the boundaries of palynology. He was an explorer rather than - he said - a specialist in a limited part of the research field. He initiated PhD projects with a remarkable variety of subjects (Supplementary Information 1). Many laboratories created their own series of publications to stimulate the exchange of publications. Today, such publications are classified as grey literature. Thomas created his The Quaternary of Colombia/El Cuaternario de Colombia. This series was distributed between 1973 (Volume 1) and 2008 (Volume 32). The large reprint collection was carefully administrated by his personal secretary.

Thomas managed to create in his research group an additional position for neotropical vegetation ecology. During reorganisations at the Botanical Institute of the University of Utrecht in 1984, Antoine Cleef was able to switch universities (Hooghiemstra 2021). Guido van Reenen, a bryologist with expertise in computer technology, came to Thomas’ group to support the introduction of computerised output. In the Colombian transect studies Guido studied elevational distributions of mosses in support to establish boundaries of the elevational vegetation belts (eg Van Reenen and Gradstein 1984; Van Reenen 2005).

Between 1976 and 1989 Thomas spent much time in Colombia which limited his involvement in the Dutch academic community. During the early 1980s he refused to become Dean of the Faculty of Biology as, he said, his commitments in Colombia and at the Free University were too demanding. The position of Dean was a rotating function and his decision was not appreciated by his colleague professors.

Thomas designed and prepared altitudinal transect studies through the Eastern, Central and Western Cordilleras of Colombia, and along the slopes of the Sierra Nevada de Santa Marta. Leslie Holdridge (1907–1999) and collaborators published a very informative book Forest environments in tropical life zones (Holdridge et al. 1971) and Thomas used it frequenly for his research planning. The four transect studies were presented under the name ‘ECOANDES’. Thomas needed a small group of reliable and physically robust fieldworkers to form the key-staff. Vegetation ecologists Orlando Rangel and Antoine Cleef, and bryologist Guido van Reenen were central in all expeditions. Many other disciplines were represented in the teams which consisted of 10 to 15 persons. The field kitchen was staffed by Thomas’ wife Anita, and Roberto’s wife Ines. WOTRO and several Colombian institutes financed this multiple-year project. The first ECOANDES transect study took place in the Sierra Nevada de Santa Marta during May to September 1977 from 520 m to 4100 m elevation along the Buritaca transect. The field stations at high elevation were reached by helicopter (Figure 3).

The second transect study in the Central Cordillera was during January to July 1980 in the Parque Nacional Natural Los Nevados from 300 to 4550 m elevation. The third transect study was carried out in the Eastern Cordillera during July to September 1981. Studies reached from the lower montane forest at 470 m to 4250 m in the Páramo de Sumapaz (Figure 4).

The fourth transect study was carried out in the Western Cordillera from January to February 1983 along the Tatamá transect from 550 to 4100 m elevation (Figure 5). Harsh field conditions required strong physical and mental health. When members of the team lost their way in the forest and had unsufficient water, drinking from bromelias made several persons ill. The panoramas in undisturbed montane slope forest and páramo were overwhelming and rewarding. Thomas’ transect studies were among the most important expeditions in Colombia’s history of biological exploration.

Many samples were shipped to Amsterdam to be studied in the frame of ongoing PhD projects. Results were published in PhD theses and in seven volumes of the book series Studies on tropical Andean ecosystems/Estudios de ecosistemas tropandinos. It was not easy to get the contributions of all co-authors in time as personal prioriries diverge after an intensive period of fieldwork. Thomas completed the last volume seven 25 years after volume one had been published.

Tropenbos International (TBI) is an organisation funded by the Netherlands Ministry of Foreign Affairs and the Ministry of Development Cooperation. TBI believes that an intercultural climate-smart landscape with high forest cover can contribute to biological connectivity and can maintain the ecological and climatic processes between the Amazon and the Andes, and providing an improved livelihood and income benefits for the inhabitants at the same time (eg Duivenvoorden et al. 2001). TBI develops training programmes about commercialisation of forest products for peasant and indigenous communities. Thomas saw an opportunity for Colombia to participate. Tropenbos Colombia has been operational since 1987 (www.tropenboscol.org). A biological station was developed in Araracuara, a former prison, very remotely located in the rainforest of the Department of Caquetá. Dozens of postgraduate students took the small aircraft to fly in three hours from Bogotá to Araracuara, admiring the panoramic views on the rainforest when there were no clouds. Tropenbos Colombia, headed by Carlos Rodriguez, published 20 studies in the book series Estudios en la Amazonia colombiana/Studies on the Colombian Amazonia. After several years this biological field station was abandoned as the guerrilla expanded their influence in the rainforest.

Several years before his retirement Thomas aquired a country estate (finca) in a rural area on the high plain of Bogotá between the villages Chia and Cota. In 1991, two years after his official retirement, he settled down there. His finca was selfsustainable to a large degree with a fruit garden, vegetables, chickens and rabbits on an area of around one hectare. Later he expanded his grounds to three hectares. Thomas built a chapel near his finca and prepared for Anita (†2009) and himself (†2010) the last resting place next to it (Figure 6).

The chapel and the stained-glass windows were designed by himself. The chapel was officially inaugurated by the Catholic Church. Their grave bears the text ‘Que amó a Anita, a sus Hijos y Nietos, a Francisco y Clara de Asis, a las rocas, las plantas, los animales, a toda la gente y al Creador de este maravilloso universo, presente in todo y en todos’. In Thomas, religion, nature, and science were united. The local campesinos (farmers) use the chapel for weddings, etc. Thomas always corresponded in hand-written letters. He was not inclined to make the change to use a computer. However, surprisingly, after residing several years in Chia the computer did enter his study. Only his fax messages continued to be handwritten.

Thomas and Anita regularly visited the Netherlands. In the 1990s Dutch universities changed their internal organisation and teaching programmes dramatically. Education of PhDs was formalised into disciplinary-organised Research Schools. Daily practice contrasted much with the previously more informal way of organising research and education. Institutes became formalised also, faculties merged, and organisatorial structures got a new masterplan. Professors lost much of their direct influence, and their personal secretary. In addition, the Hugo de Vries laboratory moved to the new buildings of the Science Park which needed much internal reorganisation and a cleaning up of paper archives and sediment samples. The Hugo de Vries laboratory became part of the Institute for Biodiversity and Ecosystem Dynamics. Thomas hardly recognised the academic atmosphere he had left ten years earlier and was happy to live in Colombia. His finca became a meeting centre where students, scientists, farmers, mayors, and even Ministers of State discussed the past and future of Colombia’s nature. He published a blueprint for an improved expansion of the city of Bogotá (Van der Hammen 1998). Areas with high biodiversity were to be included in a green network with a protected status, whereas the areas with low biodiversity were appropriate for expansion of the city.

Centuries ago, the floor of the Bogotá basin was covered by a characteristic association of trees and shrub species, the ‘high plain forest’. At present only few degraded remnants are left. With support from the Corporación Autónoma Regional de Cundinamarca (CAR) Thomas planted some 3000 juvenile trees. The selected arboreal taxa and the proportions of these native species were inferred from a pollen diagram of such forest type (Van der Hammen 1957). Information from relevees of remnants of high plain forest was also helpful (Cleef and Hooghiemstra 1984, p. 46; Cortés et al. 1999; Cortés 2003). Birds brought seeds and each year species diversity increased. During an inventarisation in 2009, the biggest trees were 15 m tall and up to 22 cm in diameter (Pérez et al. 2016) (Figure 6). Among the most important species are: Alnus acuminata (Betulaceae), Cestrum elegans (Solanaceae), Citharexylum subflavescens (Verbenaceae), Clusia multiflora (Guttiferae), Miconia squamulosa (Melastomataceae), Montanoa quadrangularis (Asteraceae), Myrsine guianensis (Myrsinaceae), Oreopanax floribundum (Araliaceae), Piper bogotense (Piperaceae), Solanum ovalifolium (Solanaceae), Vallea stipularis (Elaeocarpaceae), Xylosma spiculifera (Flacourtiaceae) and Weinmannia tomentosa (Cunoniaceae). Species of the plantation showed a good reproduction capacity. Thomas’ original experiment shows that a plantation of native arboreal species at nursery stage has the potential to reach a vegetation structure comparable to native montane forest within decades. Mayors of many surrounding villages visited the re-introduced high plain forest on its way to natural maturity (see also: Hooghiemstra et al. 2024).

Thomas had a classical career (Figure 7). After university study (1945–1951) he worked for eight years as a postdoc at the Geological Survey in Bogotá, Colombia (1951–1959).

He returned to Leiden University and became a researcher at the Institute for Geology and Mineralogy (1959–1965). He was appointed lecturer at the University of Amsterdam (1965) and obtained the title of professor in 1977. In 1976 he was also appointed to be a professor at the Free University Amsterdam. In 1989 he retired from both universities.

Thomas published in Dutch (1939), English (1951), and Spanish (1953–1962, and nearly all papers were published in local journals after 1970. After 1972 Thomas more frequently published in international peer-reviewed journals. Ten papers in journals and two book chapters are considered his key publications (Supplementary Information Table 2). After his retirement Thomas contributed as a co-author with former PhD students to a suite of papers in international journals. Although Thomas’ publication strategy served the Latin American audience well, results have reached the international literature unsufficiently (Arbeláez-Cortés 2013).

Thomas had a large network and was a broker in science. He was a superb designer of new scientific structures, such as the instigation (with colleagues) to establish the Dutch Palynological Society (1966), the series of publications The Quaternary of Colombia (1973–2008), the ECOANDES transect studies (1977–1983) and, in 1986, the initiation of Tropenbos-Colombia. Thomas was a frequently invited speaker at congresses; these activities are not shown in this overview. Thomas van der Hammen was a member of the Academy of Sciences of Colombia, and of Denmark. He was knighted by the Dutch Queen (Ridder in de Orde van de Nederlandse Leeuw), received in Colombia the prestiguous Orden de San Carlos, and was in the Netherlands decorated with the Dutch Van Waterschoot van der Gracht Medal for his palaeobotanical work. For more details see the pages 3–7 in in the tribute to Van der Hammen (Fondo FEN Colombia 1995, Supplementary Information 3) and his memoirs (Van der Hammen-Malo 2016).

After WWII, academic education and research expanded rapidly. Thomas, with an explorative mind, found a wealth of opportunities to initiate new research. Thomas introduced palynology into South America, first applied in oil exploration, and later to Quaternary environmental studies. He discovered the potential of studying the sediments accumulated in deep basins of the Colombian Andes to gather an understanding of the history of neotropical ecosystems. Haffer’s hypothesis of glacial forest refugia in Amazonia was without decisive evidence for a long time (Haffer 1969; Haffer and Prance 2001) which annoyed debaters. The word choice Paul Colinvaux used in his ‘Amazon expeditions; My quest for the ice-age Equator’ (Colinvaux 2007) ran beyond the norms of an academic exchange of arguments. However, reading Colinvaux’ sneers towards his colleagues with a smile, his adventures and considerations are fascinating to read. Thomas was a scientific explorer and charismatic person which helped him to convince decision-makers to follow his challenging ideas. In 1976 Thomas took the opportunity to integrate studies of modern neotropical vegetation in his research group. He designed the Colombian-Dutch transect studies through the cordilleras of Colombia which had many Humboldtian characteristics. These expeditions were unprecedented and hardly received the attention they deserved. With his endless energy Thomas left many tracks in science.

This paper has been written as a personal memory to my highly esteemed supervisor, colleague and friend, the late professor Thomas van der Hammen. In my career I could use the infrastructure Thomas had left. I thank Bas van Geel and Aartje Vink for comments on earlier drafts of this paper, Clara van der Hammen for support. Kees Kasse and Jef Vandenberghe provided information about Van der Hammen’s PhD students at the Free University. The artwork was made by Mick Bönnen and financially supported by IBED. I thank the Servicio Geológico Colombiano (Bogotá) and professor Sergio Gaviria for enabling me to contribute to the symposium 23–30 September 2024 in Bogotá on the occasion of Van der Hammen’s 100th anniversary.

HENRY HOOGHIEMSTRA (born 1948) studied biology at Amsterdam University. He teached and carried out research on the Quaternary history of tropical ecosystems using terrestrial and marine palynology. In 1992 he was appointed professor in Palynology and Quaternary Ecology at the University of Amsterdam. He published Holocene and Pleistocene reconstructions of the dynamic histories of the Mediterranean, Saharan Africa, and Mauritius. In the Neotropics, he studied in the area from Mexico to Ecuador the environmental and climate history in dry forest ecosystems, savanna, rainforest, montane forest, and tropical alpine grasslands (páramo). Throughout his career he worked on the analysis of sediment cores from deep sedimentary basins in the Colombian Andes which reflect almost the full Quaternary. He published over 180 papers in international peer-reviewed journals. He is a member elect of the Royal Netherlands Academy of Arts and Sciences (KNAW) in the division of Earth-Sciences. He is a regular peer reviewer for some 15 international journals.

No potential conflict of interest was reported by the author.