MONDAY, 15 JUNE 2020“It’s a sort of Tutankhamun’s tomb” says Dr. Lauren Gardiner, describing the plant collection she curates. 'I regularly feel like I’m opening the door to a treasure trove and going "Folks? Does anyone realise what’s here?”'. Cambridge University Herbarium is a little-known collection, tucked away in a basement within the Sainsbury Laboratory, on the site of Cambridge University Botanic Garden. It houses an estimated 1.1 million plant specimens that have been dried, pressed, and preserved. The Herbarium (also known by its international herbarium code CGE) forms one node in a global network of over 3,000 herbaria that collectively house an estimated 380 million specimens.
One of the largest collections in the UK, CGE is 'laden with the most extraordinary collections' says Dr. Gardiner 'and would be considered to be the national collection in many countries'. It is home to one of the finest collections of British flora and plant specimens collected by famous naturalists including Charles Darwin and Alfred Russel Wallace. The Herbarium also holds many 'type' specimens, an estimated 50,000: each one is the original specimen upon which the description of its species was based. Despite its importance, the collection has received little attention in recent decades. As a result, many specimens are uncatalogued mysteries; some have lain untouched for centuries. 'It is pretty amazing when you find something in the collection... and then you realise you have something really significant' says Dr. Gardiner. 'I quite regularly find these in this collection'. As recently as 2019, previously undocumented specimens collected by Darwin have been discovered, some still wrapped in the newspaper they had originally been collected in.
While plants collected by Darwin will excite any keen natural scientist, herbaria are much more than mere curiosities. '[Herbaria] are collected for scientific purposes. They are enormous research datasets' explains Dr. Gardiner. Individual ecological studies tend to be limited by funding cycles, conducted on only a handful of species and locations, and restricted to a few years at most. With herbaria 'others have done a lot of fieldwork over the years and you can do your research on the material they collected. You have specimens that go back over a long time period and have a wider geographic and species range than you could possibly cover in one research project'. Worldwide, the oldest herbarium specimens date from as early as the 16th Century; CGE’s collections stretch back an impressive 300 years. Crucially, this period covers the onset of industrialisation and globalisation, perhaps humankind’s most large-scale and long-term experiments. Plants have witnessed, and bear the scars of, the rapid and intense global changes that have characterised this era: climate change, pollution, urbanisation, biological invasions, and more. Herbaria offer unique insights into such changes because they match the scales at which these processes operate: over decades and centuries, across all species, everywhere.
A single specimen contains many layers of information. The preserved morphology (form) of the plant records its life-cycle stage at the time it was collected as well as characteristics of its growth. Interactions with other organisms may have left their physical mark on the specimen: disease lesions for example, or bite-marks from a hungry herbivore. The plant tissue itself holds information that can be carefully extracted with biochemical methods. Environmental pollutants and nutrients retained in the tissue can paint a picture of the plant’s growth environment. The presence of a plant’s own defensive compounds point to interactions with pests and pathogens. Even the plant’s genetic makeup, its DNA, is largely preserved. Importantly, each specimen is accompanied by metadata, usually at least the plant name, date, location, and the collector’s name. All of the characteristics of each specimen can therefore be mapped to a specific place and time, with varying degrees of accuracy. This allows changes to be tracked over time and correlated with other relevant data, such as climate and land-use.
While many specimens remain on shelves and in boxes, leveraging their potential is challenging. 'There’s an international network of herbaria and a UK network as well...We can loan material back and forth between collections' highlights Dr. Gardiner. However, physical exchange of material can only go so far. A global priority for herbaria is the digitisation of specimens. Digital records, made available via online portals, are accessible and usable by researchers around the world. However, digitisation is both slow and requires human resource. Some herbaria are finding creative ways to accelerate the process, using machine learning or engaging the public through citizen science initiatives. CGE is in the process of digitising its priority specimens —those of special historical importance and holotypes — and plans to make them freely available via portals such as the Cambridge Digital Library and JSTOR Global Plants.
Wider use of the collections is central to Lauren’s vision for CGE. 'I would like the collection to be much, much more active... particularly to see it embedded in the teaching and research of the [Plant Science] department...part of the department’s day-to-day activities...there’s a lot of research that could use the collection'. She also emphasizes the cultural importance of the collection. 'These kinds of collections were put together, particularly in the 19th century and before that, by polymaths who were interested in everything. We have botanical specimens collected by literary figures known for their writing - all sorts of people'. Other significant items in the collection include original botanical artwork, photographs and portraits. It even holds the teaching materials used by Darwin’s botany Professor, John Stevens Henslow. These materials create even more opportunities to collaborate with other collections: 'I would like to see the Herbarium’s collections represented in exhibitions at the University of Cambridge Museums and the Botanic Garden...to strengthen the links between our collections and to explore the stories behind them'.
Despite their exciting potential, the future of herbaria is under threat in many UK organisations. There are immediate threats, such as the risk of flooding and infestation by insects that eat the specimens. CGE requires careful pest monitoring and all new specimens must be frozen before entering the collection. A more serious threat is the decline in financial support that is needed to sustain herbaria. 'There were points in time when this collection was definitely at risk of being broken up' Dr. Gardiner explains. The more valuable specimens would probably have been distributed to other collections, but others could have been disposed of. 'Sometimes people think it’s not of much interest — “it’s just a bunch of dead plants”'. This attitude seems short-sighted, given that herbaria may hold the key to understanding some of the biggest threats facing our plant-life and even our planet. To safeguard the future of CGE, and other herbaria, it is critical their utility is recognised. 'I would like people to know that we’re even here', Dr. Gardiner emphasises, 'and [for them to] value the collection as the treasure it really is'.
IT’S NOT JUST A BUNCH OF DEAD PLANTS!
Metadata | Location and date of collection can be used to map changes in plant diversity and distribution over time. Records show that many plants are moving higher in altitude and towards the poles, following their climatic niches as their environment warms. Not all changes can be escaped, however: studies reveal that urbanisation and modern agriculture have caused extensive species losses. Where humans have introduced non-native 'invasive' species, herbarium records can be used to understand the causes and dynamics of these invasions. An important caveat is that collection efforts can be inconsistent across time and space. This can introduce biases into the data if they are not carefully accounted for.
Morphology | Changes to plant growth and form can be related to global change. This is used to understand how such change impacts plant-life and how plants might be adapting to new stresses. One study found that as atmospheric carbon dioxide has risen, the density of stomata (air pores) on leaves has reduced. Other research found that peak flowering advanced by 2.4 days for every 1°C increase in temperature. Changes like these could impede the interactions of plants with other organisms, upsetting the fine ecological balance. For example, if plants flower earlier but their pollinators continue to hatch at the same time, both will struggle to reproduce. At a global level, leaf gas exchange rates can affect carbon balance, and plants are a cornerstone of nutrient cycling. It is important that we know what changes are occurring
to better understand the potential wide-reaching impacts.
Chemicals | Plants accumulate and exchange a wide range of chemicals during their lifetime and these are present in herbarium specimens. Historic pollution levels can be tracked by measuring heavy metal compounds. This information can be correlated with other types of data(morphology, DNA, etc.) to understand the long-term effects of pollution. The composition of carbon and oxygen isotopes in the plant can reflect physiological changes. Studies suggest that the ratio of photosynthesis to water loss may be increasing as atmospheric carbon dioxide rises.
DNA | Samples of so-called ancient DNA (aDNA) can be retrieved from small pieces of tissue, even those which are centuries old. Segments or whole genomes of plants (and their co-preserved pathogens) can be sequenced, revealing their genetic instructions. This powerful information can be used to characterise evolutionary responses to global change. Changes to plant traits that have occurred gradually, over centuries, can now be linked to their genetic basis. Evolutionary relationships can be reconstructed, and even population dynamics — how populations have grown, shrunk, migrated, and interbred — can be revealed from the letters of code. This rich seam of information has not been widely utilised, in part owing to technical difficulties such as contamination and DNA degradation. However, modern molecular techniques are increasingly able to utilise this data in older and older specimens.
Bryony Yates is fourth year Natural Sciences student at Newnham College.
Background: John Lindley, Wellcome Collection.
Right: Cambridge University Herbarium. CGE.