Kopp, C. W., B. M. Neto-Bradley, L. P. J. Lipsen, J. Sandhar, and S. Smith. 2020. Herbarium records indicate variation in bloom-time sensitivity to temperature across a geographically diverse region. International Journal of Biometeorology 64:873–880. https://doi.org/10.1007/s00484-020-01877-1
Herbaria in the 21st Century
For over four centuries herbaria have housed “vouchers”, or expertly identified pressed plants that act as verifiable records of observations in the field. These vouchers have traditionally been created to accompany taxonomic (e.g. species classification and identification) and floristic (e.g. studies of regional biodiversity) research. Tragically, as interest in these fields has declined in lieu of more academically prized quantitative and experimental studies, so have field collections and funding for herbarium management. Yet these preserved plants are valuable for much more than (the still important) ability to “distinguish thorn from thistle” as the plant biologist Dr. Roxanne Kellar has cleverly described traditional uses of herbaria.
In juxtaposition to declining institutional support, scientific publications that rely on herbarium data have increased exponentially since the early 1900s across unanticipated research fields such as invasion ecology, plant pathology, and crop genetics. Permanent records of plant biodiversity are also crucial to land management and conservation planning. Fortunately, in recent years there has been a substantial effort to digitize herbarium specimens and share them in centralized databases that are available to researchers and the public alike. Once digitized, much of the important information connected to a specimen is immortalized and access to specimen photos and associated data is a mere click away.
Pressed plants are treasure troves of information
Plant taxonomists in today’s world use herbaria for more than just visual confirmation of species identifications; they have also developed methods to extract DNA from pressed plants which can be used to resolve complex evolutionary histories and relationships between species. Additionally, these specimens are snapshots in time that record much more information than collectors originally intended to use, including chemical signatures from the environment, ecologically important plant traits such as leaf thickness, and timing of biological events. Even simple location information can be used to learn a lot about how plants are distributed on the landscape. Collectively these data can be used for everything from tracking patterns of plant invasion to uncovering metal pollution in soils nearby a collected plant. In March 2020, a team of biologists even extracted chemical compounds from herbarium specimens of yerba santa (Eriodictyon), which they found could protect against neurodegenerative diseases like Alzheimer’s.
There are so many applications for plant specimens that the Fairchild Tropical Botanic Garden created a handy list of 72 ways that herbaria can be used. Still, despite the growing use of herbaria in botanical research, a 2013 study found that less than 2% of worldwide herbarium specimens are currently being used in ecological or evolutionary studies. That is a huge treasure trove of information yet to be unsealed!
Phenology: It’s all about timing
One of the major areas of research that has taken advantage of this information is global change biology. Phenology, or the timing of biological events like flowering and leaf growth, governs a plant’s ability to thrive and survive. Because phenology often relies on temperature cues, global change has great potential to wreak havoc on plant life cycles. Herbarium specimens can allow global change researchers to overcome common time and space constraints, as long as there are sufficient collections for a location or species of interest. For example, recent research published in 2020 by Kopp and colleagues examined over 8500 digitized herbarium specimens covering nearly 40 plant species native to the Pacific Northwest in order to determine if flowering time shifted between 1901 and 2015.
The team matched local climate records to coordinates recorded for each plant specimen and then tracked changes in the collection dates for plants that were flowering. Because they used herbarium specimens, they were able to look at these trends across a diversity of geographic locations, elevations, and vegetation types. The researchers discovered that plants generally flowered earlier as temperatures warmed over the study period and that early bloomers were the most sensitive to temperature increases. Interestingly, species at higher elevations were less sensitive to warming and western coastal species were more sensitive than those found in the eastern interior part of the study region. These results help scientists to understand how climate change and geography can interact to affect plant phenology and could guide conservation programs to protect the most at-risk species.
Applying artificial intelligence to herbarium research
Beyond tracking phenological change in flowers, herbarium specimens have been used to analyze shifts in traits like leaf size and stomatal density and distribution in response to climate change. Such studies have increased the need to carefully measure and classify a variety of organs from pressed plants. In response, scientists have begun to develop creative solutions that will make it possible to analyze thousands of specimens in much less time. Programs built using machine learning techniques, otherwise known as “trained” artificial intelligence, show promise for automatic identification, quantification, and measurement of plant organs and traits.
Some machine learning algorithms have even been successful at identifying specimens to the species level. These advances could significantly stimulate specimen-based research by overcoming painstaking hours of effort currently needed to complete these tasks.
Collecting today for tomorrow’s research
Strong evidence for the importance of herbarium collections has driven efforts to preserve specimens in the long-term (e.g. digitization), but it is crucial to recognize another threat to this unique data source. New field collections of plants are needed to keep herbaria current and to provide the next generation with specimens to analyze, yet they have severely declined over the last two decades. A lack of prestige associated with traditional taxonomic and floristic studies coupled with a misunderstanding of specimen purposes at upper administration levels has girdled collection programs. Once upon a time plant collecting was a worldwide pastime. If we can reawaken societal interest in the secrets held by the plants of the past, perhaps we can save the herbaria of the future- to the immense benefit of modern botanical research and our understanding of the natural world.