What is a voucher specimen and why do botanists collect them?

by Drew Chaney

April 4, 2022

What is a voucher specimen?

Over the past winter, the CUH field crew (Drew Chaney, Ezra Staengl, Jessie Wingo) were busy preparing voucher specimens from the 2021 field season for submission. Vouchers are plant specimens that are pressed, dried, and mounted in order to be labeled and stored in an herbarium (a collection of plant specimens used for scientific study). CUH field techs collected 119 voucher specimens in 2021, most (85) of which document new county level occurrences of their species. The remaining 34 represent additional populations of rare species and a few specimens of previously documented species for the CUH personal collection. The vouchers and associated data are used to track populations of plant species’ overall abundance, range and distribution, as well as shifting patterns in its distribution due to natural and human-caused climate change and other factors. They are also examined in order to document potential variation between individual populations of a taxa and determine taxonomic lumps and splits. 

The process of making a voucher specimen consists of several steps. Before the plant can even be collected, it must be determined that there are enough individuals present to collect without impacting the population. Next, it must be ensured that the specimen contains all parts of the plant necessary for identification. Then, it is placed in a botanical press as soon as possible, spread out so all parts are visible and not smushed together. After the specimen is fully dried, it receives a label noting the date of collection, location with GPS coordinates, number of plants in population, name of person who collected it etc. The specimen is then placed in folded newsprint with the labels, placed back in between the blotter paper, and packed in a box for shipping to the herbarium. 

Why do botanists collect voucher specimens?

Botanists collect voucher specimens for several reasons. First, understanding the distribution and abundance of a species on the landscape is an invaluable resource for conservation, and keeping physical copies with details of their location and context allows for regional genetic variation to be mapped and understood. Having a physical copy of a record provides scientists with DNA records of each individual and can help them to define a species and potentially split out new species or lump taxa together based on genetic differences that would otherwise not be noticeable by viewing the specimen with the naked eye. All of these reasons provide a powerful argument for the preservation and continued collection of voucher specimens.  

However, some have argued that the collection of physical specimens is unnecessary as well as potentially harmful to the population of the plants collected, since modern technology allows for photographic documentation of a very high quality, and have advocated for the cessation of collection, and the digitization of herbaria. However, the benefits of physical specimens far outweigh these concerns. First, strict protocols exist for the collection of specimens in order to ensure the continued survival of populations (typically only collecting a specimen if 10 or more individuals are present). For the absolute rarest plants, a photographic voucher showing all necessary parts for identification can be accepted. Second, many species can only be distinguished by minute differences in the sizes of seeds, length of hairs, etc which can be in fractions of millimeters (the genera Dichanthelium, Carex, and Pycnanthemum [more on the latter below], are textbook examples), and measurement of the actual plant material with a micrometer is crucial to reconfirm or change a past identification. Third, being able to accurately understand the relationships of species to one another,  and their context in the current landscape, as well as how their distributions and abundance shift with anthropogenic climate change, is a vital tool for learning the makeup of the ecosystems around us and restoring the very landscapes so ravaged by European agricultural systems and other extractive forces of colonialism. 

The aforementioned mountain-mints (Pycnanthemum) are a perfect example to illustrate the importance of physical specimens. Over the course of the 2021 field season, we observed members of the genus Pycnanthemum in nearly every intact grassland ecosystem we surveyed in the Virginia Piedmont. Of the 14 species recognized in Virginia, 7 were identified by the field crew over the course of 2021. In addition, multiple specimens that were unable to be firmly identified to a specific taxa and may represent hybrids were sent to the University of North Carolina for confirmation. Mountain-mints are a notoriously difficult group to identify due to the minute differences in average leaf size length, density of hairs, and shape and length of calyx lobes. In addition, many species concepts are made up of both fertile and sterile, asexually reproducing forms, and these may differ in size, coloration, and size of reproductive parts. The same “species” may consist of predominantly sexually reproducing plants in one area of its geographic range, and asexual, apomictic plants in others. All of these factors make this genus one of the most frustrating to confirm in the field, and having physical specimens to reference is absolutely crucial for understanding their status and distribution. Being able to study physical specimens enables us to see all of these subtle characters up close and personal, and even smell them (fragrance usually differs between Pycnanthemum taxa, and populations have been identified by smell).

As you can see, there are many reasons why the collection of botanical specimens is very important, and hopefully this blogpost offers some insight into what they are, the process involved in making a collection, and why botanists choose to collect. 

*anthropogenic – caused by human activity

*apomictic – producing fertile seed without sexual reproduction

*calyx – a structure of fused leafy tissue at the base of a flower or fruiting body

Special thanks to the University of North Carolina’s Derick Poindexter, Ph.D. for his enlightening conversation on Pycnanthemum taxonomy, asexual reproduction, hybridization, and the broadness of species concepts in this difficult genus. 

Stack of voucher specimens in press