Herbarium collections constitute permanent and often well-documented records of the distribution of taxa through space and time. Since their creation, their uses have dramatically expanded and with many new uses being proposed, including some for which herbaria were not initially intended for. In this paper we assess the potential of these collections on conservation biology, by providing exemplary studies that use herbarium specimens, grouped into four categories: (1) based on occurrence data, such as studies about plant extinction or introduction, or those focused on modelling their ecological niche; (2) based on the specimens themselves, such as morphological or phenological studies to evaluate the impact of climate change; (3) based in genetic data, such as phylogeographic or taxonomical studies; and (4), other applied studies. Resumen Las colecciones de herbario constituyen un registro permanente y a menudo bien documentado de la distribución de los taxones a lo largo del espacio y el tiempo. Desde su creación, sus aplicaciones se han diversificado de forma considerable, y recientemente han emergido nuevos usos, algunos no contemplados originalmente. En este trabajo evaluamos el potencial de estas colecciones para la biología de la conservación, a partir de algunos ejemplos de estudios que usan especímenes de herbario, agrupados en cuatro categorías: (1) basados en datos de presencia, como por ejemplo estudios sobre extinciones o introducciones de plantas, o modelizaciones del nicho ecológico; (2) basados en los propios especímenes, tales como estudios morfológicos o fenológicos para evaluar el impacto del cambio climático; (3) basados en datos genéticos, como por ejemplo estudios filogeográficos o taxonómicos y (4) otros estudios aplicados.

Invasive alien species are currently considered one of the main threats to global biodiversity. One of the most rapidly expanding invasive plants in recent times is Kalanchoe × houghtonii (Crassulaceae), an artificial hybrid created in the 1930s in the United States by experimental crossings between K. daigremontiana and K. tubiflora, two species endemic to Madagascar. Thanks to its large colonizing capacity (mainly derived from the production of asexual plantlets), K. × houghtonii soon escaped from cultivation and quickly spread in many parts of the world. However, its actual range is not well known due to the lack of a formal description until recent times (2006) and its strong morphological resemblance with one of its parentals (K. daigremontiana). The present study was aimed, in the first instance, to delimit the present distribution area of K. × houghtonii at the global scale by gathering and validating all its occurrences and to track its colonization history. Currently, K. × houghtonii can be found on all continents except Antarctica, although it did not reach a global distribution until the 2000s. Its potential distribution, estimated with MaxEnt modelling software, is mainly centered in subtropical regions, from 20° to 40° of both northern and southern latitudes, mostly in areas with a high anthropogenic activity. Unexpectedly, concomitant to a poleward migration, future niche models suggest a considerable reduction of its range by up to one-third compared to the present, which might be related with the Crassulaceaean Acid Metabolism (CAM) of K. × houghtonii. Further research may shed light as to whether a decrease in potential habitats constitutes a general pattern for crassulaceae and CAM plants.

Challenges in the collection of vouchers, either because of the physical properties of the material (species difficult to collect or to press, e.g., palms, cycads, toxic plants, and cacti) or because drying may have a negative effect on taxonomic characters (e.g., colour), recommend the use of photographs to complement herbarium specimens. In a series of cases (enumerated in the present contribution), however, it may be impossible to collect material and, thus, photographs may be the only option to document the presence of a plant in a given locality. We propose in this paper some guidelines for the standardization of photographs intended for deposit in herbaria.

Our analyses indicate a certain degree of consistency between the medicinal uses of plants recorded in this 18th and 19th century herbarium and the records found in the literature and in recent ethnobotanical datasets, accounting for the robustness of pharmaceutical ethnobotanical knowledge in the area considered. Data appearing on the specimen labels are numerous, pointing out the herbarium as a relevant source of ethnopharmacological information. Special attention should be paid to some original uses contained in the herbarium's labels for further investigation on plant properties and drug design.