Alexander von Humboldt (1759-1769) is often considered the father of biogeography because of his crucial work on this subject in South America, and his writings and diagrams showing the link between terrain and vegetation (Humboldt & Bonpland, 2009). However, even during the Renaissance it was becoming obvious that terrain and climate greatly influence plant life. Many of the plants of northern Europe turned out to be different from the Mediterranean plants described by ancient authors including Theophrastus and Dioscorides. Place mattered and as botanists went on field trips and collected specimens, they become more aware of local differences in plant habitats. Now plant distribution is a major focus for botanists and ecologists, and herbaria are important in documenting what grows where. Species distribution maps are a staple of floras.
Today, with the combination of GPS coordinate mapping and digitization of specimen data, it’s possible to generate distribution maps relatively easily from online herbarium data. However, there is a question as to how accurate these maps are. As with any output, the answer depends upon the input: the accuracy of the localities. There is software such as GEOLocate and the MaNIS Calculator that will generate the probability of a plant being within a particular radius. Doing georeferencing well is time-consuming, and there are millions of sheets requiring attention. Even if all the data were optimal, there is the question of what percentage of the sheets are available online? If the species distribution maps are based on online data, and that only represents, say, a quarter of the specimens, then how accurate is it? In addition, there is collection bias. In other words, rarely are regions sampled uniformly. Studies show that areas accessible via roads, rivers or railroads are more likely to be thoroughly canvased than are more remote locales, so all the error can’t be blamed on computers (Vetter, 2016). Some of the best studies are those that combine herbarium data with direct observation in an area (Martin et al., 2014; Mohandass & Campbell, 2015).
At the present there can’t be any discussion of biogeography without bringing up climate change. At the moment, interest in this topic is driving not only research on herbarium collections but also their digitization. Herbaria are among the few places where there are records that range over more than 100 years, in some cases 200-400 years, and they become more valuable by the day. Records can be studied in a variety of ways to glean information on climate and habitat characteristics in the past. The area that has been developed most substantially is phenology, the study of natural events that can be pinpointed in time. For plants this is often budding, flowering, or seed setting. Though this has not always been true in the past, it is now standard herbarium practice to collect specimens that are in flower or have seeds or fruit. Since collection data are always recorded for specimens, researchers can track flowering or fruiting over the years to detect differences that might be related to climate change. Again, if digitized and imaged specimens are used, the study is limited by the richness of the sample, but it does make for more efficient investigation. There have been several studies validating the methods used in this research (Davis et al., 2015; Spellman et al., 2016). To date, changes in flowering times have been noted in orchids (Molnar et al., 2012), eucalypts (Rawal et al., 2014), and perennial herbs (Matthews & Mazer, 2015) among others; changes were also recorded in leaf-out times for trees (Everill et al., 2014; Zohner & Renner, 2014). With projects like NSF’s iDigBio, which has resulted in a portal with access to over 75 million natural history specimens, the problems of small sample sizes are dwindling, but still significant.
Still other ways to document climate change with specimens include counting the stomata on leaves. This is obviously more time-consuming, but the number of studies in this area suggest that it’s considered a fertile area of investigation. Stomata are the pores in a leaf through which plants absorb CO2, the fuel for photosynthesis. Since global warming is precipitated by the increase in atmospheric CO2 due to fossil fuel combustion, it’s not surprising that in several species, there has been a decrease in stomatal density. Plants just don’t need as many pores to absorb the same amount of CO2. In addition, leaves can give evidence of other types of environmental change. An alteration in leaf area over the past three centuries has been noted for some species (Peñuelas & Matamala, 1990). Also, there was a marked decrease in leaf sulfur levels in the years after the clean air act was passed and an increase in nitrogen with increased use of fertilizers (Peñuelas & Filella, 2001). Someone has even studied the distance over time between leaf teeth in the mulberry Hedycarya angustifolia over the past 160 years to see if an increase in temperature had affected this trait. However, despite the painstaking analysis, researchers couldn’t find any significant change (Scarr & Cocking, 2014). This work shows how many ways climate change can affect plants, and how clever botanists and ecologists employ varied plant characteristics in collecting data on this phenomenon. Having different types of data strengthens the case for the significant impact of climate on plant growth and also on the survival of species. I’ll present more evidence for this in the next post.
Davis, C. C., Willis, C. G., Connolly, B., Kelly, C., & Ellison, A. M. (2015). Herbarium records are reliable sources of phenological change driven by climate and provide novel insights into species’ phenological cueing mechanisms. American Journal of Botany, 102(10), 1599–1609.
Everill, P. H., Primack, R. B., Ellwood, E. R., & Melaas, E. K. (2014). Determining past leaf-out times of New England’s deciduous forests from herbarium specimens. American Journal of Botany, 101(8), 1293–1300.
Humboldt, A. von, & Bonpland, A. (2009). Essay on the Geography of Plants. (S. T. Jackson, Ed., S. Romanowski, Trans.). Chicago, IL: University of Chicago Press.
Martin, M. D., Zimmer, E. A., Olsen, M. T., Foote, A. D., Gilbert, M. T. P., & Brush, G. S. (2014). Herbarium specimens reveal a historical shift in phylogeographic structure of common ragweed during native range disturbance. Molecular Ecology, 23(7), 1701-1716.
Matthews, E. R., & Mazer, S. J. (2015). Historical changes in flowering phenology are governed by temperature × precipitation interactions in a widespread perennial herb in western North America. The New Phytologist, 210(1), 157-167.
Mohandass, D., & Campbell, M. J. (2015). Assessment of Roscoea population size in the Central Himalayas based on historical herbarium records and direct observation for the period 1913-2011. Journal of Biological Records, 0022015, 10–16.
Molnár, A., Tökölyi, J., Végvári, Z., Sramkó, G., Sulyok, J., Barta, Z., & Bronstein, J. (2012). Pollination mode predicts phenological response to climate change in terrestrial orchids: A case study from central Europe. Journal of Ecology, 100(5), 1141–1152.
Peñuelas, J., & Filella, I. (2001). Herbaria century record of increasing eutrophication in Spanish terrestrial ecosystems. Global Change Biology, 7(4), 427–433.
Peñuelas, J., & Matamala, R. (1990). Changes in N and S leaf content, stomatal density and specific leaf area of 14 plant species during the last three centuries of CO2 increase. Journal of Experimental Botany, 41(9), 1119–1124.
Rawal, D. S., Kasel, S., Keatley, M. R., & Nitschke, C. R. (2014). Herbarium records identify sensitivity of flowering phenology of eucalypts to climate: Implications for species response to climate change. Austral Ecology, 40(2), 117-125.
Scarr, M. J., & Cocking, J. (2014). Historical responses of distance between leaf teeth in the cool temperate rainforest tree Austral Mulberry “Hedycarya angustifolia” A. Cunn. from Victorian herbarium specimens. Victorian Naturalist, 131(2), 36–39.
Spellman, K. V., & Mulder, C. P. H. (2016). Validating herbarium-based phenology models using citizen-science data. BioScience, 66(10), 897–906.
Vetter, J. (2016). Field Life: Science in the American West during the Railroad Era. Pittsburgh, PA: University of Pittsburgh Press.
Zohner, C. M., & Renner, S. S. (2014). Common garden comparison of the leaf-out phenology of woody species from different native climates, combined with herbarium records, forecasts long-term change. Ecology Letters, 17(8), 1016-1025.