The sheets from herbaria have more than taxonomic import. They have been used to look at aspects of physiological ecology. As mentioned ‘overleaf’, specimens taken from from the Cambridge herbarium have been used to examine how stomatal frequency has changed over the last 150 years.
The leaves of native trees in South East England now have 40% less stomatal pores than those collected at the turn of the Nineteenth Century. This seems to be a response to changing levels of carbon dioxide.
Detailed records and maps of the distribution of species over time can also be valuable in understanding how our flora is changing, the impact of invasive species and the possible effects of changing climate.
One such invasive species is the leaf miner moth (Cameraria ohridella) that affects Horse Chestnut trees. The ‘progress’ of the horse chestnut leaf miner has been reported on the web from summer 2006 to more recently, when a national survey was under way. This small, but highly efficient parasitic moth was first ‘discovered’ in trees bordering Lake Ohrid in Macedonia in 1986. It was described as species new to Europe, and has since managed to spread across almost all of Europe.
There has been a debate as to whether the moth
- was an ‘introduction’ – from an area like south east Asia, or
- whether it was a parasite that had changed its host – perhaps from sycamore or maple trees.
However, work on ‘historical’ herbarium sheets of horse chestnuts from various institutions across Europe have established that the larvae of the moth were present in foliage dating back to 1879. These early specimens were collected in Greece – a century before the moths were suspected to be present in Europe! DNA analyses of the archival DNA of various specimens indicate that the moth has its origins in the Balkans – effectively quashing the introduced species and host switch hypotheses.
The studies of this moth by David Lees (an expert on moths and butterflies at the Natural History Museum and the National Institute for Agricultural Research, France) and H Walter Lack (Botanic Garden and Museum, Berlin) demonstrate the relevance and importance of herbaria (both local and national) in studying plant – insect interactions, historical distributions of plants and their parasites, and the origins of invasive species. The leaf miner probably existed for centuries in remote valleys in the Balkans (‘home’ of the horse chestnut), but with the development of roads / transport – the moth was able to ‘escape’ and spread.*
In a similar way to tracking of the horse chestnut leaf miner, so analysis of archival DNA from 64 specimens from 11 different herbaria has helped clarify the origins of the ‘European potato’. Potatoes appeared in Europe in 1567 (Canary Isles) from South America and soon spread world wide, and across Europe.
In England, the potato began to be used more widely as a crop when it received (in 1662) an endorsement from the Royal Society, which suggested that the planting of potatoes would help prevent / offset famine. There have been two main hypotheses as to the geographical origin of the European potato; one holds that it came from the Andes whereas the other suggests a lowland Chilean origin.
Analysis of the material (which dates from circa 1600 to 1910) from the various herbaria indicates that the Andean potato was grown into the 1700’s, but the Chilean form came to dominate in early C19th (through further introductions).
The paper by Ames and Spooner (American Journal of Botany, 2008, 95(2): p 252- 257) is another demonstration of the importance of herbaria and their specimens in terms of the analysis of the origin of our modern crops and their various cultivars.
Professor David Mabberley, former keeper * of the Herbarium, Library, Art and Archives at Kew said in a recent interview: “The purpose [of a herbarium] is as a record of plants, in particular places, at particular times.” Herbaria enable scientists to create a map (in time and space) of the genetic distribution of plant material across the world, allowing them at times to reconstitute damaged ecosystems; and sometimes find relatives of ‘staple crops’ that have disease resistance.
Mabberley also added, “Who are we to destroy everything in the world? Shouldn’t we be trying to do something to conserve it?“
Why not visit your local herbarium?
For example, at the South London Botanical Institute (Tulse Hill) they have a herbarium that holds more than 40,000 sheets / specimens, a lovely garden and an excellent library.