Why are some plant species invasive in new environments, whereas others are not? An answer to this question was recently published in the journal Plos ONE in a study whose authors are Tomáš Václavík from the Department of Ecology and Environmental Sciences at the Palacký University Faculty of Science and his colleagues from the Helmholtz Centre for Environmental Research in Leipzig. Scientists arrived at the conclusion that UV radiation and the plants’ ability to protect themselves against it are significant factors for distribution of plants in certain areas. Their research combined historical and modern scientific technologies.
“Ultraviolet (UV) radiation is still an under-researched environmental factor that may help explain why certain plant and animal species are present or absent in specific regions. While UVA radiation is relatively harmless, UVB radiation is a significant stress factor on many organisms,” said Václavík. He noted that the same authors had processed data from the NASA Aqua satellite that orbits the Earth at the height of about 705 kilometres and measures the intensity of UV radiation. The available data allowed them to subsequently derive an aggregate of long-term variables for UV radiation, such as year average, seasonal occurrence, and months with the highest and lowest intensity.
Comparing plants from both hemispheres
The actual study focuses on two plants: mouse-ear hawkweed (Hieracium pilosella) and viper’s bugloss (Echium vulgare), both showing in previous laboratory investigations that they are sensitive to UVB radiation. Higher UVB levels increased the length and density of their trichomes, i.e. foliar hairs, which may potentially protect the leaf against the harmful effects of UVB. This assumption, however, was never explored in real conditions in a more extensive area. Both species are common throughout Europe, but in the Southern Hemisphere and especially New Zealand, where they had been carried in the past, they are highly invasive and in some areas they supplant the original vegetation. The amount of UV radiation is greater on the Southern than on the Northern Hemisphere, while other climatic factors such as temperatures and precipitation are at similar levels. The distribution of both species in their native range in the Northern Hemisphere and in the non-original habitat in the Southern Hemisphere therefore provides a unique natural experiment allowing scientists to study the effect of UVB on invasive plants.
“The results of the study show that foliar hairs in the herbarium specimens from New Zealand were 25% longer for Hieracium pilosella and 25% denser for Echium vulgare in records from the Southern as compared to those from the Northern Hemisphere. Although scientists could not rule out the effects of other factors, such as different genetic dispositions of the populations or microclimatic conditions, their findings suggest that the ability to protect against UVB is a potential factor determining the rate of success of invasive species in new environments,” summed up Václavík.
Source in historical herbaria
The information on foliar leaves in plants from both hemispheres were acquired from herbaria in Germany, France and New Zealand, where the department had a student on a study stay. She scanned the historical specimens, some dating back to the middle of the nineteenth century, with a special scanner. Scientists then used the images to analyse the length and density of trichomes and other morphological traits that had undergone a change. Thanks to records of specific locations of the original plant discoveries, they could relate the data to the levels of long-term UVB radiation exposure at a given place.
“The research has thus shown that the fusion of historical records and modern approaches in the form of long-distance exploration of the Earth is a promising path for future researches into the effect of environmental factors on distribution of species on the planet,” concluded Václavík.