Plant species do not just grow everywhere. Some species are common and are able grow under many different conditions and at many different places. Others are rare and only grow in very special or unusual places. In some nature reserves only common species can be found, while in some urban areas one may stumble upon a rare plant species. Why is that? What is the mechanism behind the rarity of some species? Plant ecologist Wieger Wamelink of Alterra has been investigating why some plant species are rare and today the results are published in the scientific journal PLOS ONE.
Wamelink and his fellow researchers collected more than 10,000 soil samples together with records of their associated plant species. They did this in order to research if rare plant species, such as orchids, have different preferences compared to common species like some of the grasses. At the same time, they looked at water table levels, nutrient availability, acidity and salt levels. “Our samples showed that rare species are choosier than commoner plant species,” said Wieger Wamelink. “It is not that rare species prefer certain circumstances, they simply don’t survive in many other places. This means that some rare plants can almost only be found in places where there is, for example, little nitrate in the soil, whereas other plant species are less selective. These less selective species force rare ones out of areas with high concentrations of nitrate. It is, therefore, how plant species react to specific soil characteristics that determines whether a plant is rare or not.”
Wamelink researched twenty-three soil characteristics: three relating to water table levels (spring, lowest and highest); humidity levels; soil acidity (pH, measured in two ways); the soil’s salt content; carbon and nitrogen and the ratio between the two; levels of potassium, calcium, natrium, ammonia, nitrate and phosphate; the total phosphorus content; the level of organic material and soil conductivity. “In this way we have worked with the most important soil characteristics that explain the occurrence of plant species. We did not, however, work with metals, though they are also of importance for plant species occurrence. We were able to determine the conditions under which 973 different plant species, including 190 red list species occur. Plant species are defined as rare if they appear on the Dutch ‘red list’ of threatened plant species. While mowing, and cutting as well as the climate and the propagation ability of a species all play a role it is primarily the soil characteristics that determine whether a plant species is present. These differences in preferences relate particularly to the concentrations of nitrate, phosphate, phosphorus, sodium and chlorine present in the soil, in addition to acidity.”
A number of the relevant soil characteristics impact directly on the problems currently troubling nature reserves, such as high nitrogen depositions particularly from airborne pollution, drying out of the soil and impacts of phosphate and acid pollution. This study has highlighted issues which are important for the conservation of our environment, particularly the conservation of rare plant species. Conservationists can now take soil samples and use them to decide if it is the soil that is preventing certain plants from surviving there. Wieger Wamelink: ”They can then, if so desired, take measures to improve the soil conditions. This could mean mowing more intensively or raising the groundwater level. Removing top soil layer and vegetation could also be an option. The conditions can be changed in order to make an area suitable for rare plants.”