Science - August 28, 2014

Rare plants are choosy

Roelof Kleis

Why do you see some plants everywhere while others are hard to find? What are the factors that make a plant rare? The answer is in the soil, says Alterra ecologist Wieger Wamelink.

Rare plants are choosier about the soil they grow in. If it is slightly too alkaline or contains too little calcium or magnesium, or too much phosphate or humidity, they give up the ghost. The answer sounds obvious. After all, if they were not so fussy they would not be so rare, would they? To some extent, Wamelink agrees with this. ‘You sense intuitively that rarity has something to do with choosiness. But it is not quite as simple as that. Plants can be rare for a number of different reasons. They 

might not be good at propagating themselves, for instance. Management could be a key factor, or the climate.’ But the main factor is the soil. If the soil parameters are not suitable, a species can forget it. 

And Wamelink knows the parameters of the vast majority of the plants in the Netherlands, thanks to a huge database he has amassed in the last 20 years. To do this, he researched the soil profiles of 973 plants, 190 of which were rare species. He looked at 23 characteristics, including humidity level, acidity and the availability of nutrients. This delivered 973 x 23 ‘response curves’ from which it is possible to deduce statistically which levels are optimal for a particular plant. 


Together with statistician Paul Goedhart (PRI Biometris), Wamelink came up   with a nice way of depicting this: a wheel with each spoke representing a parameter. The difference between common and rare species can be seen at a glance: the rare species are choosy and have short spokes. The evidence is equally clear statistically, says Wamlink. The rare species are significantly choosier for all 23 soil variables. In other words: per soil parameter they only tolerate a small range around their optimum level.

Give me a soil sample and I can tell you whether rare species can grow on it, and which ones
Wieger Wamelink

‘This provides the evidence that soil parameters are important if you want to conserve a particular species,’ says Wamelink, underlining the significance of his work. What is more, he can make predictions. ‘Give me a soil sample and I can tell you whether rare species can grow on it, and which ones. I can provide support for management proposals and tell you whether there is any point in certain measures.’