When I moved to the Netherlands from Colorado, USA, I brought only 2 suitcases filled with clothes and essentials. When I arrived, I had to rent a house, obtain a bank account, find new friends and cafes, and essentially recreate the life I’d left behind. It took to a while - I wished for a distant Dutch cousin who could have helped me integrate into the new culture - but eventually I was able to reassemble a home, very similar to what I had left in Colorado. 

Obviously, humans aren’t the only organisms that move. Worldwide, there is a constant flux and flow of organisms. In some scenarios, a plant or animal is able to thrive in its new home but it is unwanted and unappreciated by the original inhabitants (luckily this wasn’t my experience in Europe). This movement is termed ‘invasion’. In other cases, movements are more discreet; organisms move only a few kilometers up a mountain or a few hundred km across the landscape. These more inconspicuous movements are termed ‘range-expansions’ or ‘range-shifts’.

In Europe, climate change is an acknowledged culprit of the expansion and shrinking of habitats. Over the few decades, northern latitudes have warmed, drought events have increased, and the growing season has lengthened. Today, plants that 50 years ago were only found in southern, warmer climates, have now expanded and are fixtures in northern ecosystems. Yet, the reasons for and the implications of successful plant range-expansion remains a subject of uncertainty. As foreshadowed by my own story, important questions need to be addressed – what baggage does a plant bring along when it moves? does it have relatives in its new home? and will the plant establish similar or completely new friends?

In our recent study (Ramirez et al. 2019), we ask these questions in relation to the belowground home of plants: the plant microbiome. The bacteria and fungi that make up the plant microbiome are essential for plant success, but what role does the microbiome play during plant range-expansion? To begin to answer this question, we selected plants from three groups: range expanding plants that do not have relatives in the northern latitudes, range-expanding plants that have a close relative, and finally the native relative plants that are found across our sampling gradient.

Collection of the 11 different plant species and their microbiomes took 2 years. For those who aren’t plant ecologists, plants don’t bloom all at the same time (which is when we wanted to sample), and some plant species that thrive well in their new range are rare and difficult to find in their native range. Therefore, multiple trips and sampling excursions were planned to Greece, Montenegro, Slovenia, Austria, Germany and of course the Netherlands. Not that any of the collection team complained, all of these countries are beautiful during the growing season and our local counterparts gave wonderful guidance.

We found that plants do have a microbiome that associates to a specific species, regardless if the plant was growing in the south (original) or northern (new) range. However, for those range-expanding plants that did not have relatives, we observed that their microbiomes became more similar in the north – suggesting that they leave behind superfluous microbes and associate only with microbes are perhaps more cosmopolitan in the new home. This idea that microbes are left behind was supported by a weak trend where these plants also had fewer fungal pathogens.

This survey provides an essential view of how plant-soil-microbe dynamics are affected by range-expansion and can also be used to inform on the microbiomes of exotic and invasive plant species. Our results make clear that the baseline understanding of how the plant microbiome varies across species and the landscape needs further development, which is what we will continue to study. Find out more about our research from our group on belowground effects of plant range-expansion https://nioo.knaw.nl/en/van-der-putten-group.