Plants on the move: plant-soil interactions in poleward shifting plant species

Abstract

As a result of recent global climate change, areas that have previously been climatically unsuitable for species have now become suitable new habitats. Many plant-species are expanding their range polewards, colonizing these newly available areas. If these species are able to expand their range faster than their natural enemies they can become released from these limiting factors. A similar mechanism has been reported for invasive plant species, introduced into foreign continent, which are often found to be released from natural enemies.
An example of an invasive plant species that is introduced into a foreing contintinent is Carpobrotus edulis. This species was found to be negatively affected by the soil community collected in the native range, while the soil communities from the invaded range did not have an effect on plant performance compared to a sterilized control. I hypothesized that a similar reduction of the negative effects of the soil community can occur when plant species shift their range. This hypothesis was tested in a greenhouse experiment. I compared plant-soil feedbacks of three plant species that have recently expanded their range into The Netherlands, with three related native species. The non-native species experienced a significantly less negative effect of plant-soil feedback than the native plant species.
Concurrently with these range shifts local climate is changing and this might affect plant-soil feedback as well. In order to test this plant-soil feedbacks of six range expanding and six related native species were compared at two temperatures, 20°C and 25°C daytime temperature. While again native species showed a more negative plant-soil feedback than the non-native species, temperature did not affect the strength or direction of plant-soil feedback.
Besides pair wise comparisons between native and non-native species in the invaded range, comparisons between the native and non-native range of a range expanding plant can be used to test for effects of range shifts on plant-soil interactions. Rhizosphere soil was collected from populations of Tragopogon dubius in both the native and the recently colonized range. The soil communities from the native range had a more negative effect on plant performance than the soil communities from the invaded range as compared to sterilized controls. T. pratensis, which is native to the entire studied range, did not show this pattern.
As plant-soil interactions are the net effect of many positive and negative factors the less negative effect of plant-soil feedback can be either a result of more positive or less negative effects of the soil community. One of the mutualistic groups of organisms, the arbuscular mycorrhizal fungi (AMF) are known to be a major factor contributing to ecosystem functioning and to the maintenance of plant biodiversity and the most important soil-borne mutualists for many plants. I therefore focus on this group of soil organisms. I compared the association of T.dubius with AMF in the new part of its range with T. pratensis native to this area. Three measures for plant-fungal affinity were compared between these two plant species; the density of AMF propagules able to colonize the plant, the percentage of root length colonized by arbuscular mycorrhiza, and the composition of the resulting AMF community in the roots. This was done for four replicate soil inocula from different sites in The Netherlands. The two plant species did not differ in any of the tested factors. As there are no differences in the association with the most important mutualist the observed differences in plant-soil interaction are likely an effect of release from negative components in the soil community, e.g. soil pathogens, but further studies are needed to test this.
Alterations in biotic interactions, through climate change and range shifts, such as a release of soil-borne natural enemies, can have significant effects on the performance of plants. Predictions of future ranges and impact of range expanding plant species on invaded ecosystems can therefore not be accurately made without a thorough understanding of its biotic interactions and the way these interactions are changed by the range shifts.