Julia Dieskau, from Martin Luther University Halle-Wittenberg, discusses her article ‘Phylogenetic relationships and plant life stage but not biogeographic history mediate priority effects of European grassland plants’
Background
What factors determine the composition of plant communities? Many scientists have been exploring this question from various perspectives throughout the past century. Despite their efforts, there are still aspects that we do not fully understand. Recently, the focus has shifted towards the importance of timing and the order of species arrival. Early-arriving species could benefit from a head start and exert a priority effect hampering the growth of late-arriving species, reducing their productivity and even preventing their establishment. We hypothesise that this effect should be particularly pronounced among closely related species with more similar traits and consequently, stronger competition. However, this effect may be mitigated by certain influencing factors which could have contributed to the contradictory results of previous studies. For example, species that have coexisted over a long period may have adapted to each other in such a way that closely related species may not share traits as extensively as assumed. Additionally, the effects can depend on the life stage of the interacting species, because seedlings possess different traits compared to adult plants.
Phylogenetic tree of all experimental species with their origin (green = native early-arriving species; yellow = exotic early-arriving species; black = native late-arriving species).
The study
For a better understanding of priority effects, we conducted a pot experiment in our research greenhouse. We selected species pairs that varied in their degree of relatedness and either had the same or different origins. To create more realistic conditions and avoid planting just one individual of each species, we decided to use 10-liter boxes (with holes drilled for drainage) from the hardware store instead of traditional pots. This allowed us to establish monocultures of early-arriving species with 12 individuals per box. After an establishment phase of four months, we sowed late-arriving species into these boxes and analysed their establishment and productivity.
The author conducting the weekly counting and marking of the newly emerged seedlings of the late-arriving species (photo: Ingmar Gaberle)
One of the experimental boxes containing 12 individuals of the early-arriving species (Filipendula vulgaris) and seedlings of the late-arriving species (Onobrychis viciifolia) marked with red toothpicks (photo: Julia Dieskau).
Key findings
We observed stronger competition among closely related species. This was evidenced by reduced aboveground biomass production in adult late-arriving plants when phylogenetic distance was low. Therefore, we suggest that phylogenetic distance may serve as a valuable proxy for trait dissimilarity where the labour-intensive measurement of an extensive set of traits is not feasible. However, given the varied outcomes of previous studies on the role of phylogenetic relatedness, we believe that this relationship may be situation dependent. For instance, our findings may have been influenced by the notably stronger competition for nutrients in our greenhouse experiment compared to what might occur in natural settings, because plants were growing in pots at high densities and without additional fertilization.
Positive relationship between the phylogenetic distance among early- and late-arriving species and the strength of priority effects but no differences between species pairs of the same origin (green triangles) and pairs of different origins (yellow circles).
Regardless of phylogenetic distance, the origin of the species did not influence the significance of phylogenetic relatedness. This suggests that there may have been no adaptation of sympatrically occurringspecies to each other, or that it was not evident in our experimental setting.
Furthermore, we did not find a significant relationship between the strength of priority effects and phylogenetic relatedness in the early life stages of seedling emergence and survival. In our opinion, the dependence of the importance of phylogenetic relatedness on the life stage of the interacting species highlights the significance of timing and the order of species arrival for the assembly of plant communities. Because these results may also be relevant for the management of invasive species and restoration efforts, we believe that priority effects deserve even more attention in future studies.