OOS 53-3 - Differences in plant functional traits related to competitive ability can explain priority effect strength within a set of grasslands species

Plants compete for resources such as space and nutrients. However, competition will often be asymmetrical, since individuals will not arrive at exactly the same time. The effects of early arrivals on late arrivals are called priority effects. Likely, species will have a different experience competing with already established plants than with plants that have arrived at the same time as them. The aim of this work is to use plant functional traits to gain mechanistic understanding of the difference between symmetric competition (from simultaneous arrival) and asymmetric competition (from sequential arrival)

We performed a potted experiment with 15 grassland species (forbs, grasses, legumes). All species were grown together in all possible combinations and in all possible arrival orders, with two species per interaction. Seven functional traits were summarized in a PCA. The first axis reflected the fast-slow and leaf economic spectrum, which could also be interpreted as the competitive strategy (e.g. growth rate, specific leaf area, leaf dry matter content), and the second plant form and stature (e.g. root shoot ratio, clonal spread and plant height).

The PCA axis for competitive strategy was more explanatory for the outcome of symmetric and asymmetric competition compared to the form and stature axis. That is, we found that under simultaneous arrival, the outcome of competition mostly depended on how each species itself scored on the competition-trait axis and much less on the scores of their neighbor. In other words, species with a lower score on the trait axis experienced stronger competition, but this was almost independent of who grew next to them. Under sequential arrival however, both the score of the first and second arrival were important for the outcome of competition. There seemed to be a threshold; second arrivals were not able to compete when their score was lower or equal to the first arrival. Only when the species arriving second was the stronger competitor priority effects decreased.

Because seed dispersal is a highly stochastic process, it is quite likely that asymmetric competition (sequential arrival) is more common than completely symmetric competition (simultaneous arrival). Reflecting this in competition experiments is therefore important for improving our understanding of vegetation development. In this research I showed that plant functional traits were better able to explain competition outcomes in sequential arrival compared to simultaneous arrival.