Abstract: Competition for resources needed for survival, growth, and reproduction has long been established as an important force in shaping plant communities. Still, two topics of debate regarding competition persist: inferring competitive interactions from co-occurrence networks and identifying functional traits that can predict competitive outcomes. Species co-occurrences have commonly been examined to understand how abiotic and biotic factors (including competition) influence species distribution patterns. However, it has been suggested that co-occurrence patterns may not be adequately explained by species interactions. Evidence has also been mixed in determining which functional traits may contribute to competitive advantage. Maximum height is generally considered to be particularly important for competition. Yet, the notion of a large maximum plant height being critically important for success under severe competition falls short under examination within field studies and observed patterns in natural habitats. Here, we use a competition gradient measured by a neighbour removal experiment to test the relative importance of competition intensity, and plant height as drivers of species interactions. Specifically, we ask if pairs of plant species that significantly co-occur (positively or negatively) are likely to differ in height, or be more similar when occupying plots characterized by high competition intensity.
We found that plots in our community ranged from being characterized as highly competitive to moderately facilitative. Species in negatively co-occurring pairs that were found growing together had larger differences in their respective maximum heights; these pairs also tended to grow in plots with lower competition intensity. Pairs of positively co-occurring species that were found growing together had smaller differences in their respective maximum heights; these pairs tended to grow in plots with higher competition intensity. These patterns suggest that strong patterns of co-occurrence are, at least in part, driven by competition. Although, other factors that we did not investigate (such as dispersal and/or various abiotic factors) may also play a role. Co-occurrence patterns between species are also likely to be associated with respective differences in functional traits. Differences in height between neighbouring plants can provide an avenue for niche differentiation between species of different competitive abilities that do not typically associate with one another.
