Dominant plants control the largest proportion of resources in terrestrial ecosystems and affect other plants, environmental conditions, microorganisms and, ultimately, environmental services. Despite controlling a large proportion of resources, dominant plants often face increased environmental variability, such as wind stress or temperature fluctuation. If environmental uncertainty is a source of stress, the milder conditions under the dominant plants can equalize differences among species and reduce the predictability of the community. Therefore, removing dominant plants should increase short-term environmental variability and decrease beta-diversity in plant and microbial communities.
To test these hypotheses we identified and removed two dominant species in five old-field meadows 50 Km North of Toronto over 3 years. We contrasted the dominant removal with several other treatments including removing the same number of species from non-dominant species, and further removed, in two treatments, a similar amount of biomass randomly to match the biomass removed in the dominant and in the non-dominant removal plots. We replicated each treatment five times in each meadow in 1x1m2 plots, and measured plant cover and height, soil nutrients and microbial soil community (T-RFLP using 16s-rDNA) in each plot. We also measured microclimatic variables in a subsample of plots in each meadow.
In the dominant removal plots, preliminary results show Bray-Curtis dissimilarity before treatment, when focusing only in the new-dominant plants, was lower than expected by chance in every meadow, suggesting a stressful environment that reduces the species able to survive there. After the treatment was applied, the new-dominant plants further increased their similarity among plots along the experiment in four of five meadows, suggesting an increase in the constraints faced by these species in the community.
We also found that a larger standard deviation of temperature corresponded with plant height in control plots, and that this variability increases towards the soil level when dominant plants are removed, as expected. On the other hand, dominant removal treatments reduced the soil spatial heterogeneity among plots (i.e. orthophosphates and microbial communities).
Our results suggest opposite trends in the effect of dominant plants on spatial and temporal variability. This difference had cascading effects on other aspects of the community and therefore shed light on species co-existence, community composition and services provision.