Trade of commercial potting substrates: A largely overlooked means of the long-distance dispersal of plants

Abstract

Although long-distance dispersal (LDD) events are rare and stochastic, they are disproportionately important and drive several large-scale ecological processes; yet, we have a very limited understanding of their frequency, extent and consequences. Humanity intentionally spreads several species, which is associated with the accidental dispersal of other plant species. Although the global trade of potted plants and horticultural substrates may disperse large quantities of propagules, it has hardly been studied from an ecological point of view. We assessed the viable seed content of different types of commercial potting substrates to answer the following questions: (i) In what richness and density do substrates contain viable seeds? (ii) Does the composition of substrates influence their viable seed content? and (iii) Are there common characteristics of the species dispersed this way? We detected 438 seedlings of 66 taxa and found that 1 l of potting substrate contains an average of 13.27 seeds of 6.24 species, so an average 20-liter bag of substrate contains 265 viable seeds. There was a high variability in the seed content of the substrates, as substrates containing manure contained a substantially higher number of species and seeds than substrates without manure. Thus, this pathway of LDD is an interplay between endozoochory by grazing livestock and accidental human-vectored dispersal, implying that the diet preference of grazing animals influences the ability of a plant species to be dispersed this way. According to our results, potting substrates can disperse large quantities of seeds of a wide range of plant species over large distances. We conclude that this kind of human-vectored LDD may have complex effects on plant populations and communities; however, as this dispersal pathway is largely understudied and has hardly been considered as a type of LDD, its consequences are still unknown and further studies of the issue are of great importance.

Keywords: Growing media; Horticulture; Manure; Plant invasions; Potting soil; Traits.