An analysis of weed floras in nurseries: Do polytunnels serve as ports of entry for alien plant species?

• We tested the hypothesis that polytunnels may serve as initial point for neophytes.
• For this, we selected 15 organically producing plant nurseries in eastern Austria.
• Weed vegetation differs only little in terms of species composition and abundance.
• We conclude that polytunnels do not serve as entry points for emerging neophytes.

The introduction of alien species into new regions has become a defining feature of global environmental change. It has been argued that greenhouses and polytunnels may serve as invasion foci for emerging plant invaders of agricultural ecosystems under future warmer climates. To test this hypothesis we selected 15 organically producing plant nurseries in lowland eastern Austria. There species coverage was recorded in 73 plots inside and in 60 plots outside polytunnels. In total, 109 species were identified (including plant seedlings which could only be determined to the genus level), consisting of 27 neophytes, 6 archaeophytes and 76 native plant species. Although average weed species numbers were significantly higher inside polytunnels compared to reference plots outdoor, nonmetric multidimensional scaling revealed no significant differences in species composition. Similarly, we found no differences in the weed community indices, i.e., the proportion of species preferring low or high temperatures, low or high precipitation, and low or high soil fertility, between plots located in polytunnels and outdoor. This holds true for total species number as well as for subsets of species of different residence time. The recorded neophytes are native to Asia, Europe, North and South America at almost equal proportions, most neophytes (22 species) are introduced intentionally, while unintentionally introduced species were more abundant on average. In polytunnels, neophyte species number was dependent on the crop category only, and archaeophyte species numbers were influenced by size of the polytunnels, whereas native and total species numbers were again influenced by the crop species. In outdoor plots, archaeophyte numbers were influenced by mean annual temperature. Overall, weed vegetation in polytunnels and outdoors differs only little in terms of species composition and abundance. We thus conclude that sheltered environments (as polytunnels), which mimic warmer climates, do not serve as entry points for emerging neophytes of agricultural ecosystems.