Landscaping plants as invasive species in central Texas: Positive correlations with development proximity and age, but limited by aridity?

Background/Question/Methods

We investigated environmental factors that affect the distribution and abundance of non-native plant species in central Texas woodlands. Because many invasive woody species in this region are also landscaping plants, we hypothesized that the closer a site was to a developed area (e.g., a residential neighborhood) and the older the development, the more non-native woody species it would have as a result of ongoing propagule pressure. We also examined the effects of proximity to roads, creeks, and the city of Austin, as well as native species richness, slope, aspect, and distance to the nearest stream, on invasive species richness. Finally, we identified the most common invasive species in central Texas natural areas as well as those that are not yet common but may become so. Native and exotic species and vegetation type were recorded in the field. Other variables were calculated from maps and aerial photographs using ArcGIS and verified in the field.

Results/Conclusions

The most common non-native woody species were all landscaping species: two species of Ligustrum, Lonicera japonica, Melia azedarach, and Nandina domestica.  Photinia serratifolia, still uncommon, may be becoming a problem.

The number of non-native woody species was greater in plots that were closer to developments, closer to roads, and closer to central Austin (a surrogate for development age). In contrast, relationships between these variables and number of native woody species were much weaker.  These results support our initial hypothesis that landscaping is likely an ongoing source of propagules. If so, there may be an ‘invasion debt’ and we can expect the abundances of these non-native species to increase rapidly in more recently developed areas.

The number of  non-native woody species per plot was highest in riparian plots, while native richness was highest in non-riparian woodland plots. Both native and non-native woody species richness was lower in plots further from streams, but the effect on non-natives was greater: a drop of 36% versus 14% per 100 m. These results suggest that at least some of the common non-native invasive species may require a relatively mesic habitat and may not spread to more xeric habitats in the region