Summer Drought Reduces the Growth of Invasive Tree-of-Heaven (Ailanthus altissima) Seedlings

Tree-of-heaven (Ailanthus altissima) is an invasive species in North America that is becoming widespread in the eastern part of the continent. In the drier, western interior of North America, however, the spread of tree-of-heaven appears to be limited to areas where supplemental water is available such as in urban areas or in riparian environments. To explore the effect of summer drought on tree-of-heaven seedlings and to look for genetic variation for drought tolerance among half-sibling families, we collected samaras from 12 open-pollinated mother trees growing in Utah County, Utah, and measured the samaras' area, maximum width, length, and weight. We then planted 10 samaras per mother tree into 10-cm pots in the greenhouse, measured the height of each resulting seedling, and randomly assigned them to a watering treatment (well-watered or droughted) that lasted for 20 wk. We examined the relationship between tree-of-heaven samara size parameters and seedling height before we started our drought treatment and for additional growth parameters after our drought treatment. Several growth parameters (relative growth, final height, final dry mass, final stem dry mass, final dry root mass, and root-to-shoot ratios) were reduced significantly by drought but differed among seedlings from different mothers only for relative growth. There was no genetic variation for drought tolerance observed for any of the growth parameters. These results suggest that tree-of-heaven seedlings from this part of western North America are uniformly and negatively affected by drought and may explain the relative dearth of tree-of-heaven in the interior West.