Woody native and exotic species respond differently to New Zealand dryland soil nutrient and moisture gradients

In many New Zealand dryland grass and shrubland areas, native and exotic woody species are invading, but it is unclear what environmental factors favour native dominance. One possibility is that differences in soil nutrients and moisture, or a combination of these factors, differentially affect the growth and hence invasive potential of native and exotic woody dryland species. We tested the prediction that native woody species outperform exotic woody species under low-nutrient and dry soil conditions. In a pot experiment, we measured the relative growth rate and root:shoot ratio of six exotic and eight native species across moisture and field-derived soil nutrient gradients. Using plot survey data from the South Island, New Zealand, we then modelled the relative occurrence of exotic woody species, using derived measures of temperature, soil nitrogen, moisture and disturbance. All seedling growth responses we measured were affected by soil moisture and nutrient status, and the same set of species were fastest-growing across treatments. Contrary to predictions, native and exotic woody species performed similarly in low-nutrient soils, and native species grew faster than exotic species in soils with higher total nitrogen levels. Two native woody species (Ozothamnus leptophyllus and Kunzea ericoides) performed better than all woody exotics across all nutrient and moisture levels. Survey data showed that a higher proportion of exotic woody species were present at warm, dry sites with low levels of soil nitrogen. This study suggests that exotic woody species in New Zealand drylands are not necessarily superior in their growth rates, and that rehabilitation efforts favouring high-performing native woody species may stand the best chance of creating a native-dominated shrubland.