Do woody plants create ‘fertile islands’ in dryland New Zealand?

Woody plants in arid and semi-arid environments may enhance soil nutrient status, the so-called ‘fertile island’ effect, but this mechanism has never been tested in the drylands of New Zealand. In this study I investigated effects of Kunzea serotina, Discaria toumatou, Rosa rubiginosa, and Coprosma propinqua on soil properties in the drylands of central Otago, New Zealand. Soils had significantly higher organic matter under C. propinqua and significantly higher nitrate and phosphorus concentrations under K. serotina than soils in the adjacent open grassland. A bioassay using oat (Avena sativa) growth indicated higher fertility in soils from under K. serotina than from under grassland. A review of 28 other studies revealed that fertile island effects of woody plant species on soil nitrogen and phosphorus concentrations decreased significantly with increases in annual precipitation. The occurrence of fertile islands under only two of the four shrub species in the current study in a dry sub-humid environment is consistent with this trend of decreased fertile island effect with increased annual precipitation. The higher concentration of nitrogen in soils under woody plants such as C. propinqua may be explained by the plants depositing organic matter on the surface, but some species such as K. serotina, D. toumatou, R. rubiginosa and C. propinqua, may also preferentially establish in areas of high soil phosphorus availability. I conclude that the occurrence of fertile islands under woody plants may be due to both effects of the woody plant canopy, and the plants preferentially establishing in areas of high soil fertility.