Raoulia australis is a pioneer cushion plant, but not a facilitator, in the Old Man Mountains of New Zealand

Background/Question/Methods

Cushion plants are well known facilitator species in many environments. I tested the degree to Raoulia australis is an ecosystem engineer, facilitator, or pioneer species, in the Old Man Range of Central Otago in New Zealand.  I measured size class distribution, dieback, soil pH, soil moisture, rock affiliation, and species richness both inside and outside of 50 individuals of Raoulia australis.  Area hoops of 10, 20, or 30 cm diameter were placed on cushions and within 1 meter of cushion edge in a random direction to assess differences in species richness.  Percent cover of vegetation within and outside of cushions was also assessed.  If R. australis are ecosystem engineers, then abiotic aspects should differ within and outside of cushions.  If R. australis are facilitators, then species richness should be greater within cushions than outside of cushions.  If these trends are not apparent, then R. australis may be a pioneer species that will soon be outcompeted. 

Results/Conclusions

Density of plants was 9.68 plants per square meter.  Size class distribution showed a left hand skewed normal distribution, indicating average germination and longevity, but difficulty in persistence.  Percent dieback was not related to size of the plant (R² = 0.0393, p = 0.17).  Soil pH ranged from 6.4 to 7.1 and did not differ with plant perimeter.  Soil moisture also did not differ with plant size (R² = 0.0096).  Within cushions, average species richness was 5.9 (+/- 1.8 SE), and outside of cushions, average species richness was 5.42 (+/- 0.87 SE), and there was no difference in average species richness as tested by a T-test (p = 0.80).  Percent cover of species within cushions did not vary by size, but was lower within cushions that outside of cushions for three size classes (p < 0.05).  These results suggest that R. australis is not a foundation species at this elevation in New Zealand but is instead a pioneer species.