|dc.description.abstract||On August 6, 2012, a series of volcanic eruptions occurred on Mt. Tongariro in the central North Island of New Zealand. The eruptions included pyroclastic density currents (PDCs), a debris flow and volcanic projectiles, each of which significantly affected native vegetation. The present study investigated four aspects of vegetation dynamics in relation to the eruptions. First, the effect of PDCs on the subalpine conifer, Phyllocladus alpinus. Second, differential species sensitivity to the PDCs. Third, the species composition of the impact craters formed by volcanic projectiles, and the fourth aspect, colonisation of a newly formed debris flow.
Thirty P. alpinus individuals were sampled at twenty two plots throughout the area affected by the PDCs. The numbers of “Live” and “Dead” individuals within plots were significantly correlated with distance to the eruption, and tree height (a predictor of age). Smaller and younger P. alpinus were more likely to be “Live” while larger, older P. alpinus more likely to be “Dead”. This is most likely due to the exposure and temperatures associated with PDCs. The mean foliar cover for all “Damaged” individuals was 75.59% with these individuals significantly correlated to plot area and altitude. Increased light availability and the path of the PDCs may explain these findings.
Phyllocladus alpinus was the species most affected by the PDCs with only 57.73% foliar survival. Species sensitivity appears to be based on a combination of height and morphological features. Taller species are most affected by PDCs with small, coriaceous or glossy leaves; or narrow flexible leaves being advantageous. Stiff curved leaves, hairs, scales and a dense growth form appear detrimental to foliar survival.
Species richness and percent cover were measured, both inside and directly outside the impact craters in 2013, 2015 and 2017 respectively. Outside the craters there was an increase in species richness with each year of measurement, however, the percent cover decreased from 2013 to 2015 before increasing in 2017. This decrease is most likely attributed to a statistically significant decrease in the cover of Dracophyllum recurvum. Between 2013 and 2015 there was a decrease in both species richness and percent cover inside the craters. This trend reversed between 2015 and 2017 with a statistically significant increase in species richness, and an increase in percent cover. This suggests species survived the initial impact of the projectile but experienced subsequent dieback followed by regeneration.
Thirty 2 x 2 m sites were sampled on the debris flow, with nine sites containing vegetation. The overall colonisation rate was 0.14 species/quadrat per year, with Poa the most frequent and abundant genus. Topography, substrate particle size and proximity to seed sources appear to constrain seed dispersal and germination, resulting in varying and low rates of colonisation.
The present study highlights that species responses vary depending on the nature and magnitude of volcanic disturbance. Further research, including remeasurements, will continue to improve the understanding of mechanism/s that drive ecological succession after volcanic disturbance, and contribute to the development of future models of succession.||