Persistent effects of a tephra eruption (AD 1655) on treeline composition and structure, Mt Taranaki, New Zealand

Abstract

Tephra eruptions have significant long-lasting impacts on vegetation, and potentially explain extant vegetation patterns in volcanic landscapes. We quantified the effects of the AD 1655 Burrell Lapilli deposit, Mt Taranaki, on treeline vegetation. Where lapilli depth was 25–40 cm, a succession close to primary was initiated. Where lapilli depth was 5–25 cm, the canopy was opened, but some vegetation survived. Total tree basal area remains lower in affected vegetation (158 cf. 206 m2 ha−1), whereas total tree density is higher where a moderate disturbance stimulated regeneration (8433 cf. 6656 stems ha−1). Griselinia littoralis and Podocarpus cunninghamii dominate treeline vegetation (c. 51 and 39 m2 ha−1, respectively) within the lapilli distribution. Where disturbance was moderate, a cohort of Libocedrus bidwillii was also initiated. Weinmannia racemosa is absent from treeline vegetation within the lapilli distribution, despite being a dominant component elsewhere (basal area 48 m2 ha−1). Compositional patterns result from interspecific differences in morphology and resilience, as well as light, substrate and temperature tolerance. Light-demanding, cold-tolerant taxa were able to take advantage of the newly created open sites, whereas shade-tolerant, less hardy species lost their competitive advantage at the treeline elevation. The successional trajectory of the treeline vegetation has been set back and altered, and there is no evidence of convergence.