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What’s happening on the Claris fire scar?
by Riki Taylor (School of Environment, University of Auckland)

The 2013 fire at the Kaitoke dunes was an important event for the Barrier, burning through 116 hectares of regenerating scrub, putting much of Claris at risk, and requiring a large and expensive response by the fire service. But the fire also presented a valuable research opportunity.

My thesis, entitled ‘Post-Fire Community Dynamics in a Disturbed Landscape, Great Barrier Island’ aimed to take advantage of this opportunity, using it as a case study for the increasingly significant role fire is taking in New Zealand forests, especially where exotic fire-loving invasive plant species are present.

The initial aim of the research was to survey the post-fire vegetation at Claris to see if different plant communities had formed, and if so, to determine the main influences on the species that had become established. The results show a range of distinct communities have regenerated across the fire scar area, from relatively uninvaded areas dominated by mānuka (Leptospermum scoparium) regrowth, to plant communities comprised almost exclusively of exotic invasive woody species, either pine species or brush wattle (Paraserianthes lophantha).

Mānuka and a range of sedge species emerging among standing remains of mānuka that burned in 2013. This is one of the least-invaded communities at the Kaitoke dunes.

The other major invasive species present included hakea (both Hakea sericea and H. gibbosa) and berry heath (Erica baccans), with gorse (Ulex europaeus), and Banksia species seen in lower numbers. Common to all of these invasive plant and tree species are fire-tolerant traits that gave them a competitive advantage following the fire (see box below).

A secondary aim of the research was to determine the major influences on the establishment and composition of post-fire communities. The most influential factor was topographic position and the associated closeness of the surface to the groundwater table. For example, the mānuka-dominated community only occurred in low-lying areas inundated with water during winter. Alongside soil moisture soil nutrient levels were also influential, with high moisture and fertility associated with the most resilient areas where no invasive plants were found. Soil conditions of low moisture and fertility were consistent across the rest of the site, with the distribution of plant communities controlled mainly by the presence (or absence) of an invasive species prior to the fire.

Another aim was to assess change in community composition from before the fire, to two, and four years afterwards. This work revealed changes in the make-up of plant communities with important implications for the risk of future fire at the site.

A dense stand of brush wattle emerging beneath mature Pinus spp.
This is the most-invaded community at the Kaitoke dunes.

Of particular interest was that all of the major woody species appeared to have increased distributions across the site, implying a corresponding increase in flammable plant material which could fuel a fire. Brush wattle distribution expanded the most, and in especially high densities.

The intensity of another fire in this area is likely to be higher than the last one, with the continued spread of woody exotic invasive plant species (weeds) the ultimate outcome.

Looking forward, a range of management responses could be considered. Felling and removal of pine and wattle is an option which could be done now, although it is highly labour-intensive. More complex efforts involving replanting and control of pest mammals would require more planning, but less labour overall and could better incorporate community involvement.

Ultimately, the vegetation in the fire scar area will only return to a pre-fire state (that can act as part of the ecosystem of the island), if management efforts are made alongside consistent vigilance in preventing fire from occurring again.

Fire tolerance in plants

Several species of exotic invasive plants established in New Zealand have fire tolerant characteristics, influenced by the frequency of fires in their native ranges. Fire tolerance is exhibited through bark thickness and other vegetative insulation, above-ground re-sprouting, and underground roots and stems.

Bark thickness provides resistance to fire damage. Thick bark reduces the heat in the growing tissue beneath the bark. Species, such as pine, with thick bark do not catch fire or burn easily. Other plants are able to grow new shoots after fire from buds in parts of the plant that are below ground, such as Banksia species. Heat from a fire can also stimulate seed release from woody capsules. Seeds of many species of Casuarina, Hakea, Banksia, Leptospermum and Eucalyptus are encased in woody capsules capable of remaining dormant for years and resisting seed predation. Germination occurs after the seed coat is damaged in some way, such as through fire. Some species respond positively to chemicals in smoke. Species with this type of regeneration response are common in plant communities established after a fire.

Fire was an infrequent and relatively small part of the disturbance regime in New Zealand forests until the arrival of humans over 700 years ago - very few indigenous plants show clear adaptations to fire1. Post the arrival of humans in the mid-13th century, fire had a dramatic impact on the New Zealand landscape, reducing forest cover from around 90% to 25%.  Perhaps unsurprisingly, fire-adapted species in our flora tend to be of Australian origin and are fast growing, including mānuka (Leptospermum scoparium), kānuka (Kunzea ericoides), bracken (Pteridium esculentum) and matagouri (Discaria toumatou).

The numerous fire-resistant seedpods of Hakea gibbosa,
which open days after the death of the tree.

Notes: 1Perry, G.L.W., Wilmshurst, J.M., and McGlone, M. S. 2014. Ecology and long-term history of fire in New Zealand. New Zealand Journal of Ecology 38(2): 157-176.

Environmental News Issue 38 Spring/Summer 2017