Evidence for arrested successional processes after fire in the Waikare River catchment, Te Urewera

Anthropogenic fire has transformed New Zealand’s vegetation. Small-scale historical Māori fires in the forests of Te Urewera National Park, North Island, initiated forest successions that were dominated early on by Kunzea ericoides (Myrtaceae), and later by Knightia excelsa (Proteaceae) and Weinmannia racemosa (Cunoniaceae). Previous work in these forests suggested that the more recent of these successions, initiated after the arrival of deer in the late 19th century, have failed to recover to pre-fire composition and structure. This failure was attributed to browsing pressure that prevented palatable canopy tree species such as Weinmannia racemosa establishing at adequate densities. We tested this hypothesis by quantifying changes in forest structure, tree species composition and above-ground tree biomass between 1980 and 2010 using permanent forest plots in three successional communities. The three communities were dominated by (1) Kunzea ericoides – kanuka, (2) Knightia excelsa – rewarewa, and (3) Weinmannia racemosa – kamahi. We show that above-ground biomass increased in all communities between 1980 and 2010, and did so most strongly in the first two communities. However, tree species composition changed little over the same time except for a more than three-fold increase in tree fern abundance in the kanuka community where they are now co-dominant with Kunzea ericoides. Recruitment of trees of canopy species was very low (≤ 0.06% per year) relative to mortality (1.1–3.9% per year) in all three communities. The proportional increase in above-ground biomass observed in the kanuka community was low relative to the large proportional increase in basal area. This occurred because of the higher abundance of tree ferns in 2010 plots that have low stem tissue density. The minimal compositional change over 30 years in these communities and paucity of recruitment of trees of canopy species point to arrested succession. Without management intervention to increase tree recruitment rates of canopy species, forest successions in this region will be characterised by high tree fern abundance, low biomass at local scales, and limited transitions to tall forest communities.