Spatial and temporal dynamics in degraded forest landscapes, Coromandel, New Zealand

Abstract

Forest degradation is viewed as a serious global and local issue and there are strong calls to halt further degradation and restore previously degraded forests. In New Zealand, first Polynesian and then European arrival led to the loss and degradation of extensive areas of New Zealand’s indigenous forests through fire and logging. This exploitation phase has since largely ceased but it is important to understand whether degraded forests will recover to their original structure and composition with or without active management so that options for conservation and restoration can be generated. The Kauaeranga Valley (c. 1500 ha of dissected terrain) consisted of mixed podocarphardwood forest in the centuries preceding human settlement, after which forest degradation began with Polynesian burning, followed by partial logging, mostly for kauri (Agathis australis), and further scattered burning by Europeans from the 1870s to the 1920s. These degraded forests have partially recovered, although invasive mammalian herbivores have also had adverse effects on these vegetation communities. My thesis aimed to (1) assess spatial variation in vegetation composition across the Kauaeranga Valley forest landscape and (2) examine temporal changes in forest structure and composition in response to disturbance by fire, partial logging, and selective browsing by brushtail possums (Trichosurus vulpecula) and goats (Capra hircus). Landscape-scale vegetation patterns were described from a dataset of 103 plots occurring on varied topography and elevation (although all below 400 m a.s.l; i.e. lowland forests). Their relationships with the environment were examined using indirect gradient analysis techniques. I then assessed temporal changes in forest structure and composition in response to fire, partial logging, and selective browsing using permanent plots established between 1980 and 1993 from which I have between 21 to 39 years of change data. Compositional variation occurring across the Kauaeranga Valley forest landscape appears to be primarily related to disturbance type and elevation. Key vegetation communities identified were low-elevational forests dominated by kānuka (Kunzea robusta), tōtara (Podocarpus totara), and rimu (Dacrydium cupressinum) that were recovering from fire, higher-elevational forests dominated by tōwai (Weinmannia sylvicola) and tāwari (Ixerba brexioides) that were recovering from partial logging, and mid-elevational forests dominated by northern rātā (Metrosideros robusta), pukatea (Laurelia novae-zelandiae), tawa (Beilschmiedia tawa), tōwai, and nīkau (Rhopalostylis sapida) that were generally recovering from selective browsing, although some were influenced by fire as well. Following disturbance by fire, kānuka and tōwai were the dominant pioneers. Their decline through self-thinning and senescence initiated a period of recruitment of later-successional podocarps (e.g. tōtara, rimu) and hardwood species (e.g. tawa, hīnau Elaeocarpus dentatus). However, light-demanding podocarps were often rare which appeared to be due to shaded environments created by high tree fern densities. Since partial logging, tōwai has dominated the early stages of recovery while tree ferns were often common and again inhibited the establishment of light-demanding podocarps and favoured that of shadetolerant hardwood species which are expected to become dominant in future. Kauri regeneration is rare to absent and I have hypothesised that this is due to the highly selective and uncontrolled nature of logging which has created unfavourable conditions for recruitment and is preventing regeneration towards a pre-logged landscape. Stands affected by selective browsing are now dominated by less palatable species, however, since control operations commenced, some palatable species have shown signs of recovery. Kohekoke (Dysoxylum spectabile), which is highly favoured by brushtail possums and was dominant prior to their arrival, suffered complete mortality of all size classes in the early 1990s and has not been able to recover probably due to an inadequate seed supply. In the absence of further degradation, these forests will continue to partially recover towards their original composition and structure. However, active restoration may be needed to aid the regeneration of podocarps, kauri, and kohekohe.