Potential for Regeneration in Gaps in Pohutukawa (Metrosideros excelsa) Monoculture on Tiritiri Matangi Island, New Zealand

Abstract

In 2010 a long term study was initiated by the Supporters of Tiritiri Matangi in an attempt to increase biodiversity in pohutukawa (Metrosideros excelsa) monoculture on Tiritiri Matangi Island, New Zealand. This study involved the felling of pohutukawa to create small (1 tree) and large (2-3 trees) gaps across seven sites varying from 50% to >95% pohutukawa. A study to investigate the potential for regeneration within the created gaps was conducted. Sensor loggers were set up within two different sized gaps to periodically measure light and temperature. Light levels within both the small and large gaps were found to be significantly different from both full exposure and canopy cover, ranging between 12.4% (± 0.5) and 29.3% (± 0.9) of full sunlight in small gaps and between 16.0% (± 1.3) and 52.4% (± 2.0) in large gaps. Temperatures in the centre of the small and large gaps reached close to those of full sunlight exposure for extended periods of time. Over the small gap study period (mid December 2011 – mid January 2012) temperatures of >45°C were recorded continuously for 30 minutes or longer throughout the majority of the gap, with the exception being the northern edge of the gap. Temperatures of >45°C were recorded continuously for 30 minutes or longer in the centre of the large gap, however this study period occurred in late spring as opposed to summer (November 2011). Spatial patterns in light and temperature within the gaps will limit the potential for seed establishment and growth to specific parts of the gaps, dependent on the shade tolerance of different tree species. Bird monitoring was undertaken to find potential for seed dispersal into the forest gaps. Bird species richness was lowest in the high monoculture sites and highest in the mixed vegetation sites. Bird abundance was significantly higher in the mixed vegetation sites than in the monoculture sites. Presence of forest gaps had no significant effect on bird behaviour. Overall potential for seed dispersal into forest gaps is very low in the high monoculture areas. Selective felling and promotion of pohutukawa flowering in the monocultures are suggested as possible tools to promote regeneration and increase biodiversity in the pohutukawa monocultures on Tiritiri Matangi.