The comparative regeneration ecology of three tree species in northern New Zealand

Abstract

The objective of the study was to quantify and compare the regeneration niches of three dominant 'mature phase' canopy species in lowland Northland. The species were Beilschmiedia tarairi (taraire) and Dysoxylum spectabile (kohekohe) of the main canopy stratum, and Agathis australis (kauri) of the canopy emergent stratum. The study objective was pursued with three methodologies. Taken together those methodologies were intended to elucidate the respective regeneration strategies of the three species. The central methodology involved recording the composition and structure of the forest canopy adjacent to selected juvenile trees of the study species using a point centred sampling system. The variables measured using this method were; the distance from the selected juvenile to the nearest canopy stem, and the height, diameter at breast height, and identity of that stem. The selected juvenile was the tallest representative of its species in the undercanopy cohorts within a given (10x10m) search area. Juveniles were segregated into six size classes in the height range .5m - 5.9m. This component of the study was conducted in the southwestern lowlands of Little Barrier Island in the Hauraki Gulf and in the upper Ohae and Waiotane Stream catchments at Waipoua in western Northland. Sampling was performed across a range of forest types which in aggregate represented almost the complete vegetation succession at both study localities. In this way the response of the three study species, in terms of their respective juvenile size distributions, to variation in canopy form with the succession could be quantified and compared. The results show that the emergent, kauri, is able to produce large juveniles earlier in the succession than are the two main canopy species, taraire and kohekohe. The latter species, by contrast, are able to produce large juveniles in the middle and late successional stages where kauri juveniles are typically small. This difference is taken to represent niche partitioning between kauri and the other two species which involves the chronological displacement of regeneration optima during the succession. The results for the two study areas were similar. However the Waipoua data do show slightly less divergence in the regeneration niche between the kauri and the other two species than was found on Little Barrier Island. The second method was an experimental study of growth from seed of the three species in three light treatments (zero, low and high light) in growth cabinets where all other physical parameters were held constant. The zero light treatment was included to provide data on the contribution to growth of the seed nutritive reserve, allowing the growth contribution from photosynthesis in the lighted treatments to be determined and compared. The results show that the contribution to growth from the seed reserve for small-seeded kauri is limited whereas the seed reserve growth contribution for large-seeded taraire and kohekohe is much more substantial. In the lighted treatments kauri was found to show significantly improved growth performance in the high light compared to the low light environment whereas taraire and kohekohe did not. The results demonstrate a shade-intolerant photosynthetic physiology in kauri and shade-tolerant physiology's in taraire and kohekohe. The combination of the adaptive features of seed nutritive reserve size and relative light requirements for growth are consistent with the different regeneration niches of the study species. Shade-intolerant kauri regenerates in the well-lit undercanopy of the early succession where the seedling is less dependent for early growth on the seed reserve. Taraire and kohekohe by contrast regenerate in the more deeply shaded undercanopy environments of the middle and late succession where their large seed reserves and shade tolerant physiology's confer an adaptive advantage. The third methodology involved measurement of the light environments of juveniles in the forest on Little Barrier Island. The selected juveniles for this work were in either seedling (.5m - 1.1m) or pole (4.7m - 5.9m) size classes. Simultaneous integrated measurements of photosynthetically active radiation (PAR) were made over a five day period using photosensitive ammonium diazo paper.