Secondary succession in remnants of lowland forest on the eastern and western Hutt hills, Hutt Valley, Wellington

Abstract

Plant succession was studied in two areas of second growth forest on the eastern and western hills, Hutt Valley, lower North Island, New Zealand, about 40 years after previous research, thus contributing to two long-term ecological data sets. The two sites differed in aspect and vegetation providing an opportunity to study differences in vegetation succession between the two sites, while previous research by Druce (1957) and Croker (1949) allowed comparison of the development of vegetation over time. The secondary vegetation is broadleaved forest dominated by tawa (Beilschmiedia tawa) on the Western Hutt Hills and a mosaic of forest and scrub dominated by kamahi (Weinmannia racemosa), hard beech (Nothofagus truncata) and manuka (Leptospermum scoparium) on the Eastern Hutt Hills. Data on vegetation species composition, basal area, cover, and density in different height tiers, and soil characteristics were collected for seventy samples in Taita Scientific Reserve, on the Eastern Hutt Hills. Data on vegetation species composition, basal area, cover, and density at different height tiers for 19 samples were collected at "Woodroyd", on the Western Hutt Hills. Data were analysed using the ecological data package DECODA. The programmes TWINSPAN (Two Way Indicator Species Analysis) and DECORANA (Detrended Correspondence Analysis) were used to classify and ordinate the vegetation samples. Vectors of maximum correlation were fitted to the vegetation data for the environmental variables of soil moisture, carbon, and nitrogen content, soil pH, depth, and bulk density, site aspect and slope, and the approximate age and maximum height of the vegetation. Vegetation classification identified six main vegetation types in the Taita Scientific Reserve, associated in part with the position within the catchment. These were; Leptospermum scoparium and Weinmannia racemosa scrub; Weinmannia racemosa forest, with significant component of Nothofagus truncata; Weinmannia racemosa with Leptospermum scoparium and broadleaved species; broadleaved forest with Weinmannia racemosa and Nothofagus truncata; broadleaved forest with kanuka (Kunzea ericoides), and broadleaved forest with pukatea (Laurelia novae-zelandiae) and many ferns. When the samples were ordinated and vectors for maximum correlation with environmental factors were fitted, age of the vegetation, maximum height, soil carbon content and pH were all highly significant, suggesting that the current vegetation is attributable to a combination of all of these environmental factors, and that the patterns seen in the vegetation is not due to a gradient in any one environmental factor. Soil pH was most closely related to ordination axis one derived from DCA. Vegetation on the Western Hutt Hills is very similar in composition to the vegetation observed by Croker(1949), but largely dissimilar to the majority of vegetation in the Taita Scientific Reserve. The Western Hutt exotic plantation has a similar species composition to Nothofagus dominated vegetation, Taita Scientific Reserve, while the vegetation on the Western Hutt Hills is most similar to the broadleaved forest in the lower parts of the Taita Scientific Reserve. The development of the present vegetation at each site appears to be strongly influenced by the species present at the time of disturbance. Exotic species are unlikely to be significant in future vegetation at these sites, provided no further disturbance takes place, but may alter successional pathways. The lack of regeneration of Nothofagus truncata in the Taita Scientific Reserve or of podocarp species on the Western Hutt Hills, suggests that the vegetation may only very slowly develop from Weinmannia racemosa or Beilschmiedia tawa forest respectively, into the original vegetation types of Nothofagus truncata forest on the Eastern Hutt Hills and mixed podocarp - Beilschmiedia tawa forest on the Western Hutt Hills.