The ecology of Nothofagus menziesii in the Catlins ecological region, South-east Otago, New Zealand

Abstract

In the Catlins Ecological Region of south-east Otago, New Zealand, Nothofagus menziesii is present as a continuous body of forest surrounded by other vegetation types lacking Nothofagus, and as a number of relatively small stands isolated within other vegetation types. The structure of most Nothofagus communities in this area has not been previously described, and little was known of N. menziesii ecology, especially at the interface between vegetation types. This study was undertaken to describe Nothofagus communities and adjacent vegetation types, to determine the characteristics of the interface, and to define the factors which control the seed dispersal, establishment and growth of N. menziesii. An account is given of post-Pleistocene changes in N. menziesii forest distribution and extent. A systematic survey of nine sites was undertaken, encompassing the altitudinal and geographical range of N. menziesii in the region, and examples of interfaces with different vegetation types. Information was recorded on flora, species population structure, vegetation structure, soil depth and major nutrient levels, and site physical characteristics. Vegetation types were defined by classification analysis of species and sample plots, and by comparison of other sample plot characteristics. They were compared with similar types described by other authors elsewhere in New Zealand. Density of seed fall, seed weight, soundness and viability were recorded at intervals up to 250 m from the boundary of Nothofagus and adjacent forest types at two sites. Seedling densities were recorded at six sites to determine the effects of altitude, slope, aspect, vegetation cover and substrate composition on establishment. Seedling mycorrhizal status was investigated. Germination and establishment of N. menziesii were tested in soils from Nothofagus, Weinmannia-Metrosideros and podocarp forest types. Seedling establishment was investigated on four soil horizons and under four shade regimes. N. menziesii growth rates were determined by counts of annual growth rings on seedling stems and increment cores from trees >/ 10 cm dbh. The effects of site location, physical and vegetation characteristics on growth rate were assessed. The population structure of N. menziesii was compared between vegetation survey sites and between different altitudes. Overall population structure was described. It was concluded that during a period of post-Pleistocene podocarp dominance Nothofagus menziesii was displaced inland and to higher altitutdes, but began to replace podocarp forest after c. 4000 yr BP. The extent and distribution of N. menziesii forest were not substantially altered by Polynesian or European man. N. menziesii adjoins at least five different vegetation types in the study area. It can establish in all five, and is displacing them except on relatively warm, fertile or wet sites.