Southern South Island silver beech (L. menziesii) forests: growth rates and potential natural distribution

Every species occupies a geographic area known as its range. The range of a species is determined by the environmental niche of that species. Within the range, the environmental suitability for a species will range from low to high. Beech forests occupied a wider range 2,000 years ago, and this study explores this range before the arrival of humans in New Zealand. Relying extensively upon samples extracted from lower elevation plots, where climate extremes are less severe, this generated more complacent growth ring patterns, rendering analysis more difficult. This project is to test if changes in environmental suitability translate to predictable changes in species growth for silver beech (Lophozonia menziesii) across the lower South Island of New Zealand, (generally below the 44th parallel) by studying the relationship between growth rates across a gradient of environmental (climatic) conditions. Field data collection was undertaken on 18 different sites at 11 locations. Two core samples were extracted from 56 trees, generating 112 samples for analysis. Forest characteristics were noted, and plot photographs were taken. Cores were analysed by low powered microscope. An annual average radial growth increment of 1.24 mm was found, with a significant inverse relationship observed between annual growth and plot elevation, this lessening at higher elevations. Linkages between climate factors and plot annual growth were found, some peaks for mean air temperatures loosely matching those for total precipitation. Annual growth was found to be greater in warmer, and in drier areas, but not significantly so. Mean air temperature and total precipitation have slightly positive influences upon annual growth over the 40 year study period. Annual growth was found to be higher on south-facing slopes. Masting was considered as a biotic factor in tree annual growth. Competition from adjacent plants was found, at lower elevations, to exercise a strongly inverse effect on average annual growth but at higher elevations it was found to be slightly positive. It was found that the abiotic factors of elevation and to a lesser extent, aspect, were the major drivers of annual growth. In the past, silver beech forests, mixed with patches of New Zealand conifers, would have covered most of the study area, although patches devoid of forest occurred inland