ENVIRONMENTAL CORRELATES OF TREE ALPHA-DIVERSITY IN NEW-ZEALAND PRIMARY FORESTS

Correlations between environment and tree alpha-diversity in New Zeala nd's primary forests were examined using an extensive quantitative dat aset (14 540 plots). Generalised additive models were used to examine relationships between species richness and temperature, solar radiatio n, root-zone moisture deficit, relative humidity, lithology, drainage, and plot size for all trees (112 species), and separately for broadle aved trees (88 species), conifers (17), and the genus Nothofagus (4). Diversity both for all tree species and for broadleaved trees was pred icted to be highest on sites with high temperatures, high solar radiat ion, and high soil and atmospheric moisture, and on sedimentary and ba saltic substrates. Highest conifer diversity was predicted on sites wi th intermediate temperatures, low solar radiation; high root-zone and atmospheric moisture, and rhyolitic and Quaternary substrates, particu larly where drainage was impeded. Highest Nothofagus diversity was pre dicted for sites combining low temperatures, high solar radiation, hig h root-zone moisture but low atmospheric moisture, and on granitic sub strates. Differences in diversity between the species groups on differ ent lithologies are interpreted as reflecting both the effects of vari ation in large-scale disturbance histories, and the effects of confoun ding environmental factors associated with particular substrates. Ther e were also significant interactions between species groups: both broa dleaved tree and conifer richness were predicted to be lower on sites where one or more Nothofagus spp. - all of which have marked patchines s in their distribution - are present. Although these results are cons istent with the hypothesis that tree diversity is highest on sites con ducive to high productivity, history is also indicated as an important determinant of tree diversity in New Zealand.