Response to Reduced Irradiance of 15 Species of Native and Adventive Shrub and Tree Seedlings from Eastern Canterbury

Seedlings of fifteen species of shrubs and small trees, commonly found in open sites and early stages of secondary succession, were grown in a glasshouse under light intensities of 16% and 66% full daylight, and their growth parameters (height, number of leaves, dry weight, mean relative growth rate) recorded. Three species from open habitats, Coprosma robusta and Dodonaea viscosa, and an adventive shrub, Crataegus monogyna, had the highest mean relative growth rates in 66% daylight and 16% daylight, but their ranking for other parameters (e.g. height) was variable. Acer pseudoplatanus and Plagianthus regius had particularly fast height growth. Carpodetus serratus was the fastest growing species of early successional vegetation on weakly to moderately leached soils, and it grew as well in 16% daylight as in 66% daylight. Many species had significantly faster height growth in 16% daylight, e.g., Melicytus ramiflorus and Pittosporum tenuifolium, and several species had faster height growth rates overall, in 16% daylight, e.g., Myrsine australis, Melicytus ramiflorus, and Pittosporum eugenioides. Griselinia littoralis, from moderately to strongly leached soils, had the slowest height growth rates in both light levels. All species had lower 'root:shoot ratios and a greater proportion of dry weight in leaves in 16% daylight. Such responses may contribute to the slow establishment of these species in secondary vegetation experiencing summer drought in the seedlings' rooting zone. Growth patterns are also likely to be adaptations to factors other than light, particularly soil fertility. There is some evidence that seedlings of the earliest successional and less shade-tolerant species, especially those from weakly leached soils, grew more rapidly than those from later-successional vegetation and edaphically harsher sites. Some shade-tolerant species benefit from slight shade which concurs with their later entry into secondary vegetation.