Factors affecting growth and distribution of kauri (Agathis australis Salisb.) III. Effect of temperature and soil conditions

Abstract

Kauri (Agathis australis) seedlings were grown under controlled conditions of temperature, soil moisture, and exposure to mycorrhizal infection. High temperatures inhibited root growth; seedlings exhibited a growth optimum at 23–26°C; the evidence suggested that both trees and seedlings were dormant when the maximum temperature fell below 17°C. Seedlings were generally resistant to both waterlogging and drought: waterlogging caused a decrease in root length and branching, but did not affect the root weight; drought increased root development at the expense of the stems. In the field, seedlings were liable to be killed by drought in the first few months of growth. The "high light intensity limit" to seedling establishment previously observed in the field was apparently caused by greater solar radiation increasing soil desiccation during this critical period. Seedlings grown in sterile soil and those exposed to mycorrhizal infection showed no significant differences. The autecology of kauri is discussed in the light of current knowledge. The altitudinal and latitudinal limits could be caused by temperature limiting the rate of growth and also the length of the growing season. Kauri appears to occur preferentially on ridge-top sites because of its tolerance of poor soils and its resistance, once established, to drought. The inability of kauri seedlings to establish under cover of the mature tree is thought to be caused by root competition effects. Kauri is no longer regarded as a seral stage in the development of a hardwood climax, but as one component of a "regeneration complex".