NATURAL REGENERATION DYNAMICS OF PINUS-MERKUSII IN NORTHERN THAILAND

The effect of ground fire and subsequent exposure of mineral soil on g ermination and early development of Pinus merkusii Jungh. et de Vriese , and morphological development throughout the grass stage of naturall y borne seedlings were studied in northern Thailand, within the natura l distribution range of the species. The study was carried out with th e major aim to find out whether and to what extent the natural regener ation of mainland P. merkusii is controlled by fire, and what the main adaptive advantage is of the prolonged grass stage characteristic to the species. Finally, silvicultural implications of the findings are d iscussed. We found that the total volume of stem and tap root in the g rass stage controls the shoot growth rather distinctly: seedlings emer ge from the grass stage soon after a certain minimum volume (approx. 1 5-20 mm diameter at ground level) is reached. The growth of the tap ro ot is steady throughout the grass stage. Formation of the deep tap roo t promotes the volume growth of the stem. Two-thirds of the diameter o f the short stem is formed by cortex, which functions as a carbohydrat e store to be later utilized in the fast shoot growth. There were no s ignificant differences in germination and early development of P. merk usii seedlings between a burnt and an unburnt area. Considerable numbe rs of seedlings (11042-34250 ha(-1)) were found in stands burnt 2, 4 a nd 6 years previously. P. merkusii seeds germinate well beneath a dens e cover of grass, giving rise to an abundant ephemeral seedling stock. Mortality due to competition by grass is, however, high. Successful n atural regeneration requires a 4-6 year period without a fire. Within this period, seedlings have developed into the grass stage and are bet ter protected against seasonal drought and ground fires.