The effect of ground fire and subsequent exposure of mineral soil on germination and early development of Pinus merkusii Jungh. et de Vriese, and morphological development throughout the grass stage of naturally borne seedlings were studied in northern Thailand, within the natural distribution range of the species. The study was carried out with the major aim to find out whether and to what extent the natural regeneration 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 discussed. We found that the total volume of stem and tap root in the grass stage controls the shoot growth rather distinctly: seedlings emerge from the grass stage soon after a certain minimum volume (approx. 15-20 mm diameter at ground level) is reached. The growth of the tap root is steady throughout the grass stage. Formation of the deep tap root promotes the volume growth of the stem. Two-thirds of the diameter of the short stem is formed by cortex, which functions as a carbohydrate store to be later utilized in the fast shoot growth. There were no significant differences in germination and early development of P. merkusii seedlings between a burnt and an unburnt area. Considerable numbers of seedlings (11042-34250 ha(-1)) were found in stands burnt 2, 4 and 6 years previously. P. merkusii seeds germinate well beneath a dense cover of grass, giving rise to an abundant ephemeral seedling stock. Mortality due to competition by grass is, however, high. Successful natural regeneration requires a 4-6 year period without a fire. Within this period, seedlings have developed into the grass stage and are better protected against seasonal drought and ground fires.
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