Abstract:

To assess the effects of the Indo monsoon on litterfall dynamics and changes of litterfall along altitudinal gradients in the tropical rain forests of southwestern China, eight plots were chosen along three elevational gradients of 600, 1 100 and 1 600 m in Xishuangbanna, China. We examined the relationship between litterfall dynamics and climate, and their changes with increasing altitude. On three gradients, average annual temperature was 22.1, 20.1 and 16.6 ℃ respectively, with a mean lapse rate of -0.005 3 ℃ m^-1. With increasing altitude, annual average rainfall increased (1 532, 1 659 and 2 011 mm, respectively), while in the dry season they were similar (282-295 mm); evaporation changed slightly (1 369, 1 374 and 1 330 mm, respectively); annual average relative humidity decreased (86%, 81% and 84%, respectively) and was much more pronounced in the late dry season; and soil water content increased significantly. Litterfall production of tropical seasonal rain forest (1 072 to 1 285 g·m^-2·a^-1) was higher than in the tropical montane rain forest (718 to 1 014 g·m^-2·a^-1). Both litterfall production and CV of annual litterfall processes had a significant and negative linear relationship with altitude. Litterfall production had a significant and positive linear relationship with temperature and was inversely related to rainfall. Peak litterfall during the dry season was influenced by relative air humidity and soil water content. Peak litterfall occurred earlier in the dry season at altitudes of 1 100 to 1 720 m due to decreasing humidity with altitude whereas at higher sites (1 820 m), increasing soil moisture levels delayed the litterfall peak. Our results suggested that 1) litterfall production of the tropical seasonal rain forest coincided with those of moist tropical rain forests in Southeast Asia; 2) water stress in the dry season changed with altitude and determined the timing of peak litterfall; and 3) with increasing altitude, there was a transition from seasonality to stability in annual litterfall process.

Key words: Litterfall, Tropical rain forest, Tropical montane rain forest, Indo monsoon, Xishuangbanna