Abstract: Sap flow density of rubber trees (Hevea brasiliensis) of four ages (13 years old, young; 25 years old, middleaged; 33 years old, middle-old-aged; and 50 years old, overmature) in Xishuangbanna of Southwest China was measured by the Granier thermal diffusion probe technique from January 2013 to December 2014, and then scaled up to estimate standlevel transpiration. The results showed that, on day, month, season and year timescales, the middleaged trees had the highest mean sap flow density, being significantly higher than the other aged trees (P<0.01). The overmature stands had the highest annual transpiration rate (1212.49±42.80 mm per year), being significantly higher than those of the young and middle aged stands (P<0.01). With the increase of stand age, the sap flow density of rubber trees generally followed a parabola change, with the peak in the most vigorous middleaged stand. The transpiration of rubber stands increased with increasing stand age in general, and the overmature stand had the highest transpiration. The mean annual sap flow density of rubber stands in Xishuangbanna was estimated as 16.42±3.24 g·m^-2·s^-1 and the mean annual transpiration as 870.58±145.89 mm.

Key words: water-soluble organic matter, soil moisture, litter decomposition, amino acid, nitrogen transformation