Phosphorus (P) acquisition properties of roots may be an important factor that contributes to forest productivity, but they have not been well evaluated compared with aboveground properties of plants. In the present study, we analyzed root acid phosphatase activity and morphological properties of dominant tree species in three tropical montane forests on Mt. Kinabalu, Borneo, to investigate changes in root properties along a soil P availability gradient. We found that root phosphatase activity and specific root surface area (SRSA) increased, and root diameter decreased, with decreasing soil P availability at the community level. Root acid phosphatase was significantly positively correlated with SRSA, and negatively correlated with root diameter. Furthermore, we compared root acid phosphatase activity with leaf P concentration of a given tree species, and found a significant negative correlation between them, suggesting that root P-acquisition properties could influence leaf P concentration, and/or vice versa. The adaptive changes in the aboveground and belowground properties along the soil P gradient could contribute to the maintenance of forest productivity in the tropical montane forest in Borneo.