Chongqing (28°10' 32-13 N, 105°17' 110°11'E), the largest industrial and commercial center in Southwest China, is located in the Three Gorges Reservoir region of the upper reaches of the Yangtze River. Thus a high and healthy forest cover is required to protect the local environment and to guard the ecological safety of downstream regions. However, Chongqing was listed as one of the cities suffering from the most serious acid rain problems, and its extensive soil acidification and catastrophic scenarios of forest decline were first described in the early 1980s. Although great efforts have been made for controlling acid rain and substantial success has been obtained, the continuous economic growth, mountainous landform and low wind speed, which limit the air pollutant diffusion, will make acid rain a long-term problem. It is expected that the accumulating effect of soil acidification and increasing drought caused by climate change will further weaken the stability of the forests and endanger their multiple functions in the Chongqing region. Special forestry countermeasure such as scattering limestone powder on forest floor for alleviating soil acidification and its damage to trees could be one of effective alternative engineering measures to reduce acid pollutant effusion. Pinus massoniana is one tree species that is widely spread throughout Southwest China and is the most important native tree species for afforestation in the Chongqing region. However, the health condition of P. massoniana forests has been seriously damaged by the long-term acid rain and consequential soil acidification, heavy defoliation and high mortality of trees have been observed and reported. Former studies carried out in this region showed that applying limestone powder was helpful to recover the forest health. The recovery of the damaged fine root (≤2 mm in diameter) system, in terms of both quantity and depth distribution, should be the main focus in forest health recovery and the most direct and quickest indicator for evaluating the effect of applying limestone powder. Nevertheless, there is scare information about the response of fine root characteristics of P. massoniana to different doses of limestone powder applied under field conditions. This limits the effective and economical use of the limestone powder and the recovery of widely damaged forests. Thus a long-term field experiment to test the dose effect of limestone powder scattered on the floor of pure P. massoniana forest was set up in the Tieshanping Forest Farm of Chongqing in May 2004. The experiment was designed as a random complete block and composed of five dose treatments, i.e., scattering 0 (as control), 1, 2, 3 and 4 t/hm² of finely ground limestone on forest floor one time. All the treatments were replicated three times. The fine roots of P. massoniana were collected with root augers after a period of time, according to soil layers of humus, 0-20 cm, 20-40 cm and 40-60 cm, respectively. The fine root characteristics used here include the densities of dry weight, length, surface area, volume and the number of root tips. Monitoring results, in October 2006, indicated that applying limestone powder increased all the five densities of fine roots in all layers of the P. massoniana forest significantly (P < 0.05), compared to the control treatment, especially the fine root densities at the deeper soil layers of 20-40 cm and 40-60 cm. This would enhance the resistance of forests to the increasing drought. The fine root densities at the humus layer were also increased. This would improve the nutrient uptake ability of forests against the nutrition poorness caused by soil acidification. This meant that the limestone powder application improved both quantity and depth distribution of fine roots. Within the range of limestone powder doses used in this experiment, all investigated fine root densities increased with rising limestone powder doses, but at different rates. This could be described by the ratios of total densities of fine roots between the limestone powder treatments and the control. Here the total density meant the sum of densities of fine roots of the four layers for the same indicator. For the treatment of 1 t/hm², the total density ratios of dry weight, length, surface area, volume and the number of root tips were 1.53, 1.34, 1.39, 1.24 and 1.48, respectively, while they were 2.05, 1.83, 1.94, 1.79 and 2.04 for the dose of 2 t/hm², 2.17, 2.02, 2.02, 1.78 and 2.32 for the dose of 3 t/hm², and 2.68, 2.52, 2.62, 3.70 and 2.71 for the dose of 4 t/hm². From the viewpoint of a more effective and economical application of limestone powder in forest practice, the dose of 2 t/hm² is suggested, since this dose induced the highest increase rates of most density characteristics of fine roots. For example, the increase rates of surface area density of fine roots per ton of limestone powder were 1.0, 1.2, 0.9, 1.0 for the dose of 1, 2, 3, 4 t/hm², respectively. Here it should be emphasized that the surface area density is a more important indicator, which directly influences the uptake ability of trees for both water and nutrition. However, this dose suggestion is based on an investigation of only three growing seasons after limestone powder application. The long-term dose response and corresponding optimal practice suggestion require further monitoring of fine root growth dynamics in the future. http://www.ecologica.cn