Abstract:The high levels of acid deposition experienced in recent decades have caused a decline in health of forest tree species in the south and southwest of China, but differences in the extent of decline exist among the tree species. For Pinus massoniana and Cinnamomum camphora, the two indigenous tree species exposed to frequent acid rain and acid fogs, the former shows more visible injury symptoms of defoliation and twig death than the latter due to its higher sensitivity to acid deposition. Ecologists have focused on the chemical properties of acidified soils in Pinus massoniana forests for a long period and made a great progress. However, the information on difference in resistance to soil acidification between the two tree species, especially on fine root (≤2 mm in diameter) characteristics under field conditions exposed to long-term acid deposition, is still scarce. Therefore, we investigated the pure stands of the two tree species that grow on acidified soil under similar site conditions in Chongqing, southwestern China. The fine roots were collected according to the layers of humus, 0-20 cm, 20-40 cm and 40-60 cm by a soil core method. Contents of the exchangeable cations (K+, Na+, Ca2+, Mg2+, Al3+ and H+) and pH values in each layer were also analyzed at the same time.
The results showed that the pure Cinnamomum camphora stand had greater ability to slow down soil acidification than the pure Pinus massoniana stand due to its more exchangeable base cations (Ca2+ and Mg2+), higher base saturation (BS) and pH values and fewer exchangeable Al3+ and H+ ions in the humus layer and 0-60 cm soil layer. In the humus layer and top soil, growth indexes of fine roots of the pure Cinnamomum camphora stand were significantly higher than those of the pure Pinus massoniana stand (p < 0.05). For example, in the 0-20 cm soil layer, dry weight, length, surface area, volume and the tip number of fine roots per unit area of soil of the pure Cinnamomum camphora stand was 1.40, 291, 2.55, 2.27 and 3.48 times as much as that of the pure Pinus massoniana stand, respectively. For the two pure stands, maximum values of density indexes of fine roots were found in the humus layer, where dry weight, length, surface area, volume and the tip number of fine roots per unit volume of humus of the pure Cinnamomum camphora stand was 330, 7.55, 6.16, 4.89 and 6.89 times as much as that of the pure Pinus massoniana stand, respectively. Density indexes of fine roots of the two pure stands decreased with the increasing soil depth with the Cinnamomum camphora stand decreasing more rapidly than the Pinus massoniana stand. Analyses on the soil acidity and growth indexes of fine roots in the 0-20 cm soil layer for the two pure stands showed that fine roots of Cinnamomum camphora possess higher resistance to soil acidification. Consequently, Cinnamomum camphora can be used to improve the acidified soil and growth of Pinus massoniana forests on acidified soil.