Tree growth is a result of multiple, interacting, physiological processes influenced by an inherited genetic constitution and the ambient environment. The growth of coniferous trees is closely connected with the amount of nitrogen and other nutrients available. Air pollutants that limit carbon gain or nutrient availability may suppress growth rate and total biomass production, and thus affect the nutrient allocation pattern. An increase in the acidity of precipitation predisposes tree seedlings to a number of environmental stresses; these are reflected in seedling germination, growth, and survival. Decreased growth has been seen in response to sulfate-containing precipitation. However, considerable differences have been reported in the responses of conifer species to acid rain.
Many tree species in forest ecosystems live in symbioses with ectomycorrhizal (ECM) fungi, which provide their hosts with nutrients. Symbioses with ECM fungi are therefore very important. Soil acidification due to dry and/or wet deposition can inhibit the uptake of minerals and water essential for plant growth, and increase the uptake of toxic metals, due to poor differentiation by root meristems. However, colonization by ectomycorrhizal fungi can increase the uptake of essential nutrients and water, and reduce the toxicity of metals such as Al³⁺ and Mn²⁺.
To evaluate the effects of ectomycorrhizal colonization on the growth of Pinus massoniana Lamb seedlings grown in acidified soils, we grew masson pine seedlings with ectomycorrhizae for 210 days in acidified forest soil originating from Chongqing Municipality, Southwest China. There were two acid treatments; one at pH 3.5 and the control at about pH 5.5, and two ectomycorrhizal treatments (inoculated, non-inoculated). Simulated acid rain in combination with pH 3.5 reduced seedling biomass. Inoculation with ectomycorrhizal fungi was able to increase seedling biomass. Treatment time, acid treatment, and ectomycorrhizae all had remarkable effects on biomass, and we observed interacting effects between treatment time and acid treatment, and between acid treatment and ectomycorrhizae. Treatment time had significant effects on root, stem, and leaf biomass allocation, and acid treatment and ectomycorrhizae had obvious effects on root and leaf percentages. There were interactions between and among treatment time, acid treatment, and ectomycorrhizae. Under simulated acid rain (pH 3.5), the root-shoot ratio initially decreased and then increased. Treatment time and acid treatment had significant effects on the root-shoot ratio, while the ectomycorrhizae showed no effect. We observed interacting effects on the root-shoot ratio between acid treatment and treatment time, treatment time and ectomycorrhizae, and among these three. Compared with the control (pH 5.5), simulated acid rain (pH 3.5) initially increased leaf area, and then significantly reduced leaf area. Treatment time, acid treatment, and ectomycorrhizae had obvious effects on specific hemi-surface area, and there were also significant interactions among these factors. The effects of ectomycorrhizae on biomass allocation and leaf area was opposite to the effects of acid rain. Inoculation with ectomycorrhizal fungi encouraged the growth of masson pine seedlings, and increased specific hemi-surface area, enhancing photosynthesis and elevating biomass accumulation. Ectomycorrhizal fungi were thus able to counteract the effects of simulated acid rain, and protect the growth of the masson pine seedlings. It may be that ectomycorrhizal fungi can be used effectively to increase plant resistance to acid rain in places such as Southwest China.