In the present study, the effects of burning treatments on seedlings of five tree species were investigated. Using pot-grown 3-year-old seedlings of Fraxinus mandshurica, Juglans mandshurica, Quercus mongolica, Phellodendron amrense, and Betula platyphyla as the test materials, three burning treatment methods were applied, including T1 (leaf burn), T2 (branch burn), and T3 (stem burn). The malondialdehyde (MDA) and chlorophyll contents in seedling leaves were detected, and the net photosynthetic rate was determined. The heat energy from the fire treatments is transferred to plant tissues by conduction and radiation. Burning can damage cell membranes, resulting in lipid peroxidation and production of MDA. The product of lipid peroxidation, MDA, was used to estimate the degree of damage to the cell membranes. Lipid peroxidation can occur during organ senescence or as a result of stress damage. In T1 treatment, the MDA contents in leaves from five species were significantly higher than their respective controls, indicating that leaf cells were destroyed by fire and that membrane lipids in leaf cells had undergone peroxidation. Moreover, in T1 treatment, the chlorophyll contents were higher in the leaves of F. mandshurica, J. mandshurica, and B. platyphyla than in their respective controls, but similar in the leaves of Q. mongolica and P. amrense and their respective controls. In addition, in T1 treatment, the chlorophyll a/b ratio in the leaves from F. mandshurica and J. mandshurica was lower than that in controls, respectively. However, in Q. mongolica, P. amrense, and B. platyphyla, the chlorophyll a/b ratio was similar with controls. Also, there were higher MDA contents in leaves after the T2 and T3 treatments of Q. mongolica, the T3 treatment of J. mandshurica, and the T2 and T3 treatments of B. platyphyla.In terms of cell membrane stability, the membranes of species that were not well adapted to fire were weakened by fire damage. However, in seedlings that are well-adapted to fire, the MDA contents in young fire-treated leaves were lower than those in the control. These included the T1 and T2 treatments of F. mandshurica and the T2 and T3 treatments of P.amrense. The lower MDA contents in the burned tissues indicated that cell membrane stability is enhanced by fire treatments in young leaves and saplings from the well fire-adapted species.
Chlorophyll reflects the photosynthesis, the chlorophyll content, and the chlorophyll a/b ratio are important indicators of the physiological condition of the leaves. In all five species, the chlorophyll contents in the leaves increased after the T1, T2, and T3 treatments, except in T3 treatment of Q. mongolica and T2 treatment of J. mandshurica. The change in the chlorophyll a/b after fire treatments varied among the five species. The net photosynthetic rate, which is the most important index for photosynthesis, increased to varying extents in the five species in the T1, T2, and T3 treatments. The only exception was the T3 treatment of Q. mongolica. Fire influenced the photosynthesis of saplings by affecting physiological metabolism. The results of these experiments indicated that fire affected the growth of these tree species.