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Effects of fire and nitrogen addition on photosynthesis and growth of three dominant understory plant species in a temperate forest

Fire and atmospheric nitrogen (N) deposition have the potential to influence growth and productivity of forest canopy. However, their impacts on photosynthesis and growth traits of understory plants in forests remain largely unexplored. This study was conducted to examine the effects of burning and N addition on foliar N content, net photosynthesis and growth traits of three dominant shrub species (Vitex negundo, Lindera glauca and Symplocos chinensis) in a temperate forest in Central China. The experiment used a pair-nested design, with four treatments (control, burning, N addition and burning plus N addition) and five replicates. Leaf mass area (LMA), area-based concentrations of foliar N and chlorophyll (Nₐᵣₑₐ and Chlₐᵣₑₐ), net photosynthesis (Aₙ), stomatal conductance (gₛ), maximum photosynthetic rate (Aₘₐₓ) and maximal carboxylation rate (Vcₘₐₓ), basal diameter, height and branch length (BL) of the three species were measured. Across the three species, burning stimulated LMA, Nₐᵣₑₐ, Chlₐᵣₑₐ, Aₙ, gₛ, Aₘₐₓ and Vcₘₐₓ, and consequently enhanced basal diameter, height and BL. Nitrogen addition increased Aₙ and gₛ but did not affect LMA, Nₐᵣₑₐ, Chlₐᵣₑₐ, Aₘₐₓ, Vcₘₐₓ, basal diameter, height or BL. However, N addition strengthened the positive effects of burning on gₛ, Vcₘₐₓ, Aₙ and BL. The findings indicate the primary role of light resources in determining plant photosynthesis and growth of understory shrub species after fire and highlight that understory plants should be considered in projection of biomass accumulation and productivity of forests under environmental perturbations.