Objective Forest fire is the important disturbance factor of forest ecosystem. Shrub is an important part of the forest community. Studying the change of species diversity and leaf functional traits can provide scientific basis for the recovery of shrub layer plants after fire.

Method Taking the understory shrub layer plants of Pinus tabuliformis forest in the fire-burned land of Taiyue Mountain in Shanxi Province as the research object, for different fire severities (control, mild, moderate, and severe fire), species diversity investigations and sample collection were performed, and 4 diversity indexes (Shannon index, Simpson index, Pielou index, Margalef index) were calculated. Eight functional trait indicators (leaf phosphorus content, nitrogen-phosphorus ratio, leaf potasslum content, leaf nitrogen content, leaf organic carbon content, leaf dry matter content, leaf thickness, specific leaf area) of dominant shrub leaves were measured. Using one-way ANOVA and multiple regression analysis, we analyzed the diversity index and leaf functional trait values and studied the significance, variation patterns, and variation characteristics of differences between varied fire severities, and analyzed the correlations between species diversity and leaf functional traits.

Result (1) The Pielou index had a maximum value of 0.90 in severe burning, and its magnitude was severe > moderate > control > mild, while Shannon index, Simpson index and Margalef index all had maximum values of 1.65, 0.74 and 1.43 during moderate burning, with moderate > severe > control > light. Four diversity indexes between different fire severities had varied significant differences (P < 0.05). (2) The coefficient of variation of leaf organic carbon content (5.00%) was the smallest, and the coefficient of variation of specific leaf area (20.30%) was the largest. (3) Except for leaf potassium content, the other leaf functional traits were significantly different among varied fire severities. As the fire severities increased, the overall trend of changes in leaf thickness, leaf phosphorus content, and leaf dry matter content was gradually increasing. The overall trend of organic carbon content in leaves decreased first and then increased. (4) There were strong correlations among species diversity, leaf functional traits, species diversity with leaf functional traits.

Conclusion Fire severity has a significant impact on plant species diversity and leaf functional traits in the shrub layer, and has a complex interrelationship between leaf functional traits and species diversity.