Fire intolerance of invasive woody plants evidenced by bark thickness: implications for invasion in fire-suppressed forests

Abstract Background Frequent low-intensity fires historically shaped forest composition and structure in the southeastern United States of America (USA). However, in modern times, fire suppression has inadvertently facilitated the recruitment of mesophytic trees and potentially the invasion of non-native woody plants. In this study, we selected twelve woody broadleaved plants from forested areas of South Carolina and categorized them into three groups: (1) non-native invasive species, (2) native pyrophytic species, and (3) native mesophytic species. We used these categories to examine bark thickness—one critical determinant of fire tolerance—across a spectrum of stem sizes (16.3 mm < DBH < 69.4 mm). Results Across all species, pyrophytic species consistently exhibited the thickest bark at all measurement heights, followed by invasive species, while mesophytic species had the thinnest bark. Invasive and mesophytic species displayed similar absolute and relative bark thickness values and demonstrated comparable trends in bark thickness variation with increasing stem size. Absolute bark thickness decreased with increasing height along the stem in all groups; however, pyrophytic species exhibited a more pronounced decline in bark thickness from the ground line to breast height compared to mesophytic and invasive species. For pyrophytic species, relative bark thickness at the ground line decreased sharply with increasing stem diameter, indicating a significant early-life investment in bark development. Conclusions The congruence in absolute and relative bark thickness patterns between mesophytic and woody invasive plants along both horizontal (DBH) and vertical (height) gradients indicates that fire suppression may have helped to facilitate invasive species establishment and spread and that the restoration of historical fire regimes (e.g., repeated surface fire), suggested for reducing mesophytes, could aid in the management of woody invasive plants.