Seedling Recruitment Dynamics in the Lumnitzera–Excoecaria Zone of Kadolkale Mangrove Park, Negombo

Abstract

The landward vegetation zone of mangrove forests, characterized by transitional species assemblages and reduced tidal influence, remains comparatively under-studied with respect to seedling recruitment dynamics. This study investigated overstory structure, light availability, seedling regeneration, and herbivory pressure in the Lumnitzera–Excoecaria zone of Kadolkale Mangrove Park, Negombo, Sri Lanka (7.196°N, 79.84°E). Three 100m transects were established landward of the Avicennia zone. Overstory vegetation was assessed using the Point-Centered Quarter Method, seedling abundance was quantified in 1m × 1m quadrats at 5m intervals (n=77), and herbivory damage was scored on a standardized 0–5 scale. Canonical Correspondence Analysis (CCA) assessed relationships between overstory composition and seedling recruitment. The Lumnitzera–Excoecaria zone exhibited high species richness and stem density (2,883 individuals ha⁻¹). Lumnitzera racemosa was the structurally dominant species (Importance Value Index=222; basal area=245 m² ha⁻¹), co-dominated by Excoecaria agallocha (IVI=108). Mangrove-associated species, including Premna microphylla and Acacia auriculiformis occurred exclusively in this zone, reflecting edge effects and anthropogenic influence. Light intensity was relatively uniform (2,900–3,400 lux), likely facilitating multi-species coexistence. Seedling regeneration was dominated by E. agallocha (6,753 seedlings ha⁻¹; Regeneration Index = 71), followed by L. racemosa (3,377 seedlings ha⁻¹; RI=36). The higher seedling dominance of E. agallocha relative to its overstory representation suggests an emerging successional shift toward Excoecaria dominance warranting long-term monitoring. Herbivory severity was broader in Lumnitzera– Excoecaria than in the seaward zone, with L. racemosa recording damage across all severity classes (Scores 1–4). Canonical Correspondence Analysis confirmed overstory composition as the primary driver of recruitment patterns (Axis 1: Pearson r=0.922 for L. racemosa; r=0.890 for E. agallocha). These findings highlight the need for site-specific conservation strategies that account for compositional shifts and non-native species encroachment in the transitional landward zones of heterogeneous mangrove ecosystems.