Distinct physiological and functional responses in leaves and roots of co-occurring mangroves: contrasting invasive Laguncularia racemosa and native Avicennia marina under seasonal nutrient fluctuations

Introduction Plant invasions threaten biodiversity and exacerbate climate impacts by disrupting ecosystems. Although stressors induce antioxidant and osmoregulatory defenses, field evidence from mangroves, key coastal systems undergoing rapid degradation, remains limited. In Hainan, China, the invasive Laguncularia racemosa displaces native Avicennia marina , but their organ-specific defense responses to seasonal fluctuations remain unclear. Methods We measured antioxidant enzyme activities [superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT)], osmolytes [proline (PRO) and soluble sugars (SS)], and malondialdehyde (MDA) levels in leaves, fine and coarse roots of L. racemosa and A. marina during Hainan’s dry and wet seasons, alongside leaf area, root conduit traits, and tissue nitrogen (N) concentrations. Results While soil moisture and basic parameters remained stable, dry-season total soil N surged 4–7 fold. Leaves exhibited higher SOD and osmolyte levels than fine roots, while dry-season defenses diverged between a 9.5-fold leaf SOD increase in L. racemosa and enhanced root POD (2.6-fold) and CAT (1.6-fold in leaf, 2.2-fold in root) in A. marina . These enzyme and osmolyte patterns were consistent with seasonality rather than ontogeny. Notably, the upregulation of antioxidant defenses despite stable soil stressors, coupled with stable MDA levels and a general absence of negative enzyme–MDA correlations, suggests a plausible preventive defense model rather than reactive damage–repair. In the dry season, leaf area was generally constrained (except in mixed L. racemosa ), and root conduit traits lacked consistent increases. Leaf and root N concentrations were largely stable (except in monospecific A. marina ) and correlated more strongly with enzymes or osmolytes than with growth traits. Discussion These findings probably suggest a priority allocation toward defense over growth. In mixed stands, dry-season edaphic facilitation enabled the invasive L. racemosa to pursue a high-risk hydraulic expansion, while the native A. marina maintained a conservative, safety-prioritized stability. Together, this contrasting behavior suggests that native community structure limits invasion by forcing invaders to undertake risky physiological trade-offs to sustain competitive dominance.