Mangrove tree growth is size-dependent across a large-scale salinity gradient

Abstract

Salinity-influenced ecosystems are projected to face a tree to stand level growth reduction as a response to climate change. Although large and mature trees play a central role in defining carbon dynamics and site conditions, their eco-physiological and functional responses to increasing salinity remain poorly understood. Therefore, we test our hypotheses, i.e., large-diameter trees are predominantly contributing to above-ground biomass (AGB) stocks, whilst small-diameter trees are mainly contributing to AGB growth or gain (biomass changes over time) in higher salinity areas of the Sundarbans mangrove forest, Bangladesh. It can further be expressed by a growth dominance coefficient (GDC) that turns negative (a negative GDC indicates small trees proportionately contribute more to growth) in high-salinity areas while remaining positive in lower-salinity areas. We also hypothesized that species and structural diversity positively influence AGB stocks and gain. To test our hypotheses, we employed data from 60 permanent sample plots installed in the Sundarbans mangrove forest to estimate size-dependent functions by examining tree size, diversity, and growth dominance patterns to salinity gradients. Trees in higher salinity areas showed negative or reverse growth dominance patterns, indicating large trees contributed less to forest growth, which means smaller trees were disproportionately responsible for growth within the stand. Across the salinity zones, large-diameter (>20 cm in diameter at breast height, DBH) trees contributed primarily to AGB stocks, while small-diameter (<20 cm in DBH) trees contributed more to forest growth. We observed species diversity had no significant influence, whilst vertical diversity (height diversity) had a significant positive influence on AGB stocks and gain. Forest functioning (e.g., biomass accumulation rate) is more asymmetric (higher growth of small trees vs. low growth of large trees) in poor sites (i.e., high salinity), as poor site quality favors small trees to grow but not the large-diameter trees, which indicates large trees are more sensitive to high salinity as they lose growth. Our results indicate the size-dependent tree functions also depend on biotic and abiotic factors in mangroves. Increased structural diversity and removing mature trees to allow small trees to grow may benefit mangrove forest functioning (biomass stocks and growth), but species diversity may not.