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

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 con-ditions, 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, indi-cating 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 in-dicates 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.

Automated summary

Salinity-influenced ecosystems are projected to experience reduced tree growth at the stand level due to climate change. However, the eco-physiological and functional responses of large and small trees to increasing salinity are not well understood. In the Sundarbans mangrove forest in Bangladesh, we found that large-diameter trees primarily contribute to above-ground biomass (AGB) stocks, while small-diameter trees mainly contribute to AGB growth in higher salinity areas. Structural diversity positively influences AGB stocks and gain, while species diversity does not have a significant impact.