Adaptive responses to salinity: nutrient resorption efficiency of Sonneratia apetala (Buch.-Ham.) along the salinity gradient in the Sundarbans of Bangladesh

Sonneratia apetala (Buch.-Ham.) is a major pioneer tree species of the Sundarbans and is found to grow in the less saline zone (LSZ), moderate saline zone (MSZ) and high saline zone (HSZ) of this mangrove forest. This experiment examined the adaptive behaviour of S. apetala grown in different saline zones of the Sundarbans in terms of nutrient resorption/re-translocation efficiency (NRE). Samples of topsoil and green and senescent sun leaves were collected from S. apetala-dominated stands located in the three saline zones during the pre-monsoon, monsoon and post-monsoon seasons. Soil available nutrients (N, P and K), sodium (Na) concentrations, and NRE of S. apetala leaves from the three saline zones during the different seasons were measured in the laboratory. The average available nutrients (N, P and K); Na concentrations in the soil; and NRE of N, P and K showed significant (p<0.05) differences among the saline zones and seasons. The lowest concentrations of N (25.44 mu g/g), P (5.44 mu g/g) and K (0.036mg/g) and the highest concentrations of Na (0.50mg/g) were observed in the HSZ during the pre-monsoon season. The highest NREs of N (63.58%), P (45.86%) and K (61.30%) were found in the HSZ during the pre-monsoon season, while the lowest NREs of N (39%), P (26%) and K (35%) were found in the LSZ during the post-monsoon season. The NRE was positively correlated with the available concentration of Na in the soil. Nutrient resorption is an important nutrient-conserving mechanism for mangroves growing under salt stress. This may be the reason that the NRE was comparatively higher in the HSZ than in the LSZ and MSZ. The findings of this study support the conclusion that S. apetala has shown adaptive responses among the populations in the LSZ, MSZ and HSZ of the Sundarbans. This adaptive variation enabled S. apetala to survive and grow satisfactorily in a wider range in the Sundarbans.