Diurnal courses of stem radial water dynamics represent the sum of all internal and external conditions affecting tree water relations. Changes in stem radius due to early morning water depletion and night time refilling of storage tissues is generally well documented. This study seeks to understand the unusual daytime refilling of stem elastic storage tissues present in the mangrove species Avicennia marina (Forssk.) Vierh, which deviates from our traditional understanding of hydraulics in terrestrial trees. We explored the relationship of this pattern to other water-related physiological processes and environmental variables, and investigated the seasonal changes in the timing and time lags of peak swelling at different stem heights, in order to understand the 'peristaltic' depletion of internally stored water. Our findings show that daytime stem swelling occurs year-round, even on days when leaf water potentials dropped to values lower than -4 MPa. The amplitude of stem swelling was strongly positively correlated to daily light sums more often than to measures of water availability in air and soil, especially in winter. There was also a clear seasonal reversal in the timing and direction of the 'peristaltic' depletion of water along the stem, with an earlier onset of shrinking in the upper (median = 10:00 h) than in the lower stem (median = 12:00 h) in winter, but an earlier onset of shrinking in the lower (median = 08:00 h) than in the upper stem (median = 11:00 h) in summer. This time lag was closely correlated to daily temperature, with a clear switch in the direction of peristaltic stem shrinking at the start of the growing season. We propose that sugar loading/unloading and changes in source-sink activity play a role in the endogenous osmotic adjustment responsible for daytime stem swelling and the seasonal switch in the direction of peristaltic water storage depletion in A. marina.