Environmental effects on dynamic stem circumference variation in mangrove trees

Full description

Daily variation in the stem circumference of a diverse range of mangrove species was studied in relation to the environmental conditions in which the trees grew in five forests located in Australia and New Caledonia. The species studied were Avicennia marina, Bruguiera gymnorrhiza, Rhizophora apiculata, R. stylosa and Xylocarpus granatum. Circumference variation in tree stems were monitored with automatic dendrometer bands. The magnitude and timing of maximum diel stem shrinking and swelling were determined and analysed in relation to variation in climatic factors (air temperature, humidity and rainfall) and tidal inundation, together with the salinity of soil pore water at a depth of 30 cm below the soil surface. Stems swelled during the day and shrank at night, with no discernible effects of salinity or tidal inundation. In contrast, increases in stem circumference were highly sensitive to rainfall, while daily maximum stem swelling was strongly correlated with daily maximum temperature. Effects of defoliation on the diel patterns of variation in stem circumference were studied in two groups of mangroves: trees of Avicennia marina growing in a hypersaline mangrove system under arid climatic conditions along Giralia Bay, Western Australia, and trees of Xylocarpus granatum growing in a low salinity mangrove system along the Daintree River in the wet tropics of North Queensland. Dendrobands recorded stem circumference variation before and after defoliation. The pattern of daytime swelling was maintained in defoliated trees, indicating that processes or factors other than canopy transpiration, such as thermal expansion, influence temporary stem diameter increments. All trees fully recovered from the defoliation treatments. Please see the accompanying publication for full details (Vilas et al 2019 PLOS ONE).

Notes

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Significance statement

The results showed a common pattern of daytime stem swelling across a diverse range of habitats and mangrove species that differed in salinity tolerance, wood structure and functional leaf traits. This unusual pattern of daytime stem swelling was strongly correlated with daily maximum air temperature, while unaffected by defoliation. These observations suggest that thermal expansion may have played a greater role in stem circumference variation than expected and merits further study. In addition, rainfall events were correlated with stem swelling, underscoring the importance of stem water status to variation in circumference. More research needs to be done to understand the processes that contribute to dynamic variation in stem circumference with variation in climatic factors that affect the availability of water. Understanding these processes could enable Automatic Dendrometer Bands (ADBs) to provide a useful tool for monitoring the response of mangroves to extreme climatic events as they provide high-frequency, long-term, and large-scale information on tree water status and growth.