The Nature of Growth Rings in the Mangrove, Avicennia marina

In this study we look at the irregular growth pattern of Avicennia marina mangroves on Guam to identify the number of growth layers the tree produces each year. Using a cambial marking technique and stable carbon isotopes, we looked at the growth patterns of Avicennia marina to identify the periodicity of growth layers and determine whether this mangrove population demonstrates the same successive cambia growth as found in Avicennia trees in other locations. In July 2013, a knife was used to create a 5 cm long mark on the cambium, the layer directly beneath, on 11 trees located along the southeast coast of Guam. The marked branches were collected in August 2014 and each branch was cut into five segments for analysis. The number of layers (dark and light layers counted separately) was 6.3 ± 0.4 (mean ± SE) with a 95% confidence interval of 5.8 to 6.8 growth layers. We conclude that although there is variation in the number of layers grown since the scar, it is reasonable to assume that the Avicennia marina mangroves on the southeast coast of Guam grow approximately six growth layers, or three complete growth rings in one calendar year.