Increase in black mangrove abundance in coastal Louisiana

Black mangrove (Avicennia germinans), a subtropical species, has historically occurred in saline marsh habitat along the Louisiana coast, but their distribution has always been sparse and they seldom achieved heights of greater than 1 m. The distribution of black mangrove in Louisiana has been largely limited by freezing temperatures. Weather records show a decrease in freeze frequency and duration in the coastal zone of Louisiana. At the same time, we and others have noted an apparent, but as of yet undocumented and unquantified, increase in mangrove distribution and abundance in the state. The last hard freeze in coastal Louisiana was in 1989, so mangroves have had 20 years to grow and spread. We conducted aerial surveys along fixed transects from 2000 to 2002 and again in 2009 to determine if mangroves are indeed more numerous along the coast. For each flight we surveyed about 5,000 patches of landscape, each approximately 100 m in diameter, and classified them as to vegetation characteristics. We found that the number of such patches that were vegetated with Avicennia more than doubled from October 2001 (22 patches) to August 2002 (55 patches – Figure 1). But more importantly we found about 275 mangrove patches along the same transects in 2009, a 5-fold increase in abundance over that 6.5 year period.