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Increase in black mangrove abundance in coastal Louisiana

Louisiana Natural Resources News
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Abstract

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. In from a fixed-wing aircraft of the entire coastal area of Louisiana to document andmap the total distribution of mangroves in Louisiana. Observations were grouped into three patch sizes, each with a height modifier (>1m or <1m), for a total of 6 categories. We found 3738 patches of mangroves in coastal Louisiana (Figure 2). Almost all patches were located in southeast Louisiana, in the Deltaic Plain,except that two small patches were found in the Chenier Plain at Marsh Island inIberia Parish (one tree) and on Monkey Island in Cameron Parish (3 trees). Mangroves were absent from the currently active Mississippi River Delta andnorth of 30 degrees N latitude. Taller, denser patches, representing older colonies, were more abundant close to the coast while short, sparse patch(new colonies) were concentrated more inWe estimated a range of values for area covered bmangroves in each patch based on the size of the viewing window as obserfrom the aircraft. Based on these calculations, we estimated that, in 2009black mangrove coveredfrom 300 to 1200 ha, or uto 0.5%, of the saline marshzone of coastal Louisiana.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Increase in black mangrove abundance in coastal Louisiana
Series title Louisiana Natural Resources News
Language English
Contributing office(s) Wetland and Aquatic Research Center
Country United States
State Louisiana