The August 2005 hurricane (Katrina) affected five million acres of forest across
ID: 34662 • Letter: T
Question
The August 2005 hurricane (Katrina) affected five million acres of forest across Mississippi, Louisiana and Alabama, with the immediate damage ranging from downed trees, snapped trunks and broken limbs to stripped leaves. In addition to the immediate damage caused by the hurricane, longer term consequences included a die-off of coastal trees for several years following the hurricane.
Your assignment: Use your knowledge of nutrient absorption, exclusion of unwanted ions, and osmosis and water potentials to hypothesize why the coastal trees that made it through the hurricane died over the following months/years. Hint: Seawater flooded the inland freshwater habitats during the hurricane. Seawater is ~99% sodium chloride. Your hypothesis should include a description of any plant physiological mechanisms referenced, and an explanation of how it might be related to the die-off of the coastal tress.
Explanation / Answer
When plant cells are placed in hypotonic solution, the cells undergo selling due tomovement of solution into the cells. The plant cells have rigid cell walls, the swelling of cells increases the turgor pressure inside the cells and cell wall is pushed out but lysis is not observed. If the plant cells are flaccid they are placed in a hypertonic solution and to make them turgid, hypotonic solution must be added.
Coastal wetland plants thrive in a relatively harsh intertidal environment characterized by alternate flooding and draining of salt marshes with associated waterlogging of soils, depletion of oxygen, and production of natural toxins that inhibit plant growth. To cope with these harsh conditions, plants have a number of adaptations, including the production of aerial roots and submerged tissues that allow them to capture oxygen needed by the roots. But these adaptations are suitable only as long as the average water level remains relatively constant. Accordingly, coastal wetlands exist within a fixed elevational range, where the frequency and duration of inundation by seawater are relatively constant (McKee and Patrick, 1988). Because plants become progressively more stressed and ultimately die if they are inundated for too long (Mendelssohn and McKee, 1988), an increase in water levels due to sea-level rise can severely stress the integrity of coastal wetland ecosystems. Sea-level rise over the last several decades has reportedly led to salinity intrusion and wetland loss in a number of coastal areas around the world and in the United States, including Long Island (Clark, 1986), the mid-Atlantic region (Kana et al., 1986; Hackney and Cleary, 1987), Chesapeake Bay (Stevenson et al., 1988), and the Mississippi Delta (Salinas et al., 1986; Conner and Day, 1989; Day et al., 2000). Since sea-level rise over the last century is two to nine times lower than the projected 20-90 centimeters of sea-level rise expected over the next 100 years (Neumann et al., 2000), there is great concern for the potential loss of coastal wetlands in the United States and globally. The projected rise in sea levels under global climate change will certainly place these productive and important ecosystems under additional stress, with the very likely result of extensive dieback of plants residing in the current intertidal zone. For example, a 0.5 meter rise in sea level would inundate about 12,000 square kilometers (about 4,600 square miles) of coastal wetlands, most of which would likely be lost.
Increased freshwater input can have both beneficial and detrimental impacts on coastal systems. The benefits associated with vertical soil accretion have already been highlighted. An additional benefit could be an increase in fisheries production in coastal systems (Nixon, 1988). This could result because the nutrients in freshwater flowing into estuaries stimulate primary production, which in turn increases the energy available for organisms on which fish forage. In the Mississippi Delta, these diversions add sediments that increase accretion, lower salinity to combat saltwater intrusion, and benefit fisheries and wildlife. There is concern, however, that diversions will lead to algal blooms because of the added nutrients (see next paragraph) and that they will add pollutants, which may severely affect organisms in these areas. Thus, diversions will have to be studied and managed carefully to avoid problems. On the other hand, decreased freshwater inputs are likely to lead to less accretion, lowered productivity, and saltwater intrusion. One potential negative impact associated with an increase in freshwater runoff to coastal ecosystems is an excessive increase in nutrients. There is already considerable evidence that agricultural runoff and wastewater from human activity in tributary watersheds are degrading many coastal ecosystems.
On the negative side, hurricanes can reduce the structural complexity of coastal forested wetlands such as mangroves and tidal freshwater forested wetlands (Rybczyk et al., 1995; Stone and Finkl, 1995). The locations with the highest probability of hurricane landfall are south Florida and the Mississippi Delta. The coastal ecosystems of south Florida are dominated by mangroves and, if warming continues, the31 + + Aquatic ecosystems & Global climate change + Mississippi Delta will become increasingly dominated by mangroves. This trend, combined with increased hurricane frequency, would reduce the structure of these forests and destroy some of them. Freshwater forested wetlands (swamps) of the Mississippi Delta are slowly degrading and disappearing because increased flooding from rising water levels has largely eliminated the establishment and growth of young trees. Hurricanes can also cause tree loss; for example, Rybczyk et al. (1995) reported that nearly 10 percent of trees in a swamp forest in Louisiana were blown down during the passage of Hurricane Andrew in 1992. An increase in hurricanes would amplify the kind of damage resulting from the interaction between rising water levels and hurricanes and would hasten the loss of forests in the Mississippi Delta and elsewhere. High runoff from hurricanes can also lead to excessive nutrient loading and eutrophication problems. For example, record runoff from Hurricane Floyd into the Pamlico Sound estuary in North Carolina led to water quality problems (Paerl et al., 2000)
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