Forested Wetlands

Hurricane Andrew diminished in strength after it made landfall east of Cypremort Point. The storm turned to the north-northeast, passing over the Atchafalaya Basin where more that 450 km² (175 mi²) of cypress-tupelo and bottomland hardwood forests were at risk. The storm stayed within the levee boundaries with wind speeds gradually weakening, but wind gusts of 112 kmph (70 mph) were still recorded when the storm was 48 km (30 mi) west of Baton Rouge.

The Atchafalaya Basin contains 35 percent of the remaining bottomland hardwood forest and swamp forests of the lower Mississippi floodplain. The two dominant forest-cover types include cypress-tupelo swamps, primarily in the southeast part of the basin, and mixed bottomland hardwood forests. The southwest portion of the basin has been subjected to high rates of sedimentation since the construction of levees after the flood of 1927 and the diversion of sediment-laden Mississippi River water. The areas of new sediment are now dominated by black and sandbar willow, swamp cottonwood, and the exotic Chinese tallow.

The impact of Hurricane Andrew varied greatly with forest type, the species mixture, canopy (treetop) structure, and location relative to the storm’s path. The zone of heaviest damage to the forest extended 20 km (12 mi) east from the hurricane eyepath in the southern portion of the basin and 10 km (6 mi) east of the eyepath in the northern part. Sites exposed to wind speeds less than 120 kmph (75 mph) experienced lower levels of damage. (Figure 13)

Most of the initial loss in tree density and canopy cover was restricted to the bottomland hardwood forests. These stands lost 10 percent of the their basal area—the volume of their trunks—in areas exposed to the weaker influence of the storm. Trees in areas exposed to the full strength of the storm lost over 60 percent of their basal area.

Bottomland hardwoods in the southwest portion of the basin, dominated by willows, were especially hard-hit, with more than 85 percent of the trees in this area damaged. These stands were growing on loose soils and were quite susceptible to being pushed over. Surprisingly, cypress-tupelo stands were largely unaffected because the canopy tree species have properties, such as extensive root systems, that make them resilient to hurricane-force winds. (Figure 14)

Even though there were many fallen trees (about 10 percent of the forest volume, basinwide) overall tree mortality was initially low because, despite severe damage, many of the downed trees resprouted. Surveys conducted two years after Hurricane Andrew, however, revealed that there was a considerable amount of delayed mortality among certain tree species. Mortality for persimmon, swamp cottonwood, and sandbar willow increased from 5 to 7 percent during the first year to 25-61 percent the next year.

Despite the widespread destruction in the Atchafalaya Basin, not all tree species responded in the same manner to the catastrophic winds (Figure 15). Some species were resistant to wind and lost only individual limbs; others were susceptible to windthrown or snapped tree trunks and died. Tree species that were susceptible to windthrow are prolific sprouters and have survived. Species that initially survived and sprouted, however, appear to be highly susceptible to delayed death.

Table 1. How Hurricane Andrew Damaged Specific Types of Trees in Bottomland Hardwood Forests
Most damaged	Moderately damaged	Least damaged
sandbar willow	Red mulberry	Pumpkin ash
Swamp cottonwood	Boxelder	Deciduous holly
Black willow	Swamp red maple	Water hickory
Waxmyrtle	Swamp privet	Baldcypress
Chinese tallow	Hackberry	Water elm
Sycamore	Various oaks	Buttonbush
Swamp dogwood	Various hawthorns

A significant impact of the storm was the amount of plant material stripped and broken off the trees by the winds. This phenomenon was so widespread that it showed up on satellite imagery of the study area. Widespread defoliation (leaf removal) caused up to 41 percent of the normal seasonal leaf fall to occur in a single day. The decomposition of this large amount of organic material falling into the swamp in a short period led to extremely low levels of oxygen in the water column in the Atchafalaya Basin and a large die-off of fish. An estimated 182 million game and commercial fish died, an economic loss of $160 million. (Figure 16)

Can we predict how the forest will recover from such catastrophic disturbance? Forest recovery and the formation of a new canopy are accomplished in several ways. Defoliation of living trees by Hurricane Andrew, for instance, caused a surge of new leaf growth in the fall of 1992. Furthermore many understory trees and saplings were unaffected by the hurricane despite the loss of the canopy trees. These survivors will grow rapidly and form the new canopy. In other cases, seeds and new tree seedlings will become established and eventually form the new forest. (Figure 17)

Initial surveys suggested that heavy-seeded species such as oaks were underrepresented in the ground layer, while other species such as Chinese tallow tree and water elm were more common. It is disturbing that an exotic species such as the Chinese tallow tree has such a high representation in the understory, while the more valuable oak species commonly found in bottomland hardwood forest sites are underrepresented. This suggests that exotics species may play more of a role in recovery from disturbance than they have historically and can alter the nature and function of these forests.

Hurricanes are an important agent of forest disturbance in coastal wet forests. The recovery of the forest will depend on the previous forest cover, the type of damage, and the specific environmental conditions affecting regeneration. The harvest of the virgin cypress swamps and the building of levees earlier this century caused widespread change in the forests of the Atchafalaya Basin. Increased sedimentation has resulted in the dominance of willow stands in the southwest portion of the basin, an area that was most heavily damaged by the hurricane because of the willow's susceptibility to hurricane-force winds.

Various climate change models suggest that under global warming, tropical storm intensity might increase as much as 50 percent. If so, then the forests of the Atchafalaya Basin will be at greater risk of damage, and intensive land management may be required to direct the developing forest toward more desirable forest cover. (Figure 18)

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