Changes in forest biodiversity metrics associated with Hurricane Katrina

Recurrent tropical cyclones frequently influence warm temperate forests in
many coastal regions. If resistance to high winds (cyclone survival) and
resilience in altered environments (post-cyclone recruitment) vary with
cyclone intensity, then differences in composition, arboreal structure and
dynamics of forests should occur in these forests. We hypothesized that
major tropical cyclones should produce large, stratum-dependent effects
that result in large environmental changes. We anticipate direct
regeneration, with post-cyclone forests resembling pre-cyclone forests in
composition, but potentially shifting to long-transient alternate states
that do not readily or necessarily return to pre-cyclone states. Using a
long-term, plot-based study, we explored direct effects of major Hurricane
Katrina (sustained winds >50m/sec), as well as initial
post-hurricane changes in overstory and understory trees in oak-dominated
bottomland and cypress/tupelo-dominated swamp forests within the
Mississippi River delta. Hurricane Katrina caused high overall mortality
(14% - swamp forest, 25% - bottomland forest), but mortality was
concentrated in overstory strata (22% in swamp forests and 70% in
bottomland forests). Almost all overstory oaks were killed, while most
overstory cypress and tupelo were damaged, but survived and were reduced
to understory status. In both forests, many trees of less abundant
overstory species (red maples, sweetgum, elms, ash) survived and
constitute a slowly reforming overstory via resprouting of survivors and
recruitment of understory trees. Thus, forest overstories appear likely to
change in dominant species. Understory trees in both forests had high
survival and responded post-cyclone. Nonetheless, one usurper species,
dwarf palmetto, with high resistance and high resilience, now forms a
dense arboreal layer in the understory stratum, dominating an alternate
state with different overstory and closed understory capable of
suppressing recruitment and establishment of overstory and understory
species. Resistance and resilience of overstory and understory tree
species that vary with cyclone intensity might result in prolonged
alternate states in cyclone-frequented forests.