DOI: 10.5061/dryad.0rxwdbs5n
title: Changes in forest biodiversity metrics associated with Hurricane Katrina

description: 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 &gt;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.
description: The current study utilized one
 section of the larger transect to examine changes in forest tree
 vegetation associated with Hurricane Katrina. In 2004, 20 non-adjacent
 plots, each 100m<sup>2</sup> (10 x 10 m), were randomly
 established along several hundred m of the transect extending downslope
 from the top of the western levee of Bayou des Familles to open freshwater
 marsh. Ten plots were situated in oak-dominated bottomland forest within
 the more-upland section of the transect that was not flooded in 2004. Ten
 other plots were situated in seasonally inundated cypress and
 tupelo-dominated swamp forest. These swamp forest plots were distributed
 along the transect from upper areas only briefly flooded (1-2 months) to
 almost permanently flooded areas with numerous hummocks on which woody
 vegetation was present. Sampling was conducted four times. We
 sampled each plot during the summer (August) of 2004 and 2005
 (pre-hurricane). We revisited plots for post-hurricane field observations
 and plot relocation and remarking, as needed, in October, 2005, when we
 could access plots. We conducted our post-hurricane sampling in September,
 2006. Sampling was resumed in the summer of 2017. During our surveys, we
 systematically located and examined woody vegetation with stems
 &gt; 1.5 m tall in each plot. We recorded species
 and arboreal stratum [understory (&lt; 15m), subcanopy (15-20m), and
 canopy(&gt;20m)] of each plant on plot maps. Abundance of species was
 estimated using methodology developed by Peet et al. (1998), modified for
 only woody vegetation and using nested subplots (1, 10, and
 100m<sup>2</sup>). In all sampling years, voucher specimens of
 woody plant species were collected and deposited within the Shirley C.
 Tucker Herbarium housed at Louisiana State University in Baton Rouge,
 Louisiana. Species were identified based on Weakley and Southeastern Flora
 team (2022). Prior vegetation surveys conducted within the Preserve also
 provided useful taxonomic references (White et al. 1983, Michot 1984,
 Michot and Doyle 1999, Swarzenski et al. 2004, Nolfo-Clements 2006,
 Urbatsch et al. 2009).
description: Microsoft Excel