Doctoral Dissertations

Date of Award

6-1905

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Ecology and Evolutionary Biology

Major Professor

John C. Rennie

Committee Members

Bob O'Neill, Steve Bartell, Mac Post, Steve Nodvin

Abstract

This research forecasts the effect of variable hurricane intensity on mangrove forest structure and evaluates the role of species composition in recovery. Baseline data estimates were made along the Shark River in Everglades National Park, Florida, USA, in 1992, 32 years after Hurricane Donna ripped up the southwestern Florida coast. Hurricane disturbance and forest recovery were simulated in an individual-based model. Results were extrapolated to forecast the effects of two factors: windfall, in which 37.5%, 62.5%, and 87.5% of stand density is blown over; and species composition in which plots are categorized according to dominance by species which stump sprout [Avicennia germinans (AVGE) and Laguncularia racemosa (LARA)] vs dominance by the species not capable of sprouting [Rhizophora mangle (RHMA)]. The pre-disturbance forest was comprised of mature stems of RHMA and AVGE 50-70 cm in diameter and 24-27 m tall. Donna-related mortality in the Shark watershed ranged from 25-75% of the stems. Four stands were sampled along a 7 km transect adjacent to the Shark River and perpendicular to the path of Hurricane Donna. Mortality and recovery were inferred from diameter size-class density and basal area distributions. Analysis of the field data showed the presence of three cohorts - post-hurricane recruits, an intermediate size class of stump sprouts and young survivors, and mature survivors of the hurricane. Donna was a major hurricane and disturbance was assumed to be of high intensity. Standing-dead stems were used to estimate relative disturbance intensity among the four stands. According to the null hypothesis, disturbance intensity is the primary factor controlling stand recovery and pre-disturbance species composition has no effect. In the field: (1) the recruit cohort would dominate recovery and (2) the shade intolerant species (LARA) would be favored since available light would be plentiful. According to the alternative hypothesis, pre-disturbance species composition is the primary factor controlling stand recovery, not disturbance intensity. Assuming species composition is a mixture of all three species, in the field: (1) stems established before the disturbance would retain control of the site, and (2) the shade tolerant species (RHMA) would be favored in the post-disturbance recruit cohort. Results supported the alternative hypothesis: (1) the intermediate cohort dominated basal area, regardless of disturbance intensity, and (2) RHMA was the only species in the recruit cohort. A major conclusion of the field data analysis was that sprouting and shade tolerance are effective recovery mechanisms that unexpectedly boost residual stem density in highly disturbed stands. In the second phase of the research, simulations were used to extrapolate the field data to uniform disturbance intensities. This allowed comparison between plots with- and without sprouting. In this component of the research, the null hypothesis, in which disturbance intensity dominates recovery, would be expressed as a positive linear correlation between intensity and recruits and stump sprouts, and a negative linear correlation with survivor stems. The alternative hypothesis, in which species composition dominates recovery, would be expressed as a nonlinear relationship between these axes. In plots where sprouting did not occur, the relationship between intensity and structure was linear as predicted by the null hypothesis. In plots with sprouting, the relationship between intensity and recruits and sprouts was positive from low to moderate intensity and leveled off from moderate to high intensity, which supports the alternative hypothesis. After 32 years, basal area was 150% higher in the plots dominated by AVGE and LARA, 38 m2/ha vs 26 m2/ha in the RHMA dominated plots. Recruit basal area was two times higher in plots without sprouting. The major conclusion, when disturbance intensity is defined by the fraction of windfall stems (not mortality), is stands dominated by AVGE/LARA recover quicker than RHMA dominated stands and the impact of high intensity disturbance is similar to that experienced in moderately disturbed stands.

Download
Files over 3MB may be slow to open. For best results, right-click and select "save as..."

Share

COinS