Degree

Doctor of Philosophy (PhD)

Department

Environmental Science

Document Type

Dissertation

Abstract

Louisiana's Mississippi River Delta has lost an estimated 4,900 km² of coastal land since the 1930s through levee-induced sediment deprivation, subsidence, saltwater intrusion, and hurricane disturbance. This dissertation examines ecological, biophysical, and landscape-scale factors governing wetland restoration through four studies in Breton Sound, Louisiana, focusing on bald cypress (Taxodium distichum) establishment and land recovery.

Chapter 2 evaluated 200 bald cypress saplings at one New Land (NL4) and three Spoil Bank sites (SB1–SB3) over 22 months. New Land survival (77.9%) exceeded Spoil Banks (41.1%; p = 0.005). A storm two days post-planting drove acute mortality at Spoil Banks. Vine competition (Vicia spp.) predicted mortality (OR = 35.0, p < 0.0001), with Spoil Bank mortality reaching 87.5% versus 16.7% at the vine-free site.

Chapter 3 quantified biophysical filters governing establishment. Soil strength negatively correlated with survival (r = −0.575, p = 0.005), threshold at 5.0 kPa—below which survival averaged 62.1% versus 39.2% above (Cohen's d = 0.98). River nitrate positively correlated with survival (r = 0.517, p = 0.014). New Land (4.2 kPa) outperformed compacted Spoil Banks (5.1 kPa), yielding criteria: soil strength below 4.5 kPa, moderate nitrate, and naturally accreting substrates.

Chapter 4 examined river nitrate, porewater nitrogen, and soil strength across eleven sites over six periods. River NO₃⁻-N was negatively associated with soil strength (r = −0.289, p = 0.021; β = −0.322, p = 0.015), lowest in summer (~4.4 kPa) when nitrate peaked (median 1.53 mg/L). An NH₄⁺ hotspot of 56.7 mg N/L—370 times the site median—indicated anaerobic microsites, supporting the fertilization-weakening hypothesis.

Chapter 5 used 19 years of Landsat imagery (2006–2025) and a practitioner interview to assess post-Katrina dynamics. Land area remained stable (~6,000–6,600 ha) while vegetated cover increased 31.2%. The managed site achieved 247% vegetation gain versus the landscape-wide 18.2%, with 64.8% of area unvegetated. A diversion created ~2,800 acres of marsh, confirming connectivity and intervention drive recovery.

Across studies, success was governed by compaction, vegetation competition, storm timing, and connectivity. The 5.0 kPa threshold, nitrate–soil strength association, and 247% vegetation gain provide evidence-based guidance for integrating tree planting with sediment diversions under Louisiana's Coastal Master Plan.

Date

6-26-2026

Committee Chair

Bui, Linda H

LSU Acknowledgement

1

LSU Accessibility Acknowledgment

1

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