Abiotic Stresses to Vegetation Re-establishment in a Cutover Bog Contaminated with Seawater

Abstract

Part of a cutover bog in Pokesudie Island, New Brunswick, Canada was contaminated with seawater and was still largely devoid of vegetation 5 years after the event and was consequently chosen for study. The study area consisted of rectangular fields with cambered surface that sloped down (2%) to the drainage ditches on both sides. Across this slope zones were created: Up-, Mid- and Low- areas on either side of the centerline of fields. Two field experiments were conducted to determine abiotic stresses to plant re-establishment in terms of hydrology and peat characteristics along this cambered surface. The general objective was to identify microsites or zones that could be suitable to the introduction of wetland halophytes <em>Juncus balticus</em> Willd. and <em>Spartina pectinata</em> Link obtained from nearby salt marshes. <br /><br /> In the first experiment, cylindrical <em>J. balticus</em> sods were transplanted into the Up- and Low- areas, at 1, 3, 5, 10 and 20 d of incubation (in May 2005) with measurements made on the Outer and Inner annular sod sections, replicated over 4 blocks. Moisture (% dry weight basis (dwb)) reached maximum values 1 day after transplantation, 84±0. 05 for Outer and 103±0. 07 for Inner sod section. Salinity (dS m^-1) in sods due to ingress of sodium (Na⁺ ) and chloride (Cl^-) reached values of the surrounding peat 3 days after transplantation, 3. 52±1. 06 for Inner sod section and 4. 11±0. 99 for Outer sod section in Up-areas, and 1. 76±0. 24 for Inner sod section and 2. 57±0. 28 for Outer sod section in Low-areas. Maximum decrease in pH was at 5 days after transplantation, in Outer sod section in the Up-areas (from 5. 89 to 4. 88±0. 14) which was much higher than the pH range of 3-4 of the surrounding peat. This was due to the buffering capacity of calcium (Ca²⁺) and magnesium (Mg²⁺) in sods which did not change in concentration after 20 days of incubation. Therefore, Inner sod sections were less affected by the surrounding peat compared to the Outer sod sections, suggesting that a larger sod volume may alleviate stressful conditions for a longer time at transplantation and consequently allow greater time for adaptation. <br /><br /> In the second experiment, <em>J. balticus</em> and <em>S. pectinata</em> were transplanted on the 3 Locations Up-, Mid- and Low- areas, replicated over 10 blocks; and peat characteristics were measured at Depths 0-5, 5-10, 10-15 and 15-20 cm 5 times during the study period May-August 2005. Survival of <em>J. balticus</em> was poorest (27. 5±8. 3 %) in the Low-areas compared to 68. 5±8. 9 % in the Up- and 58. 5±8. 7% in the Mid- areas. <em>S. pectinata</em> survival was very good at all Locations (89±5. 3, 91. 6±3. 1 and 84. 2±4. 4 for Up-, Mid- and Low- areas, respectively) having better adaptation to early season waterlogged conditions. Waterlogged conditions resulted from a perched water table during the early part of the growing season (May-June) and were alleviated only upon the complete thaw of the frozen peat layer on 8 July. Thereafter, important changes in hydrology and peat characteristics occurred: water table depths decreased from -8. 5±1. 7 and -1. 6±1. 2 cm on 26 May, to -51. 5±2. 5 and -40. 7±2. 4 cm by 9 August in Up- and Low-areas, respectively; redox potentials at 12 cm depth increased from 26 June (190. 9±8, 175±10. 8 and 109. 2±29. 4 mV) to 9 August (282. 8±8, 302. 8±14. 3 and 312. 3±29. 6 mV) in the Up-, Mid- and Low-areas, respectively which showed that anaerobic conditions were maintained throughout the study period; decreased moisture content from 1256. 8±61. 9, 1667. 4±126. 3 and 1728. 6±153 on 30 May, to 851. 7±21. 2, 874. 6±47 and 1008. 2±57. 5 % dwb on 25 July) which caused increased dry bulk density (from 0. 07±0. 002, 0. 06±0. 003 and 0. 07±0. 01 to 0. 09±0. 003, 0. 09±0. 005 and 0. 08±0. 004) in the Up-, Mid- and Low-areas, respectively; and increased electrical conductivity (salinity) especially on the 0-5cm surface (from 1. 9±0. 13, 1. 8±0. 31 and 1. 5±0. 29 to 18±1. 9, 17. 5±1. 1 and 12. 2±1 dS m^-1) which also caused decreased pH (from 3. 5±0. 04, 3. 5±0. 08 and 3. 6±0. 01 to 2. 85±0. 04, 2. 85±0. 01 and 2. 9±0. 03) in the Up-, Mid- and Low-areas, respectively. Therefore, spring flooding followed by high surface salinity in summer precludes plant establishment by seeding and explains the current lack of spontaneous revegetation. Waterlogged conditions were of greater magnitude and duration at lower elevation areas unfavourable to <em>J. balticus</em> survival but salinity levels were high in the Up- and Mid-areas. <br /><br /> In the subsequent part of the second experiment, plants of <em>J. balticus</em> and <em>S. pectinata</em> grown in the study area and those collected from marshes were divided into above- and below- ground parts and accumulation of salt ions in plant tissues were determined to understand the species' salt-tolerance mechanism, as well as the accumulation of potentially toxic levels of iron (Fe) and manganese (Mn). Both plant species had similar accumulations (mmol kg^-1 dry wt,) of Na⁺ (474. 3±41 and 468. 3±31. 7, respectively) and Cl^- (314. 9±21. 9 and 310. 5±27. 5, respectively) in the above-ground parts but differed in how they managed Na⁺. <em>J. balticus</em> accumulated more Na⁺ in below-ground parts (659. 3±88. 7) and had limited transport to the above-ground parts, while <em>S. pectinata</em> accumulated and excreted Na⁺ in the above-ground parts and had less accumulation in the below-ground parts (397. 4±25. 1). <em>S. pectinata</em> maintained (313. 1±23. 8 in marsh <em>vs. </em> 292. 4±26. 2 in bog) and <em>J. balticus</em> increased (84. 2±1. 2 in marsh <em>vs. </em> 531. 2±38. 6 in bog) K⁺-selectivity in the shoots, a key requirement for survival in saline conditions. Compared with their respective marsh plants, <em>S. pectinata</em> had more salinity-tolerance than <em>J. balticus</em> primarily through its maintenance of Ca²⁺ (21. 5±1. 7 in marsh <em>vs. </em> 35. 6±3. 8 in bog) compared to a decrease in <em>J. balticus</em> (144. 7±12. 5 in marsh <em>vs. </em> 41±3. 7 in bog). Furthermore, Fe and Mn uptake in both species decreased but reached critical Fe-deficiency levels (1. 1±0. 1 mmol kg^-1 dry wt,) only in <em>S. pectinata</em> grown in drier areas. <br /><br /> It is concluded that local conditions of waterlogging (especially in lower elevation areas) and high salinity and low pH (notably in the upper elevation areas) were favourable to the survival of <em>S. pectinata</em> in all areas and <em>J. balticus</em> only in upper elevation areas. Sod transplanting may alleviate the acidity problem and depending on sod volume may delay the effects of harsh conditions of the cutover bog. However, long-term survival and growth of both species in drier areas may be constrained by deficiency in calcium in <em>J. balticus</em> and iron in <em>S. pectinata</em>.