Potential use of Uniola paniculata rhizome fragments for dune restoration

Summary

Study 1

Along the coast of the Gulf of Mexico sand dunes protect barrier islands and the mainland from storm surges and most hurricane generated waves. However, severe surges may breach even well-developed dunes, leaving foredune ridges fragmented. Perennial grasses facilitate foredune reestablishment by intercepting and stabilizing wind-blown sand within a latticework of roots, rhizomes and tillers. Along the Gulf and southern Atlantic coastal region of USA, the most abundant of these dune-building grasses is sea oats Uniola paniculata. As a potential restoration technique to assist in recovery of damaged dune systems, a study was undertaken to assess the efficacy of using sea oats rhizomes uprooted by hurricanes for use in dune planting. To test this technique, tiller emergence was observed from sea oats rhizomes replanted after submersion in seawater, freshwater rinses, exposure to air and with or without supplemental water after planting. Additionally, sea oats rhizomes uprooted by a hurricane were experimentally planted, and effects of soil salinity and moisture on emergence tested in a greenhouse (see Case 574).

Study site: The study was conducted on Eglin Air Force Base land on Santa Rosa Island, Florida, southeast USA. Trial sites were positioned on level overwashed areas, which were beach dunes before landfall in the vicinity of hurricanes Opal and Erin. Before the hurricanes foredune vegetation was dominated by sea oats Uniola paniculata. Soils consist almost entirely of pure quartz sand of a medium diameter (approximately 0.25 mm). The climate is mild and subtropical, average annual rainfall is 1,520 mm.

Exposure time: Each month from June to October 1996, sea oats rhizomes with attached culms were excavated from a natural population, cut into fragments and divided into one of four types: 1) leafy culm with basal nodes intact; 2) rhizomes with one node; 3) rhizomes with two nodes; 4) rhizomes with three nodes. All were immersed in seawater for 5 min, wrapped in nylon fish net, and left exposed on the beach for 1, 3 or 7 days before planting. Unless otherwise indicated, fragments were randomly planted 20 cm deep, 1 m apart in six rows, approximately 50 m inland from the mean high tide line. Emergence (at least one tiller per fragment) was recorded weekly from November or December and monthly thereafter.

As emergence generally declined with decreasing fragment size, only three-node fragments were subsequently used in tests. Monthly from June to October 1997 additional three-node rhizomes were planted on the beach after excavation, immersion in seawater (5 min), and 3, 5 and 7 days of air exposure.

Rhizomes were excavated in July and October 1998. As the lethal exposure threshold had not been reached in the first experiment, exposure was extended to 9 and 11 days. Also, because of variability in survival, the number of replicate rhizomes (compared with earlier years) was increased.

Storage treatment: In 1997, three-node rhizomes were immersed in seawater, exposed to air for 3 and 5 days, wrapped in a burlap sack saturated with tap water and wet daily, and planted 5 days or 7 days after initial excavation. In 1998 additional rhizomes were exposed for 3 days, wrapped in a wet burlap sack, and planted 9 and 11 days after excavation. Burlap was selected as it retains moisture and is porous enough to allow oxygen to reach the rhizomes.

Watering: In 1996 and 1997 a pot experiment using the treatments as above was conducted to determine fragment viability with supplemental water. Fragments were planted 10 cm deep in 3.8-L plastic pots filled with beach sand. A 5 cm bark mulch layer prevented sand loss through drainage holes. Pots were kept outside and the sand kept moist by watering to saturation weekly. To further determine if watering would increase emergence, all burlap–exposure–rinsing combinations were planted in watered and non-watered plots on the beach in 1998 and 2000. Rhizomes assigned to watered plots received 3.8 L of fresh water at planting and once a week thereafter for at least 5 weeks.

Hurricane Georges: On 27 and 28 September 1998 storm surge from Hurricane Georges uprooted rhizomes from sea oats populations on Santa Rosa. Three days later, 50 three-node rhizomes were collected, wrapped in wet burlap for 2 days, and planted at a depth of 20 cm, approximately 75 m inland from the mean high tide line, and 50 three-node rhizomes planted on the beach. Twenty-five rhizomes from each exposure treatment were watered.

Rinse treatment: Initial results suggested rhizome survival and tiller emergence responded to seawater and freshwater rinsing and soil conditions. To further assess the impact of rinsing, three-node rhizomes were replanted after receiving a seawater followed by fresh water rinse, or no rinse before planting. Seawater rhizomes consisted of fresh dug rhizomes rinsed with seawater. A subset of no rinse and seawater rinse, received a freshwater rinse simulating rainfall.

Emergence of sea oats tillers generally declined with increasing air exposure time, except when at least 68 mm of rain fell during exposure or immediately thereafter (e.g. as associated with Hurricane Georges). Less than 25 mm of rain during exposure throughout 1996 resulted in consistently low (≤20%) emergence for beach planted rhizomes exposed for 3 days. Lethal exposure time decreased in driest months, with beach-planted fragments failing after 3, 5 and 7 days in August 1996, September 1997 and October 1998, and after 5 and 7 days in August 1997. Rainfall prolonged viability of exposed rhizomes. Decreasing fragment size also led to reduced emergence in 1996.

Supplemental water: More potted rhizomes emerged and after longer exposure than did beach-planted rhizomes without supplemental water except when rain fell during exposure. Average emergence in 1996 after 1 day of exposure for potted rhizomes was 72% and for beach-planted 36%. Tillering of watered (pot and beach) rhizomes did not consistently decrease with exposure.

Moist storage: wrapping in moist burlap before beach planting extended viability of fragments. However, results were inconsistent and also varied with rainfall during exposure. Planting burlap-treated rhizomes without supplemental water generally resulted in similar emergence to rhizomes exposed, planted in pots, and watered unless rainfall and soil moisture were low. Planting rhizomes in pots after burlap treatment did not consistently increase emergence over planting in pots with no burlap treatment.

Emergence & rainfall: Over the first 3 years, emergence at each exposure length was compared with amount of rainfall during exposure. There was a significant positive relationship between rainfall and emergence for 3 day, 5 day and 7 day exposure. However, when emergence at each exposure length was compared with rainfall categorized as > 25 mm or < 25 mm, a significant relationship was only evident at 7 days exposure.
Rinse treatments: There was a 61% increase in emergence for freshwater rinse rhizomes compared to fresh dug and rhizomes previously exposed to salt water. Emergence of rhizomes that received salt exposure before planting (rinsed with seawater only and rinsed with seawater followed by fresh water) did not differ from fresh dug rhizomes. Tiller emergence was initially greater for rhizomes receiving 5 weeks of watering, however, after this stopped emergence equalized between watered and unwatered areas.

Conclusions: Sea oats tiller emergence declined with increasing length of air exposure and decreasing rhizome size. Tiller survival was enhanced by rainfall, watering, and rinsing with salt or fresh water during exposure and immediately after planting. Tiller emergence was reduced by soil salinity of 5,800 μS/cm but not 1,800 μS/cm. Over the 4-year study period, tiller emergence varied considerably. Rhizomes lost bud viability after 3–5 days of beach exposure, unless they recieved fresh water from rainfall, wet burlap, or watering within 3 days. Bud viability was extended by up to 11 days when supplied with water. Results from this study indicate that use of sea oats rhizomes uprooted by hurricanes is a viable restoration technique provided that they are reburied within 3–11 days of exposure.

 

Study 2

Along the coast of the Gulf of Mexico sand dunes (3 to 10 m in height) protect barrier islands and the mainland from storm surges and most hurricane generated waves. However, severe surges may breach well-developed dunes, leaving foredune ridges fragmented. Perennial grasses facilitate foredune reestablishment by intercepting and stabilizing wind-blown sand within a latticework of roots, rhizomes and tillers. Along the Gulf and southern Atlantic coastal region of USA, the most abundant of these dune-building grasses is sea oats Uniola paniculata. As a potential restoration technique to assist in recovery of damaged dune systems, a study was undertaken to assess the efficacy of using sea oats rhizomes uprooted by hurricanes for use in dune planting. As part of this study, sea oats rhizomes uprooted by a hurricane were experimentally planted, and effects of soil salinity and moisture on emergence tested in a greenhouse. Tiller emergence was also observed from sea oats rhizomes replanted after submersion in seawater, air exposure, freshwater rinses, and with or without supplemental water after planting in the field (see Case 573).

To separate the effects of rinsing, exposure, soil moisture and salinity on sea oats tiller emergence, a greenhouse experiment was conducted in conjunction with field trials (see Case 573).

Experimental design: Approximately 160 three-node rhizomes were uprooted on 26 June 2001 and 100 on 29 June 2001. Rhizome treatments included: seawater rinse (fresh dug rhizomes rinsed with seawater to simulate overwash); freshwater rinse, and air exposure (0 and 3 days). A subset of each (no rinse and seawater) received an additional freshwater rinse (simulating rainfall). Freshwater rinsing consisted of immersion in well water (salinity = 754 μS/cm = 0.377 ppt; sea strength being approximately 33 ppt) for 4 min and an additional 2-min soaking in distilled water then planting. Before planting, distilled water-rinsed beach sand (salinity 2.58 μS/cm) was dried for 3 days at 68°C and one of three salinity treatments applied:

1) soil saturated with 1.5 L of deionized water with no added salt

2) soil saturated with 1.5 L of water with a salinity of 20,000 μS/cm

3) soil saturated with 1.5 L of water with a salinity of 60,000 μS/cm (seawater salinity)

Immediately after saturation, soil salinity was 70 μS/cm (low), 1,800 μS/cm (medium) and 5,800 μS/cm (high).

The experiment had a factorial arrangement of treatments with watering (two levels), salinity (three levels), and exposure/rinse treatments (five levels).

Eight replicates of the five treatments were randomly planted 6 cm deep in each of six beach sand treatments. After planting half the trays were watered with 2.5 L fresh water once a week (moist) and half the trays were watered with 2.5 L fresh water every 2 weeks (dry). As the trays lacked drainage holes, all salts added initially remained in the sand. Tiller emergence was recorded daily for 3 months. At the end of the 3-month experiment all trays (reps) were watered (as above) and sand samples were collected daily to determine percent soil moisture and salinity during a typical watering/dry down cycle.

Consequences: With dry soil conditions and exposure of 3 days, sea oats tillering failed. Soil moisture in greenhouse planting trays dropped below 2% 5 days after watering. With or without rinsing tiller emergence dropped to no more than 13% when replanted rhizomes were only watered every 2 weeks.

Tiller emergence was generally better (ranging from 0 to 88%) for rhizomes watered weekly. Weekly watering maintained soil moisture at 8 to 1%. Exposure of 3 days reduced tiller emergence, more so in the absence of a pre-planting rinse of either salt or freshwater. Tiller emergence was not reduced at low to medium soil salinity, but was reduced significantly at a high soil salinity of 513–5,800 μS/cm.

However, soil salinity recorded on the beach site used for field trials (see Case 573) never exceeded 17 μS/cm. Although soil moisture at the beach remained between 2 and 4% during winter months with infrequent rainfall, during June moisture dropped to less than 2% even with rainfall of 7 cm just before sampling.

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