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Management Project Summary: Fire effects on 3 subtropical invasive plants in Florida and the Caribbean—Common bamboo, Natal grass, and white leadtree

TABLE OF CONTENTS

INTRODUCTION:
This report summarizes information on 3 nonnative species occurring in southern Florida and the Caribbean. Information was provided by members of the Caribbean Fire Invasives Learning Network at a meeting in December 2009, in San Juan, Puerto Rico, and in subsequent correspondence [2]. This report is designed to share on-the-ground, local expertise of managers who have worked with invasive species in specific field locations. Information from managers is supplemented by some knowledge from scientific publications, but a comprehensive search of the literature was not conducted for this report. Readers should therefore be aware of the limited scope of knowledge given here, apply it with caution, monitor treatment results, and use an adaptive approach to management.

If you have additional knowledge on the species covered here, whether consistent with these reports or disagreeing with them, please share it in an email to fmi@fs.usda.gov, with "invasive information for FEIS" in the subject line.

Common names are used throughout this summary. For a complete list of the common and scientific names of species discussed and for links to FEIS species reviews, see the Appendix.

Authorship and citation for this Management Project Summary:
Caribbean Fire Invasives Learning Network. 2011. Management Project Summary: Fire effects on 3 subtropical invasive plants in Florida and the Caribbean—Common bamboo, Natal grass, and white leadtree. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: www.fs.usda.gov/database/feis//management_project_summaries/CFILN11/all.html [].

More information on these species is available from:

COMMON BAMBOO (Bambusa vulgaris)

Most of the information in this section has been provided by Robert De Matas (Natural Resources Officer, Department of Natural Resources & Environment, Scarborough, Tobago) [2]. Distribution and site requirements for common bamboo:
Common bamboo is considered invasive throughout southern Florida and the Caribbean. It occurs in forests, riparian zones, disturbed areas, and water courses. In cultivation, it thrives best under humid conditions up to 300 ft (1,000 m) elevation, but it can also tolerate dry conditions. In the dry season, plants may become completely defoliated.

Impacts of common bamboo:
Where common bamboo forms dense, monospecific stands, it reduces available space and resources for native species and may alter ecosystem structure.

Biology and ecology of common bamboo:
Common bamboo is a large, perennial graminoid with underground rhizomes. Although it is taxonomically a grass, its habit is somewhat tree-like. It forms dense, often monospecific stands of cylindrical, jointed woody stems up to 70 feet (20 m) tall and 2 to 4 inches (4-10 cm) in diameter. It has leafy branches at nodes, with narrow leaves up to 10 inches (30 cm) long.

Common bamboo can grow very rapidly, especially during the rainy season. In Tobago, one population of common bamboo increased from 85 to 135 stems in 14 days; during the same time, stems grew to heights of 10 to 14 feet (3-4 m). Common bamboo sprouts from rhizomes after it is cut. It can also reestablish from cut stems lying on the ground, perhaps sprouting from stem nodes.

Fuel and fire regime information for common bamboo:
Common bamboo stands contain a mixture of dead and live fuels. If a fire occurs during the rainy season, only about 20% of the plant burns. In the dry season, common bamboo stands have more dry material and less green material than in the rainy season, but still only about 40% of the plant is likely to burn.

Information on the distribution of common bamboo is insufficient for determining the specific plant community types that may be invaded and describing their historical fire regimes.

Response of common bamboo to fire:
Fire-related information in this report is mainly from Tobago, where most fires are caused by arson and occur during hunting season, October through February. In this region, common bamboo does not burn readily. Even in the dry season (December through June), fires do not generally burn common bamboo stands completely. In a common bamboo stand with 300 stems, dry-season fire top-killed stems along the edges but left many living stems in the interior. Stems that had been top-killed by fire resprouted from rhizomes.

Common bamboo resprouting after fire. Some clumps were burned and later were mechanically removed by excavators. Photo by Robert De Matas.

Control of common bamboo:
Managers in Tobago have reduced common bamboo abundance by cutting it, then burning the cut material so it does not regenerate from stem nodes, then cutting sprouts repeatedly so the plant eventually dies. A possible management strategy would be to cut only the exterior stems of a stand so fire can spread through the interior and top-kill these stems. Repeated cutting of sprouts would then be needed to kill the entire plant.

Farmers in Tobago have suggested that okra plants inhibit common bamboo growth. If okra is planted soon after a common bamboo stand is burned, the bamboo rhizomes may fail to resprout.

NATAL GRASS (Melinis repens)

Most of the information in this section has been provided by the following professionals working with fire and Natal grass in Florida pine rocklands and sand pine scrub: Rosi Mulholland (Florida Park Service, Clermont, FL, USA), Joe Maguire (Miami-Dade Parks, Miami, FL, USA), Keith Bradley (The Institute for Regional Conservation, Miami, FL, USA), Steve Morrison (Conservation Program Manager, The Nature Conservancy, Babson Park, FL, USA), David Knowles (Park Warden, Bahamas National Trust, Abaco, BS) [2]. Distribution and site requirements of Natal grass:
In central and subtropical Florida, Natal grass occurs in disturbed uplands, drained hydric pine and cypress prairies, pine rocklands, scrub habitats, flatwoods, firebreaks, and sandhill restoration areas [2]. As of 1999, Natal grass occurred in 49% of nature preserves in southern Florida [1]. In pine rocklands, Natal grass establishes and spreads on both limestone and sandy soils but may be more invasive on sandy soils, especially after disturbance [2]. It may be more persistent in pine rockland forests of southern Florida than in those of central Florida [8]. Natal grass also occurs in sand pine scrub; experiments in the Ocala National Forest, central Florida, indicated that it establishes and persists more successfully along roadsides than in the interior of sand pine scrub (Table 1) [3]. Natal grass may be less invasive in white sand scrub than in yellow sand scrub, possibly responding to the slightly greater nutrient content of soils in white sand scrub [2].

Table 1. Frequency (%) of seeded Natal grass 6 months after planting and 4.5 years after planting [3]
Proximity to unpaved road* Roadside Interior
Year measured 1995 1999 1995 1999
Road and roadside soil type:
~Native sand		 25-75 0 0-25 0
~Sand amended with clay 50-100 0-25 70-100 0
~Sand amended with crushed limestone and clay 0-50 0-33 50-100 0
*Roadside measurements were taken within 3 feet (1 m) of road edge; interior measurements were taken at least 30 feet (10 m) from edge of same road, where soils were not ammended.

Natal grass does not persist on soils that retain moisture and is inhibited by flooding (as little as 1 month of inundation). It is also inhibited by shade and cold, although seeds can germinate after freezing [2].

In the Bahamas, Natal grass occurs in farm areas and where farms have been abandoned. It is especially prevalent on limestone substrates, particularly in areas that have been fertilized for agriculture. Natal grass also occurs on sand dunes in the Bahamas. It is not considered invasive in natural areas of the Bahamas [2].

Impacts of Natal grass:
Natal grass establishes and regenerates most successfully on sites that have been mechanically disturbed, including road edges and firelines, but it can also spread from edges into the interior of undisturbed areas. It can thrive in drained sites, mowed firebreaks, fire-created openings [2], and mined areas [9]. It can form a monoculture in disturbed areas, reducing native species, particularly graminoids. In Florida pine rocklands, the species richness of native graminoids was negatively associated with density of Natal grass (P<0.001), while richness of trees, shrubs, and vines was not significantly associated Natal grass density [8]. On phosphate-mined uplands in Florida, first-year survival of planted Beyrich threeawn and lopsided indiangrass was greater where Natal grass had been removed [9].

Biology and ecology of Natal grass:
Natal grass is a short-lived grass ("annual to facultative perennial" [3]) that reproduces from small, wind-dispersed seeds. It does not have rhizomes [8]. In Florida, it can reproduce year-round; 3 or 4 seed crops may be produced in a single season [2]. Experiments showed that germination of Natal grass seed improved with storage, possibly indicating an afterripening mechanism. Fresh seed showed 55% germination, seed stored for 15 weeks showed 80% germination, and seed collected from the duff layer showed 95% germination. Germination occurred at temperatures between 70 and 90 °F (20-35 °C), with optimum germination at 90 °F (30 °C). Seedlings emerged from seeds buried 1 inch (2 cm) deep, though most seedlings emerged from seeds buried 0.2 inch (0.5 cm) deep or less. Natal grass required "fairly moist conditions" to germinate, suggesting that most germination is likely to occur during rainy periods [7]. Natal grass can grow rapidly, maturing from seedling to seed-bearing adult in 8 weeks [2].

Fuel and fire regime information for Natal grass:
Fuel properties of Natal grass are similar to those of many native grasses, including wiregrasses. In the understory of pine rocklands, Natal grass co-occurs with native grasses and does not affect fuel loading or spatial arrangement. However, in dry areas and scrub habitats that historically supported discontinuous grass cover mixed with shrubs, Natal grass creates continuous fine surface fuels unlike those in natural stands. The dried grasses accumulate over winter, creating greater fuel loads in spring [2].

Natal grass burns readily, especially in winter and spring. Thick patches of dry Natal grass may provide a flashy fuel bed that burns "hot" and spreads fire rapidly. In the Bahamas, however, Natal grass is not considered a particularly flammable fuel [2].

Table 2 describes presettlement fire regime characteristics of some plant communities in central and southern Florida where Natal grass occurs:

Table 2. Fire regime information on some vegetation communities where Natal grass occurs. This information is taken from the LANDFIRE Rapid Assessment Vegetation Models [6], which were developed by local experts using available literature, local data, and/or expert opinion. This table summarizes fire regime characteristics for each plant community listed. The PDF file linked from each plant community name describes the model and synthesizes the knowledge available on vegetation composition, structure, and dynamics in that community. Cells are blank where information is not available in the Rapid Assessment Vegetation Model.
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
(years)		 Minimum interval
(years)		 Maximum interval
(years)
Palmetto prairie Replacement 87% 2 1 4
Mixed 4% 40    
Surface or low 9% 20    
Pond cypress savanna Replacement 17% 120    
Mixed 27% 75    
Surface or low 57% 35    
Pocosin Replacement 1% >1,000 30 >1,000
Mixed 99% 12 3 20
Pine rocklands
Mixed 1% 330    
Surface or low 99% 3 1 5
Sand pine scrub Replacement 90% 45 10 100
Mixed 10% 400 60  
*Fire Severities—
Replacement: Any fire that causes greater than 75% top removal of a vegetation-fuel type, resulting in general replacement of existing vegetation; may or may not cause a lethal effect on the plants.
Mixed: Any fire burning more than 5% of an area that does not qualify as a replacement, surface, or low-severity fire; includes mosaic and other fires that are intermediate in effects.
Surface or low: Any fire that causes less than 25% upper layer replacement and/or removal in a vegetation-fuel class but burns 5% or more of the area [4,5].

Response of Natal grass to fire:
Natal grass is tolerant of fire. It spreads into burned areas from wind-dispersed seed and establishes readily in openings created by fire. In pine rocklands, Natal grass populations increased after severe summer fires, possibly because these fires prevented regeneration of native species, including saw palmetto. In pine rocklands and sand pine scrub, organic matter accumulates in the soil after many years without fire. Fires in these communities tend to be severe when the organic soil layers burn. Areas where woody debris has accumulated or been piled can also burn severely, increasing the potential for invasion by Natal grass [2].

Control of Natal grass:
Any treatment to control Natal grass requires repeated followup, at approximately 6- to 8-week intervals, since seeds germinate readily and new plants rapidly produce seed [2]. Hand-pulling can be effective for controlling Natal grass. Most other control techniques in natural areas are likely to cause substantial collateral damage to some native species. Here are examples from field managers [2]:

On reclaimed phosphate-mined uplands in Florida [9], postemergent application of imazapic caused severe damage to Natal grass, causing nearly total necrosis 6 weeks after an August application. The same application had mixed effects on native species, causing little damage to some species (Beyrich threeawn and creeping bluestem, for example), but causing severe damage to lopsided indiangrass [9].

The following management approaches are being investigated as of 2009: No treatment of invasive plants in a natural area is effective unless seed sources outside the area are also eliminated. Currently no knowledge is available on how far seed of Natal grass can disperse, so the minimum width of "buffer strips" to prevent spread is unknown [2].

WHITE LEADTREE (Leucaena leucocephala)

The information in this section has been provided by professionals working with fire and white leadtree in Puerto Rico, Dominican Republic, and southern Florida [2].

Distribution and site requirements of white leadtree:
White leadtree is invasive in many subtropical and tropical areas, including Puerto Rico, Dominican Republic, and Florida. It has been widely promoted for tropical forage production and reforestation, although managers in Puerto Rico report that it is not a good forage plant. White leadtree populations can spread easily. The species is difficult to eradicate once established. White leadtree became invasive in southwestern Tobago as a result of the removal of mangroves and subsequent back-filling of the area by land developers.

White leadtree can tolerate a wide range of rainfall (from 20 to 140 inches/year (500 - 3,500 mm)) and can withstand dry seasons 6 to 8 months long. However, it is not frost hardy and grows poorly in relatively cool tropical highland sites. White leadtree also grows poorly on acid soils with high aluminum saturation.

Impacts of white leadtree:
White leadtree establishes in open areas and tends to grow in dense, monospecific thickets, especially in coastal and riverine habitats and in disturbed areas, rendering extensive areas inaccessible. Where white leadtree becomes established, native and endemic species are less likely to occur. However, it may not be invasive in undisturbed, closed forests, and it may be kept in check by browsing deer. On a Florida island with many deer, for example, it shows little impact (as of 2009).

Biology and ecology of white leadtree:
White leadtree is a perennial shrub or tree that reaches up to 20 feet (6 m) tall. It produces flowers year-round, often in "massive" quantities. It is able to sprout after being cut.

White leadtree grows better in light than shade. Because of this property, it thrives in openings caused by hurricanes, fire, and land clearing.

Fuel and fire regime information for white leadtree:
White leadtree itself does not seem to increase fuel loads or change the spatial distribution of fuels in the tree canopy. However, grass can grow under white leadtrees, whereas it does not grow well under native forest cover. Dense grass under the white leadtree canopy may increase the quantity and continuity of fine surface fuels; these changes have potential to increase fire spread and continuity.

Information on the distribution of white leadtree is insufficient for determining the specific plant community types that may be invaded and describing their historical fire regimes.

Response of white leadtree to fire:
Managers have observed white leadtree sprouting after top-kill by fire.

Control of white leadtree:
In Florida, managers have controlled white leadtree by burning it, then applying herbicide. Attempts to kill it with fire alone or herbicide alone have failed.

APPENDIX: SPECIES INCLUDED IN THIS SUMMARY

Common name Scientific name
Beyrich threeawn Aristida beyrichiana
common bamboo Bambusa vulgaris
creeping bluestem Schizachyrium scoparium var. stoloniferum
lopsided indiangrass

Sorghastrum secundum
rose Natal grass Melinis repens (Rhynchelytrum repens)*
okra (ochro) Abelmoschus esculentus
saw palmetto Serenoa repens
white leadtree Leucaena leucocephala
wiregrass Aristida spp.
*Names in parentheses are from the sources cited.

REFERENCES:

1. Bradley, Keith; Gann, George D. 1999. The status of exotic plants in the preserves of southern Florida. In: Jones, David T.; Gamble, Brandon W., eds. Florida's garden of good and evil: Proceedings of the 1998 joint symposium of the Florida Exotic Pest Plant Council and the Florida Native Plant Society; 1998 June 3-7; Palm Beach Gardens, FL. West Palm Beach, FL: South Florida Water Management District: 36-41. [53998]
2. Florida and Caribbean Fire and Invasives Learning Network. 2011. Workshop: Information gathering on invasive species. Notes from the 2nd meeting of the Florida and Caribbean Fire and Invasive Learning Network; 2009 December 8-10; Isla Verde, Carolina, Puerto Rico. Alamonte Springs, FL: The Nature Conservancy, Florida Chapter. [Smith, Jane Kapler, compiler]. Unpublished report on file with: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory, Missoula, MT. 8 p. [81645]
3. Gordon, Doria R.; Greenberg, Cathryn H.; Crownover, Stanley H.; Slapcinsky, Jodi L. 2005. Effects of unpaved road soil on persistence of three non-native grass species. Natural Areas Journal. 25(3): 257-262. [81028]
4. Hann, Wendel; Havlina, Doug; Shlisky, Ayn; [and others]. 2010. Interagency fire regime condition class (FRCC) guidebook, [Online]. Version 3.0. In: (Frames Fire Research And Management Exchange System). National Interagency Fuels, Fire & Vegetation Technology Transfer (NIFTT) (Producer). Available: http://www.fire.org/niftt/released/FRCC_Guidebook_2010_final.pdf. [81749]
5. LANDFIRE Rapid Assessment. 2005. Reference condition modeling manual (Version 2.1), [Online]. In: LANDFIRE. Cooperative Agreement 04-CA-11132543-189. Boulder, CO: The Nature Conservancy; U.S. Department of Agriculture, Forest Service; U.S. Department of the Interior (Producers). 72 p. Available: http://www.landfire.gov/downloadfile.php?file=RA_Modeling_Manual_v2_1.pdf [2007, May 24]. [66741]
6. LANDFIRE Rapid Assessment. 2007. Rapid assessment reference condition models, [Online]. In: LANDFIRE. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Lab; U.S. Geological Survey; The Nature Conservancy (Producers). Available: http://www.landfire.gov/models_EW.php [2008, April 18] [66533]
7. MacDonald, Greg; Stokes, Courtney. 2009. Biology and management of natalgrass (Melinis repens) in Florida. Final Report. Gainesville, FL: University of Florida, Department of Agronomy. Unpublished report on file at: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Forestry Sciences Laboratory, Missoula, MT. 4 p. [81687]
8. Possley, Jennifer; Maschinski, Joyce. 2006. Competitive effects of the invasive grass Rhynchelytrum repens (Willd.) C. E. Hubb. on pine rockland vegetation. Natural Areas Journal. 26(4): 391-395. [65076]
9. Richardson, Steven G.; Bissett, Nancy; Knott, Cathy; Himel, Kate. 2003. Weed control and upland native plant establishment on phosphate mined lands in Florida. In: Cannizzaro, Patrick J., ed. Proceedings of the 30th annual conference on ecosystems restoration and creation; 2003 October; Plant City, FL. Tampa, FL: Hillsborough Community College: 126-138. [81029]
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