Role of Fire in the Germination Ecology of Fountain Grass (Pennisetum setaceum), an Invasive African Bunchgrass in Hawai‘i

Field and laboratory studies were carried out to test factors expected to be relevant for the germination of fountain grass: (1) light; (2) emergence of fountain grass seedlings from depths of 0, 2.5, and 5 cm; (3) fire passing over exposed and buried seeds; (4) laboratory heat treatment mimicking exposure to grass fire. Both fire in the field and heat applied in the laboratory killed fountain grass seeds. In the laboratory, some seeds were killed after exposure to 75 C for 3 min, and all seeds were killed at 100 C. During the prescribed burns, temperatures at the soil surface reached at least 204 C, but temperatures at depths of 2.5 and 5 cm showed no measurable change. Light is not essential for germination of fountain grass seeds, and seedlings can emerge from depths of at least 5 cm. Both of these traits contribute to the invasive capacity of the species. Because fountain grass seeds are killed at temperatures in excess of 100 C, the species depends on its ability to resprout and quickly set seed after fire for population growth and spread. Seeds buried beneath the soil may escape exposure to fire, and substrate heterogeneity may provide refuge from temperature extremes experienced during fire. The morphology of fountain grass seeds likely inhibits burial in the soil for the most part, but there are several potential burial mechanisms. Prescribed burns could prove to be a useful tool for fountain grass control in large, degraded sites where fountain grass has invaded but only when coupled with additional control measures. Invasion of native ecosystems by alien grasses has become a worldwide phenomenon (D’Antonio and Vitousek 1992), and island ecosystems are particularly vulnerable to alien plant invasion (Loope and Mueller-Dombois 1989, Simberloff 1995). Alien plant species threaten the native biota of Hawai‘i in rising numbers (Denslow 2003, Asner et al. 2008). The Hawaiian Islands now play host to many alien grass species including fountain grass, Pennisetum setaceum (Forssk.) Choiv. Most of these alien grasses were brought in as ornamental species or fodder for livestock, and some have quickly spread and now dominate large portions of previously native ecosystems (Smith 1985, D’Antonio and Vitousek 1992). Several of the grass species problematic in Hawai‘i are a concern throughout the Indo-Pacific (D’Antonio and Vitousek 1992). Clearing of lands for agriculture or cattle ranching usually prompts the introduction of alien grasses, and the ensuing grass/fire cycle facilitates their spread into neighboring native ecosystems (D’Antonio and Vitousek 1992). In Hawai‘i, these grasses also colonize barren lava flows (Hughes et al. 1991, Williams and Black 1994), creating contiguous fuel loads interconnecting otherwise isolated forest patches. As a result, the likelihood and propagation of fire is greatly increased. Many of these invasive grasses evolved in habitats prone to frequent fire and have Pacific Science (2011), vol. 65, no. 1:17–25 doi: 10.2984/65.1.017 : 2011 by University of Hawai‘i Press All rights reserved 1 Research funding was provided by the Joint Fire Science Program, the Charles H. Lamoureux Fellowship in Plant Conservation, and the West Hawai‘i Wildfire Management Organization. Manuscript accepted 12 April 2010. 2 Botany Department, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i. 3 Institute of Pacific Islands Forestry, U.S. Department of Agriculture Forest Service, Hilo, Hawai‘i. developed mechanisms for regeneration after fire (Vogl 1975). The native Hawaiian flora is thought to have evolved under infrequent exposure to fire (Vogl 1975) and is less capable of regeneration following fire (Smith and Tunison 1992). When alien grasses colonize an area, the difference in regenerative ability initiates a vicious cycle (D’Antonio and Vitousek 1992). Each fire further reduces the coverage by native flora and thereby clears land for the expansion of the grass (D’Antonio and Vitousek 1992). Furthermore, the presence of grass chronically alters the processes in ecosystems such as Hawaiian dry forests by competing for light, nutrients, and water (Blackmore and Vitousek 2000, Cabin et al. 2002, Cordell et al. 2002, Mack and D’Antonio 2003, Litton et al. 2006, Cordell and Sandquist 2008, Litton et al. 2008). Fountain grass, a C4 bunchgrass native to North Africa, was introduced to Hawai‘i as an ornamental during the early 1900s (Wagner et al. 1999). This drought-tolerant, rapidgrowing species has escaped from cultivation in Australia, Fiji, North America, South Africa, and Hawai‘i (Chippindall and Crook 1976, Williams et al. 1995, Milton et al. 1998). Fountain grass has spread throughout the islands of Hawai‘i, particularly on the island of Hawai‘i, and it has become the dominant cover in many dry, leeward areas (Williams and Black 1994, Goergen and Daehler 2002, Poulin et al. 2007). Fountain grass is particularly problematic in Hawai‘i because it invades lava flows, disrupting primary succession (Tunison 1992) by preventing colonization of native species. Fountain grass forms monotypic stands with large amounts of dead mass that fuels fires (Tunison 1992). The fires fueled by exotic grasses are extremely damaging to the native vegetation because many native Hawaiian shrubs and trees burn readily and do not regenerate well, if at all, after fire (Smith and Tunison 1992). In contrast, individual fountain grass culms regenerate rapidly, flower, and set seed within a few months after fire (Goergen and Daehler 2002). Although fountain grass may resprout from cut or burned culms, it lacks vegetative reproduction and is therefore reliant upon seeds for establishment of new individuals and population growth (Goergen and Daehler 2001). Knowledge of basic germination requirements and seed ecology of fountain grass is lacking. Factors such as light requirement, depth of emergence, and heat/fire tolerance may affect seedling recruitment and play a role in the further spread of this invasive grass. It is of interest then to investigate which aspects of its seed and germination ecology may aid in the invasive capacity of fountain grass. Herbaceous plants have two primary modes for survival following the passage of fire; either they resprout from underground storage tissues or recruit new individuals from a seed bank (Pausas et al. 2004, Paula and Pausas 2008). During a fire, seeds may be killed unless protected by some kind of adaptation or by an insulating layer of soil (Baskin and Baskin 1989). Protected seeds may either be stimulated to germinate or remain dormant or quiescent (Baskin and Baskin 1989, Paula and Pausas 2008). Seeds can avoid the effects of fire through a number of mechanisms including burial in the soil either by animals or with the aid of hygroscopic awns (Ernst et al. 1992). Because grassland fires move quickly, soil temperatures may remain unchanged even in the upper layers, thereby sparing buried seeds from temperature extremes (Gillon 1983). In contrast, seeds resting on the soil surface can be completely destroyed by direct combustion or exposure to temperatures generated during fires (Ernst et al. 1992). The objectives of this study were to (1) determine whether fountain grass seeds require light for germination; (2) determine the potential depth of seedling emergence from soil; and (3) examine the ability of seeds to survive high temperatures and/or fire. materials and methods