Carrera-Martínez et al. Fire Ecology (2020) 16:14 Fire Ecology https://doi.org/10.1186/s42408-020-00075-z FIELD NOTE Open Access Effects of fire on native columnar and globular cacti of Puerto Rico: a case study of El Faro, Cabo Rojo Roberto Carrera-Martínez1, Jorge Ruiz-Arocho1,2, Laura Aponte-Díaz1, David A. Jenkins3,4 and Joseph J. O’Brien5* Abstract Background: Many Puerto Rican ecosystems evolved without a regular fire regime. As such, many native plants lack adaptations necessary to survive even low-intensity fires. Human-caused fires are increasing in frequency, in part promoted by the presence of invasive grasses. During the afternoon of 19 February 2014, a wildfire burned a large portion of the dry forest in El Faro Natural Reserve, Cabo Rojo, Puerto Rico. Using a previously monitored population, we documented the mortality and morphological damage on three columnar and semi-epiphytic cactus species (Pilosocereus royenii [L.] Byles & Rowley, Harrisia portoricensis Britton, and Leptocereus quadricostatus Britton) and one globular species (Melocactus intortus [Mill.] Urb.) in relation to canopy cover and cactus growth strategy. Results: Our results suggest that M. intortus and H. portoricensis were more susceptible to fire damage. Cacti growing in areas dominated by grasses and shrubs were significantly more likely to burn than those growing under a closed tree canopy. Apart from L. quadricostatus, cacti that survived at least one year after the fire had lost spines, developed scarring tissue in some or all ribs, and few were able to regrow. Conclusions: Because extensive tissue damage and environmental alterations were observed during the post- fire survey, which occurred between 14 to 19 months after the fire, we argue there will be significant delayed mortality and that the mortality estimates presented here are conservative. Keywords: closed canopy, disturbances, grasses, mortality, refuge, survival Resumen Antecedentes: Muchos ecosistemas de Puerto Rico evolucionaron sin un régimen de fuegos regular. Como consecuencia, a muchas plantas nativas les faltan las adaptaciones necesarias para sobrevivir aún a fuegos de baja severidad. Los incendios causados por humanos han incrementado sus frecuencias, en parte promovidos por la presencia de pastos invasores. Durante la tarde del 19 de febrero de 2014, un incendio quemó una gran parte del bosque seco en la Reserva Natural del Faro, en Cabo Rojo, Puerto Rico. Usando una población relevada previamente, documentamos la mortalidad y daños morfológicos sobre tres especies de cactus (Continued on next page) * Correspondence: [email protected] 5USDA Forest Service, Center for Forest Disturbance Science, 320 Green Street, Athens, Georgia 30602, USA Full list of author information is available at the end of the article © The Author(s). 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Carrera-Martínez et al. Fire Ecology (2020) 16:14 Page 2 of 10 (Continued from previous page) columnares y semi epifíticas (Pilosocereus royenii [L.] Byles & Rowley, Harrisia portoricensis Britton, and Leptocereus quadricostatus Britton) y una especie globular (Melocactus intortus [Mill.] Urb.), en relación a la cobertura del dosel y las estrategias de crecimiento de los cactus. Resultados: Nuestros resultados sugieren que M. intortus y H. portoricensis fueron más susceptibles al daño por fuego. Los cactus creciendo en áreas dominadas por pastos eran significativamente más probable que se quemaran que aquellos creciendo debajo del dosel cerrado de árboles. Aparte de L. quadricostatus,loscactus que sobrevivieron al menos un año después del fuego habían perdido las espinas, desarrollado tejido chamuscado en algunas o todas sus nervaduras, y unos pocos fueron capaces de rebrotar. Conclusiones: Dado el extensivo daño y alteraciones ambientales que fueron observadas durante el relevamiento post-fuego, que ocurrió entre 14 y 19 meses posteriores al incendio, argüimos que habrá una significativa mortalidad retrasada y que las estimaciones aquí presentadas son conservadoras. Introduction maximus on Mona Island could drive some populations of Fire influences virtually all terrestrial ecosystems and, in the endemic columnar cacti Harrisia portoricensis Britton many cases, regular fires are needed to maintain com- to extinction in less than 60 years (Rojas-Sandoval et al. munity structure, productivity, and biodiversity (Bond 2016). In addition, when compared to invasive grasses, and Keeley 2005). However, the alteration of a fire re- Puerto Rican native grasses generate limited fuels, generally gime can also radically alter ecosystem function and not enough to naturally combust (Thaxton et al. 2012b). composition resulting in habitat loss (Janzen 1988; Miles Tropical dry forests in Puerto Rico are home to 24 et al. 2006; Jolly et al. 2015; Monmany et al. 2017). In species of cactus, of which five are native columnar, many ecosystems on the island of Puerto Rico, fires are semi-epiphytic, or globular cactus inhabiting the main almost exclusively the direct or indirect result of recent island (Liogier 1994; Carrera-Martínez et al. 2018). human activity (Burney et al. 1994; Méndez-Tejeda et al. These dry forests are more likely to ignite under normal 2015; Monmany et al. 2017) and cause severe impacts climatic conditions than tropical wet or rain forests on ecosystems and negatively affect public health (Aide (Monmany et al. 2017) and, at least in Puerto Rico, fire et al. 1995, 2000; Grau et al. 2003; Miles et al. 2006; frequency in these forests is expected to increase (Van Thaxton et al. 2012b; Wolfe et al. 2014; Jolly et al. 2015). Beusekom et al. 2018). Cactus species are historically For example, fire delayed the recovery of forests in aban- considered fire intolerant (Benson and Walkington doned agricultural fields and landslides by promoting the 1965), with average mortality considerably higher than presence of grasses and delaying colonization by woody other succulent species (Thomas 1991). Previous studies species (Walker and Boneta 1995;Grauetal.2003). on Central and North American cactus taxa have shown Tropical dry forests are especially threatened by an in- mixed post-wildfire cactus survival varying from >90% to creased fire frequency that promotes the persistence of <30% within the same species (Benson and Walkington invasive species (Janzen 1988; Miles et al. 2006). The dry 1965; Bunting et al. 1980; Thomas 1991, 2006; Thomas forests of Puerto Rico are located in the south and and Goodson 1992; Vermeire and Roth 2011; Thomas southwestern lowland regions of the island (Murphy and et al. 2017). This is likely due to the inherent variability Lugo 1995; Murphy et al. 1995) and are generally con- in wildfire behavior. However, no studies have focused sidered biological hotspots (Monsegur Rivera 2009). on the effects of fires on Puerto Rican cacti, even though They are also susceptible to the deleterious effects of they are generally referred to as keystone species fire. In these ecosystems, native woody plant species lack (Valiente-Banuet et al. 2002; Carrera-Martínez et al. the morphological adaptations necessary to survive even 2018), and at least two species (H. portoricensis and low-intensity fire events (Wolfe et al. 2014). Further- Leptocereus quadricostatus Britton) are considered to be more, fires in the dry forests of Puerto Rico can also endangered (US Fish and Wildlife Services1990; Gann promote the invasion of exotic fire-promoting grasses and Taylor 2013). Protections were extended to H. por- like Megathyrsus maximus (Jacq.) B.K. Simon & S.W.L. toricensis after it was extirpated from the main island Jacobs and Cenchrus ciliaris L., suppressing native (US Fish and Wildlife Service 1990), and it has been re- grasses (Thaxton et al. 2012b). Megathyrsus maximus in- cently reintroduced in Los Morrillos Peninsula in south- vasions in Puerto Rico are associated with the decline of western Cabo Rojo (O. Monsegur, US Fish and Wildlife native flora diversity and richness (Rojas-Sandoval and Service, Puerto Rico, USA, personal communication). Meléndez-Ackerman 2012;Rojas-Sandovaletal.2012; The objective of this study was to document the impact García-Cancel and Thaxton 2018). In particular, M. of the El Faro fire on four cactus species (Pilosocereus Carrera-Martínez et al. Fire Ecology (2020) 16:14 Page 3 of 10 royenii [L.] Byles & Rowley, Melocactus intortus [Miller] juveniles (O. Monsegur, personal communication). The Urban, L. quadricostatus and H. portoricensis). Here, we: site soils are mostly shallow, with many patches of ex- (1) provide observations on the morphological conse- posed limestone typical of coastal dry forests. For more quences of fire injuries; (2) confirm the negative effects of details on Puerto Rican dry forests, including the El Faro fire on cacti survival; (3) determine which habitats (closed site, refer to Murphy and Lugo (1995) and Murphy et al. canopy versus grass-dominated open canopy) had the (1995), and for Puerto Rican columnar cacti habitat pref- highest cacti mortality; and (4) determine if morphology erences, refer to Carrera-Martínez et al. (2018). (globular versus columnar cacti) affected post-fire mortal- ity. We expected to observe a higher mortality on areas Wildfire dominated by grasses and shrubs than under closed can- The wildfire at El Faro occurred on the afternoon of 19 opies, as the latter have lower fuel reserves (Thaxton et al.