Invasion of a Southeastern Pine Savanna by Japanese Climbing Fern
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CASTANEA 76(3): 293–299. SEPTEMBER 2011 Scientific Note Invasion of a Southeastern Pine Savanna by Japanese Climbing Fern Ellen R. Leichty,* Becky J. Carmichael, and William J. Platt Department of Biological Sciences, Louisiana State University, 107 Life Sciences Building, Baton Rouge, Louisiana 70803 ; Lygodium japonicum (Thunb. ex Murr.) Sw. is most that remain are degraded (Platt 1999, an invasive fern in the southeastern United Varner et al. 2005, Means 2006). Invasion by States. This perennial fern, which produces L. japonicum thus could have major conse- twining fronds from underground rhizomes quences for these fire-frequented ecosystems (Clarke 1936), is native to open forests and containing species rich and endemic ground- forest edges in temperate, subtropical and cover vegetation (Sorrie and Weakley 2006). tropical regions of eastern and southeastern Mesic pine savannas may be particularly Asia, East Indies, and northern Australia vulnerable to invasion by L. japonicum. Lygo- (Singh and Panigrahi 1984, Munger 2005, dium japonicum tends to be intolerant of Willis et al. 2006). Lygodium japonicum was extreme drought and/or flooded conditions introduced to North America as an ornamen- (Bargeron et al. 2008), suggesting that old- tal; the first known wild populations were growth mesic pine savannas with mixed recorded in Georgia in 1903 (Pemberton and sandy-clay soils ideal for fern growth may be Ferriter 1998), but this fern is now widespread susceptible to invasion (e.g., the Wade Tract; in the southeastern United States. Japanese Platt et al. 1988). Such pine savannas also climbing fern occurs in human-modified tend to have pronounced biodiversity at small habitats, as well as forests and woodlands scales (Peet 2006, Platt et al. 2006, Carr et al. (Langeland and Burks 1998, Rosen et al. 2009). Might invasion by a species capable of 2003), where it often forms dense mats that forming mats on top of existing groundcover grow on and cover native species (Gagnon et plants compromise the high biodiversity of al. 2005, Zeller and Leslie 2004). Some mesic pine savannas? management plans have emphasized the Lygodium japonicum has been present at Girl dearth of scientific data on invasions by Scout Camp Whispering Pines in eastern climbing ferns (e.g., Ferriter 2001). (Tangipahoa Parish) Louisiana for more than Lygodium japonicum has been noted as two decades. This upland, mesic site, de- present in pine savannas. Nonetheless, there scribed in Platt et al. (2006), historically is minimal documentation of invasion of pine contained pine savannas with a two-layered savannas by this fern in the literature, physiognomy resembling that described in although it has been designated a threat to Gilliam et al. (2006). The overstory contained southeastern pine savannas (Munger 2005, predominantly longleaf pine, Pinus palustris Stocker and Hupp 2008) and invasion of pine Mill. (Noel et al. 1998). Groundcover vegeta- savannas has been hypothesized to be facil- tion was dominated by warm-season grasses, itated by fires (e.g., Wade et al. 2000). Pine especially bluestems [Schizachyrium scoparium savannas (sensu Platt 1999) have been re- (Michx.) Nash, Schizachyrium tenerum Nees], duced to less than 3% of their original extent but also contained high biodiversity of her- by fragmentation and fire suppression, and baceous plants mixed with shrubs and lianas (Platt et al. 2006). Pine savannas at Camp *email address: [email protected] Whispering Pines have experienced different Received June 8, 2010; Accepted February 18, 2011. land uses during the past century: logging in 293 Castanea cast-76-03-10.3d 10/8/11 14:01:18 293 Cust # 10-029R2 294 CASTANEA VOL.76 the 1920–30’s and 1960–70’s, cattle grazing greater local abundance in shrub thickets until the 1960’s, and fire suppression in the than in open pine savanna. 1980’s after becoming a Girl Scout Camp. We further asked ‘‘Is this species invading Nonetheless, pine savanna vegetation persist- the herbaceous groundcover of pine savannas ed in upland areas and along slopes, inter- at Camp Whispering Pines?’’ In 2006, 128 mixed with thickets containing native and one square meter circular plots were random- exotic hardwoods, as well as Lygodium japoni- ly located in herbaceous groundcover without cum (Platt et al. 2006). Lygodium japonicum within three different Camp Whispering Pines has been under- regions of Camp Whispering Pines. Plots were going restoration since the early 1990s. censused annually in the summer from 2006 Prescribed fires during the spring-summer through 2009. Stem counts were recorded for transition, when lightning fires would have all plants within each plot. Plants were occurred naturally, have been implemented identified to species and placed in the follow- in alternate years from 1992–2010. Differ- ing categories: fern, forb, graminoid, vine, or ent sections have been burned in prescribed woody. Complete census of all plots did not fires each year over this 18 yr period. occur each year; some plots were destroyed by Cover of hardwood shrubs has been greatly salvage operations after Hurricane Gustav, reduced as a result of the biennial fires, and time constraints reduced sampling in but shrub thickets remain scattered in the some years. Lygodium japonicum occurrence landscape (Thaxton and Platt 2006, Pass- increased over time in these plots. The more 2009). percentage of plots that contained L. japoni- This note compiles data from two different cum increased, especially after fires in 2006 experimental field studies that documented and 2008 (Figure 1). More than twice as invasion of mesic pine savanna by L. japoni- many plots contained the fern after four cum. One study examined effects of grani- years as when the plots were installed. These vores and herbivores on species composition results indicate that the herbaceous ground- and abundance in pine savanna ground- cover in pine savannas at Camp Whispering cover. The other study explored the combined Pines is being invaded by L. japonicum. In influence of hurricane, fire, and animal addition, data suggest that frequent fire may disturbances on species composition and facilitate local spread of Japanese climbing abundance in pine savanna groundcover. fern. During the course of both field experiments, Understanding responses of L. japonicum to changes in the presence and abundance of L. fire is important because fire is an integral japonicum offered an opportunity to examine part of restoring and managing pine savan- invasion in this pine savanna. nas. We used 23 one square meter plots in We first asked ‘‘Is Lygodium japonicum more shrub thickets at Camp Whispering Pines to likely to occur in disturbed than undisturbed assess responses to prescribed fires. These areas of pine savannas?’’ We documented the plots were burned in 2006 and 2008. We occurrence of L. japonicum at 20 randomly clipped and removed all fronds of L. japonicum selected sites at Camp Whispering Pines. In eight times beginning in 2006. We clipped 2006, ten sites were located along margins of L. japonicum at the time that the plots were shrub thickets (presumed local sites of past burned in the spring, and again 3 months disturbance) and a similar number were and 15 months after spring fires, during the located in nearby herbaceous groundcover. late summer when annual growth of fronds Three one-meter square plots were installed at had slowed. Fronds were clipped the year of each site. Lygodium japonicum occurred in all fire (2006, 2008), as well as the year without sites with shrub thickets (10/10), but in only 2 fire (2007, 2009); as a result, biomass col- of 10 of the sites with herbaceous ground- lected during the 3 and 15 month post-fire cover. On average, the fern occurred in 2.3 of sampling periods represented only the current the 3 plots in each shrub thicket, but in 0.3 of season’s growth. All underground rhizomes 3 plots in the herbaceous groundcover sites were left intact. The fern fronds were oven where it was present. Japanese climbing fern dried at 75uC and weighed to the nearest thus occurred at higher frequency and at 1/100th gram. Castanea cast-76-03-10.3d 10/8/11 14:01:35 294 Cust # 10-029R2 2011 LEICHTY ET AL.: 295 Figure 1. The occurrence of Lygodium japonicum in pine savanna groundcover at Camp Whispering Pines, Independence, Louisiana over a four year period. Aboveground growth of L. japonicum fronds restored to Camp Whispering Pines. Past was greater in the year of fire than in the year open-range grazing also may have altered without fire. More aboveground biomass of the groundcover in some unknown way (e.g., the fern was produced the year of fire than the reduction of cover of bunchgrasses) that now year without fire (Figure 2). These differences is facilitating invasion by L. japonicum in occurred in both pairs of years (2006/2007: frequently burned areas without high densi- t 5 2.98, df 5 22, p 5 0.007; 2008/2009: t 5 ties of shrubs. Although almost twenty years 2.07, df 5 22, p 5 0.051). Furthermore, of prescribed fires have reduced shrub thickets mechanical removal had no long term effect to remnant patches, L. japonicum rhizomes on the mass of L. japonicum fronds in plots; already established in these areas survived total dry weight mass of L. japonicum did not prescribed fires that opened the habitat. differ between the two successive sampling Frequent prescribed fires during the transition periods. These data thus show little effect of from spring to summer, essential for restora- long-term removal of aboveground biomass, tion and management of diverse groundcover but increases in above-ground biomass im- in mesic pine savannas like Camp Whisper- mediately after fire. We thus propose that ing Pines, do not appear to deter invasion by frequent fires should not deter invasion of L.