Large-scale disturbances (e.g., hurricanes) produce non-uniform distributions of woody (tree stumps, branches) and fine (bark, needles) fuels in pine savannas. These non-uniform fuels produce local heterogeneity in fire intensity, which should influence invasion by exotic species, such as Japanese climbing fern (Lygodium japonicum). We hypothesized that increasing/decreasing fuel loads would decrease/increase survival and re-growth of L. japonicum genets through damage of rhizomes and thus the potential to produce additional fronds. Lygodium japonicum plots were randomly selected in different fire blocks in a frequently burned, second-growth longleaf pine savanna (Camp Whispering Pines, Tangipahoa Parish, LA) prior to 2007 prescribed fires. Individual genets of L. japonicum were selected that were <1m or >5m from stumps. One of three treatments (increased, decreased, or unaltered fine-fuels) was randomly applied to circular 1m2 plots around each plant. We measured fuel consumption of the experimental treatments to estimate the effects of fuel treatments. Fronds were counted and mapped and percent cover of ferns was estimated before prescribed fires. Measurements were repeated one month, six months, one year and two years after fire. Data were analyzed with ANOVA, and ANCOVA with pre-fire frond number as a covariate, and conducted using PROC MIXED using SAS.
Differences in fuels affected both fuel consumption and responses of L. japonicum. Fine-fuel addition plots had higher consumption of fuel than did either fine-fuel removal or unaltered control plots, which were not significantly different. In fine-fuel addition plots, immediate post-fire numbers of fronds were reduced compared to pre-fire numbers of fronds. This effect was transient; two years later frond numbers in fuel addition plots were similar to those in the pre-fire census. Post-fire numbers of fronds were slightly increased compared to pre-fire frond numbers in fine-fuel removal plots two years later. Unaltered plots demonstrated an increase at the two year census. The observed responses of L. japonicum suggest sensitivity to fire intensity. More below ground structures may survive low than high intensity fires. Further, low intensity fires may stimulate increased frond production by top-killing above ground fronds. Our results suggest that low intensity ground fire may facilitate invasion of L. japonicum. Large-scale disturbance conditions had no direct effect on L. japonicum; the location relative to stumps had no effect on fire intensity or response of L. japonicum. The intensity of the fires used in prescribed fire management thus may influence the invisibility of pine savannas by herbaceous exotic species.