Understory fern community structure, growth and spore production responses to a large-scale hurricane experiment in a Puerto Rico rainforest

Ferns are abundant in most rainforest understories yet their responses to hurricanes have not been wellstudied. Fern community structure, growth and spore production were monitored for two years before and five years after a large-scale experiment that simulated two key components of severe hurricane disturbance: canopy openness and debris deposition. The canopy was opened by cutting branches above 3 m and the fallen leaf and woody debris was either added to or removed from the forest floor resulting in four treatments that were replicated three times in a factorial design. Of the 16 fern species observed during the experiment 12 were present before treatments were applied. All but two of the 16 species had densities \u3c250 ha-1; Thelypteris deltoidea (2258 ha-1) and Cyathea borinquena (1521 ha-1) were by far the most common ferns. Under simulated hurricane conditions (open canopy and debris deposition) abundance levels for both T. deltoidea and C. borinquena were highly resilient and returned to pre-disturbance conditions within three years; therefore the resident (non-pioneer) fern species continued to dominate after disturbance. However, several other variables had increasing or decreasing responses that had not returned to pre-treatment levels by the fifth and final year of the experiment. Four pioneer species appeared in low abundance almost immediately after the canopy was opened, including three native species that spread via spore germination and the invasive Nephrolepis brownii that spread by runners. Debris deposition resulted in high mortality for 7 of 12 fern species, with C. borinquena among the species little affected and increases in recruitment following mortality of T. deltoidea under open canopy. Individuals of both T. deltoidea and C. borinquena responded to higher light levels with growth spurts reflected in up to two- to three-fold increases in leaf production and emergence of longer leaves. Spore production rates for both T. deltoidea and C. borinquena had been low in the undisturbed pre-treatment forest, but more than doubled in the years that followed canopy opening. Hurricane impacts to this tropical forest alter the fern community by (1) debris deposition burying individuals and initially reducing some population sizes, and (2) canopy openness overwhelming the negative effects of debris deposition and stimulating growth and reproduction that can last \u3e/=5 years. Changes resulting from hurricane disturbance that affect the ferndominated herbaceous layer may ultimately influence structure and function of the long-term understory plant community and consequently the habitat of litter- and soil-dwelling organisms