Responses of the mangrove fern Acrostichum danaeifolium Langsd. & Fisch. (Pteridaceae, Pteridophyta) to disturbances resulting from increased soil salinity and Hurricane Georges at the Jobos Bay National Estuarine Research Reserve, Puerto Rico

Abstract

Mortality of plants in a mangrove forest dominated by the tree Avicennia germinans began to appear at the southern coastal edge of the forest in the central watershed of the Jobos Bay National Estuarine Research Reserve in Puerto Rico in 1977. The dieback was attributed to salinities in excess of 70 ppt and gradually continued north until some tree mortality was observed in 1995 at the forest’s landward interface with agricultural land. Mortality of the common understory mangrove fern Acrostichum danaeifolium occurred at a much slower rate than tree mortality. Remediation at the forest’s edge was recommended in 1997 and a study to monitor A. danaeifolium at 4-month intervals was begun in January 1998 to determine how the ferns would respond to hydrologic change. Hurricane Georges passed over the reserve in September 1998, so the effects of this disturbance were assessed as well. In May 1998, 4 months after tagging individual plants, the mean number of leaves plant⁻¹ at the impact site was only half that for plants in a control area in the nearby undisturbed eastern watershed. Fertile (spore-bearing) leaf production in both watersheds was very low accounting for plant fertility of 7% or less. Short leaf lengths in the impact site combined with smaller plant leaf counts and much lower leaf production rates in one area of the impact site had resulted in biomass production at the control site that was 18 times greater than in the impact site. After Hurricane Georges plant mortality in the impact area increased with individuals of A. danaeifolium found dead on every monitoring date except September 1999. However, when means for all variables measured in both sections of the forest were compared within 8 months after the hurricane there was an increase in all growth characteristics except sterile leaf length. Relative plant fertility increased from 10% before the hurricane to over 50% by May 2000. The fertile leaf production rate of these plants increased nine-fold by May 1999, and though it declined slowly thereafter it was still almost three time pre-hurricane levels at the end of the study almost 2 years later. Biomass production rates, though very low in the 4 months after the hurricane, had increased to five times pre-hurricane levels by the end of the study in January 2001. Efforts at remediation of the hypersaline conditions that were completed in May 1999 at the landward edge of the impact area appeared to have little effect on the ferns. For A. danaeifolium, plant mortality and decreases in plant size characteristics were the most notable responses to high salinity levels. In contrast, increases in fertile leaf and biomass production rates were the most significant response to hurricane disturbance.