Ferns are an important component of many tropical ecosystems, but the life-history details of few species are well understood.  The temporal pattern of frond production and mortality, as well as the timing of spore production, would be of particular relevance in any consideration of climate sensitivity or long-term climate change.  I studied thirty-one individual Saccoloma moranii ferns (Family Saccolomataceae) in lowland old growth tropical rainforest along the CES and SURA trails at the La Selva Biological Station in Costa Rica.  This species is a terrestrial understory fern that is usually found above the flood lines of streams.  Each fern was checked monthly between June 2008 and June 2009 for new fronds and dead fronds.  I attempted to determine the cause of mortality for all dead fronds.

Results/Conclusions

The number of fronds per plant at the end of the study year (mean = 10.6, sd = 4.6) was significantly less than the number of fronds per plant at the beginning of the year (mean = 11.9, sd = 4.3) (Wilcoxon Signed Rank Test 143.5, P < 0.001).  The number of new fronds produced per plant (mean = 6.3, sd = 3.1) was significantly exceeded by the number of fronds that died (mean = 7.6 sd = 3.0) (Wilcoxon Signed Rank Test 143.5, P < 0.001).  Although new and dead fronds were produced in every month of the year, the maximum die-off of fronds occurred in April 2008, after a severely dry March.  The maximum number of new fronds was produced the following month in May.  Cause of mortality was usually but not always evident.  Every plant in the study population had sori on at least one frond at some point during the study interval.  These data indicate that Saccoloma moranii ferns make a substantial energetic investment in new fronds and yet may end the year with fewer fronds than at the beginning.  Long-term changes in rainfall pattern, particularly in the strength of the dry season, could potentially affect the population dynamics of these ferns.