Soil charcoal in old-growth rain forests from sea level to the continental divide

Soil charcoal is an indicator of Holocene fires as well as a palaeoecological signature of pre-Colombian land use in Neotropical rain forests. To document rain forest fire history, we examined soil charcoal patterns in continuous old-growth forests along an elevational transect from sea level to the continental divide on the Atlantic slope of Costa Rica. At 10 elevations we sampled 1-ha plots, using 16 cores/ha to collect 1.5-m deep soil samples. We found charcoal in soils at every elevation, with total dry mass ranging from 3.18 g/m(2) at 2000-m elevation to as much as 102.7 g/m(2) at 300 m. Soil charcoal is most abundant at the wettest lowland sites (60-500 m) and less at montane elevations (> 1000 m) where there is less rainfall. Between 30- and 90-cm soil depth, soil charcoal is present consistently and every 1-ha plot has charcoal evidence for multiple fire events. Radiocarbon dates range from 23,240 YBP at 1750-m elevation to 140 YBP at 2600 m. Interestingly, none of the charcoal samples from 2600 m are older than 170 yr, which suggests that forests near the continental divide are relatively young replacement stands that have re-established since the most recent localized volcanic eruption on Volcan Barva. We propose that these old-growth forests have been disturbed infrequently but multiple times as a consequence of anthropogenic and natural fires.