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Background: Fire is an important disturbance that influences species composition, community structure, and ecosystem function in forests. Disturbances such as hurricanes and landslides are critical determinants of community structure to Caribbean forests, but few studies have addressed the effect of paleofire disturbance on forests in Puerto Rico, USA. Soil charcoal is widely used to reconstruct fire history. However, the occurrence and frequency of paleofire can be underestimated due to charcoal decay.

Results: We reconstructed the fire history of subtropical dry forests of Puerto Rico based on the analysis of soil macrocharcoal numbers adjusted by the negative exponential decay function of charcoal. Twenty-one fire events occurred over the last 1300 yr in the subtropical dry forest of northeastern Puerto Rico, and 10 fire events occurred over the last 4900 yr in the subtropical dry forest of southeastern Puerto Rico. The average turnover time of charcoal in these subtropical dry forest soils of Puerto Rico was 1000 to 1250 yr. Soil charcoal decay leads to an underestimation of one to two undetected fire events during the Late Holocene in the subtropical dry forests of Puerto Rico. The peak of paleofire events for subtropical dry forests in northeastern and southeastern Puerto Rico was broadly similar, occurring between 500 to 1300 calibrated years before present (cal yr BP; before present is understood to mean before 1950 AD). Fire frequency of the subtropical dry forests in Puerto Rico decreased after the immigration of Europeans in the past 500 yr. The fire that occurred between 4822 and 4854 cal yr BP can be interpreted as either a natural fire or a new record of a native peoples settlement in southeastern Puerto Rico. Fire became a frequent disturbance in the subtropical dry forest of Puerto Rico after the development of cultigens by native peoples.

Conclusions: Our data suggested that fire was a frequent disturbance and human activity was likely a dominant cause of these paleofires in the subtropical dry forests of Puerto Rico.