Pine trees, wind and limestone degradation, South Florida, USA

Exposed limestone is subject to numerous pedogenic processes, many which cause surface degradation. The uprooting of windthrown trees is a major pedogenic process, as demonstrated in South Florida pine forests. The effects of Hurricane Andrew’s winds on two forest stands are documented; the Pine Island forest is rooted in exposed level Late Pleistocene limestone bedrock, and Lostman’s Pines in quartz sand overlying the level mid-Pliocene limestone bedrock. Windthrown trees made up 19% and 29% of Pine Island and Lostman’s Pine, respectively and windthrown tree pit areas are 0.5% and 0.65% of the hectare plots. Pine Island pits are smaller, deeper and vertical walled in contrast to the larger, shallower pits at Lostman’s Pines. Pine Island pits are persistent features after 25 years. Evidence of past windthrown trees covered 75% of the Pine Island study site. Assuming a hurricane frequency of one in 87 years, the rotation period (time to disturb 100% of surface area) is 18,270 and 12,756 years at Pine Island and at Lostman’s Pines respectively. Since limestone deposition 6.9 rotation cycles occurred at Pine Island with a degradation of 10.32 cm per cycle (the average depth of a pit), or a potential limestone surface degradation of 0.71 m, equivalent to approximately 7.1% of the entire Miami limestone section. There is no regolith build-up because the dissolution rate of the limestone clasts is greater than the rotation period. The average potential rate of limestone surface erosion by windthrown trees is 5.6 to 8.5 mm 1,000 -1 years.