Elevation promotes long-term survival of Pinus elliottii var. densa, a foundation species of the endangered pine rockland ecosystem in the Florida Keys

Abstract

The pine rockland community in southern Florida, which supports numerous state- and federally listed flora and fauna species, is considered endangered. Without its foundation species, Pinus elliottii var. densa, habitat and species diversity are lost to tropical hardwood hammock. Here, we investigate the landscape factors that contribute to the long-term persistence of pine rocklands on the 2 islands that contain the largest remaining habitat in the Florida Keys: Big Pine Key and No Name Key. Plot-level biophysical data and island-scale remotely sensed vegetation data were collected from pine rockland savannas and examined with multi-dimensional analysis. On both islands, vegetation plots located at higher elevations contain the greatest basal area and the oldest and largest-diameter trees. In contrast, the lowest-elevation plots contain more standing dead trees and downed logs. Change-detection analysis of the normalized difference vegetation index between 2001 and 2011 supports the notion that habitat areas at lower elevations experience more vegetation change compared to those at higher elevations during the time period (p < 0.05). Trees rooted at higher elevations frequently exceeded 200 yr in age, which is uncommon in this highly disturbed sub-tropical region. Although elevation range on the 2 islands was only ca. 0 to 3 m, these results demonstrate that higher elevations can act as a refuge for the salt-intolerant P. elliottii var. densa during storm surge events, which promotes long-term development of old-growth savanna structure and aids the long-term persistence of pine rocklands in the Florida Keys.