Effects of Hurricane Andrew on stands of slash pine (Pinus elliottii var. densa) in the everglades region of south Florida (USA)

Few hurricanes affect intact stands of subtropical pines. We examined effec ts of winds in the eyewalls of Hurricane Andrew, where wind speeds were > 2 00 km h(-1), on all remaining large mainland stands of Pinus elliottii var. densa (south Florida slash pine) on limestone outcroppings (rocklands) in the everglades region of southern Florida. We measured densities and sizes of trees and assessed damage and mortality in plots in old-growth stands in the Lostman's Pines (LOP) region of Big Cypress National Preserve and in s econd-growth stands in the Pines West (PIW) and Long Pine Key (LPK) regions of Everglades National Park. We also examined age-size relationships using sections from trees killed by the hurricane in LOP and LPK. We used the da ta to predict effects of recurrent hurricanes on the structure and dynamics of the old-growth stand and to compare effects of hurricanes on old- and s econd-growth stands. Slash pine was resistant to hurricane winds. Most trees in stands (68-76%) were not severely damaged; mortality in the three regions averaged 17-25% s hortly after the hurricane and 3-7% during the following year. Mortality wa s positively associated with tree size; mean tree sizes decreased and size- selective thinning occurred in all stands. Nonetheless, local mortality ran ged from 3-4% to 50-60% among plots in all stands. Such local variation in mortality resulted from clustering of large trees, especially in old-growth stands, and from microbursts during the hurricane, which affected all stan ds. Recurrent, intense hurricanes are predicted to kill larger trees, slowl y opening new patches and increasing sizes of extant patches, thus resultin g in almost continual presence of openings suitable for recruitment in old- growth stands. Age-size relationships also indicated that large trees in ol d-growth stands may survive 2-3 centuries. The combination of frequent open ings and wind resistance of large trees is predicted to result in old-growt h stands that are highly uneven aged, with trees locally distributed in sim ilar-aged patches. The extent to which such stands deviate from demographic equilibrium, as well as turnover rates within stands, are likely to increa se as the frequency of recurrent, intense hurricanes increases. Damage and mortality differed in old- and second-growth stands. Large trees were more, but small trees less likely to be damaged in old- than second-g rowth stands. In contrast, mortality was significantly lower in old- (LOP: 16.9% +/- 3.1 [mean +/- s.e.]) than second-growth stands (PIW: 22.5% +/- 2. 0; LPK: 25.2% +/- 2.7). Total hurricane-related mortality was 30-60% higher in second- than old-growth stands. Size class structure, more uneven in ol d- than second growth stands prior to the hurricane, diverged even more aft erwards. Hurricane Andrew removed more large trees, but opened fewer patche s suitable for recruitment in second- than old-growth stands. Thus, second- growth stands did not more closely resemble old-growth stands after Andrew , and size class distributions were not likely to shift towards those in ol d-growth stands. Moreover, rapid growth of both current large trees and sma ll trees in the newly opened patches should result in second- growth stands being susceptible to future hurricanes. Management that shifts structure a nd dynamics towards old-growth stands will require changes in patch dynamic s so that growth rates of trees in open patches are slowed and they become less susceptible to wind damage. High intensity prescribed fires may slow t he growth of small trees, eventually resulting in second- growth stands con taining larger trees more resistant to frequent, intense hurricanes.