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

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Few hurricanes affect intact stands of subtropical pines. We examined effects of winds in the eyewalls of Hurricane Andrew, where wind speeds were >200 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 second-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 data 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 second-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% shortly after the hurricane and 3-7% during the following year. Mortality was positively associated with tree size; mean tree sizes decreased and size-selective thinning occurred in all stands. Nonetheless, local mortality ranged 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 stands. Recurrent, intense hurricanes are predicted to kill larger trees, slowly opening new patches and increasing sizes of extant patches, thus resulting in almost continual presence of openings suitable for recruitment in old-growth stands. Age-size relationships also indicated that large trees in old-growth stands may survive 2-3 centuries. The combination of frequent openings and wind resistance of large trees is predicted to result in old-growth stands that are highly uneven aged, with trees locally distributed in similar-aged patches. The extent to which such stands deviate from demographic equilibrium, as well as turnover rates within stands, are likely to increase 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-growth 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 old-than second growth stands prior to the hurricane, diverged even more afterwards. Hurricane Andrew removed more large trees, but opened fewer patches 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 old-growth stands. Moreover, rapid growth of both current large trees and small trees in the newly opened patches should result in second-growth stands being susceptible to future hurricanes. Management that shifts structure and dynamics towards old-growth stands will require changes in patch dynamics 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 the growth of small trees, eventually resulting in second-growth stands containing larger trees more resistant to frequent, intense hurricanes.

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Plant Ecology

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