Discussion

The long-term ecological roles of hurricanes at the Harvard Forest and Luquillo Experimental Forest can be compared in terms of the spatial and temporal distribution of disturbance events (hurricane wind damage) and the corresponding ecosystem response. At a continental scale, the locations of New England and Puerto Rico relative to hurricane patterns in the North Atlantic basin account for historical differences in hurricane frequency and intensity. Hurricane frequency and maximum intensity are significantly higher in Puerto Rico, and average storm duration is longer (because storms move more slowly). As a result, hurricane impacts are both more frequent and more severe than in New England. At a regional scale, gradients of hurricane damage result from track patterns and the tendency for hurricanes to weaken as they pass over land (especially over mountains) or over cold ocean water. In New England these gradients extend from southeast to northwest, and in Puerto Rico from east to west. At a landscape scale, the interaction between local topography and constrained peak wind directions creates a landscape-scale gradient of impacts within the larger regional gradient. In the gently rolling terrain of central New England, only scattered areas were found to have long-term protection from the most damaging hurricane winds. In the mountainous terrain of the LUQ, more extended areas were found to have such protection. At smaller scales, the random nature of hurricane gusts contributes to spatial heterogeneity in patterns of wind damage.

Historical records for New England and Puerto Rico extend back roughly 400 and 500 years, respectively. Hurricane frequency and maximum intensity were higher in Puerto Rico than in New England, but otherwise the temporal distributions were similar. For example, we found no clear evidence of centennial-scale trends for either region. In both regions we found the same multidecadal variation that is well documented for North Atlantic hurricanes in general (Neumann et al. 1987), though the specific patterns were different in the two regions; there is a growing understanding of how such variation is linked to other global climatic factors (Gray et al. 1997). Individual hurricanes tended to be clustered in time, whereas the most extreme events were also the most rare.

A critical factor controlling the impact of hurricanes on ecosystems is the relative length of hurricane return intervals and vegetation life spans. For example, if hurricane damage at a given site occurred only at a millennial scale (comparable to tornadoes in southern New England; Fujita 1987), then we might expect those impacts to be negligible for most of the intervening period. On the other hand, if hurricane-force winds occurred every year (as is the case in some wind-swept alpine areas), then we might expect to find heath or shrub communities that were highly resistant to wind damage. In southern and coastal New England, most trees on exposed sites experience some wind damage from hurricanes during their lifetimes, whereas the maximum size and life span of susceptible species may be limited. In Puerto Rico, most trees on exposed sites experience significant wind damage from hurricanes during their lifetimes, and the maximum size and life span of many species, and possibly the distribution of some species, may be limited.

The mixed hardwood forests of central New England and the Tabonuco forests of the LUQ both exhibited remarkable resiliency to wind damage. In both cases, despite major structural reorganization, there was rapid regeneration of canopy cover through releafing, sprouting, or recruitment, which helped to reduce impacts on soil moisture, temperature, and nutrient cycling processes. A large number of damaged trees (even uprooted trees) survived, at least for a few years. Nutrient retention was high despite initial pulses. Long-term impacts on species composition depended on initial composition and the extent of damage. In general, the effects of individual storms remained visible longer in New England, where growth and decomposition rates are slower than in Puerto Rico. Because of their reduced stature, heavily damaged stands are naturally protected from subsequent wind damage for a period of years or decades.

Many interesting questions remain to be answered for these two regions; a few of these questions are outlined here.

Cumulative impacts of major damage. The cumulative impacts of major wind damage (bole snap or uprooting) are not well understood, and field studies are difficult because return intervals are often measured in decades. In this case a modeling approach that utilized meteorological data from historical storms, topographic data for the study site, damage data from past hurricanes, and information on ecosystem response might be useful. Such an approach could also be used to explore the effects of land use or climate change. The accuracy of such modeling efforts might well improve in the future with improvements in our understanding of topographic control of hurricane winds, the response of individual species to a range of wind speeds, and ecosystem response to wind damage in other forest communities.

Cumulative impacts of minor damage. The cumulative impacts of more frequent minor wind damage (defoliation and branch break) are also not well understood. The creation of new foliage, branches, and sprouts appears to be an important adaptive response in many species, but one that cannot be repeated indefinitely at short intervals. Presumably, the impacts of such damage are more significant when combined with other stresses such as drought, disease, or insect outbreak. Future field studies, especially in tree physiology, may shed light on this question.

Interactions with other disturbances. As mentioned previously, hurricane wind damage may be combined with other direct hurricane impacts (e.g., river floods or saltwater inundation) or secondary impacts (e.g., landslides or fires), whose effects sometimes rival or exceed wind damage, at least at a local scale. Hurricanes may also precipitate extensive logging operations whose long-term effects far surpass those of wind damage alone (Foster et al. 1997). Hurricane wind damage is also strongly dependent on previous disturbance history; for example, agricultural and logging activities in New England and Puerto Rico in recent centuries have strongly affected the impacts of hurricanes on forests in those regions (Foster et al. 1999; Boose et al. 2001). Many of these questions could be explored in future modeling efforts.

Prehistoric hurricane record. In general, the historical-modeling method described in this chapter provides a relatively high degree of accuracy and spatial resolution in its hurricane reconstructions. What it does not provide is millennial-scale data that could be invaluable for studying the frequency of the most intense (and rare) hurricanes, as well as the possible effects of climate change. Because our present understanding of hurricane meteorology is not sufficient to predict the effects of climate change on hurricane frequency and intensity on theoretical grounds alone (Emanuel 1997), there is a growing interest in investigating the relationship between hurricanes and climate in the past. Recently, several new techniques have emerged for studying past hurricanes on a millennial scale, for example, stratigraphic analyses of salt marsh deposits. The historical-modeling method can be used to help calibrate and test such methods for recent centuries (Donnelly et al. 2001).

Acknowledgments The author thanks D. Foster, D. Greenland, and an anonymous reviewer for helpful comments on the manuscript. The research was supported by grants from the National Science Foundation (DEB-9318552, DEB-9411975, and DEB-9411973) and is a contribution from the Harvard Forest and Luquillo Long-Term Ecological Research Programs.

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