Hurricanes in temperate and tropical regions can differ significantly, and in fact, many that form never reach the temperate latitudes. Those that do reach these latitudes are usually less severe because: 1. Hurricanes require the warm tropical ocean water as a primary source of energy and weaken when they reach the cooler oceans of the north. 2. The Coriolis force (see def. below) increases with latitude and thus, similar pressure gradients cause lower wind speeds at higher latitudes. 3. Cool, dry air masses found in temperate regions convert the internal dynamics of the hurricane to a less severe, extratropical storm. Thus, even though hurricanes in temperate regions move considerably faster and create larger asymmetry in the surface wind fields (causes the highest winds to be concentrated on one side of the storm), the range of damaging winds and potential destruction is diminished (Boose et al. 1994).
Even though hurricanes that reach temperate regions tend to be less severe and move faster, they can still cause extensive damage to forests and are the most devastating natural disturbance within New England, US (Merrens and Peart 1992). Impacts due to a hurricane are mediated by the interaction of biotic, edaphic and historical factors with meteorological and stochastic patterns (Foster and Boose 1992), but these interactions can be different at different spatial scales. At the landscape level, much of the wind damage is controlled by the interplay among various factors such as: topographic position and aspect, the local meteorology of the storm, the vegetation structure and composition, and the history of the area such as prior disturbances and human activity (Foster 1988; Foster and Boose 1992). For example, differences in topography can cause the acceleration of wind over ridges with strong turbulence on the lee sides, wind channeling up valleys and around swells, and variations in exposure due to wind shadowing during which upper slopes may be affected by the higher wind velocities associated with hurricane occurring at higher altitudes (Boose et al. 1994).
In contrast, the amount damage observed within a given site is largely a result of the forest composition and structure (Foster and Boose 1992), and somewhat by the site's position in the landscape relative to wind breaks, variations in local topography, and wind gusts (Boose et al. 1994; Foster and Boose 1992). Species differ in susceptibility with conifers considerably more susceptible than hardwoods because of differences in shoot and root structure, strength of the wood, anchorage, and canopy position (Foster 1988). Thus, the proportion of each group in an area may significantly influence a stand's susceptibility to wind damage (Foster and Boose 1992). The structure of the stand also contributes significantly to a stand's response to the winds of a hurricane. In general, older stands are more susceptible to damaging winds because of the increase in canopy size and the greater wind speed found at the taller heights, the greater length of turning motion, and the increased canopy surface roughness found in stratified and vertically differentiated stands which causes greater wind turbulence and incursion of air to accelerate blowdown (Foster 1988). Also, the placement of understory trees relative to overstory trees can affect the amount of damage to a stand, hence even if damage to the stand is slight, several understory trees can be crushed by windthrown overstory trees and falling debris (Foster 1988).
Damage incurred in a stand can range from slight defoliation of individual trees to the blowdown of entire stands (Foster and Boose 1992). However, much of the damage is caused by trees snapping and consequential death of the snag (Foster 1988). Both large and small gaps can be created within an area by tree-snapping, extensive defoliation, and/or blowdown by the catastrophic winds of a hurricane. This in turn can affect the successional patterns of a stand (Foster and Boose 1992). Similar to gap dynamics in northern hardwoods, pioneer species can invade large gaps and smaller gaps can be filled by undamaged understory species via vertical recruitment. As a consequence a higher density of both tolerate and intolerant species can be found within a disturbed stand compared to an undisturbed area (Merrens and Peart 1992). Surviving canopy trees have shown dramatic increases in growth with radial growth sometimes increasing three- to six-fold following a hurricane (Merrens and Peart 1992). Also, conifers are more susceptible to damage than hardwoods because of their shallow root systems, full crowns, and often dominant canopy position. Hence, hurricane winds could directly affect the species composition of a stand by eliminating many of the overstory conifers (Foster 1988).
Along with the vegetation, the site characteristics of an area can be affected by a hurricane. Uprooting of trees can cause the mixing of upper soil layers, impede soil development, expose mineral soil, create microtopography and microenvironmental variation, and remove or expose buried seed pools (Foster 1988). Rains that reach an area before a storm can completely saturate the soil which can affect the dynamics of the soil hydrology and increase the amount of windthrown trees (Foster 1988). Hence, hurricanes not only affect the composition and structure of a stand directly through the damage of trees and formation of gaps, they can indirectly influence an area by changing the soil chemistry and physical properties and creating microenviroments.