Hurricane Wilma's Effect on the Gumbo Limbo Hammock

Hurricane Wilma struck Boca Raton from the West, rather than from the East. That, in itself, is not necessarily an anomaly, since late season storms often come from the Gulf and cross the peninsula to the Atlantic. But it means that salt spray and storm surge were NOT factors in the destruction. But the severity of the storm and the fact that Boca Raton was on the "right front", or strongest quadrant of the eyewall, were major in terms of effect. Wilma's eye was so large that, at one time, it covered almost all of Palm Beach County. When Wilma came ashore, it was a strong Category II with winds over 110 mph. Moving at 25 mph, Wilma didn't have time to lose much strength as it passed over the peninsula, in fact, there is some indication that this large storm gained some strength as it neared the Atlantic coast. The winds in the hammock were the strongest so far this century, exceeding 100 mph. Of all the storms to effect Gumbo Limbo since 1984, it has been the only one where the eye passed over the site. That means that hurricane force winds blew from every direction. The effect of that factor cannot be underestimated.

Since Gumbo Limbo was dedicated in 1984, it has seen the effects of five hurricanes: just on the fringes of Andrew (1992), Irene (1999), Frances (2004), Jean (2004) and Wilma (2005).We were just on the fringes of Andrew and that powerful storm had a negligible effect here; a few large limbs down and leaf loss in the upper canopy. Irene was not a hurricane when it reached us, but a full tropical storm with torrential rains. The loss of branches from Ficus trees from Irene was considerable as was leaf loss. Frances and Jean were so near in time and their paths so nearly identical, they might be considered as one storm. Their combine effect at Gumbo Limbo was one of heavy damage to brittle trees, but surprisingly little to the stronger woods of the hammock. Frances and Jean, however, did significantly weaken the hammock's top canopy by removing upper limbs of strangler fig, one of two high canopy trees, the other being the paradise tree. That fact played an important part during Wilma in the following year.

Going back beyond the Center's dedication in 1984, you have to refer to the hurricanes of the 1940s to find storms as destructive as the storms of 04-05. Dr. Dan Austin, who wrote the text for our boardwalk guide, believes that the westward "set" of the hammock's largest black ironwoods was caused by that decade's storms. This demonstrates the longevity of hurricane effects in a hammock.

The site for Gumbo Limbo Nature Center was chosen because of its pristine maritime tropical hammock. Let's call it M-T hammock for short. The name "hammock" is applied to a forest with an evergreen canopy where the trunks of the trees are big enough to hold a hammock and the right distance apart to easily sling one and sleep in it. Florida was explored by sailors and when they came ashore, the brought their hammocks with them. At bed time, the kind of place they looked for came to be called what they slept in. Types of hammocks are named either geographically or by the dominant vegetation, or both. Thus, "maritime" hammocks are found by the sea and "tropical" hammocks contain vegetation characteristics of lower latitudes. Gumbo Limbo's seaside hammock contains many of the same species, but it is not diverse as, hammocks in the Florida Keys and the Caribbean.

Rare everywhere north of the Keys, ours is one of only three pristine M-T hammocks in Palm Beach County. Since the habitat and its trees evolve in hurricane country, they show adaptations to wind damage both collectively and individually. In a very real sense, the hammock is self-protective wind protective  from winds; trees are windbreaks for other trees and the whole can withstand more than the individual trees can. There are really four features to a hammock's collective wind protection; substrate, overall shape, layering of canopies and intertwining of tree.

Our kind of hammock is found on substrates ranging from the solid rock of the Keys to the deep sandy soils characteristic of our area. Roots of hammock trees tend to be shallow in either case, but trees grow taller in deep soil. In a natural situation, hammock forests are surrounded by shorter, denser, natural communities. The canopy  of the hammock comes down and blends with the lower one of the surrounding vegetation. This gradual rise in height creates an aerodynamic shape which wind flows over, rather than attacks. The Gumbo Limbo Hammocks has significant aerodynamic  weakness in this regard, like the opening along A1A. Like tall forests everywhere, M-T hammocks are stratified, that is, there is an upper canopy and subcanopies below it. The tallest layer is supported by canopies of shorter trees and so forth. This has a tendency to slow uprooting of the taller trees because the shorter ones "catch" them. The "leggy" growing trees intertwine up through the canopy toward the light and add to this protection up to a point, stitching the various canopies together. Past the "point" in wind velocity, the intertwining becomes a liability in terms of the overall resistance of the hammock to wind damage. Suck intertwining creates a "domino" effect in catastrophic storms.

All four of the collective factors were at play in the destruction Wilma wrought on the Gumbo Limbo Hammock in 2005. But the individual abilities of the hammock trees to withstand wind damage also played a part. To understand why this happened, you need to consider the "strategies" which tropical trees exhibit vis-à-vis windstorm damage. Some species, like gumbo limbo and strangler fig have brittle wood. To these species, a destructive event like a hurricane becomes a reproductive opportunity. Fallen branches of these trees sprout and grow into new trees. In a minimal hurricane, like Irene in 1999 and Frances and Jean in 2004, these are the trees which show the most damage. After all, it is to their advantage to do so. As such trees shed their brittle branches, they become less adapt to uproot or experience catastrophic damage. Ideally, they are progressively pruned back to a state which is then both resistant to further damage and ready to resprout with the greatest leaf surface possible.

At the other end of the spectrum are the real tropical hardwoods; these are the trees like mastic and black ironwood. These slow growing species don't break up, even in minimal hurricanes. Their wood is just so hard that "ordinary" windstorms don't faze them. Black ironwood, for example, has about twice the density of the hardest oaks and is the densest wood that grows in North America. As wind speeds go up, however, these hardwoods grudgingly give way. In their case, however, yielding may mean uprooting or snapping of the main trunk, both catastrophic in terms of their post hurricane recovery. This effect is even more dramatic when the tree in question is very tall. That brings us to a special case in Gumbo Limbo's hammock, the paradise trees.

Paradise trees, Simarouba glauca, have medium strong wood and the species is one of the few M-T hammock trees that show regular rings. They also provides a third to a half of the shade in our hammock. The fact that Wilma uprooted and broke off so many large paradise trees (when the 2004 storms didn't) is due to a number of factors. Since they are among the tallest of the hammock trees and since their branches don't break off easily, there is full resistance to the stronger winds, the ones flowing over the top of the hammock. The tall trunks just mean there is more leverage, multiplying the effects of the wind force. Then there is the fact that Frances and Jean had previously :lowered" so many of the paradise trees' equal in height, big strangler figs. Finally, the fact that Wilma's winds blew in every possible way. Wilma had every chance to make Gumbo Limbo's paradise trees fail. When they failed, it was either because the wind strain broke the main trunk, or the roots could not hold in the loose sand and the tree uprooted.

Visitors to our boardwalk today will be astonished at the number and size of paradise trees, black ironwoods and mastics that are torn down. Due to extraordinary effort by our staff, the boardwalk was quickly cleared. But we'll be seeing Wilma's effects for a long, long time.

For more information, please contact:
Gumbo Limbo Nature Center
1801 N Ocean Blvd
Boca Raton, FL 33432-1946
T: 561-338-1473
I: www.gumbolimbo.org 

VISIT PALM BEACH provides Gumbo Limbo Nature Center and Boca Raton residents and visitors with kayak tours, kayaking lessons and nature oriented bicycle tours. Immediately after the storm, staff members of Visit Palm Beach volunteered and helped clear the massive amounts of natural debris that significantly clogged the water trails and waterways in Gumbo Limbo and its sister park, Rutherford Park, the site of VPB's Mangrove Creek Kayak Tour.