September 2008

HURRICANES
'Tis the Season

 

 The "season" has arrived this year,
 When circular wind patterns do appear,
 Off Africa's coast where palm trees sway,
 In the gentle breeze that builds each day.
 A cloudy shape that we recognize,
 As it slowly begins to organize,
 A floating wheel of wind and rain,
 Can this be the birth of a hurricane?

 Hurricane Season, Dr. Doug Shanahan


Hurricane, showing eye

 

If it does become a hurricane, it will have an enormous, high, spinning funnel with a quiet center zone of lowest barometric pressure called the "eye". The highest wind speeds will be around the eye wall. and moving outward from the eye will be concentric bands of squalls, thunderstorms, and even mini-tornadoes. We saw these bands even with tropical storm Fay (August 18-20) that dropped ~ 5 inches of rain on the Refuge and as much as 30 inches as it stalled off Cape Canaveral.

SO, WHAT IS A HURRICANE?
 
Technically, a tropical cyclone is a storm system with a low pressure center with numerous thunderstorms that produce strong winds and flooding rains. Such a storm in the Atlantic Ocean is called a hurricane ("hurican" is the Caribbean Amerindian word for storm god). Such a storm in the Pacific Ocean is called a typhoon. Both typhoons and hurricanes have rotating winds, hence they are cyclonic. And both must have sustained winds of at least 74 mile per hour to be a category 1 storm and winds in excess of 155 miles per hour to be a category 5 storm.


Hurricanes in south Florida are variable in their intensities and timing and are only predictable in a general way. During the season the local fishermen's jingle comes to mind: "June - too soon, July stand by, August - look out you must, September remember, and October all over." Even hurricanes of the same strength can vary enormously in their size, rate of movement, and amount of rain generated. And just like lightning, hurricanes can and do strike twice, and even in the same month! Refuge staff will remember Frances on September 5th 2004 and Jeanne 20 days later!! Frances was weaker than Jeanne but was slow and large. With its soaking rains, Frances probably saturated the soil making the trees less stable when the more intense Jeanne followed.

HURRICANES ARE AWESOME, WILD, SCARY, & WONDERFUL


Just how awesome is a hurricane? An average hurricane releases 70 times the yearly energy consumption by all of humanity. This is equivalent to exploding a 10 megaton nuclear bomb every 20 minutes.

 
If you are away from buildings, power lines or non-native trees, then being in a hurricane is only wild and scary. The impact of wind-driven raindrops feels like stinging wet pellets shot from a gun. The sound of an approaching squall line starts like drumbeats, becomes a howl, and continues as a roar. It is so loud that you cannot even hear a nearby train or traffic along an expressway. So even in an urban setting it is akin to a wilderness experience.


Here are some paraphrased reminiscences of an elder Miccosukee, Buffalo Tiger, during an Everglades hurricane.

Our chickees [Seminole houses] were blown down or blown away. The wind was blowing so hard we could hardly walk. A still-standing old big tree had only a few remaining limbs, and they were all beat up. After the wind had changed direction of blowing -- from the southeast, northeast, west and then southwest -- we knew that the hurricane was going to be finished. As the wind lessened, the snakes were crawling around trying to get away from the water, but the 'gators and turtles were able to see us coming after them in the deep water and quickly submerge, and a big dark bird with its tail like two sharp points sticking out came flying by low.

This was a frigate bird that had been carried by the storm from its usual ocean habitat. In fact frigate birds have been seen as far north as Indiana after hurricanes!

CAN ANIMALS ESCAPE HURRICANES?


Refuge marsh with little danger to animals during a hurricane

 

Land animals move and behave so as to enable them to avoid the worst winds and rains of hurricanes; frogs and toads "love" rain. To avoid wind and the heaviest rain, small land animals get into small, protected places in dense vegetation or in treeholes or in burrows of larger animals. Large strong-flying pelagic birds like frigates and boobies may be carried ahead of storms. But what do small, weak-flying songbirds do? These birds have a special tendon-muscle arrangement that locks their toes around a perch even while asleep. Thus they hold onto a perch even in strong winds. After a hurricane we rarely see injured or even-rain-soaked birds, or damaged dragonflies or butterflies; they must be doing something right. Soon after the heavy rains of tropical storm Fay, grackles, jays and doves appeared at our feeder and engaged in a veritable feeding frenzy. I presume that they got pretty hungry during the two days of torrential rains.


An interesting phenomenon is the "hurricane birds" that stay in the quiet eye of the storm as the storm travels. They can only escape their "cage" when the hurricane greatly weakens. Hurricane spotter planes occasionally see great flocks of larger birds, like gulls, in an eye. If flying birds get too close to a hurricane they can be drawn into the eye by winds that act like a whirlpool. But if they are not too close they may be able to sense the lowering barometric pressure and avoid the storm.


Reports that pets and wild animals have a sixth sense for or can hear ultrasound from a distant hurricane are not taken seriously by most scientists. As a hurricane gets close, it would not take any special senses to detect increasing wind speed and rain intensity and move to safety. But where is safety in the open marsh in the middle of the Refuge where even tree islands provide little dense cover? What ibises, herons, and egrets do in the marsh during a hurricane is another mystery of the marsh! Maybe they have no problems.

ARE PLANTS ADAPTED TO TOLERATE HURRICANES?


Hurricane tracks since 1816

 

Since plants cannot move, we might expect that long-lived plants in south Florida will certainly experience hurricanes (see map) and thus will have evolved adaptations. Of all plants, trees are tallest and so are most exposed the highest wind speeds.


In south Florida, even adapted trees can be severely damaged by intense hurricanes. With a category 5 hurricane, like Andrew in 1992, all large shrubs and trees were defoliated and/or blown over.


Devastated hammock after Andrew

 

Andrew devastated Homestead and wiped out all the weather and water measuring devices in the Everglades. It had great, but temporary, effects on tree hammocks. In the hammocks, Andrew defoliated all the large trees and broke or partially uprooted about a quarter of the trees.

 

 

 

Vines cover broken trees for years after Andrew

 

For 2-8 years afterward, vine growth exploded and covered everything. But today the vines are no longer dominant and the tree canopies have re-grown. Careful observations show new trunks that have grown from leaning and broken trees.


 

At the Refuge in 2004, during category 2 and 3 hurricanes, cypress trees in the swamp lost branches and even some tree tops. But almost all of these trees survived and re-sprouted. Today the crowns of even the most damaged trees have regenerated.


In the interim the increased light in the understory stimulated great growth, flowering, and fruiting of shrubs, flowers, air plants and ferns. Now, the diversity of plant species is greater than before the hurricanes.


The woody plants on the Refuge tree islands also did relatively well in 2004. In a study of 74 Refuge tree islands, most swamp bay, some dahoon holly, and a few wax myrtle had snapped branches and some canopy loss. The tallest trees sustained the most damage. The only island with severe damage had abundant climbing fern, that appeared to cause the whole canopy of the island to collapse.


In contrast the marsh plants sustained almost no damage. Stems of bushes, grasses, sedges (e.g. sawgrass), rushes, and ferns are flexible enough that they sustained little damage. In addition they always re-grow from the below-ground growing tips even if tops are burned or blown down (see June column on fires). In the marsh interior it was hard to tell that two hurricanes had even occurred!

TREE RESISTANCE AND RESILIENCE


Why is it that native trees do so well and non-native trees do so poorly in hurricanes? Some natives show resistance to wind, a safety adaptation, and others show resilience, a fast re-growth adaptation. The mode of adaptation involves leaf size and retention in wind, twig and branch flexibility and retention, density of limbs and branches in the canopy space, trunk stiffness, and trunk-root stability.


Our native live oak demonstrates the resistance mode; it only sustains minor damage even in major hurricanes. It has a combination of small, widely spaced leaves and flexible, widely spaced branch tips that offer little wind resistance. If the wind increases in speed, then the tree progressively sheds its leaves, terminal twigs, and branchlets. Live oaks also have a combination of widely spread huge branches, relatively short stature, dense trunk wood, and roots that extend far out from the trunk, all of which prevent trunk breakage and uprooting.


Coconut palms along our shores also demonstrate resistance adaptations, and cabbage palms are almost as resistant. As wind speeds increase, the many long leaflets on each palm frond fold together and the fronds progressively bend and fold together until they look like a little girl with her long hair streaming out behind. Wind resistance of the palm leaves decreases further as the leaflet tips shred. Finally the trunk may bend a bit in the most intense hurricanes but, despite their amazingly small root mass, it is rare to see either species of palm blown over.


Among our native species, the gumbo limbo of hammocks and tree islands demonstrates the resilience mode. Almost every tree of any size is contorted due to multiple episodes of major limb loss and re-growth.


Note the tiny leaves and open spaces between limbs that offer no resistance to hurricane winds

 

Our native cypress demonstrates a combination of resistance and resilience. Its thin leaves are easily shed and the very open canopy, with widely spaced branches, offers little wind resistance. The strongly buttressed trunk virtually precludes tip-up even in water-logged soils. And, despite frequent branch and trunk breakage, it re-sprouts and repairs itself. If the Refuge cypress had not been logged in the past, it is certain that huge old trees would have survived many hurricanes and would have grown to over two meters in diameter and over 1,000 years of age. Examples of such trees are seen along the wild and scenic upstream portion of the Loxahatchee River near the Refuge.

 
Tree species with large and/or dense leaves, brittle branches, low density trunk wood, and shallow roots suffer the most defoliation, branch breakage, and uprooting. Alien examples include Tabebuia, Eucalyptus, African Tulip Tree, and Banyan Fig. Native north Florida species that suffer major damage in south Florida hurricanes include sycamore, magnolia, and sweet gum.


So get out and look at the different trees in your neighborhood and see if they are adapted to withstand hurricanes. You can get a clue as to what adaptations a tree has by some simple observations. You can also take a branch and flex it until it breaks and pull at leaves and branch tips until they break. And, as a thunderstorm approaches and wind speed increases, see whether the branches and leaves flex to reduce area exposed to the wind.

 

Based on your observations can you predict which species will suffer the most damage in high winds and why? Then you can look forward to the next hurricane to get outside and test your predictions.

A REQUEST FROM DR. TOM


I would like to hear from readers about their neat observations and any questions about the subjects of my monthly columns. I promise to answer. Click below.

Tom Poulson

 

REVIEW QUESTIONS  

a. Any cyclonic tropical storm with flooding rains, thunderstorms, and mini tornadoes is a hurricane.

b. During a hurricane, humans and wildlife are relatively safe in the marsh / tree island mosaic in the center of the Refuge.

c. By simply observing leaves, twigs, branches, trunks, and roots we can predict whether and how tree species will respond to hurricanes.

d. The species diversity and health of the Refuge cypress swamp community is enhanced by hurricanes.