Trees of the Lower Suwannee Region: A Natural History

Part I. Longleaf Pine – A Fire Specialist

The lower Suwannee River and Cedar Keys region, including two outstanding National Wildlife Refuges, boasts more species of trees—as many as 130 species—than all but a few other places in North America. Some of our trees are widely distributed in eastern North America, some are restricted to the southeastern coastal plain, and some reach their greatest prominence locally. A few years ago our Friends of the Lower Suwannee and Cedar Keys National Wildlife Refuges organization designed a tee-shirt featuring six iconic species of trees occurring on the refuges. Our series of blog posts will begin by considering each of these six species. This, the first, will consider longleaf pine.

Perhaps the most celebrated of the trees on our refuges and elsewhere in the region is longleaf pine (Pinus palustris). It may seem a paradox that individual longleaf pines are relatively rare and difficult to find, both on Lower Suwannee refuge lands and in the immediate vicinity. Another species, slash pine (P. elliottii) is much more abundant. Probably slash pine was always the dominant pine on the islands, but its current dominance on the adjacent mainland is largely as the result of human activities. 

Distribution of Longleaf Pine Savannas. Once covering much of the Florida peninsula and the southeastern coastal plain, longleaf pine savannas have largely disappeared. However, small clusters of longleaf pines remain throughout much of their former range, and it is believed that changes in land use and much time may ultimately result in restoration.

Longleaf pine is a large, slow growing tree that may take 100 to 150 years to reach full growth at more than 100 feet in height, and individuals may live for 500 years. It is intolerant of shade and normally occurs in so-called savannas as widely spaced individuals separated by much ground-level vegetation. Longleaf pine savanna historically covered much of the uplands in the lower Suwannee region and throughout much of the Florida peninsula, although documentation is scarce. Early loggers did not distinguish between the species of southern pines and lumber from longleaf, slash pine, and loblolly pine (Pinus taeda) was all marketed simply as “southern yellow pine.”

All three species mentioned above occur throughout the mainland and on the refuge, and all can be seen along the first third of the right (eastern) part of the Tram Ridge Trail.

 

Pine Flatwoods Restoration. In this image the refuge’s Tram Ridge Trail passes through an area where some of the features of the natural longleaf pine savanna have been recreated by selective thinning of slash pine trees that were originally planted in close rows. Wide spacing of the trees permits sunlight to reach diverse low growing plants, which in turn support a diversity of wildlife species.

Most in evidence are slash pines. Planted in close rows 40-50 years ago, they have been thinned by refuge managers to more resemble the longleaf pine savanna that would exist under natural conditions.

Loblolly pines require more water than slash pines, and a few individuals can be seen on the left side of the trail as it approaches an extensive swampy hammock. Loblolly pine has the shortest leaves of the three species and the smallest cones. The bark of older trees is broken into large plates that have a dark red brown coloration. The leaves of the much more abundant slash pines are longer than those of loblolly, the cones are larger, and the plates on the bark are yellowish-brown in color. To my eye the highlights on loblolly bark are reminiscent of dark chocolate, while those of slash pine are more the color of milk chocolate. Finally, the two species can be distinguished by the shapes of their crowns; loblolly crowns tend to be more open, spreading upward vase like whereas slash pine crowns are rounded and umbrella like.

Cones from Five Species of Pines. The two large cones at the top of the image are from longleaf pines, those at the left center are from slash pines, and the pair to their right are from loblolly pines. The pair of cones on the right in the lower row are from sand pine (Pinus clausa); unlike those in the image, some sand pine cones in the Cedar Keys area remain closed, releasing their seeds only when the wax holding them closed is melted by a hot fire. The pair of cones at the lower left are from spruce pine (P. glabra), a species reported from nearby, but not from Levy or Dixie counties.

As the Tram Ridge Trail nears State Road 24 and begins to turn to the left one encounters a small cluster of longleaf pines. They have thick bark less broken up into plates than the other two species, much longer leaves (up to 15 inches in length). their limbs branch irregularly, and any shed pinecones beneath them will be relatively huge, 6-8 inches long.

 

Cluster of Longleaf Pines. These trees are part of a small cluster of longleaf pines on the Tram Ridge Trail, just beyond where it closely approaches State Road 24. A few young trees are nearby, but it may take several more decades or longer before these trees reproduce abundantly.

All three species (and pines in general) are intolerant of shade, and depending on species, all are to some extent tolerant of fire. Longleaf stands out in its extreme tolerance for fire and for drought. Not only is it tolerant of fire, it is said to be promoted by fire. When fires occur frequently longleaf will eliminate its competitors. Longleaf seedlings go through a so-called grass stage in which little above ground growth is evident save for a grass like clump enclosing a growing shoot. Almost fireproof, the grass stage lasts from three to seven years in which most growth occurs underground in the formation of a deep taproot that penetrates up to 12 feet deep to reach groundwater. Once this root system is established the tree grows rapidly, in a few years becoming tall enough that vulnerable parts are beyond the reach of most fires and a thick, fireproof bark has formed. All potentially competing trees are less resistant to fire and are eliminated. Presumably, it is the extensive root system of longleaf pines that appropriates available groundwater and keeps adult trees widely spaced.

Grass Stage. This longleaf seedling in the “grass stage” was part of a restoration effort on the Lower Suwannee refuge. Longleaf pines in this stage are relatively resistant to fires but are said to be grazed by wild hogs (Sus scrofa).

 

Surviving a Fire. The “grass stage” longleaf seedling shown here was completely engulfed by fire on the San Felasco State Preserve, but the meristem was undamaged, and growth had resumed at the time this image was made.

 Longleaf pine savannas with their sparse shading and wide openings to the sun support a huge diversity of low growing plants, many of which have their own adaptations to fire. The disturbance to ground-level vegetation caused by frequent fires ensures that competition among understory species does not lead to monocultures. Because of the diversity of understory vegetation longleaf pine savannas have been recognized globally for their significant biodiversity. One animal species abundant in longleaf pine ecosystems is the gopher tortoise (Gopherus polyphemus), a well-known keystone species that contributes significantly to biological diversity by constructing burrows which are relied upon by a large variety of other animals, some of which are found nowhere else. 

Gopher tortoise. This common animal of longleaf pine savannahs is considered a "keystone species" because its burrows provide shelter for many rare animal species.

Why is longleaf pine no longer abundant on the lower Suwannee refuge, and why throughout the south do longleaf pine savannas now occupy less than 5% of their former geographic range? First, longleaf pines are poorly suited for commercial timber production. Originally used as a source of turpentine, after development of mechanized transport they became an attractive source of lumber. However, slow growing, slow to mature and reproduce, difficult to plant, and intolerant of crowding, longleaf pines were economically far inferior to slash pines, which produce reliable crops that can be harvested on as little as a twenty-year rotation. Second, regular fire suppression led to elimination of the competitive advantages enjoyed by longleaf. A third factor possibly important on the Lower Suwannee refuge is the abundance of wild hogs (Sus scrofa), which eat longleaf pine seedlings. Although wild hogs were introduced by the first Spanish colonists and have probably been present throughout Florida for hundreds of years, fire suppression and the invasion of pine savannas by hardwoods favor hog populations by producing acorns and hickory nuts. So, the enhanced hog populations and the invading hardwoods work in tandem against longleaf pine savannas.

A long-term goal of managers of the Lower Suwannee National Wildlife Refuge is to restore its landscapes and habitats to their natural condition, ideally involving removal of the closely planted slash pines and replacement with longleaf pines and an understory of wiregrass (Aristida stricta). Wiregrass is highly flammable helps to maintain the fires on which it and longleaf both depend. This strategy is used on the refuge but on a relatively small scale owing to cost. As a more practical alternative, slash pines planted in the past are thinned and the understory is regularly burned or mechanically treated in order to reclaim the physical characteristics and ground-level diversity found in a natural longleaf pine savannas. Such restoration is seen along the southern third of the Tram Ridge Trail and on much of the refuge’s uplands.

Certain scrub communities in the Lower Suwannee region support sand pine (Pinus clausa), a mid-sized wispy tree of little economic value. Sand pine has disjunct populations on the northern Gulf Coast, the Cedar Keys and nearby mainland areas, and in the sand ridges of the central Florida peninsula. The inland populations, particularly in the Ocala National Forest, form dense continuous stands. With some other pines the central Florida stands have closed, so-called serotinous cones—their cones are sealed by a wax that melts and releases the enclosed seeds when subjected to a hot fire. Sand pines in the lower Suwannee region have both serotinous cones and others that open normally as in most other pines. Those along the upper Gulf Coast have no serotinous cones. Sand pines with serotinous cones are obviously adapted to fire, but their adaptations are far different from those of longleaf. They are adapted to infrequent but devastating fires. Such fires will almost surely kill the trees, completely wiping out the entire parental generation. Thus, trees in stands tend to be even aged, with the new generation completely replacing the old one. Highly flammable, they promote fires that reach the crown by retaining dead branches in the lower parts of the trunk to act as “fire ladders.” Dispensing with an entire generation is an unusual life history strategy that perhaps may be related to extreme intolerance of shade that makes young sand pines intolerant of the shade created by their parents. Or maybe it is a strategy to eliminate the many potential competitors less well adapted to persist after catastrophic fires.

Stand of Sand Pines. Sand pine (Pinus clausa) occupies old sand dunes in our area in communities that often include scrub oaks. Populations occur in the coastal panhandle, and in the sandhills of the Ocala National Forest. Panhandle pinecones open spontaneously, but in central Florida the cones are serotinous—sealed by a waxy substance. Seeds are released only when opened by a hot fire. Some cones in the Cedar Key area open normally and others are serotinous. Sand pines promote fires that kill adult trees and release the seeds that will be founders of the next generation. Stands tend to be even-aged as in this image made at the Cedar Key cemetery. Dead branches extending from the trunk, form “fire ladders” that carry flames to the crowns.

 

 

Salt Marsh Voles. Part IV. the Hurricane and the Aftermath

 

My interest in the Long Cabbage Islands and the voles intensified a few years later when Hurricane Hermine came ashore in the Florida panhandle on September 2, 1916. The storm surge was variously reported at between seven and 11 feet in the Cedar Key area, and it certainly completely engulfed the Long Cabbage Islands. 

What happened to the voles? Were they wiped out? Or were some individuals able to cling to a few protruding cabbage palms until the water subsided? And if some survived, were their tunnels and other workings in the mats of grass much disturbed, or even the grasses washed away? Had local populations, or maybe the entire subspecies, been extirpated? 

Tide gauge at Cedar Key intended to measure storm surge. Levels registered during Hurricane Hermine were variously reported between seven and ll feet, with most clustering around nine feet. If comparable surge heights occurred at the Long Cabbage Islands, they might well have been as tall or taller than the largest cabbage palms on the islands.

 I don’t know that these questions have ever been answered, but I suspect that the vole population pulled through. The favored habitats of voles are grasslands (and salt marshes are a kind of grassland). Much of their geographic range to the north is dominated by forests. Grasslands in forested areas are relatively rare and ephemeral, mainly resulting from disturbances like fires and blowdowns. Where grassy patches exist, they are subject to the gradual encroachment of trees and are almost sure to disappear in a few decades. Voles may be examples of “fugitive species”—biota whose habitats are unstable and short-lived. When their patches of grassland disappear, they must move on. They are adapted to find and rapidly exploit new habitat patches created by disturbances.

Looking into vole population biology sheds some light on their adaptations. Their tendency to “wander,” territoriality among breeding females, and the outward pressure produced by overproduction of offspring all favor expansion into newly available habitats. And they are prodigious reproducers, rivaling or outperforming rabbits. Female meadow voles studied in Pennsylvania reach reproductive maturity at 20 days of age, have from 2-11 offspring per breeding attempt, and can breed again in as little as three weeks, having as many as 12 litters in a year. The mathematics are simple, but the results are startling, nevertheless. In the unlikely event that breeding were left unchecked, in a year a single pair of voles could potentially produce more than 4,000 descendants.

So, the biological adaptations of voles fit well with unstable habitats because they allow pushing into newly available habitat patches and rapidly populating them. A few survivors from the storm surge overtopping the Long Cabbage Islands could rebuild the population to former levels in a relatively short time. Or descendants of survivors from other less damaged habitats might rapidly push into the newly vacant habitat.

Clearly glaciers have not directly influenced Florida landscapes in the past tens of thousands of years, and probably never. However, as is attested by the Florida salt marsh vole, the indirect influences of an ice age lasting until 10,000 years ago are clearly visible. And these effects may be dwarfed by changes in sea level on the scale of hundreds of thousands of years, in which the Florida peninsula expanded and shrunk as glacial ice alternately withdrew water from the oceans and released it. Salt marshes would have been repeatedly destroyed and recreated in response to moving shorelines. Perhaps sometime during past millennia suitable habitats and populations of the vole were distributed much more extensively.

 

 

Salt Marsh Voles Again. Part III. New Information

Back in 2010 I put up two blog posts about the Florida salt marsh vole, a unique and unusual inhabitant of the Lower Suwannee National Wildlife Refuge. The earlier posts can be found here and here. New information, new questions, and some ideas about answers suggest it is time for yet another series of posts. They begin with field notes from a paddling trip undertaken ten years ago, almost to the day.

 Field Notes: Long Cabbage Islands – September 25, 2011

I am beginning a paddling trip in the Gulf waters bordering the Lower Suwannee National Wildlife Refuge, with a group ground-truthing a series of kayak routes laid out on refuge waterways. While verifying the maps, we hope also to learn about important natural features to help in preparing informative narratives to go along with the maps.

We leave from the small gravel launch ramp at Shell Mound, glide past Hog Island—a chain of narrow islands that are the site of a prehistoric cemetery—and paddle north by northwest. Billowing clouds punctuate the bright blue sky, waves are minimal, and the view toward the horizon in every direction is stunning.

Despite the picture-perfect day, when we reach the Long Cabbage Islands, we find them underwhelming. Today the tide is relatively high, at 3.5 feet, and there is little to be seen that one could properly call islands. I would describe them as barely exposed sand dunes. Indeed, the tides tomorrow, on September 26 are forecast to be nearly 5 feet, and little of the islands should be visible then except for emergent spikes of smooth cordgrass (Spartina alterniflora) and occasional partly submerged black mangrove trees (Avicenna germinans). That these low islands are usually not completely submerged is attested to by a few mostly solitary cabbage palms (Sabal palmetto) clinging onto the highest patches of sand. Despite their precarious relationship with the open waters of the Gulf, the wispy, patchy islands are clearly not devoid of animal life. We see a few horseshoe crabs (Limulus polyphemus) copulating in the shallow water. And on the dunes are abundant shorebirds of several species, probably foraging for the countless horseshoe crab eggs that provide them with copious food. Other abundant animals are snails—marsh periwinkles (Littoraria irrorata)—we see on many of the stems of smooth cordgrass. These snails are said to always remain above the waterline except when venturing down below the surface to lay their eggs. Aware of this, I briefly wonder what they do when an unusually high tide overtops all the cordgrass stems.

Our return from the Long Cabbage Islands is uneventful. I think we will retain some pleasant memories of today’s paddle; it has been a great outing. The islands are a nice—or at least interesting—place to visit if not stay awhile. Importantly, the event has offered some interesting perspective on a marginal habitat that might be swallowed up with only minimal sea level rise.

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View of the Long Cabbage Islands showing their scant elevation. The cabbage palms (Sabal palmetto) and black mangrove (Avicennia germinans) trees seen in this image would be the only possible refuges available in severe storm surges.

 

Salt grass (Distichlis spicata) patch on the long cabbage islands. Thick mats of this grass are believed to be the primary environmental feature favored by the Florida salt marsh vole.

Despite the pleasantness of the visit to the Long Cabbage Islands, we felt no need to return. Other projects took center stage, and these islands were soon all but forgotten. 

That visit was to be forgotten for only a few months, however. The islands grabbed my attention when their name came up in a report of a survey underway by University of Florida scientists. They were looking for the endangered Florida salt marsh vole (Microtus pennsylvanicus dukecampbelli)—an animal formerly known from only two nearby locations also in Levy County, Florida. I knew about the project, knew some of the students and professors involved, and realized that the vole has an interesting story, much of which I had related in blog posts back in 2010.

 Studies culminating in 2013 were successful in finding voles in several new locations through a trapping technique that “captured” them photographically in an ingenious device able to verify their presence without the need to disturb them. Of interest to me was that the Long Cabbage Islands were an area in which voles and another rare animal and likely predator, the salt marsh mink (Neovison vison lutensis) were relatively abundant.