Trees of the Lower Suwannee Region: A Natural History

Part VI. Southern Magnolia: Another Competitor

Florida’s lower Suwannee River and Cedar Keys region, including two outstanding National Wildlife Refuges, boasts more species of trees—as many as 130—than all but a few places in North America. Some of our trees are widely distributed in eastern North America, some are restricted to the southeastern coastal plain, and others 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 final installment, focuses on southern magnolia.

Southern magnolia (Magnolia grandiflora) is an iconic southern species that, although nowhere dominant on the Lower Suwannee refuge, can be expected to increase in abundance as the vegetation of moist hammocks matures. These trees may share dominance with other species in older forested stands. In our area southern live oak (Quercus virginiana) is an expected co-dominant in such mature communities.

When grown in full sunlight as in ornamental plantings, southern magnolia trees acquire a pyramidal shape, but when grown in sites having a forest canopy, they are more columnar in shape. Bearing leaves on the lower branches is a sign of shade tolerance; shade intolerant species usually lack branches on the lower trunk because leaves on the upper branches shade them out. The large, flat leaves of magnolias are efficient at capturing sunlight. As in many broad-leaved evergreen trees the thick waxy coating of magnolia leaves presumably helps to prevent excessive moisture loss in the winter season, but it likely also restricts the transmission of sunlight to the chloroplasts and limits shade tolerance. When exposed to full sun, leaf undersurfaces have a reddish appearance caused by tiny hairs, and these also might be an adaptation to prevent water loss.

Like our other iconic species magnolias possess a blend of tolerances that allow them to persist in a variety of situations. Relatively fast growth and prolific seed production are other characteristics of southern magnolias that are more common in pioneers than in climax adapted species.

Southern magnolias generally occur on sites with moist soils, but they are intolerant of inundation and thus require good drainage. When found near riverbanks and seepage areas they are usually far up on the banks where drainage is good. They do have extensive root systems which convey some degree of drought tolerance. Adult trees have bark that is relatively resistant to fire, and their extensive roots promote regeneration by sprouting after adults are top killed by fires.

One other species of magnolia is found in the region. Sweetbay magnolias (Magnolia virginiana) are often smaller than southern magnolias and tend to occur in wetter areas, on the edges of ponds and ditches for example. The leaves are somewhat narrower and less glossy than those of southern magnolia, and they have a distinctive silvery hue on the undersides.

Southern magnolia is sometimes called “Bull Bay” and together with sweetbay it is one of five kinds of trees possibly occurring in the area that have the word “bay” in their common names. The two magnolias are clearly related as are two of the others, red bay (Persea borbonia) and swamp bay (P. palustris), although neither of the latter two is related to the magnolias. A fifth species, loblolly bay (Gordonia lasianthus), is not closely related to any of the others. All five, however, are evergreen and vaguely similar in appearance. Their common names seem to come not from their associations with specific habitats but from their resemblance to one another. 

 

Distribution of Southern Magnolia. A strictly southeastern species, southern magnolias occur throughout the coastal plain, with a few outliers in upland areas. Like some other southern trees, planted magnolias can survive far outside their native range, although they rarely reproduce in such sites. When grown in the north they are frequently damaged by freezes.

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Southern Magnolia. This tree was photographed on the Lower Suwannee Nature drive not far from its junction with the McCormick Creek Road. In early May new leaves are emerging and can be distinguished from older leaves by their lighter green color. As they mature, they will become darker and develop thick waxy cuticles. Leaves generally remain on trees for two years

 

 

Southern Magnolia Blossom. These trees have some of the largest flowers in the plant world. In those grown as ornamentals the blossoms are usually conspicuous, but in those growing the wild they are often further up in the crown and out of sight. Flowers average 6 - 8 inches (15 - 20 cm) wide and are responsible for the species name, grandiflora.

 

Southern Magnolia Seeds. The colorful seeds are attractive to birds and thus provide an effective dispersal mechanism.

 

Shade-grown Southern Magnolia. This magnolia is growing in the shade under a canopy of southern live oaks, pignut hickories (Carya glabra), and other species. Note that its shape is more columnar than the pyramidal shape of magnolias grown in the open.

 

Trees of the Lower Suwannee Region: A Natural History

Part V. Eastern Redcedar – a Pioneer

Florida’s lower Suwannee River and Cedar Keys region, including two outstanding National Wildlife Refuges, boasts more species of trees—as many as 130—than all but a few places in North America. Some of our trees are widely distributed in eastern North America, some are restricted to the southeastern coastal plain, and others 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 fifth installment, focuses on eastern redcedar.

 

Of the species chosen as icons of the refuges, eastern redcedar (Juniperus virginiana) is a distinct outlier because it occurs over much of North America. Far from being restricted to Florida’s Gulf Coast or southeastern lowlands like the other trees highlighted, eastern redcedar is common throughout much of the east. Redcedars occur from central Florida, as far north as Maine and southern Canada, and as far west as western Kansas and Nebraska. Some botanists have argued that redcedars in Florida and southeastern coastal regions should be distinguished as a separate species, the southeastern redcedar, Juniperus silicola. Others regard these southern populations instead as a variety of J. virginiana, arguing that is the subspecies silicola. The Latin name silicola roughly translates as “sand dweller,” and at least in our region redcedars appear to be most at home on old sand dunes. I follow the Atlas of Florida Vascular Plants in regarding our redcedars as a variety of the widespread virginiana. 

Eastern Redcedar Distribution. Unlike most of the iconic trees of our region, eastern redcedar is widely distributed in the east and even extends to the Midwest and southern Canada. The red area in the map shows the distribution of the variety silicola, which some botanists consider to be a species distinct from the wider ranging Juniperus virginiana. The map suggests the forms are separate species because they do not grade gradually into one another as one would expect if the two forms were mere varieties.

Eastern redcedar is considered an undesirable, even invasive weed in parts of its range, particularly in the west where in the absence of fire it invades tallgrass prairies. So how can this tree be regarded as an icon of this part of Florida? One reason is its close historical association with the region and its economic development. Redcedar trees are the namesake of the Cedar Keys, and it is said that Atsena Otie, a Native American name associated with one of the islands in the archipelago translates as “Cedar Key.” So, in ancient as well as modern times, redcedars were a defining feature of the islands. Following settlement by Europeans, the early economic development of the region depended in part on exploitation of redcedar lumber for use in the pencil manufacturing industry. And despite the large-scale elimination of adult trees by lumbering, their descendants are still much in evidence on the islands and nearby mainland.

As with all biological species, redcedar trees are good at some things and not-so-good at others. They are good at invading dry areas. Tolerant of drought, they can become established on areas of bare or sparsely vegetated ground. But they are intolerant of shade, and do not grow well in competition with more shade tolerant trees. The foregoing qualities distinguish them as a “pioneer” species—able to invade new, often harsh habitats by producing abundant offspring and thereby getting ahead of less hardy but more shade tolerant competitors. They thrive on well-drained soils and not so well when soils are permanently saturated Nevertheless, they are somewhat tolerant of occasional inundation by salt water and may grow in high spots surrounded by marsh. They readily invade new areas, producing abundant cones which are dispersed widely by birds. 

 

Redcedars on Shell Mound. Redcedar trees are abundant on shell mounds and shell middens throughout the region. In this image made on the Lower Suwannee refuge's Shell Mound Unit, redcedars are growing around the margins of the mound avoiding the shade provided by the oaks in the center of the mound, seen in the upper left of the image.

Besides the key environmental factors of moisture and sunlight, one more driving force explains the current distribution of redcedar trees; they are intolerant of fire, being sensitive even to low intensity fires. Their thin bark, shallow roots, inability to sprout, and highly combustible evergreen foliage, which may extend to the ground, makes them vulnerable to fire. 

Redcedar at the Summit of Shell Mound. This tree has taken a foothold at the pinnacle of the mound and is probably hundreds of years old. Note the sparseness of leaves in the crown—a common feature in pioneer species owing to the upper leaves shading out lower ones.

The islands and certain headlands of the Cedar Keys region offer some distinct if not unique advantages for redcedar trees. The old sand dunes that make up the islands and other coastal areas have good drainage and often lack enough soil moisture to favor the establishment of competing tree species. Shade may not be a problem in sparsely forested islands with fringing beaches, making it difficult for other trees to shade out redcedars. Most importantly, the islands are protected from the frequent fires that affect most other habitats on Florida’s Gulf Coast. Redcedars also occur in local estuarine swamps which, although probably not as favorable as islands, are also protected from frequent fires. Shell mounds—either purpose-built structures or waste heaps left by ancient Native Americans—are abundant in our area. Their importance for redcedars should not be minimized. The purpose-built Shell Mound on the refuge and numerous other shell middens share qualities with islands, particularly protection from fires, and likewise provide good habitats for redcedars to become established and persist.

The abundance of redcedar trees on the Cedar Keys and nearby sites made them an attractive place for those in search of commercially exploitable sources of cedar lumber. The local concentration of redcedars and access to shipping may have made the Cedar Keys superior economically to exploiting sources of cedar lumber in other places where trees were scattered widely and water and rail transport were not readily available.

 Some aspects of the ecology of redcedar trees don’t fit those commonly associated with pioneer species. They are relatively slow-growing and given adequate protection are long-lived. And their lumber is commercially valuable; the heartwood becomes mpregnated with resins and is resistant to decay. It has been used for pencils, lining closets and storage trunks (because volatile compounds released by its resins repel clothes moths), and fence posts. Native Americans used redcedar posts as boundary markers; the place-name Baton Rouge came from French-speaking settlers in Louisiana.