Silviculture is “the art and science of controlling the establishment, growth, composition, health, and quality of forests and woodlands to meet the diverse needs and values of landowners and society on a sustainable basis” [The Dictionary of Forestry, 1998]. 

The two cornerstones of silviculture are silvics and ecology.  Silvics provide descriptions of individual tree species in the context of sciences basic to understanding how trees grow.  Ecology explains the relationships between trees, other living organisms and their environment: soil, climate, and the land itself.  The silvicultural research program at UF/IFAS North Florida Research and Education Center (NFREC) is focused on nutrient and vegetation management in commercial southern pine forests.  The extension component of our activities covers all aspects of silviculture described below.

Southern Pines 

Ten hard “yellow” pine species occur in the southern United States: longleaf, loblolly, slash, shortleaf, sand, spruce, pond, pitch, Virginia, and table-mountain pine.  Longleaf pine is one of the most distinctive tree species in North America, and the pillar of the open, park like, native forests, which dominated some 60 million acres (24 million hectares) of the lower South before European settlement.  This pine species is valued both for its ecological importance and high value timber qualities.  Loblolly pine is the most important timber producing species throughout the South, but in Florida this commercial importance is rivaled by slash pine.  Shortleaf pine predominates in the northern part of the Gulf Coastal Plain and its range merely reaches Florida.  Sand pine has some marginal commercial value for pulpwood production and as a Christmas tree.  Beside these five species, spruce and pond pines also grow in Florida.  Pitch, Virginia and table-mountain pines can only be found further north. 

Forest Regeneration and Establishment 

Natural regeneration of the hard southern yellow pines occurs only when seeds falling on the forest floor come in contact with exposed mineral soil and adequate sunlight is available during following growing seasons for seedling germination and establishment.  Because all three conditions (emphasized in bold) may not occur simultaneously under undisturbed, natural conditions, stands of southern pines often do not naturally regenerate to the same species.  Consequently, under large old pines, one finds oaks, hickories, and other broadleaf species.  The resulting hardwood forests represent the climax stage in ecosystem succession, i.e., this collection of species that can grow and reproduce naturally in perpetuity in the absence of wildfire, hurricane, or other disturbance, which could again re-create conditions required for natural pine regeneration.

Artificial regeneration techniques have been devised in part to overcome difficulties associated with year-to-year variability in seed production by different pine species.  To assure good survival and new forest establishment, sites are prepared by mechanical, chemical and/or prescribed fire treatments.  Forest regeneration is then accomplished by either direct seeding or planting of seedlings.  Direct seeding is similar to natural regeneration in many ways, except that a specific quantity of seed of a desired pine species is broadcast over the known area of land.  Planting of seedlings further affords a way to control not only species composition, but also initial tree density and arrangement of trees – rows or random placement – in the resulting stand.  Herbaceous weed control (chemical, hand-cutting and/or mowing treatments) may be needed, especially if a site is insufficiently prepared before tree seeding or planting, to assure minimal competition during stand establishment.  A tree stand is properly established if sufficient number of “free-to-grow” seedlings per unit area is past the stage of juvenile mortality that could be caused by frost, drought, weeds or animal browsing.

Vegetation Management 

Forest vegetation management aims at controlling the growth of undesirable plant species, so sunlight, moisture, and soil nutrients are available to trees and other desirable vegetation (e.g., wiregrass in longleaf pine forests).  In Florida, sound forest vegetation management for timber production is often compatible with management for wildlife, recreation, and biodiversity.  The most common tools for controlling competition from unwanted plants are herbicides, fire, and mechanical operations, such as mowing, cutting, bedding on wet sites, scalping on old-fields and pastures, and roller chopping prior to prescribed burning.  Any one of these treatments can be used alone or in combination for site preparation before forest regeneration.  This is also the time when competition-reducing treatments can be applied most effectively because of the absence of vulnerable young pine trees.  Later on herbaceous weed control, conifer release from hardwood competition, as well as understory hardwood and shrub control are achieved by proper herbicide applications and/or prescribed fire.

Forest Fertilization 

Current recommendations for intensive pine timber management on nutrient deficient sites include fertilization near time of planting (age 0-4) (lbs/acre of element): 40-50 N plus 50 P, and during mid-rotation (age 7 -14): 150-200 N plus 25-50 P.  Alternatively, pine straw producers may fertilize pine plantations every five years between ages 6 and 16 with (lbs/acre of element) 200 N, 50 P and 80 K.  Another aspect of forest fertilization and nutrient management in Florida, and throughout the South, involves recycling of large quantities of animal waste generated by poultry and hog operations.  Pine plantations could serve as animal waste recycling sites, if there is an associated fertilization benefit that could be realized in an environmentally sound manner.  One concern currently being addressed through research at NFREC is the question of shallow groundwater contamination with nitrates in relation to forest fertilization practices.  Another aspect addressed by research at UF/IFAS School of Forest Resources and Conservation is the relationship between disease incidence (pitch canker) and sites receiving nutrient additions either from inorganic or organic sources.

Forest Improvement 

High-quality forests start with the use of genetically improved seedlings at the time of forest regeneration.  These seedlings are the result of genetic selection and tree improvement programs that started in Florida and elsewhere in the South in the 1950s.  In the natural forests, superior parent trees were identified on the basis of bole quality, general tree form, and disease resistance.  Open and controlled pollination in seed orchards among the selected individuals followed by progeny testing allowed for continuous genetic improvement through subsequent cycles of selection and testing.  Significant genetic gains were achieved for slash, loblolly, longleaf and sand pines.  For example, second generation improved slash pine plantations are expected to produce 35% more wood and have 60% less disease, which results in 45% more harvestable wood. 

In a more classical sense, stand improvement activities means tree cuttings that take place between stand establishment and final harvest.  Intermediate cuttings may take the form of a pre-commercial thinning, commercial thinning(s), pruning, sanitation harvest, improvement cuttings, pest control, or salvage operations.  Usually at least one commercial thinning is necessary to produce high-quality timber.  The other intermediate stand management operations are performed on an as needed basis.

Ecosystem Restoration and Conservation 

Much of Florida and the southeastern U.S. was dominated by longleaf pine forests in the pre-settlement times.  Out of the remaining 3.3 million acres of longleaf pine in the Southeast, about 30% is found in Panhandle region of Florida.  Besides longleaf pine, wiregrass is the other key plant component of the native longleaf pine ecosystem.  Together, they form a fire dependent sub-climax forests that can exist “in perpetuity” provided frequent low-intensity burns occur.  Wiregrass and pine straw fueled fires, either prescribed or natural, keep hardwood competition in check, create conditions for continuous pine regeneration, and stimulate wiregrass to produce viable seed.  The longleaf pine-wiregrass forests are thought to be among the most diverse ecosystems in the world, outside of the tropics.  They provide habitat to federally endangered red-cockaded woodpecker, state-endangered Florida mouse, as well as “species of special concern”, such as the gopher tortoise and Sherman’s fox squirrel, among many other species. 

Because of the ecological importance, highly appealing aesthetics, and a potential for high-quality timber production, longleaf pine ecosystems are in the center of restoration and conservation efforts in the region.  To be successful, these efforts need to take into account current forest or land conditions, which dictate different restoration and conservation measures.  On sites where longleaf pine and wiregrass are present, proper herbicide treatment and/or reintroduction of periodic fire may adequately release both species from hardwood or other competition and restore desired ecosystem conditions.  In other cases, artificial longleaf pine regeneration and/or ground cover restoration by seeding or planting may be necessary.  Since successful pine establishment may hinder subsequent ground cover restoration, the sequence of events must be carefully thought out depending on site conditions.  Conservation of existing longleaf pine ecosystems requires active management, mainly periodic (every 3 to 4 years) fire.  Existing native groundcovers should be protected from any mechanical or chemical disturbances, such as intensive site preparation before tree planting.

Forest Health 

Many forest health problems are expressed as insect infestations and/or fungal disease infections.  However, when these ailments occur, they are often a mere consequence of and underlying primary stress, such as nutrient deficiencies, chemical toxicity that might result from previous fertilization or pesticide practices, drought, tree-on-tree or understory competition.  Trees may also be weakened by prescribed fire that was too hot, or applied at less than optimal time.  Stressed trees fall prey to primary insect attacks and/or disease infections, and then secondary insect attacks. 

Southern pine beetle, Ips bark beetles and black turpentine beetle are often the first invaders attacking weakened pines.  Among these, southern pine beetle causes the most damage, as infestations can kill hundreds and even thousands of acres when the beetle is in the “outbreak” status.  Other insects of ecological importance include the Nantucket pine tip moth, which destroy seedling terminal shoots causing loss of wood quality, and pine web worms, which can defoliate and kill young pine plantations. 

There are four groups of fungi-caused diseases affecting southern pines: cankers, needle blights, heartrots, and butt rots.  Spindle-shaped cankers on stems, branches, and the bases of needles are symptoms of a fusiform rust, the most devastating disease occurring on slash and loblolly pines in Florida.  In the past, trees were often infected with the fusiform rust when still in a nursery, today these infections are much less common because of treatments available to nursery managers.  Pines of all ages are susceptible to pitch canker; however, the disease is most common in trees 10 years of age or older.  The most conspicuous symptom of the pitch canker is copious outflow of resin without obvious insect attack.  It is important to point out that diseases like fusiform rust and pitch canker are under genetic control and hence some genotypes have inherently more resistance or susceptibility.  This genetic variability is the basis for tree improvement programs through cycles of selection and testing (see: “Forest Improvement” above), and disease resistance is one of the traits selected for in subsequent tree generations.  Site hazard ratings research indicates that drier upland sites with significant oak populations tend to be of greater risk for fusiform rust than other sites.  Also, intensive management tends to increase the incidence and severity of this disease.  Needle blights affect pines from seedling to maturity.  Infected foliage dies and falls to the ground, while the disease may go undetected, mistaken for seasonal needle cast.  Fungal fruiting bodies, or conks, on the trunk surfaces of older pines are symptoms of an ongoing decay in the “heartwood” (inner, darker wood column in the stem center).  Across the South, these heartrots cause more wood volume loss in standing timber than any other disease.  Butt rots cause similar wood decay; however, wood destruction is usually limited to the lowest few feet of the trunk.

Wildlife Considerations 

The value of a southern pine forest as a habitat for game and other wildlife species depends on the developmental stage of the forest.  Different plants associated with each stage provide optimal food and cover for different animal species.  Mourning doves may frequent cutover sites for about 5 years, as they favor large open areas.  Fox squirrels, which loose their dens during clearcutting, may return to the site in about 5 years after stand establishment.  Quail are most numerous 4 to 10 years after harvest, and they prefer open forest conditions typical for longleaf pine-wiregrass forests thereafter.  The needs of deer are best met for 6 to 10 years after clearcutting and subsequent forest regeneration.  As stands close and exclude ground vegetation and shrubs on which deer depend for browse, the number of animals that could be supported dwindle.  Optimum conditions for cottontail rabbits occur at about the same time as for deer.  Rabbits could also be pests in young pine plantations by nipping on planted seedlings. Gray foxes are most abundant about 15 years after regeneration.  Ideal red-cockaded woodpecker habitat consists of mature, sparse pine forest with open understory, and trees with red heartrot for building of nesting cavities.

Sustainable Forest Management 

Today’s southern pine forests are managed for a variety of benefits, including aesthetics, wildlife habitat, and recreation, as well as timber production.  These objectives are seldom mutually exclusive, although usually one of them takes center stage depending on a landowner’s preferences.  Silviculture, a discipline within forest science, offers a set of tools that make achievement of the forest management objectives possible.  Sustainable forest management strives to maintain healthy ecosystems, improve the balance among alternative forest values and conserve biodiversity.  For more information on forest management in Florida visit University of Florida/IFAS, School of Forest Resources and Conservation (SFRC) website (http://www.sfrc.ufl.edu/), or call (850) 875-7145 [Minogue, P., pminogue@ufl.edu]

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