Well developed forest showing a dense A layer at the forest floor, a tall C layer with few understory trees, and the dispersed canopy typical of temperate forest. Fernbank Stream drainage, north-east Atlanta)
A. The Principal Focus.
Mature, southeastern, hardwood forest is primarily structured by the older trees; these contribute most of the biomass to the more-or-less continuous canopy (Level D). The understory is comprised of the near vertical trunks and stems of the taller trees as well as younger trees and smaller tree species adapted to lower light levels (Level C). Below this is the shrub layer that includes small tree saplings as well as shrubs (Level B). Level A comprises the herbs of the forest floor but includes ferns, and the vital tree seedlings that are the forest of the future (see Kricher & Morrison, 1988, for a simple introduction to southeastern forest).
Invasive vines climb into the trees of the understory and canopy. They compete with the trees for soil nutrients and water, cause mechanical damage, contribute to a higher risk of disease, and a higher risk of being knocked down by the wind (‘wind-blow’).
Trees overwhelmed by English Ivy. Inman Park, north-east Atlanta
Ivy climbs up and overwhelms smaller trees, its new growth cascading from near the top of the tree and bending it over. Wisteria can overwhelm some of the largest trees. Both wisteria and kudzu kill trees at the forest’s edge. The action of the vines nearer the ground in both the B layer and the A layer shuts down the all-important secondary succession by which the forest regenerates itself. The invasive vines also exclude the majority of native herbs, shrubs and tree seedlings. If the standing forest is seen as the forest of the present, then the tree seedlings and saplings of the A, B, and C layer can be recognized as the forest of the future; the latter is being extinguished by the invasive species. Invasive shrubs act in concert with the invasive vines. By itself, privet can so dominate the B layer that it completely shuts out the native tree seedlings and saplings.
Privet dominating the B layer in part of the Fernbank Forest, north-east Atlanta.
Moreover, privet “bushes” and large privet trees provide convenient “stepping stones” for the vines to climb up into the larger, native trees.
Large vines threaten the established trees. Ivy can completely encase the tree trunk to 80 – 90% of the height of even the tallest trees, and both wisteria and kudzu can grow completely over an entire tree. The School sees the removal of these vines from the small, medium, and large trees as a first priority, and as the first phase of the conservation effort. A machete is used to cut away the vines from ground level to a height of about 3 m. without damaging the tree. The root systems of the vines are removed to a distance of about 0.7 m of the tree trunk, and the debris piled so as to prevent erosion and to shade out the vines beyond the cleared circle. Vines at the periphery of the tree are also cut. These activities effectively kill the entire infestation of vines throughout the tree. ‘Cut-Treat’ herbicidal treatment, as recommended by Miller (2003), may be used to kill root systems at the base of the tree when they cannot be removed mechanically without damage to the tree itself. “Cut-Treat” involves the very careful application of a herbicide to the freshly cut stump so that the herbicide is absorbed into the root system, kills the roots, and then degrades. There is no threat to other plants in the immediate area which is a problem posed by using spray treatments. Extreme care is taken to recognize and cut around tree seedlings and saplings proximate to the tree and extend the cleaned area at the base of the principal tree to include, and hence protect, these contributions to the forest of the future.
The second phase is to cut, pull, and/or ‘Cut-Treat’ the invasive shrubs that are shutting out the seedlings and saplings of the native trees. Again, extreme care is taken to recognize and cut around the seedlings and saplings of any native tree species that have managed to survive the competition with the invasive vines. Some of these tree species, such as Box Elder (Acer negundo) and Laurelcherry (Prunus caroliniana), are natives but are regarded as weed species. In the first instance, these are left alone to prevent erosion and encourage conditions at floor level conducive to the germination and growth of the more shade-loving native tree seedlings.
Follow-up treatments constitute a third phase. The invasive species have an extensive seed bank in the soil and some of the larger root systems, especially those of kudzu and privet, may require a second ‘Cut-Treat’ effort. A third ‘Cut-Treat’ effort may be needed where kudzu or wisteria has been established over a long period. These forest conservation efforts can only be successful in the long-term by effective and determined follow-up in conjunction with a planting effort. The planting of native herbs, ferns, shrubs, and trees in cleaned areas “jump-starts” a renewed secondary succession, and constitutes a fourth phase. Planting is not, as yet, a School activity but is enabled by the cutting and ‘Cut-Treat’ work of phases 1 - 3. In the Atlanta area, “Trees Atlanta is taking great steps in this re-planting process.
B. The Larger Picture
Insularization, or the breaking apart of intact major habitats into smaller and smaller areas, is one of the principal mechanisms by which biodiversity is being lost worldwide; forest communities undergo faunal and floral collapse. Smaller areas of habitat are more and more susceptible to the external threat of invasive species the smaller the areas become (a species area effect). Large tracts of existing, mature forest can act as refuges for the fullest range of native species. The larger these areas can be the more effective they become as refuges (see articles in Soule & Wilcox, 1980). The urban forest of Fernbank constitutes such a refuge. This is particularly the case since it is already closely connected to other forested areas; the Deepdene woodland, the extensively wooded area of the Frazier Center, and the more distant woodlands on the property of Emory University. The forest also has connections with the semi-continuous woodlands on the private properties of residents, especially those abutting the principal drainages of the area. The woodlands about the streams can act as important biological corridors as they can connect otherwise isolated patches of woodland and, hence increase the forested area. Neighboring woods and trees on the properties of residents, when cleaned of invasive species, contribute to a buffer zone around the refuge and may contribute to an extension of a refuges effective area.
Schematic map of the Fernbank Forest area of Northeast Atlanta. The forest refugium of specific interest is centered at the Fernbank Museum of Natural Histpry (F.M.N.H.; dark and light green area, mid-center). The biological corridors are defined by broken green lines and the light green areas around the water courses (blue). The Buffer Zone is indicated by the yellow lines. The forest in and about Emory University to the north (Lullwater Park & Wesley Woods) constitute another core refugium.
It follows from this rationale that the ecological focus of Schools activity is to prevent the degradation of actual or potential refuge areas, contribute to the establishment of strong biological corridors, and improve the effectiveness of the surrounding buffer zones.
A particular concern of the DFFS is the status of the woodpeckers (Picidae) and the other bird species that forage on the trunks and major branch systems of the woodland trees (e.g., Nuthatches, Sitidae). Their susceptibility to extinction is exemplified by such species as the Ivory Billed Woodpecker (Campephilus principalis, Gallagher, 2005; Hill, 2007) and the Red-cockaded Woodpecker (Picoides borealis, McFarlane,1992), as well as other picidiform species. A number of small and large species of the woodpecker habit are excluded from their tree trunk and branch habitat by the infestations of invasive vines, and can be viewed as “indicator species” of forest health. When the first work began on the grounds of the Fernabnk Museum of Natural History there were no observations of woodpeckers and only the occasional distant calls of the Pileated woodpecker (Dryocopus pileatus) were heard. At one point, however, during Phase 2 work in May of 2007, a Downy woodpecker (Picoides pubescens), a Hairy woodpecker (P. villosus), and a Red-bellied woodpecker (Melanerpes carolinus), as well as a nuthatch, were all seen within a six minute period, and a Pileated woodpecker was observed not far distant; all these birds were on tree trunks that had been cleaned by Machete man.
Bibliography
Gallagher, T. 2005. The Grail Bird. Houghton Mifflin, Boston.
Kricher, J. & Morrison, G. 1988. A Field Guide to Eastern Forests: The Peterson Field Guide series, 37. Houghton Mifflin, New York.
MacFarlane, H. W. 1992. A Stillness in the Pines: The Ecology of the Re-Cockaded Woodpecker. Commonwealth Fund Book Program (republished 1994, Norton, New York).
Miller, J.H. 2003. Nonnative Invasive Plants of Southern Forests: A Field Guide for Identification and Control. General Technical Report SRS-62, Southern Research Station, Forest Service, USDA.
Radford, A.E., Ahles, H.E., & Bell, C.R. 1964. Manual of the Vascular Flora of the Carolinas. University of North Carolina, Chapel Hill.
Soule, M. E. & Wilcox, B.A. (Eds.) 1980. Conservation Biology: An evolutionary-ecological perspective. Sinauer, Sunderland, MA.