Arboriculture & Urban Forestry 36(3): May 2010 Table 1. Examples of annotations included in the literature review. Gilman, E.F., and G. Knox. 2005. Pruning type affects decay and structure of crapemyrtle. Journal of Arboriculture 31:38–47. Established Lagerstroemia × Natchez trees were topped, pollarded or unpruned for four years. Topping resulted in more dead stubs and discolored wood than pollarding which had limited decay development. Recommended developing pollards rather than routine topping. Nice photos. Florida US. Neilsen, W., and E. Pinkard. 2003. Effects of green pruning on growth Pinus radiata. Canadian Journal of Forest Research. 33:2067–2073. Crowns of 6- to 8-year-old, plantation Monterey pine were raised. Rais- ing to 45% of tree height had no effect on growth which was reduced with greater crown removal. Suggests maintaining a live crown ratio of 55%. Tasmania Australia. Schwarze, F., J. Gruner, M. Schubert, and S. Fink. 2007. Defence reactions and fungal colonization in Fraxinus excelsior and Tilia platyphyllos after stem wounding. Arboricultural Journal 30:61–82. Describes the anatomy of the barrier zone (= CODIT wall 4), suggesting that differences in this zone account in part for species differences in de- cay resistance. Strong within species variation in discoloration associated with both increment borer holes and chain saw cuts to the stem. Also isolated fungi from around the wounds. Excellent photos. Good discus- sion of fungal development and tree response. Freiburg Germany. Forest tree pruning was represented by Mayer Wegelin’s papers (1936; 1952), the silvicultural textbook of Smith et al. (1996), and a Hanley et al. (1995) volume on pruning of conifers. Also refer- enced were resources written in German (Hoster 1993; Dujesiefken 1995; Pfisterer 1999; Stobbe et al. 2002a; Stobbe et al. 2002b), and French (Drenou 1999; Austad and Hauge 2007). Palms were referenced through Broschat and Meerow (2000). For the most part, books were included as general references to the topic of pruning but lacked extensive references to the scientific literature. Journal articles comprised 152 of the 201 citations. For- ty-four journals were referenced, published in 12 countries (Table 2). Journal of Arboriculture (51), Arboriculture & Ur- ban Forestry (12), and Arboricultural Journal (9) were the most frequently referenced. Journals cited originated in Eu- rope (25), North America (17), and the Asia-Pacific region (2). More than half of the journals (24) were focused on for- estry and forest science. Another 10, such as American Jour- nal of Botany, were oriented to the traditional plant scienc- es. Three journals were horticultural in focus; another five were oriented to arboricultural and urban forestry. Two jour- nals, Trees—Structure and Function and Tree Physiology, crossed lines among forestry, arboriculture, and horticulture. Citations arose primarily from English language journals (113 of 201). Some journals, notably Arboriculture & Urban Forestry, Arboricultural Journal, Canadian Journal of Botany, Canadian Journal of Forest Research, and Journal of Arbori- culture may provide abstracts in languages other than English. Also included were citations in French, German, and Italian. A small fraction of the citations had not undergone the nor- mal peer-review process. Four citations were reports of the USDA Forest Service, all authored by Shigo (Shigo and Lar- son 1969; Shigo and Marx 1977; Shigo at al. 1979; Butin and Shigo 1981). Such reports are normally reviewed by other sci- entists within the agency. Articles in Arborist News, such as 111 Fraedrich and Smiley (1996) and Guggenmoos (2007), receive technical review. The nature of the review for books, industry standards, extension publications, conference proceedings, and book sections was unknown. The main reason for including ma- terial that had not been peer-reviewed was to highlight a specific pruning topic. This will be discussed in the following section. A list of all authors was compiled. The most frequently cited au- thors were Alex Shigo of the United States and Dirk Dujesiefsken of Germany. Both were noted 13 times. Authors cited with four or more references included Ed Gilman, Jason Grabosky, Brian Kane, Dan Neely, and Tom Smiley of the United States; Karen Barry and Eliza- beth Pinkard of Australia; W. Liese, D. Eckstein, Francis Schwarze, and Horst Stobbe, of Germany; and Francesco Ferrini of Italy. MAJOR TOPICS IN PRUNING RESEARCH Research topics were identified by the frequency with which keywords were applied. The following discussion high- lights a portion of the literature included in the bibliography. The dominant theme of the literature review was wound- ing, the tree’s response and possible treatments to affect that response. Wounding and the tree response, to it were to- gether noted as keywords in 30 of the 201 citations. They were often linked to compartmentalization (24 citations), de- cay (25), and wound dressing (10). O’Hara (2007) provided a review of the literature on this topic, emphasizing wound response and the goal of producing clear wood in timber. Modern research activity in this area might begin with Shigo and Larson’s (1969) photographic summary of the patterns of discolor- ation and decay in hardwoods of the northeastern U.S. This report focused on the relationship of external appearance to wood quality. It was observational in nature, rather than founded in experimen- tation. One finding was that covering pruning wounds with “dress- ings” neither improved closure nor reduced the presence of decay. In 1977, Shigo and Marx released their seminal report Com- partmentalization of decay in trees, which introduced the CODIT concept. Shigo et al. (1979) then reported on the relationship of flush cuts to the development of internal decay and other defects in black walnut (Juglans nigra). The authors noted, “When prun- ing is done late in the life of a tree, care must be taken not to remove the branch collars that form about the bases of dead and dying branches.” Also in 1979, Shortle expanded on the compart- mentalization model with very well-illustrated paper. He posed the “heartrot” concept, describing how external wounds allow decay fungi to enter and become established in the tree. Devel- opment of the CODIT model culminated with two publications: How tree branches are attached to trunks (Shigo 1985) and Com- partmentalization: A conceptual framework for understanding how trees grow and defend themselves (Shigo 1984). As noted previously, the vast bulk of this work was observational in nature. Shigo was neither the only scientist interested in tree re- sponse to wounding nor the first to examine it. For example, foresters have long had an interest in tree response to pruning and wounding (McQuilkin 1950; Herring et al. 1958; O’Hara 2007). Von Aufsess (1975) noted the formation of a protective zone at the base of branches. Neely (1970; 1979) observed that production of callus (i.e., woundwood) at the margins of prun- ing wounds was related to tree vigor, as measured by growth. Research on the topic of tree wound response and its man- agement continued through the following decades. Experiments ©2010 International Society of Arboriculture
May 2010
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