Abstract
Sweetgum, Liquidambar styraciflua L. (Hamamelidales: Hamamelidaceae), is a species of interest for short-rotation plantation forestry in the southeastern United States. Despite its high levels of resistance to many native insects and pathogens, the species is susceptible to generalist defoliators during outbreak epidemics. The objective of this field study was to evaluate the potential impact of defoliation on sweetgum growth and productivity within the context of an operational plantation. Over three growing seasons, trees were subjected to artificial defoliation treatments of various intensity (control = 0% defoliation; low intensity = 33% defoliation; moderate intensity = 67% defoliation; high intensity = 99% defoliation) and frequency (not defoliated; defoliated once in April of the first growing season; defoliated twice, once in April of the first growing season and again in April of the second growing season). The responses of stem height, stem diameter, stem volume, crown volume, total biomass accumulation, and branch growth were measured in November of each growing season. At the end of the first growing season, when trees had received single defoliations, significant reductions in all growth traits followed the most severe (99%) defoliation treatment only. After the second and third growing seasons, when trees had received one or two defoliations of varying intensity, stem diameter and volume and total tree biomass were reduced significantly by 67 and 99% defoliation, while reductions in stem height and crown volume followed the 99% treatment only. All growth traits other than crown volume were reduced significantly by two defoliations but not one defoliation. Results indicate that sweetgum is highly resilient to single defoliations of low, moderate, and high intensity. However, during the three-year period of the study, repeated high-intensity defoliation caused significant reductions in growth and productivity that could have lasting impacts on yield throughout a harvest rotation.
Highlights
Managed short-rotation hardwood plantations in the southern United States offer opportunities to meet a diversity of management objectives that cannot be met through natural stand management alone (Robison et al 1998; Romagosa and Robison 2003)
At the end of Year 1, when the trees had received a single artificial defoliation treatment of varying intensity the previous April, stem height was significantly affected by the main effects of block and defoliation intensity, while stem diameter, crown volume, stem volume, and total biomass were affected significantly by defoliation intensity alone
Among the varying defoliation intensities, stem height and crown volume decreased with increasing intensity, while stem diameter increased slightly from 0% to 33% defoliation and decreased with increasing intensity (Table 2)
Summary
Managed short-rotation hardwood plantations in the southern United States offer opportunities to meet a diversity of management objectives that cannot be met through natural stand management alone (Robison et al 1998; Romagosa and Robison 2003). Sweetgum was recognized early on as a prime candidate for short-rotation applications in the southern U.S due to its wide site adaptability, potential for genetic improvement, responsiveness to intensive silvicultural inputs, and high level of native pest resistance (Kormanik 1990; HRC 1997; Steinbeck 1999). There are approximately 8,000 ha of sweetgum plantations in cultivation across the southern U.S established as sawtimber, pulpwood, and bioenergy crops (Wright and Cunningham 2008)
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