Available on-line at: http://www.newzealandecology.org/nzje/ 202 NEW ZEALAND JOURNAL OF ECOLOGY, VOL. 31, NO. 2, 2007
Factors influencing brushtail possum (Trichosurus vulpecula) damage in a Pinus radiata plantation on the Coromandel Peninsula, New Zealand Marco A. Jacometti1*, Chris M. Frampton2 and Graham J. Hickling3 1
National Centre for Advanced Bio-Protection Technologies, PO Box 84, Lincoln University, Lincoln 7647, New Zealand 2 Department of Medicine, Christchurch School of Medicine and Health Sciences University of Otago, PO Box 4345, Christchurch, New Zealand 3 Department of Forestry Wildlife and Fisheries, University of Tennessee, Knoxville, TN 37966, USA * Author for correspondence (Email:
[email protected]) Published on-line: 3 December 2007 ___________________________________________________________________________________________________________________________________
Abstract: Damage by introduced brushtail possums (Trichosurus vulpecula) to Pinus radiata trees was assessed in 41 compartments of a commercial forestry plantation on the Coromandel Peninsula, New Zealand. All the trees assessed were less than 3 years old. Possum damage in the compartments was low (median prevalence 3.3%) but highly variable (range 0–30%). Eight of 37 measured habitat factors differed significantly (P < 0.05) between the sites with damaged and undamaged trees. The best predictor of mean damage was stand age, but this explained only 21% of the variation in damage among compartments. Including both stand age and New Zealand bracken (Pteridium esculentum) cover improved the model significantly and explained 36% of variation in damage. Damage was apparently unrelated to compartment size, distance from the compartment boundary, and possum den-site availability. Surprisingly, the relationship between browse damage and a trap-catch index of possum abundance was weakly negative (rS = −0.53, P = 0.05). The dense understorey associated with young pine stands tends to increase possum damage to associated P. radiata trees, but the possums in such stands may be less mobile at ground level and thus less easily trapped. Assessment of stand age and understorey characteristics, together with visual inspection for early signs of damage, is likely to be more cost-effective than possum surveys for identifying forest compartments at risk from possum browse. ___________________________________________________________________________________________________________________________________ Keywords: browse damage; introduced mammals; plantation forest
Introduction Most commercial forestry plantations in New Zealand are composed of compartments of even-aged Pinus radiata D. Don. Introduced brushtail possums (Trichosurus vulpecula Kerr) living in or adjacent to these plantations commonly use P. radiata as a seasonal food source, which subjects the trees to browse and secondary physical damage. Direct feeding damage includes pine-needle consumption, which typically represents less than 10% of the annual diet, but this can increase to over 40% during autumn (Clout 1977). Bark consumption in winter and early spring (Clout 1977; Fitzgerald 1981) and female and male cone consumption are also important. Female cones are eaten in the late summer and autumn, when they can comprise more than 10% of the diet, and male cones are eaten from June to September (Harvie 1973; Warburton 1978), when they can contribute up to 70% of the diet (Clout
1977). Indirect damage comprises mainly leader or lateral breakages that occur as possums move through the canopy (Clout 1977; Keber 1988; Jacometti et al. 1997). The potential economic consequences of such damage have been discussed by Griffiths (1985), Keber (1988) and Butcher (2000), but the prevalence of possum damage is known to be highly variable within and among New Zealand plantation forests (see references in Jacometti et al. 1997). Variables such as stand age (Jacometti et al. 1997; Butcher 2000), understorey composition, food and den-site availability, and possum abundance (Clout 1977; Keber 1988) also impact on damage levels. Possum control and understorey management could reduce this damage, but both activities are costly. It would be advantageous, therefore, if forest managers could identify those forest compartments within a plantation most at risk from possums, so that any management activities could be targeted there.
New Zealand Journal of Ecology (2007) 31(2): 202-207 ©New Zealand Ecological Society
JACOMETTI ET AL.: POSSUM DAMAGE IN A PINE PLANTATION
The relative importance of stand age, understorey composition, food and den-site availability, and possum abundance is uncertain, so this study investigated a range of site factors that might have predisposed P. radiata trees to possum damage at Tairua Forest on the Coromandel Peninsula, New Zealand. This analysis extends the study by Jacometti et al. (1997), which described the prevalence of various types of possum damage in the plantation in summer 1996/97.
Methods Possum damage was assessed in Tairua Forest, Coromandel Peninsula (37°10´ S, 175°51´ E), between November 1996 and January 1997; the forest is described in more detail in Jacometti et al. (1997). Damage assessment was restricted to stands of P. radiata aged 3–20 years. Possum damage is common in younger trees but is not easily distinguished from damage by other herbivores; trees over 20 years of age are rarely damaged (Clout 1977). Forest compartments less than 1 ha in size, which made up
