61 SWEETAPPLE ET AL.: MISTLETOE AND POSSUM CONTROL New Zealand Journal of Ecology (2002) 26(1): 61-71 ©New Zealand Ecological Society Mistletoe (Tupeia antarctica) recovery and decline following possum control in a New Zealand forest P. J. Sweetapple 1 , G. Nugent 1 , J. Whitford 1 and P. I. Knightbridge 2 1 Landcare Research, P.O. Box 69, Lincoln 8152, New Zealand. (E-mail: sweetapplep@landcare.cri.nz) 2 Department of Conservation, Private Bag 701, Hokitika __________________________________________________________________________________________________________________________________ Abstract: The condition of 79 plants of the loranthaceous mistletoe Tupeia antarctica in a podocarp-hardwood forest in the central North Island, New Zealand, was monitored over 4 years during a period of increasing possum density, following previous possum control. Mistletoe comprised 1.2% of total possum diet during the three years following possum control. Incidence of possum browse on mistletoe plants increased from 2.6% of plants when the trap-catch index of possum density was < 3%, to 75.9% of plants when trap-catch rates reached 4.6%. Mistletoe foliage cover declined from 49.8% to 15.6% and mean plant size declined by about 55% over the same period. The mistletoe population was dominated by plants with large haustoria, located in heavily shaded locations in the lower crown of their Carpodetus serratus hosts. Most plants established more than 20 years ago, and the current potential for recruitment of new individuals into the population is severely limited by possum browsing and the senescent nature of the mistletoe population. Intensive management of host crowns and possum populations will be necessary to ensure the long-term viability of mistletoe at the study site. __________________________________________________________________________________________________________________________________ Keywords: impacts; mistletoe; population status; possums; Trichosurus vulpecula; Tupeia antarctica. loss or modification, insect browsing, over-collecting (de Lange, 1997; Norton, 1991; Norton and Reid, 1997), and the decline in pollinating and seed-dispersing bird species (Ladley and Kelly, 1996; Ladley et al., 1997a; Robertson et al., 1999). However, while the causes of mistletoe decline and the relative importance of the role of possums remain unclear (de Lange, 1997; Norton, 1997), there are now a number of reports of improvements in mistletoe vigour following reductions in possum densities, or protection from possum browse (e.g. Courtney, 1997; King and de Lange, 1997; Ogle, 1997; Sessions et al., 2001). In this paper we document such recovery and, in addition, show that the recovery persisted only while possums remained at low densities. Between 1990 and 2000 we undertook a study of possum diet and impacts, before and after an aerial- 1080 possum control operation in August 1994, in podocarp-angiosperm forest in the central North Island. The presence of mistletoe species in this area was not detected prior to possum control, but within one year of the control operation regrowth of the mistletoe Tupeia antarctica was noted on large haustoria (gall-like growths) on the trunks of some putaputaweta (Carpodetus serratus) trees within the study area. Subsequent searches revealed that T. antarctica was locally common in parts of the study area and, from Introduction Five indigenous extant species of loranthaceous mistletoes have a wide distribution in New Zealand, but their distribution and density have declined markedly since European settlement, particularly in the North Island, Nelson, northern Westland, and Southland (Ogle and Wilson, 1985; Ogle, 1997; Overmars, 1997). Possums (Trichosurus vulpecula) have been widely blamed as a major cause of this decline (Ogle and Wilson, 1985; Barlow, 1987; James, 1990; Webb et al., 1990; Clark, 1993). However, the evidence linking possums to mistletoe decline is largely circumstantial (Ogle, 1997), and the few quantitative studies undertaken have been equivocal. Wilson (1984) concluded that possums were having a detrimental impact on Peraxilla species at his Nelson Lakes study site, whereas two other studies drew the opposite conclusion. Possums exerted minimal browsing pressure on Peraxilla and Alepis mistletoes at four sites in the eastern South Island (Sessions, 1999) and on Peraxilla colensoi near the Haast River, South Westland, (Owen, 1993; Owen and Norton, 1995) where mistletoes were common. Besides possums, other factors potentially contributing to the decline of mistletoes include habitat