Paired sites, one subject to extensive control (treatment) and one with no control (non-treatment), were located at Coromandel (treatment: 2300 ha, 36°56′S, 175°37′E; non-treatment: 1200 ha, 36°58′S, 175°39′E) and Urewera (850 ha, 38°17′S, 177°11′E; 850 ha, 38°14′S, 177°11′E) in the North Island, and Haast in the South Island (1300 ha, 44°03′S, 168°47′E; 1300 ha, 44°06′S, 168°39′E). Each pair of sites had similar topography, altitude and forest composition (assessed using the Recce method; Hurst & Allen 2007). Extensive possum control was carried out at: Coromandel, 1995, 1999 and 2003 prior to the study and during the study in 2006; Haast, 1995–96 and 2000–01 prior to the study, but the next control operation was delayed until after the study period; and Urewera, 1993–94, partially in 1996–97, 1998–99 and 2002–03 prior to the study, and during the study in 2006–07.
Experimental protocols were based on techniques used by DOC to assess the abundance of possums (Trap-Catch Index (TCI); National Possum Control Agency 2010) and their impacts on tree canopies (Foliar Browse Index (FBI) and Foliage Cover Index (FCI); Payton, Pekelharing & Frampton 1999) and survival. Additional data were collected to characterize foraging patterns at a local scale because previous studies found significant site differences in possum impacts (Duncan et al. 2011). The experimental design included measurements of tree size using diameter at breast height (DBH), local availability of other food sources and possum browse at the patch scale (Fig. 1).
In each area, two tree species with foliage preferred by possums and one with foliage not preferred (Owen & Norton 1995; Sweetapple 2003; Sweetapple, Fraser & Knightbridge 2004) were selected as ‘indicators’ (Table 1) to assess the outcomes of extensive possum control. A criterion for selecting species was that they were relatively common and widespread throughout the area. Non-preferred species acted as experimental controls for non-selective disturbances such as storms, drought and earthquakes. Smaller numbers of a third highly preferred species were selected from sparse populations at Coromandel (Dysoxylum spectabile) and Haast (Fuchsia excorticata).
Table 1. Preferred and non-preferred tree species at each site, annual tree mortality at non-treatment and treatment sites [sample size n of trees that were tagged at the start of the study (Time1) and relocated during the remeasurement period (Time 2)]. Trees with unrecorded browse data (FCI and/or FBI), or that could not be relocated, at Time 2 were not used for analysis
|Area||Species||Abbreviation||Possum||Non-treatment sites||Treatment sites|| P a |
|preferred||Mortality (%)|| n ||Mortality (%)|| n |
|Coromandel|| Weinmannia silvicola ||WEISIL||Yes||0·18||225||0·28||213||0·525|
| Olearia rani ||OLERAN||Yes||2·17||221||0·94||238|| 0·014 |
| Dysoxylum spectabile ||DYSSPE||Yes||6·20||84||0·0|| 21|| 0·008 |
| Knightia excelsa ||KNIEXC||No||0·25||237||0·09||206||0·359|
|Haast|| Weinmannia racemosa ||WEIRAC||Yes||0·0||229||0·41||247||0·957|
| Schefflera digitata ||SCHDIG||Yes||15·53||200||12·16||197|| 0·040 |
| Fuchsia excorticata ||FUCEXC||Yes||21·20||102||19·73|| 3||0·5|
| Nothofagus menziesii ||NOTMEN||No||0·60||202||0·57||212||0·5|
|Urewera|| Weinmannia racemosa ||WEIRAC||Yes||6·87||218||0·11||237|| <0·001 |
| Beilschmiedia tawa ||BEITAW||Yes||0·33||229||0·40||249||0·621|
| Knightia excelsa ||KNIEXC||No||0·11||222||0·12||211||0·5|
Preliminary power analysis indicated that 200 trees of each species per site would be required to have an 80% chance of detecting a treatment effect in annual tree survival where the difference in annual mortality is ≥2% and the interval between measurements is ≥4 years (B. Reddiex unpublished data). Initial measurements (Time 1) and remeasurements (Time 2) were carried out in 2004 and 2009, respectively, at Coromandel and Haast, and in 2006 and 2010 at Urewera. Sampling transects, consisting of 5–15 plots at 50-m intervals, were located randomly in each site with randomly selected bearings. Transects and plot centres were marked and GPS coordinates recorded. Within 20 m of each plot centre, up to four trees of each indicator species at least 10 m apart where possible were permanently marked using numbered tags. Trees were included if they measured ≥10 cm DBH (1·4 m above the ground), and if the canopy was visible from the ground.
Browse damage was recorded using Foliar Browse Index (FBI) in five categories, 0–4, indicating that 0%, 1–25%, 26–50%, 51–75% or >75% of leaves, respectively, showed evidence of possum browse. Foliage cover was recorded using Foliage Cover Index (FCI), the fraction of sky occluded by leaves, observed from beneath the centre of the tree crown looking up. Occlusion was recorded in 10% categories, that is, 0·05 = 0–10%, 0·15 = 10–20%, etc. For each indicator tree, DBH, FCI and FBI were recorded at Time 1, and again at Time 2 for those trees still alive. At Time 2 the status (alive or dead) of each tree was recorded.
At each plot, the closest indicator tree to the plot centre was selected as the ‘focal tree’. The DBH, Alive/Dead status and FBI of all trees (with DBH ≥ 10 cm) within 5 m of the focal tree were measured (irrespective of species). These ‘neighbourhood trees’ were used to characterize a patch of forest that could be selected by foraging possums.
Possum Trap-Catch Index (TCI) was obtained at each site as near to Times 1 and 2 as possible following the standard protocol of randomly located lines within each area, each line consisting of 10 or 20 traps set for three nights (National Possum Control Agency 2010). TCI lines were located independently of the transects for measuring browse impacts. DOC measured TCI on treatment sites after each control operation; we used DOC trap lines that overlapped our study area to calculate mean TCI values (6–18 lines per site). In the non-treatment areas, we measured TCI using 7–10 lines per site, with some additional DOC measurements at Haast.