Survey of mistletoe (Loranthaceae) abundance surrounding Lake Rotoiti’s southern beech forests
March 17th, 2018
Abstract: In New Zealand, mistletoe is an ecologically important endemic species, threatened by native bird loss and damaging possum browsing. In this study we were interested in the effect of possum-control on the abundance of the three species of nothofagus southern beech-reliant mistletoe in Nelson Lakes National Park’s Lake Rotoiti area, hypothesizing that there would be a significant difference between abundance within the controlled and less-controlled sides of the lake. Population surveys were conducted within three sites on both controlled (East) and less-controlled (West) sides, and within these surveys, zero mistletoe were found. Nonetheless, this survey was informative of Lake Rotoiti’s pest-control program and evolving ecosystem.
Keywords: mistletoe, southern beech forests, pest-control, possum predation, Lake Rotoiti
Mistletoe is an epiphytic semi-parasite well known for its role in the holiday season. Beyond this cultural fame, mistletoe is an ecologically significant organism that grows on trunks and large branches, using both photosynthesis and roots to extract nutrients from host plants. Though there are nine mistletoe species in New Zealand, the southern beech nothofagus forests around Lake Rotoiti support red peraxilla tetrapetala, yellow alepsis flavida, and scarlet peraxlilla colensoi mistletoe species (Norton and Reid 1997). Unlike more familiar North American pest mistletoe, declining endemic mistletoe is an ecosystem threat in New Zealand.
As an important indicator species, mistletoe presence is reflective of the extent of forest hosts, the presence of specialized native pollinators, and the health of the plants themselves (Keey 2005). Though these plants were abundant in the late 19th century, currently their range is restricted to the southern and eastern South Island (Norton and Reid). The main threats to the plant are both deforestation and predation by the introduced brushtail possum, while reduction in native pollinators may also be a factor (Robertson et al, 1995).
Research has shown that areas with possum control demonstrate a significant increase in mistletoe population (Sweetapple et al 2002; Sessions et al 2001). An area on the North Island with no mistletoe prior to intervention, showed mistletoe growth within one year after aerial 1080 possum control (Sweetapple). However, there is not extensive research on the importance of the plant’s abundance or scarcity, or the effectiveness of management strategies (Norton and Reid). The current conservation strategies range from specific banding and caging mistletoe hosts and specimen, to broad possum traps and spraying.
Lake Rotoiti is a fitting site for this study, with abundant beech forests and clearly delineated strict predator controlled and less controlled areas. Accordingly, this study focused on the extent of mistletoe around Lake Rotoiti. Our hypothesis is that there will be a significant difference between the abundance of mistletoe in the controlled (East) and less-controlled plots (West). Therefore our null hypothesis is that there will be no significant differences between the two plot types.
A mistletoe recovery group within the Department of Conservation (DoC) found a distribution of 20-30 individuals per hectare in possum-controlled areas (Keey, 2005 ). Based on this existing research, we could expect one mistletoe per 400 m2 plot in the possum controlled areas. This study adds to the sparse research on the extent of mistletoe in the Nelson Lakes area, as well as the relative effectiveness of different possum-control techniques for mistletoe persistence.
In this study, we began by selecting six survey sites. Three pest controlled sites on the East side of the lake, and three less controlled sites on the West side. Pest-controlled sites are defined as sites within the area of intensive DoC trapping and Friends of Rotoiti trail trapping, while less-controlled sites are in the area with only Friends of Rotoiti possum-trapping along the trails. On both sites we had one site close to the lake shore (>50m), one site mid-distance from the shore (200-400m), and one far site from the shore (500-700m). Distance from lake was used as a control for site selection on both sides, not because proximity to the lake has an effect on mistletoe presence.
We chose sites by walking along the trail until we reached an area of dominant beech forest (greater than 70% of tree type beech) at the respective near, mid, and far distances from the lake, determined using the handheld Garmin GPS, at which point we would designate a plot directly adjacent to the trail. Our plots on the controlled West side of the lake were located off of the Honeydew, St. Arnaud and Black Hill Walk for near, mid, and far distance plots respectively, and our less-controlled sites on the East side of the lake were located off of the Lakeside Walk for near and mid-distance sites and Robert Road for our far distance site. The mid-distance plot on the Black Hill Walk was chosen instead of a mid-plot on the Honeydew Walk based on recommendation from the Lake Rotoiti DoC office that mistletoe was spotted along this track. Unfortunately they did not have any more detailed mistletoe information beyond this.
Figure 1. Map of North end of Lake Rotoiti. The pest-controlled area is on the East side, the near site starts at 1, mid 2, far 3. The less-controlled West side starts on the South side of the Butler River, plots marked in blue with the same number coding system.
We marked each plot coordinates on the GPS, and then designated twenty by twenty meter plots. Using a tape measure and flagging tape, we walked twenty meters off trail perpendicular to the track to begin, and formed a square plot from there. We chose twenty by twenty meters as a plot size based on DoC’s procedures for surveying canopy tree or plant populations (Greene, 2012).
Once our sites were chosen, our surveying process started with informal written observations of the site, noting tree type, diameter at breast height (DBH) in inches, percent canopy and relative ground cover, Black Sooty Mould presence, bird presence, and any notable features of the plot.
To survey the mistletoe, each of the four researchers stood at a corner of the plot looking inwards and scanned the plot, two with Kathmandu binoculars and two without, for five minutes looking for mistletoe. As mistletoe grows as a bush on the trunk and large branches of mature beech trees, it is easily recognizable even outside of the January bloom period.
If any mistletoe was found, the other group members would confirm a correct identification. We chose five minutes so as to ensure a thorough scan of the plot within our given time constraints.
After our survey of all six sites we found a total of zero mistletoe specimen. If we had found mistletoe we would have conducted a t-test to determine the statistical significance of any difference of our findings between the treatment and control sites.
This is not to say that there are no mistletoe around Lake Rotoiti, as a DoC employee told us of a recent sighting around the Black Hills Walk area and the Honeydew Walk interpretive signs discuss mistletoe’s presence and importance.
In addition to our mistletoe surveying, we took observations at each site, as written below.
|Controlled Site 1||Honeydew Loop, 29 m from shore, mistletoe: 0||
|Controlled Site 2||Black Hill Walk, 356 m from shore, mistletoe: 0||
|Controlled Site 3||St. Arnaud Trail, 607 m from shore, mistletoe 0||
|Less controlled Site 1||Lakeside Track, 22 m from shore, mistletoe: 0||
|Less controlled Site 2||Lakeside Track, 225 m from shore, mistletoe: 0||
|Less controlled Site 3||Mt. Robert Road, 658 m from shore, mistletoe: 0||
Figure 2. Survey observations at each site. At each site we noted a range of observations including tree type, DBH in inches, height in meters, percent canopy cover, understory density, black sooty mould presence, bird presence, and notable features.
These observations were noted as a record of each site, so that we may be able to use the differences noted in our data analysis. We did not want to differentiate plots based on pest-controlled and less pest-controlled only.
With our results of zero mistletoe per site, we cannot reject our null hypothesis that there is no significant difference in mistletoe abundance or health between pest-controlled and less-controlled areas of Lake Rotoiti. This finding contradicts previous mistletoe research in which possum-control is shown to significantly affect mistletoe abundance (Keey, 2005; Sweetapple et al., 2002). We cannot be sure whether these results are due to lack of mistletoe abundance around Lake Rotoiti or confounding variables in the study. Nonetheless, our null results are not unimportant.
As discussed in the introduction, mistletoe is an indicator species for overall ecosystem health. One mistletoe researcher argues, “Mistletoe will only survive if we protect its hosts, the animals that pollinate and disperse its seeds, and the mistletoe plants themselves. They are an important indicator of the health of the forest” (Keey). While our study did not yield results, it still contributes to the gaps in mistletoe research, especially in regards to the effectiveness of different pest-control strategies. Research shows that control is most effective for mistletoe when it is protected tree-by-tree, by banding or caging (Norton and Reid). Lake Rotoiti’s pest control program uses primarily traps which is not ideal for mistletoe persistence. This may have affected our results, however there were also many other factors involved in our study.
Although we followed DoC’s surveying procedures, these methods are most effective over a longer study period in order to ascertain useful mistletoe data (Greene). The majority of mistletoe research takes place over hectares and over months or years. We were not able to replicate the methods of previous mistletoe research due to the three day time limit of our study.
Another constraint of our survey was that we were confined to plots next to trails. Mistletoe is pollinated by birds, which may be deterred from congregating or pollinating next to sites of disturbance and human activity. We know that there is mistletoe in the Lake Rotoiti area from conversations with DoC and formal information provided by the DoC offices. However, to avoid bias in our study we wanted to ensure that we were not explicitly looking for mistletoe and creating plots around those specimen, but rather randomly selecting sites to survey. Therefore, with the time and methodological constraints of our study we were unable to survey any. If we were to conduct this study again we would allow for more time so that we could survey larger plot sizes and in more diverse areas.
Despite the study’s weaknesses, an important strength of our surveying techniques was our attention to observation even beyond mistletoe presence. One important observation in our plots was the high prevalence of Black Sooty Mould, a fungus that utilizes the honeydew nectar excreted by burrowing insects in the Southern Beech tree bark (Dhami et al., 2013). As a possible harm to mistletoe, this observation led us to research indicating that the fungus does not negatively affect tree productivity unless it cover’s the tree’s leaves thus decreasing it’s photosynthetic activity. Based on this knowledge we know that the presence of black sooty mould in our sites, though noteworthy, did not have an effect on mistletoe abundance in our survey.
In our observations we also noted stark differences in tree type and distribution between the East and West sides of the lake. Because we were searching for dominant beech forests as hosts for mistletoe we noticed that beech was much younger and much more sparse within dominant new-growth Kanuka on the West side of the lake. This observation led us to find that this forest difference was not due to predator control but rather due to persistent fires and grazing in this area and only recent replanting in the last 50 years (DoC). And so while this research was tangential to our hypothesis, it was interesting to see the differences in the younger and older beech forests, such as the extent of canopy cover, and mixed tree type, which may have affected mistletoe should it be there.
In conclusion, though we did not survey any mistletoe and were unable to reject our null hypothesis, looking for endangered mistletoe encouraged us to notice important ecosystem differences in vegetation in the controlled and less-controlled areas, as well as differences in bird and fungal presence. These factors are important indicators of mistletoe health, just as mistletoe is an important indicator of ecosystem health.
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