Will Kauri Survive? Resilience of Ancient Kauri Populations to the Modern World.

By Toby Elliot

Kauri (Agathis australis) is one of New Zealands most prominent, notable and exceptional tree species. It is a taonga species for Māori, as kauri are considered irreplaceable ancestors, and their health is often used as a sign of the wellbeing of the ngahere (forest), as well as the plants and animals within it [1,2]. Kauri is also economically important, as its valuable timber was logged and used for various projects [3]. However, recently kauri has gained economic value through tourism. Visitors to the Northland region often visit prominent kauri, such as Tāne Mahuta, and learn more about their rich history and the fascinating ecology of these important trees [4].

A large healthy kauri tree. Photo by Toby Elliot.

Kauri is an integral part of the many ecosystems that it inhabits — its leaf litter creates acidic and nutrient-poor soils that can promote the growth and survival of some species, while also inhibiting others [5,6,7]. Therefore, kauri can create distinctive vegetation communities composed of kauri and a suite of associated species — such as Corokia buddleioides — with competitive advantages in kauri forests [8,9]. However, the value of kauri for materials and human-mediated habitat clearance resulted in a rapid decrease in its range since the arrival of humans to New Zealand. Less than 1% of the original kauri forest area remains [1], and many of the largest and oldest kauri trees have been lost. Although large areas of secondary (planted) kauri forest now exist, kauri now has a new microscopic enemy: kauri dieback (Phytophthora agathidicida). Furthermore, climate change is predicted to cause a variety of modifications to Northern New Zealand. It is expected to impact aspects such as temperature and rainfall patterns and the frequencies of disturbances, such as fires [9,10], therefore casting further uncertainty over the long-term survival of kauri.

Kauri dieback is a fungal pathogen that affects kauri roots and kills trees of all ages by essentially ringbarking them [2]. P. agathidicida has been found in many A. australis forests throughout its present range [14], making it a potential imminent threat to the survival of these important trees. Infected trees typically have bleeds at their bases that do not appear to be caused by physical damage. They also typically have thinning canopies that degrade over time [2]. Kauri dieback is primarily spread through soil and water, and the movement of humans between forest patches can facilitate the spread of kauri dieback over long distances. Pig activity serves as a secondary disease pathway [12].

Canopy of an infected tree. Photo by Toby Elliot.

Various methods are in place to contain kauri dieback, slow its spread, and give researchers time to develop effective ways to treat infected trees. These methods include track closures, spray stations for people to wash their shoes before and after entering kauri forests, and strategies to control pig numbers [2,12]. The primary treatment method for infected trees is through the injection of phosphite into infected kauri trunks, which

can temporarily control kauri dieback and reduce mortality rates in infected trees [13]. A permanent control method, however, is currently absent. Additionally, little research is present on the effect of kauri dieback on kauri population dynamics (e.g. growth rates, death rates, recruitment rates), which can be used to predict the long-term survival of kauri in a particular forest, and as a species as a whole.

Mairehau (Leionema nudum) is common within kauri forests. Photo by Toby Elliot.

For my PhD, I will be attempting to explore kauri population dynamics, how they might be impacted by kauri dieback and climate change, and assess the survival of kauri as a species. To do this, I will investigate the population dynamics of kauri under 'normal' conditions by creating various population models, which can predict long-term changes in forest compositions using this demographic data. I will then analyse how these factors might change with kauri dieback and under conditions and disturbance regimes predicted to occur under future climate change scenarios. One of the methods I am using to achieve this is using permanent plots, which are plots within kauri forests where trees were measured and had tags with unique codes. The idea is that one can go back and re-measure these plots after a few years, as they can give invaluable information regarding how much the trees grew between measuring times, and which new trees came into the plot or have died between measurements. Some of the plots that I will be using are in the Waitakere Ranges, which are heavily affected by kauri dieback. I will also look at how these dynamics change in different regions, which can help see what forests are most likely to suffer more into the future, especially if they are infected with kauri dieback.

I hope that my research can shed some light on the severity of kauri dieback, and identify stands that are most at risk of being lost to this terrible disease. This identification, hopefully, will allow for more targeted containment and control measures to protect these incredible trees and the unique forests that they help create.

Basal bleeding, which commonly appears when trees are infected. Photo by Toby Elliot.

References

[1] Steward, G. A., & Beveridge, A. E. (2010). A review of New Zealand kauri (Agathis

australis (D. Don) Lindl.): its ecology, history, growth and potential for management for

timber. New Zealand Journal of Forestry Science (New Zealand Forest Research Institute Ltd

(trading as Scion)), 40.

[2] Bradshaw, R. E., Bellgard, S. E., Black, A., Burns, B. R., Gerth, M. L., McDougal, R. L., Scott, P. M., Waipara, N. W., Weir, B. S., Williams, N. M., Winkworth, R. C., Ashcroft, T., Bradley, E. L., Dijkwei, P. P., Guo, Y., Lacey, R. F., Mesarich, C. H., Panda, P. & Horner, I. J. (2020). Phytophthora agathidicida: research progress, cultural perspectives and knowledge gaps in the control and management of kauri dieback in New Zealand. Plant Pathology, 69(1), 3-16.

[3] Steward, G. A., Kimberley, M. O., Mason, E. G., & Dungey, H. S. (2014). Growth and

productivity of New Zealand kauri (Agathis australis (D. Don) Lindl.) in planted forests. New

Zealand Journal of Forestry Science, 44(1), 27.

[4] Boswijk, G. (2010). Remembering kauri on the 'Kauri Coast'. New Zealand

Geographer, 66(2), 124-137.

[5] Jongkind, A. G., Velthorst, E., & Buurman, P. (2007). Soil chemical properties under kauri

(Agathis australis) in the Waitakere Ranges, New Zealand. Geoderma, 141(3-4), 320-331.

[6] Wyse, S. V., Macinnis-Ng, C. M., Burns, B. R., Clearwater, M. J., & Schwendenmann, L.

(2013). Species assemblage patterns around a dominant emergent tree are associated with

drought resistance. Tree Physiology, 33(12), 1269-1283.

[7] Wyse, S. V., Burns, B. R., & Wright, S. D. (2014). Distinctive vegetation communities are

associated with the long‐lived conifer Agathis australis (New Zealand kauri, Araucariaceae)

in New Zealand rainforests. Austral Ecology, 39(4), 388-400.

[8] Wyse, S. V. (2012). Growth responses of five forest plant species to the soils formed beneath

New Zealand kauri (Agathis australis). New Zealand Journal of Botany, 50(4), 411-421.

[9] Wyse, S. V., & Burns, B. R. (2013). Effects of Agathis australis (New Zealand kauri) leaf

litter on germination and seedling growth differs among plant species. New Zealand Journal

of Ecology, 178-183.

[10] Sansom, J., & Renwick, J. A. (2007). Climate change scenarios for New Zealand rainfall. Journal of Applied Meteorology and Climatology, 46(5), 573-590.

[11] Watt, M. S., Kirschbaum, M. U., Moore, J. R., Pearce, H. G., Bulman, L. S., Brockerhoff, E. G., & Melia, N. (2019). Assessment of multiple climate change effects on plantation forests in New Zealand. Forestry: An International Journal of Forest Research, 92(1), 1-15.

[12] Bassett, I. E., Horner, I. J., Hough, E. G., Wolber, F. M., Egeter, B., Stanley, M. C., & Krull, C. R. (2017). Ingestion of infected roots by feral pigs provides a minor vector pathway for kauri dieback disease Phytophthora agathidicida. Forestry: An International Journal of Forest Research, 90(5), 640-648.

[13] Horner, I. J., & Hough, E. G. (2013). Phosphorous acid for controlling Phytophthora taxon Agathis in kauri glasshouse trials. New Zealand Plant Protection, 66, 242-248.

[14] Waipara, N. W., S. Hill, L. M. W. Hill, E. G. Hough, and I. J. Horner. "Surveillance methods to determine tree health distribution of kauri dieback disease and associated pathogens." New Zealand Plant Protection 66 (2013): 235-241.