The Port Hills are a highly valued geographical feature of Chirstchurch. Located southeast of the city, they are home to a wide range of activities, including rock climbing and mountain biking, as well as being popular among walkers and joggers. Vegetation throughout the Port Hills is varied, containing a range of tussockland, pine forestry blocks, native scrub, farmed grassland, gorse and broom scrub and small pockets of remnant forest.
On the 14th of February 2024, over 700 hectares of land was ravaged by wildfire in the Port Hills of Christchurch, New Zealand. Over 80 residents were evacuated, and around 130 firefighters with 12 helicopters were involved. Drought conditions and vegetation structure contributed to this event, but could the damage caused by the blaze have been reduced? Could grazing these hills with livestock have reduced the amount of tall dry grass present which fuelled the fire, or could different pasture or shrub species have helped to reduce the flammability of the Port Hills.
A recent paper from Lincoln University’s own Tanmayi Pagadala, Azharul Alam, Tim Curran and Tom Maxwell has highlighted the differences in flammability between different pasture, crop, weed and shrub species found commonly on farms throughout Canterbury.
A good range of scientific work is available which has investigated the flammability of various plant species in New Zealand, but this has been mainly focused on species in natural areas (both native and exotic), rather than in agricultural environments. Gorse, eucalypts, pines and long grass are well known to be extremely flammable, so why is it that certain areas of the port hills were allowed to return to their same fuel rich state following the 2017 blaze which destroyed over 1600 hectares? It must be acknowledged that efforts were made to replant some of the previously burnt areas in green firebreaks and others in less flammable native species, which were shown to survive the previous blaze in well-established areas.
Species that regrow following a fire are often also very flammable (eg. gorse and pine). Unfortunately, a significant proportion of the burned land was in pines for forestry, which has since been replanted and will likely create another significant fire risk for the foreseeable future. Continuing the efforts of plant firebreaks of less flammable tree species throughout the Port Hills, as well as within pine forestry blocks, should not be underestimated.
Individually these breaks may not appear significant, but a thorough network of them throughout the Port Hills could be exactly what is needed to slow the spread of the next blaze and allow firefighters to gain control sooner. Minimising the presence of long, rank grass could also help to slow the spread of the burn.
Could additional efforts be made in to reducing the presence of long rank grass through the addition of cattle to grazed areas which would trample and eat this dry plant material? Or perhaps planting more drought tolerant, water-efficient forages which can be grazed down during dry periods to minimise the fuel loading of grasslands could be beneficial.
There is a huge range of flammability in different crop and pasture species common to Canterbury farming systems. Assessments carried out on Lincoln University’s trusty ‘plant BBQ’ tested 47 different plant species and varieties common throughout Canterbury farms (see table below), including cereals, forage crops, fruit crops, forage herbs, forage grasses, forage legumes, vegetable crops, weeds and a range of wine grape varieties.
Unsurprisingly, the majority of forage and pasture species showed very low flammability, as did some vegetable crops and wine grapes. Cereal crops behaved as expected, showing high flammability as they matured and dried off. Surprisingly, apple trees, pears and raspberries showed a high degree of flammability.
The slope of the Port Hills, and an average annual rainfall of 700 mm, means that using low flammability crops like potatoes or peas is not practical. There are, however, a range of pastoral species that show the potential to be beneficial in reducing the flammability of farmland. Forage crops, herbs, legumes and grasses all showed very low flammability scores, which is due to their high moisture content and quality traits meaning they carry very little dead material (the ideal fuel for fires).
Knowing these flammability scores in addition to the the drought tolerant traits of species, such as lucerne, cocksfoot, red clover, plantain and chicory, raises the question: why are these species currently not implemented throughout the fire prone Port Hills as a method of reducing fire risk? Yes, these forages will become flammable if they are allowed to turn to a reproductive state. However, their drought tolerance and palatability will allow them to be well grazed during dry periods and not contribute to the fuel loading of hills anywhere near the amount that browntop and other native grasses will.
The photos above paint a picture of a potential solution to the Port Hills fire woes. At the very least, an effort should be made to ensure that flammable biomass throughout the Port Hills is minimal. Minimising the presence of flammable species, such as gorse and pines, through manual removal or switching to planting less-flammable alternatives, such as native shrubs, are some potential solutions.
Preventing the planting of pine plantations near the city and other populated areas seems like another fairly logical solution to reducing the fire risk in populated areas, as does surrounding these potential high-risk areas with low flammability and native shrub species. Another area of focus could be to focus more on the management of cattle and/or sheep to intensively graze the hillsides and ensure that a bank of highly flammable fuel does not build up over time. Intensive grazing will not only prevent grass banks from building up, but the ‘hoof and tooth’ activity from grazing may also prevent other flammable species, such as gorse and broom, from re-establishing.
Functional firebreaks could also be of huge benefit to these hillsides. In areas where tractor access is possible, consideration should be given to the establishment of drought-tolerant, low-flammability species, such as red clover, chicory, or lucerne. These will create ‘green zones’ throughout the hillsides that could slow the spread of the next inevitable fire, especially compared to the current vegetation which is prone to turning to a dry, reproductive state over summer.
To wrap up, logic suggests that previous fires in 2017 and 2024 on the Port Hills, in combination with the presence of flammable vegetation, make another blaze in the future almost inevitable. The findings from recent research on the flammability of pasture and crop species commonly found on Canterbury farms, combined with modern grazing regimes present a real opportunity to significantly reduce the fire risk on the Port Hills. The use of firebreaks planted with native, low flammability species around high risk areas such as pine forestry blocks, along with the protection of existing pockets of native scrub/forest should also help to reduce the fire risk on the Port Hills.
This article was prepared by Master of Science postgraduate student Kaylee Spain as part of the ECOL608 Research Methods in Ecology course.
Reference article:
Pagadala, T., Alam, M. A., Maxwell, T., & Curran, T. (2023). Measuring flammability of crops, pastures, fruit trees, and weeds: A novel tool to fight wildfires in agricultural landscapes. Science of the total environment, 906(1). https://doi.org/10.1016/j.scitotenv.2023.167489