By Molly Shaw
Last September many shelter belts in Canterbury were blown down or damaged, and land owners now have an opportunity to re-assess the reasons for having shelter in their fields. Sure, there has been a trend toward cutting down shelters in irrigated pastures the past few years. But as climate change research predicts that Canterbury will be a hotter, drier place in the next few decades, the economic picture may not always look like it does now.
Farmers expect shelter belts to benefit them in some or all of the following ways:
- Wind breaks to reduce drought stress on pasture (even farmers with access to irrigation can’t run it for free)
- Shelters for livestock during southerlies
- Habitat for beneficial species that subsequently decrease pest pressure in crops
- Some degree of nutrient scavenging to reduce leaching into water sources, particularly for shelter belts lining streams or ditches
For the ecologically minded, you can add the less tangible benefits of preserving native species, aesthetic appeal, etc. Landowners prioritize one or more of the above benefits, take into account establishment cost and plant growth rate, and then choose shelterbelt species accordingly. In the past, species like pines, macrocarpa, gorse and poplar have come out on top.
Now we have one more characteristic to take into consideration: flammability. The tendency for vegetation to facilitate fire spread will become more and more important in Canterbury’s hotter, drier future.
Wild fires are most likely to break out and spread at a devastating rate when we’ve had a hot, dry summer. It’s easy to think back to this past summer, cool and wet as it was, and wonder what we’re worrying about. But think back one more year to the heat and drought of the 2012-2013 summer—that’s the type of summer predicted to become more common in the upcoming few decades.
Flammability isn’t rocket science. Remember your basic scout fire building skills? Fires catch best in super dry, lightly packed, twiggy material, and these qualities vary between plant species. Fire spreads quickest in plants that:
- Hold more dead material (dead plant matter is drier than live and requires less energy to ignite)
- Hold fuel, especially small twigs, evenly spaced along the branches, facilitating fire spread
- Contain oils and resins—sparks flying ahead of the fire can ignite these substances easily
For example, young gorse isn’t rated as highly flammable but older plants are highly flammable because they can be made up of 65% dead material arranged neatly along the branches. And manuka and kanuka burn with high intensity probably due to the essential oils they contain and their small leaves which are continuously spaced along their branches.
In wet conditions or if you irrigate your shelter belts, then species choice probably isn’t so important when it comes to flammability—regardless of what it is, if it’s wet then it generally won’t burn. But if you don’t have irrigation, or it doesn’t reach to the shelter belts, then read on.
In 2001, the NZ Fire Service Commission harnessed the experience of about 60 fire managers via surveys, asking them about their experience with fires in different native NZ species. The information gathered is somewhat subjective, but trends did emerge, see figure 1.
Figure 1: Flammability guide for 42 NZ native trees and shrubs .
|Botanical Name||Maori/European Name||Flammability class|
|Cyathea and Dicksonia spp.||tree ferns||Moderate/High|
|Dodonea viscosa||ake ake||Moderate/High|
|Phormium cookianum and P. tenax||flax/harakeke||Moderate/High|
|Metrosideros umbellata||southern rata||Moderate|
|Podocarpus dacrydioides||kahikatea/white pine||Moderate|
|Cordyline australis||ti kouka/cabbage tree||Low/Moderate|
|Hebe salicifolia and H. stricta||koromiko||Low/Moderate|
|Melicytus lanceolatus||mahoe wao||Low/Moderate|
|Nothofagus menziesii||tawhai/silver beech||Low/Moderate|
|Coprosma grandifolia||raurekau, kanono||Low|
|Macropiper excelsum||kawakawa/pepper tree||Low|
|Psuedopanax arboreum||five finger||Low|
Tim Curran, ecologist and lecturer at Lincoln University, has been researching flammability of plants grown in NZ and was able to add comments about non-natives used for shelter belt species. He classifies some of the most common shelter belt species as highly flammable, such as:
- Eucalyptus, e.g. manna gum E. viminalis (high in natural oils)
- Pines, such as radiata pine, especially when it retains dead material
- gorse (especially old gorse hedges with high levels of dead material)
Deciduous species such as willow and poplar tend to have leaves with higher moisture content and therefore are quite a bit less flammable.
Dr. Curran’s recommended low-flammability species for shelter belts are:
- Broadleaf (Griselinia spp.)
- Coprosma species
- Pseudopanex (five-finger, lancewood)
- Pittosporum eugenioides (lemonwood)
Admittedly, these species are slower growing than the standard pines and macrocarpa often used for shelter belts. A compromise might be to plant the slow-growing natives interspersed with fast-growing non-natives, then remove the big trees as the native ones mature. It is important to realize that if the weather is hot and dry enough, even the low flammability species are likely to burn.
Dr. Curran’s research team is currently conducting flammability tests of NZ shelterbelt species in order to make objective recommendations for “green fire break” plantings, lines of low-flammability species that could serve as fire breaks in the greater Canterbury landscape.
References and further reading
Fogarty, L.G., A flammability guide for some common New Zealand native tree and shrub species, New Zealand Fire Service Commission Research Report. 2001. http://www.fire.org.nz/Research/Published-Reports/Documents/89fa12a030b48531cf396dcdba52c6e2.pdf
Berry, Z.C., Wevill, K., and Curran, T.J., The invasive weed Lantana camara increases fire risk in dry rainforest by altering fuel beds. Weed Research, 2011. 51(5): p. 525-533. http://dx.doi.org/10.1111/j.1365-3180.2011.00869.x. Please email [email protected] if you would like an ePrint of this paper
Lantana fuels rainforest fires ABC Science (web page)
Shelter and nature conservation in Canterbury – a practical guide ECan (PDF file)