The Government’s goal of planting 1 billion trees by 2028 has, for some, caused concern about a landscape painted dark green with radiata pines. However, the initiative could also be the opportunity to both increase the numbers of some forestry species which are currently less common in New Zealand, and to explore the potential of a variety of other species which are just beginning to make their mark here.
I sat down with Pete Gatehouse from the Central Canterbury Farm Forestry Group to talk about some of the less common tree species that have potential in New Zealand. When asked about how these new species can be incorporated into our existing forestry model, Pete was strongly of the opinion that “it’s about having the right tree in the right environment… the days of radiata being forced to fill every possible need are behind us.” He also believes that mixed-species forestry lots ought to be more prevalent in the future, and that we should be looking to make better use of species which have proven themselves in Europe. One strength of mixed forests is that they are less vulnerable to pests and diseases, while another is that compared to a monoculture forest, they are better able to support a diverse ecosystem within them.
New Zealand is uniquely well suited to growing trees, and has the potential to produce a large number of timber species. This makes it quite ironic that in reality, we have one of the most narrowly based forestry industries worldwide, with the vast majority of production coming from radiata pine. This reflects the strengths of radiata pine in New Zealand: it is easy to establish, easy to grow, and easy to process; it’s tolerant to a wide range of sites, produces versatile timber, and has a short 20-30 year harvesting rotation; and, there’s a century of experience and research about growing it in New Zealand. When it comes to quickly sequestering large amounts of carbon it’s hard to beat. But, radiata pine isn’t resistant to forest fires or wind, and although versatile, its timber is not particularly special. If we’re willing to accept slightly longer rotation times and to utilise species which are more site-specific, a number of alternative timber species will become viable, many of which have superior wood properties compared to Pinus radiata.
With that in mind, here are some of the trees we might be seeing more of in the future. This list only represents a tiny fraction of alternative species which could be more widely utilised in New Zealand. Some will undoubtedly be familiar; others, like the Dawn Redwood, are far less common.
Given it is already New Zealand’s number two plantation species, Douglas Fir can hardly be considered an alternative. However, it earned its position on this list because of a few interesting properties, one of which might increase the range of sites on which it can be planted in the future.
Douglas Fir is an excellent framing timber, and can be used untreated for internal framing. This is allowed due to it being far more moisture resistant than radiata pine, and because it is difficult to CCA treat Douglas Fir timber.
Douglas Firs are generally thought of as a species better suited to wetter, cooler sites with reliable summer rainfall. Sites with a rainfall between 1000-1500mm are typically considered suitable. However, pockets of Douglas Fir have proven to grow successfully in lower rainfall areas, including at McHughes Forest Park near Darfield on the Canterbury Plains (annual rainfall 787mm).
This ability to grow in lower rainfall sites than expected is likely connected to Douglas Firs being an ectomycorrhizal species. A mycorrhiza is a relationship between a soil fungus and a plant root. The fungus benefits because the tree provides it with a food source. The tree benefits because the fine fungal hairs provide a much larger surface area to absorb water and nutrients from the soil. This allows the tree to extract more moisture from the soil than would otherwise be possible. However, to benefit from this relationship, the tree has to pick up the fungal spores, either as a seed or a seedling. The increasing prevalence of these pores in New Zealand soil may be responsible for the emergence of Douglas Firs as an invasive pest species now able to successfully self-seed in the high country.
The Douglas Fir is also a root-grafting species- that is, a forest of Douglas Firs join their roots together, effectively making them one combined organism. This has positive implications for the durability of the forest, but is not a particularly well researched area. One resulting benefit of root grafting may be increased resistance to being blown over, especially in shallower soils. Another anecdotal benefit is the ability of the forest to share nutrients, meaning that no individual tree is disadvantaged because of slightly worse conditions in the exact spot where it was planted.
Coastal Redwood and Sierra Redwood
Sequoia sempervirens and Sequoiadendron giganteum
Early redwood plantings in New Zealand were often failures. This was likely due to poor site selection and poor planting stock. With good management however, redwoods growing in New Zealand can show better growth rates than in their native California. Whakarewarewa Forest near Rotorua is a superb example of a successful Coastal Redwood forest in New Zealand.
The Coastal Redwood has fern-type leaves, while the Sierra Redwood has foliage more similar to a macrocarpa. Both are heavily buttressed, gigantic trees capable of growing taller than 100m. The Coastal Redwood originates along the Californian coast, while the Sierra Redwood can be found in California’s mountain ranges. Both species have a characteristically spongy bark which is fire resistant- in fact, in their natural habitat, redwoods benefit from forest fires, as they remove competition for seedlings.
Redwoods are also able to coppice from stumps. This ability to send up new shoots means that a plantation doesn’t have to be replanted after each harvesting rotation- instead it can be left to regenerate naturally, resulting in lower long-term costs. Coppicing is beneficial on steep hill sites, as the existing root systems remain alive, and therefore continue to prevent erosion, after harvesting.
Redwood timber has good natural durability when used above ground, and can be used untreated in outdoor applications. Shrinkage is low, and so it is possible to use it wet. It is not strong enough to use as a structural timber, but its warm red color makes it well-suited for use in outdoor furniture, decking, and joinery.
The third member of the redwood family, the Dawn Redwood, was only known from the fossil record until it was rediscovered in a small population in China in the 1940’s. It has since been sent to Botanic Gardens worldwide to ensure its survival, and has proven to grow well in a range of climates. It is a deciduous conifer best suited to damp, shady locations, and although it doesn’t attain the enormous heights of its relatives, it is capable of growing 50 metres tall.
Although it is not currently a commercial species, there may be value in its willingness to grow in wet environments. Pete Gatehouse grafted a specimen from the tree which was sent to the Christchurch Botanic Gardens, and milled it at maturity. He found that, like other redwoods, it produces a pleasingly colored, highly figured timber.
The macrocarpa is unquestionably the best known cypress in New Zealand. In general, these trees produce good timber with excellent working and finishing properties, attractive appearance, and good durability. Unfortunately, cypresses are vulnerable to canker dieback, although the disease is less problematic on shadier, cooler sites, such as those found in the South Island. The Swamp Cypress (Taxodium distichum) is an interesting tree in the same family as macrocarpa. Although it is not currently grown commercially in New Zealand, it is a well known timber species in the Southern United States.As the name suggests, it has an affinity for particularly wet environments- it can even grow with its roots submerged year-round.
Its timber is ground durable, resistant to water, and good for furniture. Pete Gatehouse is growing it at Kumara on the West Coast, and thinks it may have the potential to fill in the gaps where other timber species simply can’t grow.
Last on the list for today is the Chestnut tree. Although it may be better known for its nuts, Chestnut wood is particularly hard and durable. Chestnut posts are increasingly being utilised as an alternative to treated pine posts in vineyards, due to the latter tending to leach copper, chromium and arsenic into the surrounding soil over their lifetime. Pete Gatehouse has conducted some rudimentary rot tests which indicate it has the same in ground durability as H4 treated pine. Chestnut is also another coppicing species. This trait is exploited in Europe, where several shoots are grown from a harvested stump, and themselves cut and sold as untreated garden stakes. And of course, the chestnuts themselves can provide an income, or can form a supplementary part of a diet for grazing livestock.