Thursday, August 11, 2022

Forest giants a viable carbon option for NZ

In certain parts of New Zealand, says researchers, majestic redwoods put radiata pine in the shade when it comes to carbon capture – both in the short term and for potentially hundreds of years to come.
Dr Michael Watt’s work has shown that, in the right environment, redwoods put radiata pine in the shade when it comes to carbon sequestering.

Redwoods, the towering natives of California, account for a minuscule 1% of New Zealand’s plantation forest, despite the well-known Rotorua stand being there for over 100 years as an example of the tree’s potential. Redwoods dwarf their radiata pine cousins in height, but those same pines eclipse redwoods for popularity, accounting for 90% of NZ’s plantation plantings.

A carbon modelling study by Scion researcher Dr Michael Watt and colleague Dr Mark Kimberley shows that redwoods’ capacity for carbon sequestration makes them an appealing option as a plantation species in some parts of NZ, exceeding pines’ ability to store carbon on suitable sites.

Watt says while pine trees are good candidates for early “heavy lifting” in carbon sequestration, their declining growth rate after 30 years could limit this country’s ability to hit its carbon reduction targets come 2050. 

“Our modelling work, which by and large was relatively conservative in terms of estimates, found on average by age 40 redwoods growing in the North Island have sequestered 1943 tonnes of CO2 per hectare, against pines’ 1864t CO2/ha,” Watt said.

“In the most ideal sites in Taranaki, Bay of Plenty and Waikato, redwoods reached 4000t CO2/ha, far exceeding the 1750-2500t CO2/ha achieved by radiata pine at those sites.”

But redwoods are only just warming up over their first 40 years, maintaining high growth rates and by default carbon sequestration for hundreds of years afterwards. 

Some individuals in the species include some of the oldest living trees in the world, reaching ages of 2200 and heights of more than 110m.

The researchers’ modelling work enabled them to create a granular map of NZ that highlights areas where the trees are most suited to grow and their expected growth rates. 

Areas most suited to redwood are generally warmer and wetter, and include parts of Bay of Plenty, Waikato and Taranaki. The trees do not perform well in drier, cooler areas such as the east coast of the South Island.

Watt also points to redwood’s harvest value as another reason for growing the species.

“Like cypress species, redwoods have significantly greater value than pines due to their appearance-timber premium, often double that of radiata pine. They can often be used as a replacement for imported western red cedar,” he said.

Modelling reveals redwoods’ ability to sequester high amounts of carbon in select areas of NZ.

As the Government looks to limit carbon forestry to natives-only, the researchers’ work also highlights how NZ may miss a valuable sequestration opportunity by keeping non-natives, including redwoods, limited to short-rotation plantings alone. 

Watt says the concern for NZ’s carbon targets is the sawtooth-like pattern of sequestration and drop after harvest at year 30 that characterises pines. 

“Both volume and carbon of redwood planted as permanent forests will continue to increase rapidly after age 30. Establishment of a significant sustainably logged redwood resource in New Zealand would ramp up the carbon capture for NZ forests and at the same time supply high value appearance-grade timber to the market,” he said.

Their forward projections show that carbon differences between redwood and pine diverge even more at older ages and higher stand densities in the North Island. For instance, when redwoods were grown at high density, by age 50 they had sequestered on average 2804t of CO2 per hectare, compared to pines at 2296t CO2/ha.

Watt says the work has also highlighted the significant disparities between actual sequestered carbon and the Ministry for Primary Industries’ “look up” table estimates of carbon capture for the species.

“The look-up tables are too conservative in their estimates of redwoods’ sequestration. Our study found carbon to be at least twice as high as the values in the look-up tables for all North Island regions,” he said.

This has flow-on effects for farmers who are planting trees, as they will usually plant less than 100ha, and under the regulations have to use the look-up table estimates. 

Watt describes the issue as critical, as the tabled values of carbon are often used by landowners to make decisions about species selection – and at the current time these will favour the establishment of pines.

Look-up data for redwoods is also not regionalised to allow for major environmental differences along this long skinny country. 

“They should reflect the wide variation in growth for redwood (and other alternative species) throughout the country, so that incentives are in place to establish the right tree in the right place,” he said.

The researchers hope their work prompts and assists government policy-makers to revise current look-up tables and also consider longer rotations and non-native permanent carbon forests in the Emissions Trading Scheme forestry regulations going ahead now. 

“Redwoods offer a very viable option in some places to natives, which are extremely expensive to establish, and slow to sequester. Natives will not do the job of achieving net zero in this generation. Redwoods would be cooking with gas come 2050, through to 2100,” he said.

More: Downloadable maps on redwood growth potential are freely available at here. Type in “redwood” to the search engine. 

People are also reading