By Eloise Gibson
Pinus Radiata sequesters carbon at a much higher rate in NZ than much-preferred native trees. So scientists propose an unconventional solution to get the best of both.
To measure how much carbon is in a tree, you first have to kill it.
You slice up the trunk, branches, twigs, leaves and roots and dry the dismembered tree parts in an oven. Then you weigh them.
“It takes a long time,” says Euan Mason, a professor at the University of Canterbury’s School of Forestry. “I did some in 2012 with two students, and in six weeks I think we did 25 trees.”
Sacrificing trees like this is expensive, but researchers need these measurements.
Typically, about half a tree’s dry weight is carbon, which you can multiply by roughly 3.7 to work out how much carbon dioxide the tree has sucked from the atmosphere.
Once enough trees of different ages and species have been dissected, the results are used to help build computer models estimating how much carbon is in a hectare of living forest, or an entire country’s worth of trees.
Forest owners can use models like this to see how much money they can claim for carbon credits under the Emissions Trading Scheme. Similar estimates tell the Ministry for the Environment that New Zealand’s forests removed 24 million tonnes of carbon dioxide equivalent from the atmosphere in 2017, enough to offset 29 percent of the country’s greenhouse gas emissions.
Most of this CO2 was absorbed by Pinus Radiata, a species much-loved by commercial foresters for its astonishing rate of growth, but seemingly little-loved by anyone in the general population.
Radiata became the nation’s wood crop after most of our ancient Kauri forests were destroyed by indiscriminate logging in the 1880s. (“I wouldn’t call it forestry, because it was just pillaging,” says Mason).
Permanent indigenous forest still covers a much larger area than pine – almost quarter of the country, compared with 6.6 percent in wood plantations. But old-growth forests on conservation land are excluded from the tallies of New Zealand’s carbon sinks and emissions. (This sounds less insane after you find out that mature forests often reach a steady state, sucking about the same amount of CO2 they are losing from dead wood.)
For such peaceful beings, trees have sparked some heated arguments lately: how many we should plant, where and what kind. One point on which no one disagrees is that New Zealand needs to hold on to its old, indigenous forests: mature forest in the conservation estate holds about twice as much carbon per hectare as tree plantations do. After all, our ancient forest has centuries to hoard it.
But the question of what to plant in the next few decades is different, and even forestry scientists can’t agree. The basic points are common ground. We face a climate emergency. The Government, like others around the world, is committed to being carbon neutral by 2050. Trees can help.
But do we want maximum carbon-sucking, fast, or do we value other attributes more, or is there some way to have it all?
A bizarre insight from our kauri means we should view forests as ‘super-organisms’.
Kiwi scientists have been astonished to find how kauri stumps can keep themselves alive by feeding off water from neighbouring trees.
The AUT researchers behind the ground-breaking discovery say it should mean we view trees not as individuals, but members of a forest ecosystem that’s essentially a “super-organism”.
Further, their findings could have big implications for tackling the disease killing kauri across the upper North Island.
In the new study, published in iScience this week, AUT’s Dr Martin Bader and Associate Professor Sebastian Leuzinger described how trees surrounding kauri stumps offer them a form of life support, possibly in exchange for access to larger root systems.
It was an insight the pair stumbled across while hiking in West Auckland, and spotting an unusual-looking stump.
“It was odd, because even though the stump didn’t have any foliage, it was alive,” Leuzinger said.
They decided to investigate how the nearby trees were keeping the tree stump alive by measuring water flow in both the stump and the surrounding trees belonging to the same species.
They found that the water movement in the tree stump was strongly negatively correlated with that in the other trees.
These measurements suggest the roots of the stump and surrounding conspecific trees were grafted together, Leuzinger said.
Root grafts can form between trees once a tree recognises that a nearby root tissue, although genetically different, is similar enough to allow for the exchange of resources.
“This is different from how normal trees operate, where the water flow is driven by the water potential of the atmosphere,” Leuzinger said.
“In this case, the stump has to follow what the rest of the trees do or else use osmotic pressure to drive water flow, because since it lacks transpiring leaves, it escapes the atmospheric pull.”
But while root grafts are common between living trees of the same species, the pair were interested in why a living kauri tree would want to keep a nearby stump alive.
“For the stump, the advantages are obvious— it would be dead without the grafts, because it doesn’t have any green tissue of its own,” Leuzinger said.
“But why would the green trees keep their grandpa tree alive on the forest floor while it doesn’t seem to provide anything for its host trees?”
One explanation, Leuzinger said, is that the root grafts formed before one of the trees lost its leaves and became a stump.
The grafted roots expand the root systems of the trees, allowing them to access more resources such as water and nutrients.
They also increased the stability of the trees on the steep forest slope.
Lawmakers have called for a ban on the “mining” of an ancient New Zealand timber resource after a government report Monday showed that half of it might have already disappeared.
The report showed that an estimated 30 percent to 50 percent of swamp kauri logs – massive logs of New Zealand’s native kauri hardwood that have been preserved in peat land for thousands of years – have been removed from the ground.
It was one of three reports on swamp kauri, which is found in the far northern Northland region, published by the Ministry for Primary Industries (MPI) on Monday.
The reports provided information on the scientific and cultural values of swamp kauri and its distribution and remaining volume.
“This is the first time an attempt has been made to assess the swamp kauri resource,” MPI director general of regulation and assurance Bryan Wilson said in a statement.
The reports also said that swamp kauri held significant value for New Zealanders, due to its age, appearance, and its cultural properties.
“They also highlight swamp kauri’s scientific value in helping to understand the natural history of New Zealand, and its contribution to understanding the effects of climate change,” said Wilson.