Science1 publisher2 min readPublished
Severe kauri dieback costs Waitākere forest stands about 1 tonne of carbon per hectare a year
Resurveyed Waitākere Ranges plots show severely diseased kauri stands lost about a tonne of carbon per hectare a year while healthy stands gained 0.8 tonnes. The study counted only living trees, so how much of that carbon reaches the air is still unknown.
The Scientist · Science desk
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What happened
- Co-author Toby Elliot remeasured Waitākere Ranges kauri plots first surveyed between 2011 and 2014, recording canopy condition, tree deaths and newly established trees.
- Where kauri were present, the researchers' modelling put them at about half of the living carbon, with the share rising in more carbon-rich forest.
- The modelling suggests severely affected, carbon-rich stands may never regain their earlier stocks and instead settle with less living biomass.
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Why it matters
- exposure Carbon estimates that treat mature kauri forest as a steady sink are most exposed where dieback is severe, because the stands holding the most carbon showed the largest changes.
- cost At the measured rates a severely affected hectare falls about 18 tonnes of carbon behind healthy forest each decade, a loss to the living store that counts whether or not the dead wood has decayed.
- decision Managers now have a carbon reason, alongside the cultural one, to put mature, carbon-dense kauri stands first when deciding where protection effort goes.
Severe dieback opens its 1.8-tonne gap in two ways [6]. Declining trees grow less, and trees that die take their carbon out of the living store [16]. About 56 percent of the gap is carbon lost from living vegetation. The other 44 percent is growth that never happened [18]. The stands in that comparison began at an average of 169 tonnes of carbon per hectare, so a tonne a year is roughly 0.6 percent of that average starting stock [19]. Stands showing only signs of dieback sat close to even, losing about 0.1 tonnes per hectare a year [5].
The carbon figures are calculated from standing trees. Living biomass came from each tree's size and the typical wood density of its species [4]. Nobody assigned the disease, so the comparison runs across stands the pathogen reached to different degrees, and other differences between those stands could vary with severity too. The researchers do report a gradient: the more severe the dieback, the less carbon a stand gained and the more it lost [12].
The pathogen, Phytophthora agathidicida, is a soilborne water mould that infects kauri through the roots and disrupts their uptake of water and nutrients [1]. Hundreds to thousands of kauri have died across the upper North Island, with some of the heaviest losses in Northland and the Waitākere Ranges [2]. The researchers wrote that "the impact of kauri dieback depends on which trees die, as well as how many" [14]. Apply kauri's average half-share of living carbon to the 248-tonne average for mature plots and you get roughly 124 tonnes per hectare in kauri alone [20].
For carbon accounting, the study supports a narrower claim than release to the atmosphere. It measured living vegetation only. The researchers say they cannot tell how much of the lost carbon stays in dead wood, litter and soil, or how quickly it returns to the air [11]. I think the evidence is enough to treat severely diseased kauri stands as a weaker sink in any forest carbon estimate. The article does not give the number of plots or scale the rates beyond the Waitākere Ranges [3], so there is no regional figure yet to revise an estimate by. The researchers say longer-term measurements of the other carbon stores will be needed [15].
What to watch
- Measurements of dead wood, litter and soil carbon in the same Waitākere plots, to show how much of the lost biomass carbon stays in the forest.
- Any attempt to scale the plot rates to the area of kauri forest in each dieback severity class across Northland and Auckland.
- A further resurvey testing the model's prediction that severely hit, carbon-rich stands settle at lower living biomass.