Science1 publisher2 min readPublished
Cell-by-cell counts in 50,000 conifer samples rank growth rate above season length
Fifty thousand wood samples, counted cell by cell, show a warmer climate lengthening the conifer growing season while slowing the rate at which wood is laid down. The samples span four continents.
The Scientist · Science desk
What happened
- An international team including researchers from Switzerland's WSL analyzed more than 50,000 wood samples from conifer forests on four continents, and published the results in Science.
- The rate at which trees produced new xylem cells influenced the annual cell count more strongly than the length of the growing season did.
- Warming lengthened the season, but above an average annual temperature of around 6 C cell production slowed. The authors attribute that slowdown to reduced water availability.
- Boreal forests grew both longer and faster as temperatures rose, while in Mediterranean and temperate forests the gain from extra days was often offset or outweighed by slower cell production.
- One contributing site, the Loetschental in the Canton of Wallis, has been sampled every week of the growing season for 20 years at several elevations.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- constraint A projection that turns warming into extra growing days needs a second term for how fast cells form, because past a temperature the second term works against the first.
- capability Weekly cell counts give modellers a quantity they can be tested against directly, instead of inferring season and rate together from a finished ring.
- exposure Boreal stands currently gaining from warmth are the ones whose gain is conditional on staying cool enough, and they are warming.
- decision Anyone booking a future forest sink now has to decide whether cell counts are an acceptable proxy for stored carbon. The study's own authors say cell counts are not, on their own.
An annual tree ring records the year's total. A wide ring can come from a long season or a fast one, and width alone cannot tell you which. Repeat sampling separates the two: samples collected through the growing season at each partner institute, then processed, photographed under the microscope and analyzed cell by cell [4]. What the team tracked was xylogenesis, the formation of new xylem cells in the outermost ring [3].
"By looking at the samples under the microscope, we can see exactly when trees are actively forming wood," said Antoine Cabon, a forest ecologist at WSL and the Spanish National Research Council [5][18]. "This allowed us to observe when this process starts and stops, and at what rate it occurs." [6]
The spread in rate is wide. At the slow end a tree added one new wood cell a month; at the fast end, two cells a day [8]. Two a day is about 60 cells in a 30-day month, roughly sixty times the slow end [9]. Season length, by comparison, moves by weeks between sites and years.
Temperature pulls the two terms in opposite directions past a point. Warmer conditions generally extended the season [10]. The account attributes the slowdown at the warm end to reduced water availability and calls that explanation likely. The inference rests on observed rates. No watering experiment was run [11].
The study counts cells. Carbon mass is a further step, and Patrick Fonti, the WSL dendrochronologist who coordinates the long-running Swiss sampling, made that point himself [19]. "Although carbon storage depends on more than xylem cells alone," he said, "slower wood formation ultimately reduces the capacity of forests to act as carbon sinks." [22]
The paper's stated target is an expectation, that longer growing seasons necessarily improve forest carbon storage and so help mitigate warming [23]. The phys.org account does not name the models that carry that expectation or say how much their projections would move [24].
Repeat weekly sampling of the same trees is the rare part of this. "Such long-term and frequent observations are extremely rare and valuable," said Fonti [16]. "By following the same trees over many years, we can reconstruct how wood formation responds to changing environmental conditions." [17]
Cabon puts the forward case as a prediction: "As temperatures continue to rise, even boreal forests may eventually experience slower wood formation and lose the benefits of a longer growing season" [20]. The published paper is titled "Growth rate overrides the benefit of extended growing season from boreal to semiarid conifers" [2].
What to watch
- Whether the full Science paper reports how many sites and species sit behind the 50,000 samples, and the uncertainty on the 6 C turning point.
- Whether groups building forest carbon projections add a wood-formation rate term and publish how much the projected sink moves.
- Whether boreal monitoring sites in the network warm past 6 C mean annual temperature. That would test Cabon's prediction directly.