Hvad lærte man stadig om, hvordan træer vokser

Hvad vil der ske med verdens skove i en opvarmende verden? Vil øget atmosfærisk kuldioxid hjælpe træer til at vokse? Eller vil ekstreme temperaturer og nedbør holde væksten tilbage? Det hele afhænger af, om trævækst er mere begrænset af mængden af ​​fotosyntese eller af de miljømæssige forhold, der påvirker træcellevækst - et grundlæggende spørgsmål i træbiologi, og et, som svaret ikke var godt forstået før nu.

En undersøgelse ledet af forskere fra University of Utah, med et internationalt team af samarbejdspartnere, finder ud af, at trævækst ikke synes at være generelt begrænset af fotosyntese, men snarere af cellevækst. Dette tyder på, at vi er nødt til at gentænke den måde, vi forudsiger skovvækst på i et skiftende klima, og at skove i fremtiden måske ikke vil være i stand til at absorbere så meget kulstof fra atmosfæren, som vi troede.

"Et træ, der vokser, er som et heste- og vognsystem, der bevæger sig fremad ad vejen," siger William Anderegg, lektor ved U's School of Biological Sciences og hovedforsker af undersøgelsen. "Men vi ved dybest set ikke, om fotosyntese er den hest, der oftest er, eller om det er celleudvidelse og -deling. Dette har været et langvarigt og vanskeligt spørgsmål på området. Og det har enorm betydning for forståelsen af, hvordan træer vil reagere på klimaændringer."

Undersøgelsen er publiceret i Science .

Kilde vs. sink

Vi lærte det grundlæggende i folkeskolen – træer producerer deres egen mad gennem fotosyntese, tager sollys, kuldioxid og vand og forvandler det til blade og træ.

Der er dog mere i historien. At omdanne kulstof opnået fra fotosyntese til træ kræver, at træceller udvider sig og deler sig.

Så træer får kulstof fra atmosfæren gennem fotosyntese. Dette er træernes kulstof kilde . De bruger derefter det kulstof på at bygge nye træceller - træets kulstof synk .

Hvis træernes vækst er kildebegrænset, så er det kun begrænset af, hvor meget fotosyntese træet kan udføre, og trævækst ville være relativt let at forudsige i en matematisk model. Så stigende kuldioxid i atmosfæren burde lette den begrænsning og lade træer vokse mere, ikke?

Men hvis i stedet træernes vækst er synkebegrænset, så kan træet kun vokse så hurtigt, som dets celler kan dele sig. Masser af faktorer kan direkte påvirke både fotosyntese og cellevæksthastighed, herunder temperatur og tilgængeligheden af ​​vand eller næringsstoffer. Så hvis træer er synkebegrænsede, skal simulering af deres vækst inkludere synkens reaktion på disse faktorer.

The researchers tested that question by comparing the trees' source and sink rates at sites in North America, Europe, Japan and Australia. Measuring carbon sink rates was relatively easy—the researchers just collected samples from trees that contained records of growth. "Extracting wood cores from tree stems and measuring the width of each ring on these cores essentially lets us reconstruct past tree growth," says Antoine Cabon, a postdoctoral scholar in the School of Biological Sciences and lead author of the study.

Measuring carbon sources is tougher, but doable. Source data was measured with 78 eddy covariance towers, 30 feet tall or more, that measure carbon dioxide concentrations and wind speeds in three dimensions at the top of forest canopies, Cabon says. "Based on these measurements and some other calculations," he says, "we can estimate the total forest photosynthesis of a forest stand."

Decoupled

The researchers analyzed the data they collected, looking for evidence that tree growth and photosynthesis were processes that are linked, or coupled. They didn't find it. When photosynthesis increased or decreased, there was not a parallel increase or decrease in tree growth.

"Strong coupling between photosynthesis and tree growth would be expected in the case where tree growth is source limited," Cabon says. "The fact that we mostly observe a decoupling is our principal argument to conclude that tree growth is not source-limited."

Surprisingly, the decoupling was seen in environments across the globe. Cabon says they did expect to see some decoupling in some places, but "we did not expect to see such a widespread pattern."

The strength of coupling or decoupling between two processes can lie on a spectrum, so the researchers were interested in what conditions led to stronger or weaker decoupling. Fruit-bearing and flowering trees, for example, exhibited different source-sink relationships than conifers. More diversity in a forest increased coupling. Dense, covered leaf canopies decreased it.

Finally, coupling between photosynthesis and growth increased in warm and wet conditions, with the opposite also true:that in cold and dry conditions, trees are more limited by cell growth.

Cabon says that this last finding suggests that the source vs. sink issue depends on the tree's environment and climate. "This means that climate change may reshape the distribution of source and sink limitations of the world forests," he says.

A new way to look forward

The key takeaway is that vegetation models, which use mathematical equations and plant characteristics to estimate future forest growth, may need to be updated. "Virtually all these models assume that tree growth is source limited," Cabon says.

For example, he says, current vegetation models predict that forests will thrive with higher atmospheric carbon dioxide. "The fact that tree growth is often sink limited means that for many forests this may not actually happen."

That has additional implications:forests currently absorb and store about a quarter of our current carbon dioxide emissions. If forest growth slows down, so do forests' ability to take in carbon, and their ability to slow climate change.