Carbon Capture, Climate Levels Trigger Earlier Shedding of Leaves

(Courthouse News photo / Chris Marshall)

(CN) — The change of seasons can be marked by autumn leaves falling from a tree or sprouting from buds as spring draws near. But like many natural occurrences, the cycle has been altered by climate change, according to a study that found European trees may shed leaves earlier as they absorb more carbon from the atmosphere.

Climate change has brought about an earlier start to the period in which deciduous trees — those that produce leaves annually — expand their branches and grow leaves, also called a growing season. 

The growing cycles of temperate forests and the shedding of leaves — also called senescence — play major roles in regulating climate systems, carbon cycles and surface temperatures worldwide.

With the expanded growth season, trees are capturing more carbon and once a tree has enough carbon stored, it could shed its leaves earlier in autumn, according to the study published Thursday in the journal Science.

“As a result, primary productivity of temperate forests is expected to increase, with each day advance in spring leaf-out translating to an increase in net carbon uptake of 45 kg (99 pounds) carbon per hectare forest and each day delay in autumn senescence translating to an increase of 98 kg (216 pounds),” the study said. 

But scientists have long speculated that the shedding of leaves would occur later and later under climate change.

Previous studies found that in decade after decade, as warmer climates stretched longer into the year, leaves stayed on branches for more time. 

That so-called productivity was believed to have the potential to curb climate change because trees would have more time to capture atmospheric carbon through photosynthesis.

But the trend is changing.

Researcher Deborah Zani and colleagues found that tree productivity or growth is limited by the tree tissue’s ability to use and store carbon.

To study climate’s impact on senescence, the team led by Zani, with the Institute of Integrative Biology in Switzerland, used data gathered between 1948 and 2015 on productivity of central European trees and results from climate manipulation experiments.

Researchers found that in growing seasons extended by elevated carbon dioxide levels, trees shed their leaves earlier, not later, in the year. Temperature levels and varying quantities of natural light also impacted senescence.

The roots, trunk and branches of trees have limited ability to store carbon captured by leaves for long periods. 

Study authors used their observations to build projection models for autumn senescence over the next century, though their findings are specific to tree species in the European region.

“Although autumn warming is likely to increase over the rest of the century, our model forecasts that there might be slight advances, not delays, in autumn senescence dates,” the study said. “These results highlight physiological constraints on growing-season lengths and plant productivity in a warming, CO2-enriched world, which has direct implications for future carbon-cycle and climate projections.”

The authors write that results stifle prior predictions that extended growing seasons will lead to increased carbon uptake in forests, though the study says more research is needed to understand this effect in other regions of the world.

“Furthermore, given that ecosystems with high nutrient availability and minimal carbon-sink limitation, such as regions dominated by nitrogen-fixing trees, are not expected to show the observed negative relationship between seasonal productivity and senescence dates, a major research challenge will be to generate a thorough spatial understanding of the extent of sink limitation to forecast plant phenology and forest productivity over space and time,” the study said.

Researchers did not immediately respond to requests for further comment on the study.

In a related commentary on Zani’s study, Christine Rollinson of the Morton Arboretum in Illinois said the findings show that forest preservation and even expansion cannot be the sole method for carbon sequestration on a global scale.

“The study of Zani et al. shows that the forest carbon sink is limited in ways that are not yet fully understood,” Rollinson said in a Perspectives post published Thursday in Science. “A diverse portfolio of climate actions that include emissions reductions and tree conservation and planting is essential to mitigate anthropogenic carbon emissions and climate change.” 

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