Trees show that growing is not putting on weight!

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An international consortium led by scientists of the French National Institute for Agricultural Research (INRA) has shown that mechanisms involved in tree growth are in action at different times. First, the tree gets bigger due to the production and enlargement of woody cells; then the cell walls are reinforced, which increases the tree’s mass. These results, published in Nature Plants on 26 October 2015, suggest that the effects of climate change may alter the second phase and in turn modify carbon sequestration in wood, a climate change issue.
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A large amount of the carbon needed for tree growth comes from the carbon dioxide (CO2) that is present in the atmosphere. Forests capture every year around 15% of CO2 emissions originating from human activity. One of the lead authors of the study, Henri Cuny, comments: “Forests represent the first long term reservoir for carbon sequestration”.

Stem-girth increase and woody biomass production are two key processes of forest ecosystems, driving respectively stand growth in volume and carbon sequestration into wood. “Until now these two processes were considered as totally synchronous”, says Cyrille Rathgeber, another lead author of the study.

INRA scientists, in collaboration with international teams, demonstrated that in coniferous forests of the northern Hemisphere, woody biomass production lagged behind stem-girth increase by about one month over the growing season. They also showed that these two biological processes exhibited differential sensitivities to climatic conditions. Indeed, the seasonal dynamics of stem-girth increase matched the photoperiod cycle (change in daytime and nighttime lengths), whereas those of woody biomass production closely followed the seasonal course of temperature.

These new results show how difficult it is to properly infer the seasonal dynamics of carbon sequestration from external measurements of stem size change (e.g., dendrometer records). The amount of biomass produced from photosynthetic carbon cannot be approximated through changes in tree size on the short term. “Even though stem size doesn’t evolve in the fall, the forest can still capture carbon, thus increasing the wood mass at an even volume”, adds Henri Cuny.

This work helps to better quantify the seasonal balance of terrestrial carbon, providing insights and methods to link forest–atmosphere exchanges and woody carbon sequestration at weekly time scales. Modelling the dynamics of the processes that drive carbon fluxes in different forest ecosystem components (leaves, stems, soil, necromass, plant genitalia) is a key issue, especially for assessing how climate change impacts these ecosystems.

The researchers suggested that the forecasted climatic changes might shift the phase timing of stem size increase and woody biomass production in the future and affect carbon sequestration in forest ecosystems.

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