Ecosystems

Changing seasons change tree growth, changing carbon sequestration by forests

An earlier spring changes the growing conditions for trees, and not always for the better.
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Changing temperatures are changing the growing seasons. For example, there’s a strong link between temperatures and leaf phenology, such as the first appearance of leaves as buds on branches. But how do these changes affect tree growth? A research group led by Professor Liang Eryuan from the Institute of Tibetan Plateau Research has found that trees may benefit from an earlier start of the growing season in cold, humid areas but not in dry areas.

The team examined 3,451 tree-ring chronologies and daily climatic data for 1948–2014 to find a relationship between the start of the thermal growing season and tree growth across the Northern Hemisphere outside of the Tropics. The researchers used a collection of annual tree-ring width data from the International Tree-Ring Data Bank, the world’s largest public archive of tree ring data.

Tree rings in a cracked trunk. The wood is almost black and white with strong concentric striping. Dark cracks run to the centre of the tree, giving the image a hint of looking like a dartboard.
Tree rings. Image: Canva.

“Previous studies have shown that the onset of wood formation is closely related to the fulfilment of critical temperatures and photoperiod. Therefore, we analyzed how annual tree-ring width relates to thermal thresholds for growth start on a hemispheric scale,” said Prof. Liang in a press release.

The onset of wood formation in the cambium is the main factor that directly or indirectly triggers all subsequent phases of xylem maturation and determines the period of xylem growth. The xylem is the tissue in trees that carries water and nutrients up to the leaves. Small advances in cell division can therefore lead to substantial increases in xylem cell production, which eventually results in broader growth rings.

Therefore, tree growth in cold climates would be enhanced throughout the entire growing season by an earlier onset of cambial activity or by higher growth rates at the peak of the growing season.

Annual ring width is usually correlated with monthly or seasonal climate data. After extensive processing of daily climate data and conducting different analyses, the researchers found distinct spatial patterns of tree growth in response to an earlier growing season onset. Still, not all trees benefited from warmer weather.

Areas where tree growth benefits from an advanced spring, are generally located at the higher latitudes (above 60°N), central Europe, as well as in eastern and western coastal North America. These cool and humid regions are not strongly limited by water availability during the growing season, and the rising temperatures helped by removing some cold stress from the plants.

The regions showing adverse effects of advanced springs on tree growth were mainly located on the Colorado Plateau and the Tibetan Plateau, which have cold and dry conditions. Here, forests are typically limited by factors, such as low temperatures and drought events and poor soil fertility. The increased growing season also exposed the trees to drier weather, leading to increased drought. Also, as one day isn’t always warmer than the next, an early spring could lead to young growth being exposed to a late frost.

In their article, Gao and colleagues write: “We found that the impact of shifts in the timing and duration of [the start of the thermal growing season] could be detected in tree rings at regional to hemispheric scales. Our study thus allows for the further exploration of the impact of climatic trends and variability on tree growth. Such information is essential for integrating information regarding the responses of foliage and stems to climate change and for predicting future vegetation performance. Explaining the influence of plant phenology on carbon sequestration solely on the basis of the perspective of foliar phenology and photosynthesis seasonality (which drive carbon uptake) is insufficient.”

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Gao, S., Liang, E., Liu, R., Babst, F., Camarero, J.J., Fu, Y.H., Piao, S., Rossi, S., Shen, M., Wang, T. and Peñuelas, J. (2022) “An earlier start of the thermal growing season enhances tree growth in cold humid areas but not in dry areas,” Nature Ecology & Evolutionhttps://doi.org/10.1038/s41559-022-01668-4.

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