Vegetation cycles play a big role in the world’s carbon fluxes: green leaves absorb carbon dioxide to produce carbohydrates for plant growth, whilst browning leaves mark the end of this process. The later the onset of colouration, and the longer the growing season, the greater the expected carbon uptake – and the less carbon left in the atmosphere. While a lot of studies have focused on the timing of new growth in spring, say Lingling Liu and Xiaoyang Zhang of South Dakota State University, US, fewer have focused on when leaves turn orange.

"Determining the long-term peak-colouration timing and its complexity greatly improves the calculation of the vegetation growing season length, which [in turn] facilitates more reliable measures of carbon dynamics in vegetation–climate interactions models," said Liu.

Liu, Zhang and colleagues took satellite measures of the greenness of deciduous and mixed forests in the central and eastern US from 1982 to 2014. They converted the data to a measure of brownness, and used a computer model to remove patches of "noise" such as snow and cloud. Then they determined the onset of peak colouration for each satellite pixel for each year to look for trends, while exploring possible links with temperature and precipitation.

Over the study period, peak colouration became earlier in 40% of forests and later in 35% of forests, the researchers found. For the remaining 25%, which were mainly north of 25°, the situation was more complex, with a trend towards later colouration before the year 2000 and earlier colouration after. This complex timing seemed to be caused by late summer and autumn temperatures for just over half of this portion, but by late precipitation for a little over 12%. "These indicate that the timing of peak colouration was controlled by different factors across the region," said Liu.

Liu believes the results emphasize how nuanced the timing of colouration is – and therefore how a better understanding is crucial for the improvement of climate models. "In the era of warming climate, the end of the vegetation growing season is not necessarily going to be later," he said. "The long-term trend is complex under climate change."

Now the researchers plan to explore a longer time series of satellite data on foliage colouration, to understand long-term variations associated with both climatic and non-climatic variables.

They published their findings in Environmental Research Letters (ERL).

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