More drought brings earlier autumn leaf fall, Northern Hemisphere study finds

The progression of the coloring of green leaves to the reds, oranges and yellows of the fall season can be a spectacular view to see, but this process is also an important part of the development cycle of a plant.

The internal and external forces, such as phytohormones and environmental constraints, trigger this annual process at a given time towards the end of summer – considered as the date of autumn of foliar senescence (DFS) in more scientific terms. Currently, the plant is starting to move the vital nutrients from its leaves to reproductive parts, such as seeds or on storage organs.

Previous research has revealed that the warmer falls due to climate change are linked to a delay in the DFS, causing subsequent autumn seasons, but the relationship between the DFS and the environmental factors is complex. Although the warmer weather can lead to a delay in the senescence of the leaves, a lack of water from the drought seems to cause an anterior DFS. However, the specific drought thresholds that trigger the senescence of anterior autumn have remained vague in previous research.

To better understand the pre-season drought thresholds (PDT) triggering an anterior DFS (PDT-DFS), researchers from the University of Yunnan, recently conducted a study focused on the senescence of autumn sheets through the northern hemisphere above a 30 ° latitude, using a 1951-site level combination and later, 1982–2021 and Drirvigh indexes. They sought to determine where the PDT-DFS occur, how it is affected by heat waves and which are the pilots of PDT-DFS.

Their analysis indicated that an earlier DFS is associated with higher drought thresholds. These drought thresholds are essentially a certain level of drought that transforms the region under drought conditions, causing faster senescence of leaves.

The researchers identified the values of PDT-DFS as drought thresholds for different quantiles of early senescence at −2.59, −2,30, −1.80 and −1.63, in which the lower thresholds mean more severe drought for senescence. They found that deserts and shrubs are most sensitive to pre-season droughts. Research is published in Nature communications.

The study authors explain: “When the PDT-DFS approaches −0.5, a lower drought intensity is necessary to trigger an anterior DFS. Conversely, when the PDT-DFS approaches −5.0, a higher drought intensity is necessary to trigger an anterior DFS.”

There is also a clear difference in the effects between the heat waves that occur day and night. Their analysis has shown that waves of daytime heat amplify the effect of droughts on previous senescence more than heat waves that occur at night. This is partly due to the increase in perspiration in the stomates of a plant, which further increases water loss and uses water from the soil.

“This increase in increased water at high temperatures requires stress of substantial drought to vegetation, thus reducing the intensity of the drought necessary to trigger an anterior DFS. On the other hand, the waves of night heat exert a less immediate impact on the humidity levels of the soil even if they always contribute to the overall temperature stress. Closed during fresh night periods, which explains the amount of perspiration and sweating.

Certain factors contribute to a reduced sensitivity to early senescence induced by drought, such as increased resilience of ecosystems in the form of mechanisms such as deep roots that access more underground water, or index of the leaf area – a value that describes the canopy of the plant, which can provide protection against excess heat. These attributes can protect ecosystems, but they can also be reduced by drought. Thus, they could temporarily delay the DFS, but once they have been hampered, the senescence of early leaves can still occur.

The study also examined future trends using simulations of climate models, which revealed a likely increase in sensitivity to pre-season droughts in most regions of the northern hemisphere.

The study authors note an aggravation effect over time, claiming: “Compared to estimates in 1982-2021, more than 51.09% of the areas showed higher drought thresholds in 2061-2100. with warming. “”

It is clear to understand the effects of drought and how these thresholds can be used to predict the responses of ecosystems to climate change and drought. The senescence of anterior leaves means less carbon absorption and shorter growth seasons and, as droughts increase, these effects have the potential to cause even more difficulties.

Additional research can overcome some of the limits of this study, such as the limitation of the northern hemisphere and the possible sensitivity overestimated of satellite data compared to the ground observations. Improved models could improve the predictions of the responses of ecosystems, as well as contribute to a better understanding of the resilience mechanisms to combat the effects of severe drought.

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