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The progression of leaf coloring from green to the reds, oranges, and yellows of the fall season can be a spectacular sight to behold, but this process is also an important part of a plant's developmental cycle.

Internal and external forces, like phytohormones and environmental stresses, trigger this yearly process at some point near the end of summer—referred to as the autumn date of foliar senescence (DFS) in more scientific terms. At this time, the plant begins to relocate vital nutrients from its leaves to reproductive parts, like seeds, or to storage organs.

Previous research has found that warmer autumns due to climate change are linked to a delay in DFS, causing later autumn seasons, but the relationship between DFS and environmental factors is complex. While warmer weather might cause a delay in leaf senescence, a lack of water from drought seems to cause earlier DFS. However, the specific drought thresholds that trigger earlier autumn senescence remained unclear in prior research.

To gain further insight into pre-season drought thresholds (PDT) triggering an earlier DFS (PDT-DFS), researchers from Yunnan University, recently conducted a study focused on autumn leaf senescence across the Northern Hemisphere above a latitude of 30°, using a combination of site-level data from 1951 and later, satellite data from 1982–2021, and drought indexes. They sought to determine where PDT-DFS occurs, how it is affected by heat waves, and what the drivers of PDT-DFS are.

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

The researchers identified PDT-DFS values 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 is needed for senescence. They found that deserts and shrublands are most sensitive to pre-season droughts. The research is in Nature Communications.

The study authors explain, "When the PDT-DFS approaches −0.5, a lower drought intensity is required to trigger an earlier DFS. Conversely, when the PDT-DFS approaches −5.0, a higher drought intensity is needed to trigger an earlier DFS."

There is also a stark difference in the effects between heat waves that occur during the day and at night. Their analysis showed that daytime heat waves amplify the effect of droughts on earlier senescence more than heat waves that occur at night. This is partially due to the increased transpiration in a plant's stomata, which further increases water loss and uses up water from the soil.

"This heightened water demand under high temperatures imposes substantial drought stress on the vegetation, thereby lowering the drought intensity required to trigger an earlier DFS. In contrast, nighttime heat waves exert a less immediate impact on soil moisture levels even though they still contribute to overall temperature stress. This is primarily because the stomata are nearly closed during the cooler nighttime periods, which reduces the amount of transpiration and conserves soil moisture," they explain.

Some factors do contribute to a reduced sensitivity to drought-induced early senescence, like increased ecosystem resilience in the form of mechanisms like deep roots that access more groundwater, or leaf area index—a value that describes the plant canopy, which can provide protection from excess heat. These attributes can protect ecosystems, but they too can be reduced by drought. So, they might temporarily delay DFS, but once they have been hindered, early leaf senescence can still occur.

The study also looked at future trends using climate model simulations, which revealed a likely increase in sensitivity to pre-season droughts in most areas of the Northern Hemisphere.

The study authors note a worsening effect over time, saying, "Compared with the estimates in 1982–2021, more than 51.09% of the areas showed higher drought thresholds in 2061–2100. This indicates that the future DFS will be more sensitive to pre-season droughts in most of the areas over the Northern Hemisphere. These results offer a perspective for decision-makers to address the nonlinear relationship between water deficiency and vegetation growth with warming."

There is a clear need to understand the effects of drought and how these thresholds can be used for predicting ecosystem responses to climate change and drought. Earlier leaf senescence means less carbon uptake and shorter growing seasons and, as droughts increase, these effects have the potential to cause even more hardship.

Further research can overcome some of the limitations of this study, such as limitation to the Northern Hemisphere and the possible overestimated sensitivity of satellite data compared to ground observations. Improved models might enhance predictions of ecosystem responses, as well as contribute to a better understanding of resilience mechanisms to combat the effects of severe drought.

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