Wu Chaoyang
The research on vegetation phenology changes, led by the Institute of Geographic Sciences and Natural Resources Research of the Chinese Academy of Sciences, has made important progress recently in collaboration with a number of universities and institutions at home and abroad.
The study found that in the high latitudes of the northern hemisphere, the weakened wind speed will reduce the evaporation of the vegetation canopy, alleviate the effect of drought, and then delay the time node of vegetation deciduous.
Today, researcher Wu Chaoyang, a member of our special research team, chatted with readers about their new discoveries.
Han Wudi Liu Che wrote in "Autumn Wind Ci": "The autumn wind rises, white clouds fly, and the vegetation falls yellow, and the geese return south." Putting aside the sad autumn feelings, we know that the ancients have long regarded wind and vegetation phenology (vegetation yellow falling). ), animal phenology (Goose Returning from the South) are linked together.
The so-called phenology refers to the time during which the cyclic phenomena of different organisms appearing repeatedly in the annual cycle (such as the germination, flowering, fruiting of plants, the migration of migratory birds, the hibernation of certain animals, etc.) in nature. The research on phenology can Let us understand more clearly the impact of climate change on the ecosystem.
For plants, the signs of the arrival of autumn are mainly the discoloration and falling of leaves, that is, the autumn phenology period.
So, what trend has the autumn phenology shown in recent years?
What are the main factors affecting the change of phenological period in autumn?
What role does the change of wind speed play in it?
Researchers Wu Chaoyang and Ge Quansheng from the Institute of Geographic Sciences and Natural Resources Research of the Chinese Academy of Sciences, together with Dr. Wang Jian from Ohio State University and other international scholars, have carried out active exploratory research on the above issues, opening up a new perspective on the relationship between climate change and vegetation phenology.
1 The autumn phenology of vegetation suffers from "procrastination"
Many friends living in Beijing believe that autumn is the most intoxicating of the four seasons of the year.
As soon as autumn falls, all around you are full of bright colors. It is so beautiful to watch the colorful autumn leaves while enjoying the juicy fruits.
However, this good season is not only short-lived, it also comes later and later.
China Weather.com reported in 2020 that through monitoring, Beijing officially entered autumn on September 15, which was 6 days later than the average September 9.
According to big meteorological data, from 1981 to the present, the autumn in Beijing has come more and more late.
In the 1980s and 1990s, the average time of autumn in Beijing was still in early September. Since the beginning of this century, the average date of autumn in Beijing has been postponed to mid-September, and the latest is even delayed to late September.
Except for Beijing, the entire northern autumn is getting shorter and shorter, and many cities suffer from "procrastination" in autumn. For example, Lanzhou has been postponed from August 8 in the 1980s to August 17 in recent years. Hohhot also gradually delayed from August 1 to August 11.
Similar to the late arrival of autumn we feel, the arrival of autumn vegetation is also delayed.
The senescence and shedding of leaves in autumn vegetation is an indicator that vegetation is sensitive to climate change, which is the arrival of autumn, and reflects the ecosystem's response to climate change.
The physiological functions of cells, organs, or the entire plant in plants decline and tend to die naturally. We call it senescence.
Senescence is an active and orderly developmental process controlled by plant genetics. For example, the leaves of deciduous trees in autumn show a variegated color from yellow to red, and then the fallen leaves fall apart.
Vegetation leaf shedding refers to the process of separating plant cells, tissues or organs from the plant body. It is also a biological phenomenon. For example, street trees in northern cities will shed their leaves in autumn, which is an adaptation of plants to the external environment.
Scientists using satellite observations also found that the autumn phenology of vegetation in the middle and high latitudes of the northern hemisphere showed an overall postponement trend.
2 The effect of temperature increase during the day or night is completely opposite
The conditions for vegetation senescence are intricate, mainly including: light, temperature, moisture, nutrition and plant hormones.
Light can reduce the degradation of protein, chlorophyll and RNA, thereby alleviating leaf senescence; low temperature or high temperature may interfere with the operation of calcium, or affect protein degradation and chloroplast function, thereby accelerating leaf senescence; drought in areas with less water will increase Protein degradation increases the rate of respiration, destroys the chloroplast lamella structure, and promotes leaf senescence.
In addition, the lack of nutrients will also promote leaf senescence.
In previous studies, scholars have paid more attention to the spring phenology of vegetation. Meteorological factors such as temperature, rainfall, and radiation are the main driving factors that affect the spring phenology of vegetation, which can effectively explain the changes in vegetation spring phenology (such as the vegetation leaf period).
Compared with the spring phenology of vegetation, the research on the change mechanism of autumn phenology is slightly lacking.
Global temperature rise is a commonplace issue. It is undeniable that temperature rise is one of the most important factors affecting vegetation phenology. However, some scholars have found through research that temperature contributes far less to changes in autumn phenology than spring phenology.
Regarding the increase in global temperature, scholars mainly use the average temperature throughout the day to assess the impact of temperature changes on the phenological period of vegetation, thus ignoring the possible differences in the impact of temperature changes during the day and night.
Starting from this entry point, our research team combined the observational data of different time and space scales to analyze and explore the response of vegetation in the northern hemisphere to the increase in temperature during the day and the increase at night.
Interestingly, the data at different scales have reached the same conclusion: if the increase in the maximum temperature during the day delays the autumn phenology of vegetation, the increase in the minimum temperature at night will advance the autumn phenology of the vegetation.
The study further found that the drought effect caused by temperature increase during the day and night has completely opposite characteristics.
As in common sense, the increase in temperature during the day will increase the degree of drought, while the increase in temperature at night will alleviate the degree of drought.
Such an opposite mechanism to drought provides mechanistic support to explain the opposite effect on the autumn phenological period.
Another important finding of this study is that the existing vegetation autumn phenology model overestimates the vegetation autumn phenology period in the northern hemisphere at the end of this century (2080-2100), especially in the northern arid regions.
Such overestimation further affects scientists' judgments on future climate change.
For example, if the actual autumn phenology of vegetation is half a month earlier than previously predicted, this means that the vegetation will stop photosynthesis half a month earlier, which will increase the concentration of carbon dioxide in the atmosphere.
At the same time, the increased carbon dioxide will further increase the greenhouse effect, causing the temperature to rise further.
Therefore, in the context of global warming, taking full account of the difference between daytime temperature and night temperature change is of great significance for analyzing the response of vegetation to climate change in the autumn phenology period.
3 Weakening wind speed delays the autumn phenology of vegetation
As mentioned earlier, there are many factors affecting vegetation senescence, including light, temperature, moisture, nutrition and plant hormones, but why is it said that wind speed changes the deciduous period of vegetation in autumn?
We know that the evaporation rate of water is mainly affected by three factors: the higher the temperature, the faster the evaporation; the larger the surface area, the faster the evaporation; the faster the air flow above the liquid surface, the faster the evaporation.
Then, under the background of rising global temperature, it can be reasonably inferred that changes in wind speed will affect the moisture of vegetation through evaporation.
The research we conducted learned from multi-scale data (ground observations, flux data, satellite products, etc.) that wind speeds have gradually weakened in the entire high latitude area of the northern hemisphere in the past 30 years.
The weakened wind speed is partly due to the increase in vegetation coverage, and more plants can act as a buffer to reduce the wind speed.
As the wind speed decreases, the evaporation effect of the vegetation canopy will also decrease, and more water will be retained in the vegetation and soil, thus alleviating the drought in most parts of the north.
Research combined with site observations, manual observations and satellite observations have all proved that the weakening of wind speed will delay the autumn phenology of vegetation.
The study further combined soil moisture data and vegetation water content data, which proved that wind speed regulates the evapotranspiration of soil and vegetation, thereby changing the drought conditions for vegetation growth.
A series of effects caused by drought to promote leaf senescence and shedding are reduced. On the one hand, alleviating drought will weaken the protein degradation of vegetation. At the same time, the respiration of plants will also be reduced, and the senescence of natural leaves will also be delayed.
On the other hand, under drought conditions, the deciduous condition of trees is also more serious. This is to reduce transpiration and loss of water, otherwise the trees will gradually wither and die.
Therefore, leaf shedding is an important protective response of plants in response to drought.
When the drought is relieved, the shedding of vegetation leaves is also relieved.
This picture is a schematic diagram.
The data are from 1981 to 2020 when the autumn begins in Beijing, and the statistical site is the Southern Suburb Observatory.
China Weather Network Mapping
4 New discoveries are of great significance to long-term ecological protection
After discovering the important influence of wind speed on the autumn phenology of vegetation, our study further compared the contribution of wind speed, temperature, rainfall and radiation to the interpretation of phenological changes.
Studies have shown that the impact of wind speed on the autumn phenology of vegetation is comparable to the well-known elements of temperature, and is much higher than the impact of rainfall and radiation.
This further emphasizes that the impact of climate change on the growth process of vegetation is complex, and different climate and environmental factors need to be fully considered to enhance the understanding of the driving factors of vegetation phenology.
In view of the declining wind speed, further in-depth exploration of the response and adaptation mechanism of vegetation autumn deciduous period changes to wind speed and improvement of the existing vegetation autumn deciduous period prediction model are of great significance to long-term ecological environmental protection and adaptation strategies to respond to global changes.
Our research integrates wind speed factors to further improve the current mainstream vegetation autumn phenology model and improve the prediction accuracy of the model.
The model predicts that the autumn phenological period of vegetation in the future will be about 12 days earlier than previously predicted.