Three-dimensional dynamic characteristics of vegetation and its response to climatic factors in the Qilian Mountains

Highlights

• The increase trend of NDVI in the northwest is greater than that in the southeast.

• NDVI fluctuated greatly and the growth rate was greater at low-altitude.

• Vegetation in the QLMs is mainly affected by the time-accumulation of PRE and the time-lag of TMP.

• Compared with PRE, TMP was the dominant factor of the greening in the QLMs.

Abstract

Understanding the trend of vegetation change and its reaction to climate variation is important for revealing the mechanism of ecosystem behavior. However, current research rarely systematically analyzes the time effects of climate variation on vegetation dynamics (time-lag and time-accumulation effects), especially in arid and semi-arid mountainous terrain. The typical mountainous terrain—the Qilian Mountains was taken as the study area, and the spatiotemporal changes and vertical zonality distributions of the normalized vegetation index (NDVI) were explored. This study explored the time-lag and time-accumulation effects of the NDVI response to climate factors (precipitation, temperature), identified the main controlling factors that influence the variation of NDVI. The results show that in the growing season from 2000 to 2019, the NDVI represented an overall upward trend, especially in the northwest, and the growth rate of NDVI at low-altitude was greater. The time-accumulation effect of precipitation has an obvious effect on vegetation, especially on deserta and meadow; and the time-lag and time-accumulation effects of temperature have an obvious influence. Regarding the climate-vegetation response mechanism, this study finds that considering the optimal time effect is of great significance. In addition, compared with precipitation, the temperature has a more significant promotion effect on vegetation growth in the Qilian Mountains. The above results indicate that when the existing climate models study vegetation-climate interactions, considering the time effects of vegetation response to climate is of great significance for accurately monitoring vegetation dynamics under environmental changes.

Introduction

Vegetation, as an important part of the earth, plays an indispensable role in providing organic matter for terrestrial organisms, regulating the carbon cycle and promoting energy exchange (Guan et al., 2018). Climate change will directly affect vegetation growth; on the other hand, vegetation changes will also feedback to climate change by regulating water, exchanging energy and carbon dioxide concentration (Bonan, 2008, Craine et al., 2012, Wang and Alimohammadi, 2012). In recent years, climate change, especially global warming, has caused frequent events, such as flooding, high temperature, and drought, which would cause harm to terrestrial ecosystems (Zhou et al., 2014). The increasing warming and humidification of the high-altitude climate over time also affects the growth of vegetation (Liu and Chen, 2000, Palazzi et al., 2016, Yao et al., 2018). As a consequence, it is necessary to understand the vegetation behavior mechanisms, that is, to study the variations and the reaction to climate dynamics (Zhou et al., 2018, Piedallu et al., 2019, Zhao et al., 2019, Piao et al., 2020).

When climate change exceeds the capacity of vegetation, vegetation will react with a feedback mechanism (Wang et al., 2012), that is, the dynamic reaction to climate variation may not be co-instantaneous, and there may be a time-lag or time-accumulation effect between them (Wen et al., 2018). The time-lag effect refers to the influence of climatic conditions occurring before a specific time on the current vegetation growth, that is, the vegetation growth is significantly affected by climatic conditions in a previous month (Braswell et al., 1997, Wu et al., 2015, Chen et al., 2014). The time-lag effects are different because of the different vegetational forms and climatic factors. For example, the lag time of AT_max (monthly mean maximum temperature) on the plant communities in the middle and high latitudes (Tibetan Plateau and Brazilian Plateau) is relatively long (>12 months) in the global terrestrial ecosystem (Wen et al., 2019). The best lag time of EVI (Enhanced Vegetation Index) response to different factors (precipitation, radiation) is also different (2 months and 1 month, respectively) (Anderson et al., 2010). The time-accumulation effect represents that based on the time-lag effect, vegetation growth is significantly affected by the cumulative climatic conditions of the previous month and this month over a period of time (Vicente-Serrano et al., 2014, Ding et al., 2020). It is found that in semi-arid areas, the effects of accumulated temperature and water for a period of time on carbon exchange of plants, soils and ecosystems may be more significant than the current climate conditions (Shim et al., 2009). Vegetation dynamics are mainly affected by soil respiration (Janssens et al., 2001), the gas exchange of plants (Patrick et al., 2009), and ecosystem productivity (Reichmann et al., 2013).

In recent years, most studies have only linked NDVI to the time-lag effects of climate during the growing season. For example, Kong et al. (2020) found that the temperature and precipitation on the Loess Plateau have the longest lag time for the NDVI of the grassland. Li et al. (2021a) revealed that the time-lag effect between NDVI and climate factors is significantly different under different land types in plateau areas. However, the above researches mainly focused on the influence of precipitation and temperature on vegetation growth in a single month in the past, without considering the influence of accumulated precipitation and accumulated temperature in the previous months, and failed to fully capture the relevant characteristics of vegetation growth and climatic factors. In fact, the lag and accumulation of time effects of the vegetation response to climate variation coexist (Guo et al., 2017, Guo et al., 2019), and it is always better to consider the lag and cumulative effects at the same time than to consider one effect alone (Ding et al., 2020). Especially in arid and semi-arid areas, most studies focus on the effect of climatic factors on vegetation during the same period (Yuan et al., 2019, Zhu et al., 2020), or only the time-lag effect is considered (Ning et al., 2015), and there is a lack of research considering the combined effect of time-lag and time-accumulation effects.

The Qilian mountains are widely considered an important ecological security barrier and important water source-producing region in Western China. Its ecological environment changes are closely related to the local and even western China's ecological security construction (He et al., 2018b). In recent years, due to the combined effects of global warming and human activities, vegetation in the Qilian Mountains has undergone obvious fluctuations (Hao et al., 2018, Gao et al., 2019b). In particular, meadow ecosystems are more fragile and more susceptible to the change of external conditions because of the cold and dry environments, thus affecting the stability of plateau ecosystems (Zeng et al., 2013, Li et al., 2014). On account of this, this study aims to ascertain the temporal and spatial dynamics of vegetation in the study area and reveal its response to climate variation. The research aims were: (1) to analyze the spatiotemporal changes of NDVI during the growing season and explore its vertical distribution characteristics; (2) to explore the time effect of climate on vegetation factors (time-lag effect, time-accumulative effect, comprehensive effect); (3) to quantitatively analyze the relationship between vegetation change and climate factors (temperature, precipitation) on the basis of the determined time effect; (4) to analyze the response of different climatic factors to vegetation and identify the main controlling climate factors on vegetation change.