1. Introduction
The IPCC (Intergovernmental Panel on Climate Change) Fifth Assessment Report states that most parts of the world are experiencing climate change, especially sustained warming (the global average surface temperature increased by 0.72 °C from 1951 to 2012) [
1,
2]. Climate change includes meaning climate change and extreme climate change [
3,
4]. Some studies have shown that the impacts of extreme weather events are more obvious and direct than climate averages with increased frequency and intensity of extreme climate events [
2,
5]. Based on global climate change, more and more scholars devote themselves to research the dynamics of vegetation and its response to climate change [
5].
As an opportunity to study seasonal changes in vegetation, status vegetation phenology not only determines the length of vegetation canopy photosynthesis but also plays an important role in the carbon balance of terrestrial ecosystems [
6,
7,
8]. Climate change is one of the main drivers of vegetation growth, the effect of extreme climate changes vegetation dynamics appears to be more pronounced than climate change mean state [
9,
10]. The occurrence of extreme events can disrupt vegetation activity, advancing the flowering period of some species or even causing some species to fail to complete their flowering cycle [
11]. Some researchers studying the Tibetan Plateau found that the end of the growing season (EOS) of vegetation is more influenced by extreme temperature events than extreme rainfall. An increase in the heat index of extreme temperature events leads to a delay in vegetation EOS, whereas an increased cold index can lead to an early end to vegetation EOS [
12], with warmer days and nights similarly causing delayed fall phenology in Inner Mongolia [
13]. A study of vegetation phenology in the United States found that an increase in daily maximum temperature had its main influence on an earlier vegetation start of the growing season (SOS) [
14]. By studying the effects of drought, frost, heat, and humidity on fall phenology in Greenland, Xie et al. found that an increased cold index caused forests to end the growing season early, while increased heat index and drought stress delayed the dormancy of vegetation [
15]. This shows that extreme climate events have a greater impact than averaged climate values on vegetation phenology in climate-change-sensitive areas. Therefore, the occurrence of extreme events may alter vegetation dynamics and carbon balance, which may lead to a shift from carbon sinks to carbon sources, such as the European heatwave event in 2003, Hurricane Katrina in 2005, and the 2019 Australian bushfires [
16,
17].
The Loess Plateau is located in the arid and semi-arid region of Northwest China, with a fragile ecological environment and pronounced sensitivity to climate change, so it is very important to study the response of the Loess Plateau to climate indices [
18]. Previous scholars quantified the phenology of the Loess Plateau based on different types of data and found that the advancement of vegetation SOS and delay of vegetation EOS on the Loess Plateau in recent years were jointly regulated by mean temperature and total annual precipitation [
19,
20]. Recent studies have pointed out that there is a significant correlation between the normalized difference vegetation index (NDVI) and extreme temperature index in the Loess Plateau region [
21]. However, these studies were either limited to the study of mean climate effects on phenology or to the analysis of vegetation response to climate extremes, whereas the sensitivity of vegetation community-scale phenological changes to extreme climates has been rarely studied at the Loess Plateau. The Loess Plateau forms an obvious environmental gradient with its unique geographical conditions. Most studies have shown that the total annual precipitation, annual average temperature, and extreme precipitation indices of the Loess Plateau show a decreasing trend from southeast to northwest [
3,
18,
22]. The resulting spatial heterogeneity of energy and water distribution also leads to a distinctive distribution pattern of vegetation, which is low in the northwest and high in the southeast, with the transition from northwest to southeast being mainly from grassland to forest. Therefore, it is important to understand the key factors regulating vegetation phenology and the response of different vegetation types to extreme events in arid and semi-arid areas.
In this study, we mainly used the SOS and EOS data from 2001 to 2018 and seven extreme climate indices to analyze the spatial differentiation of different vegetation phenology’s sensitivities to climate indices. The purpose of this study was to analyze the temporal and spatial changes in vegetation phenology, the spatial and temporal changes of extreme climate indices, and the sensitivity of the response of vegetation phenology to climate extremes. The present study will provide a scientific basis for local response to extreme climate events, disaster prevention and mitigation, and ecological environment protection.
4. Discussion
The Loess Plateau is the largest loess region on the earth. Its climate is characterized by drought, little rain, strong wind, and more sand, which leads to dusty and sandy weather in this region and its surrounding areas [
33]. In the past two decades, the government of the People’s Republic of China has taken a series of measures to improve its fragile ecological environments, such as returning farmland to forest, sealing sand for forest and grass cultivation, and vegetation coverage has been effectively improved [
34]. This paper indicates clearly that SOS advanced and EOS delayed, and the growth period of the whole vegetation showed an increasing trend. Moreover, in the context of climate change (increasing temperature, increasing precipitation, and frequent extreme events), vegetation phenology on the Loess Plateau has undergone significant changes [
19], and phenology also affects climate change to a certain extent [
35]. For example, when Chang et al. [
36] studied the relationship between the frequency of sandstorm and air humidity as well as plant phenology in the Minqin desert area, they believed that the wet and warm climate, vegetation SOS advance, and vegetation EOS delay could reduce the frequency of sandstorm days.
Most studies have shown that extreme climate events have an impact on vegetation phenology. Extreme temperature events affect the growth and development of vegetation through effects on the activities of photosynthetic and respiration enzymes [
15,
37]. Extreme precipitation events restrict growth by affecting the water content or availability in the soil in which the vegetation is growing [
38]. Piao et al. found that vegetation in northern areas was more affected by high temperatures than by precipitation [
39]. Through experimentation, Nagy et al. found that phenology was affected by extreme climate indices [
13], observing that the flowering date of temperate vegetation was nearly one month earlier, and some vegetation species could not complete their flowering cycles, but the phenology of the vegetation studied was not affected by heavy precipitation events.
In the current study, we found that both extreme temperature events and extreme precipitation events affected vegetation phenology, with the sensitivity of phenology to extreme temperature events being greater than that to extreme precipitation. The possible cause of this finding is that the Loess Plateau is in an arid and semi-arid region. Vegetation has adapted to the extremely dry environment, resulting in a weak relationship between vegetation phenology and extreme precipitation events. Although, to some extent, phenology is affected by heavy precipitation events [
15]. Among the three vegetation types studied in this paper, the autumn forest phenology responds to the influence of the RX5day index by ending the early growing season. The possible reason is that forest vegetation sites have soil with higher soil water contents than those sites supporting shrubland and grassland, and further increases in more water input will form an anaerobic environment in the plant rhizosphere, thus preventing the growth of vegetation.
The sensitivity of the response of the phenology of different vegetation types to extreme climate events is complex. We found that the sensitivity of phenology to the DRT index was negative. In other words, when the daily temperature difference causes the advance of vegetation phenology in the spring, it will indirectly lead to the early end of the fall vegetation growing season. The possible reason is that the rates of increase in TXx and TNx indices were higher than those of the TXn and TNn indices in the years under study. Unusual warming events may prematurely induce plant activity, but this situation is not conducive to the growth of vegetation, as it would lead to the subsequent early end of the vegetation growing season, thus shortening the vegetation growth cycle because plants need exposure to lower temperatures before dormancy ends. Another possible reason is that when vegetation encounters environmental stress, its own constantly changing signals, mainly including hormones and signal molecules, will control phenology, namely germination, leaf senescence, and growth stagnation. Vegetation ends its growing season early to avoid damage to itself caused by environmental stress. In addition, the seasonal trajectories of vegetation activities are likely to be more sensitive to extreme climates [
40,
41].
The high temperatures in the spring of the ecosystem of arid regions in Australia will seriously delay or blocks phenological cycles [
42]. Siegmund et al. found that high temperature usually had a strong negative effect on flowering in four shrub species, a finding which was similar to the sensitivity response of spring phenology to extreme temperature [
5]. The TNn and TXn indices had a significant delaying effect on fall phenology delay in the Tibetan Plateau, while Hong et al. considered that warm days and warm nights were the main factors affecting the phenological delay of vegetation in the fall [
15]. The above results differ from our findings in that fall forest, shrubland, and grassland phenology were all delayed by TXx and TNn indices. Although the evidence that phenology is affected by extreme weather is increasing, with the increasing frequency of extreme events [
2], vegetation phenology in sensitive areas is also increasingly affected by it. The focus of future research should become how to deal with the significant impact on ecosystems caused by extreme events.
In addition, vegetation phenology is both a periodic and a continuously dynamic process, which is restricted by numerous influencing factors, which interacted with one another [
25]. Previous studies have generally believed that climate change is the most important factor affecting vegetation phenology [
19,
43]. In this current study, we also focused on the phenological change affected by extreme climate indices, but, in the process of analysis, we found that the advance of vegetation SOS by extreme climate indices may indirectly lead to the advance of vegetation EOS; after the vegetation EOS is affected by extreme climate in the current year, it may also lead to changes in vegetation SOS in the following year. Therefore, in this paper, we carried out makes a correlation analysis between vegetation SOS and vegetation EOS. Our results showed that there was a correlation between vegetation SOS and vegetation EOS on the Loess Plateau (
Figure 10), with the areas with significant correlations being mainly distributed in the areas with the high correlation coefficients. The reason for this result may be that vegetation needs its own stored carbohydrates and nutrients with one another growth and development, and that delay in vegetation EOS may cause a longer storage period in which to store more reserves [
44,
45]. These reserves can then be supplied to power shoot development and growth of vegetation in the second year, thus promoting the advance of vegetation SOS [
46]. Of course, these explanations are only from the perspective of nutrition. The exact contribution between vegetation SOS and vegetation EOS is difficult to define, and there is no direct evidence to prove any relationship between vegetation SOS and vegetation EOS on the scale of the community ecosystem. Therefore, this is an open question, which requires exploration of the mechanism of the interaction between large-scale vegetation SOS and vegetation EOS and the phenological differences among secondary vegetation types, in combination with remote sensing and vegetation nutrition.