**1. Introduction**

Vegetation phenology refers to periodically recurring growth patterns [1], and sheds a unique light on how ecosystems respond to climate change [2–4]. Shifts in phenology trends can affect the carbon budget, water flux, and energy balance from a regional to global scale [5]. Regional warming in alpine regions has led to several significant phenology changes, including advancement of the start of the growing season (SOS) in spring and a delay of the end of season (EOS) in autumn, as well as extensions of the growing season [6]. Phenology changes in turn provide strong feedback to climate systems, which can affect the regional carbon and water cycles [7]. The advancement of SOS and its

**Citation:** Yang, Y.; Qi, N.; Zhao, J.; Meng, N.; Lu, Z.; Wang, X.; Kang, L.; Wang, B.; Li, R.; Ma, J.; et al. Detecting the Turning Points of Grassland Autumn Phenology on the Qinghai-Tibetan Plateau: Spatial Heterogeneity and Controls. *Remote Sens.* **2021**, *13*, 4797. https://doi.org/ 10.3390/rs13234797

Academic Editors: Alfredo Huete, Xuanlong Ma, Jiaxin Jin, Xiaolin Zhu, Yuke Zhou and Qiaoyun Xie

Received: 12 October 2021 Accepted: 23 November 2021 Published: 26 November 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

controls have been detected in numerous satellite data and observations [8,9]. However, emerging evidence has shown that autumn phenology may contribute more strongly to the growth season length extension than spring phenology, leading to an increase of biomass accumulation [10,11]. Autumn phenology plays a critical role in carbon and nitrogen cycling [12]; it is thus important to track the spatial dynamics of autumn phenology to obtain more accurate information regarding growth season length variations and improve the modeling of biochemical cycles at vegetation-climate intersections [13,14].

With its distinctive geographical and hydrothermal condition, the Qinghai-Tibetan Plateau is regarded as one of the planet's most vulnerable alpine and cold ecosystems because of its strong sensitivity to climate change and has thus become a hotspot of international research [15]. Some degree of consensus has been achieved in recent decades regarding EOS changes on the Qinghai-Tibetan Plateau. Previous studies have shown an overall lengthening of the growing season and extension of the EOS on the Qinghai-Tibetan Plateau due to the warming and increasingly humid climate [16,17]. Studies based on a limited number of phenological observations reported that the EOS exhibited advancement trends on a regional scale [18]. Moreover, EOS changes have significantly affected the gross primary productivity (GPP) and evapotranspiration (ET) of alpine and cold ecosystems [15]. Some evidence has demonstrated that EOS is not only controlled by climate conditions and human activities [19,20] but also depends on the previous growth stage (i.e., SOS, annual peak growth time) [21,22], which make the EOS variation controls complex and difficult to constrain. Additional studies are therefore required to more clearly reveal the mechanism of EOS changes.

The major challenge of EOS studies arises from the poorly understood control mechanism. Previous studies have recognized that warmer temperatures and inadequate autumn solar radiation enhance vegetation growth [22,23]. Daytime and nighttime temperatures have different impacts on the alpine grassland EOS. However, the effects of higher preseason precipitation or longer sunshine duration on the EOS changes remain unclear [6]. The intersection of a wide variety of climate variables complicates this interpretation. Furthermore, some evidence has shown that human activities (especially grassland grazing) can alter vegetation phenology [24,25], but the superimposed effects of ecological protection and grazing make this effect difficult to quantify.

Recent advances in time-series analysis have demonstrated that ecosystem status changes are gradual but ultimately lead to qualitative changes [26]. The concept of turning points has opened a new research direction of ecosystem status change. Land cover changes, extreme climate, and human disturbances often occur abruptly and can result in ecosystem status changes [27,28], whereas increasing human pressure or grazing may more gradually change the ecosystem. Some previous studies demonstrated that the trend rates of EOS changes tend to vary over long periods, whereas turning points (sometimes referred to as breaking points) are more distinct, with different change rates occurring before and after these points [29]. A turning point of the Qinghai-Tibetan Plateau has traditionally been defined in the year 2000 or the entire study period is taken as a whole [20], but notable variations can be detected at the pixel-scale, which have not been previously reported.

This paper investigates the Qinghai-Tibetan Plateau as a study area to (1) detect the existence of EOS turning points in different subregions, (2) quantify the determined climatic factors before and after the turning points, and (3) explore the contribution of climate change and human activities (grazing, economic development) to the EOS turning points. The detection of EOS turning points at the pixel and regional scale not only enriches the understanding of the EOS controls on alpine and cold grassland but also provides further details to reveal the EOS change mechanisms over different periods and their controls on the Qinghai-Tibetan Plateau.

#### **2. Materials and Methods**
