Thermal growing season and response of alpine grassland to climate variability across the Three-Rivers Headwater Region, China
Graphical abstract
Introduction
The thermal growing season, which refers to the entire period during which plant/vegetation growth can occur (Carter, 2008, Linderholm, 2006), is closely correlated with changes in temperature. It is widely recognized that the changes in the thermal growing season is a highly sensitive indicator of the terrestrial ecosystem response to climate change (Shen et al., 2012), and previous studies have suggested that a climate-induced change in the length of the thermal growing season (tGSL) has already occurred (Schwartz et al., 2006, Walther et al., 2002). The changes in tGSL can also regulate climate change through the exchange of carbon and water with the atmosphere and variations in albedo (Liu et al., 2014). Currently, tGSL is of substantial interest, as any variation in this parameter can highly affect ecosystem function and carbon sequestration, possibly resulting in changes in the amplitude of the annual cycle of CO2 (Goulden et al., 1998, Keeling et al., 1996, Myneni et al., 1997, Piao et al., 2007, White et al., 1999). Therefore, the indices of the thermal growing season, such as start (tGSS), end (tGSE), and tGSL, are important parameters for terrestrial ecosystems, and their study is important for investigating climate change.
During the recent decades, an increasing number of studies described the changes in tGSL across many regions of the world (Feng and Hu, 2004, Linderholm et al., 2008, Liu et al., 2010, Menzel, 2003, Peñuelas and Filella, 2001, Sparks et al., 2009). A significant extension of the thermal growing season was observed throughout major parts of the Northern Hemisphere mid-latitudes during the 20th century, associated with increasing temperatures (Frich et al., 2002). However, large discrepancies exist in the magnitudes and trends of tGSL reported in different studies, owing to differences in species, study areas, study periods, and data sets. Linderholm et al. (2008) reported a general increase in tGSL in the Greater Baltic Area, while the most considerable change was recorded in the spring. In China, Liu et al. (2010) found that tGSL increased by 6.9–8.7 d between 1955 and 2000, mainly due to the earlier spring. Similarly, a study in China indicated that tGSL increased by 2.3 d per decade in northern China between 1951 and 2007, mainly due to the earlier tGSS (−1.7 d per decade) (Song et al., 2010). Furthermore, regional scale studies based on meteorological data from high altitudes have found a significant positive trend in tGSL (3.29 d per decade), caused by an earlier tGSS (Dong et al., 2012). Several recent studies have reported the different role of tGSS and tGSE in the extension of tGSL in the Northern Hemisphere, with a combination of earlier tGSS and delayed tGSE in Eurasia, and a significantly delayed tGSE in North America (Barichivich et al., 2012). These studies helped us to better understand the changes in tGSL across different scales, investigation periods, and data sets.
It should be noted that numerous studies have investigated tGSL (Liu et al., 2006), mainly focusing on the spatiotemporal variations of the thermal growing season (Dong et al., 2012, Linderholm et al., 2008, Song et al., 2010) or the possible mechanism underlying the changes in the thermal growing season (Irannezhad and Kløve, 2015). However, few studies have reported the changes in the pattern of tGSL or the variability in the trend of tGSS and tGSE during different periods in the recent decades. Additionally, the relationship between the thermal growing season and the response of alpine grassland to climate variability (expressed by actual growing season) that significant influences the net carbon uptake in terrestrial ecosystems is also not fully understood. A previous study investigated the relationship between the thermal growing season and the timing of biospheric carbon uptake, reflecting the actual growing season and found that the extension in tGSL did not lead to an extension in the period of biospheric carbon uptake (Barichivich et al., 2012). Thus, the relationship between the thermal growing season and the actual growing season needs further investigation, in order to acquire more robust conclusions.
In the recent decades, a significant warming trend was observed on the Tibetan Plateau (Zhang et al., 2013), which is known as the Earth's third pole and is regarded as a very sensitive area to climate change (Liu et al., 2008). Meanwhile, the Tibetan Plateau is the highest and largest plateau on Earth and has significant spatial differences in vegetation types and climate conditions. Thus, studies on the relationship between the thermal growing season and the response of the actual growing season to climate variability in sub-regions of the Tibetan Plateau are needed. The primary objective of this study was to investigate the spatiotemporal pattern and trend of the thermal growing season across the Three Rivers Headwater Region (TRHR) between 1960 and 2013, and further discuss the response of alpine grassland to climate variability. To achieve this goal, we first investigated the spatiotemporal variability of the thermal growing season based on air temperature. Then, we used the partial least squares (PLS) model to explore the response of the alpine grassland actual growing season to climate variability using field-observed phenology data records from three agro meteorological stations.
Section snippets
Study area
The TRHR located in the hinterland of the Tibetan Plateau is the watershed of the Yangtze, Yellow, and Lancang Rivers (Fig. 1). The region is known as the ‘Chinese water tower’ because of its high average altitude of more than 4000 m a.s.l. (Liu et al., 2006, Zheng, 1996). Generally, the annual mean temperature in the TRHR is less than 10 °C, and a negative trend of precipitation exists from southeast to northwest. The main vegetation type is alpine grassland, including meadow and steppe (Liu et
Changes in temperature
Both the mean annual and seasonal temperature showed a significant positive trend across the TRHR between 1960 and 2013, while the highest change occurred between 1986 and 2013. During the latter period, the most significant increase in temperature occurred in the winter (1.03 °C per decade, p < 0.01), followed by the summer (0.78 °C per decade, p < 0.01) and spring (0.73 °C per decade, p < 0.01), while the increase in the fall was the lowest (0.57 °C per decade, p < 0.01) (Fig. 2). Spatial differences in
Role of tGSS and tGSE in extension of tGSL
Temperature variability resulted in significant changes in the thermal growing season indices. Previous studies have documented that the extension of tGSL is mainly attributed to earlier tGSS in Eurasia and delayed tGSE in North America (Piao et al., 2007). For instance, Linderholm et al. (2008) reported an increase in tGSL, mainly attributed to an earlier tGSS, in the Greater Baltic Area between 1951 and 2000, using mean daily temperature. Similar results were also obtained in northern China (
Conclusions
We found a significant extension in the thermal growing season in the TRHR between 1986 and 2013. This change was the combination of earlier tGSS and delayed tGSE that were weakened between 2000 and 2013 compared to the period between 1986 and 1999, results that were in agreement with the changes in seasonal temperature. Our results also suggested that earlier aGSS was associated with the increasing winter and spring temperature, earlier aGSE with the increasing summer temperature, and delayed
Author contributions
Xianfeng Liu and Yaozhong Pan designed research; Xianfeng Liu performed research; Xianfeng Liu, Yaozhong Pan Xiufang Zhu, Wenquan Zhu, Jinshui Zhang and Donghai Zhang analyzed data; Xianfeng Liu, Yaozhong Pan, and Xiufang Zhu wrote the paper.
Conflicts of interest statement
The authors declare no conflict of interest.
Acknowledgments
This work was supported by the Major Project of High Resolution Earth Observation System, China.
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