Responses of net primary productivity to phenological dynamics in the Tibetan Plateau, China
Introduction
Since the 1950s, global mean air temperature has increased by 0.6 °C, and the warming trend was more rapid at higher latitudes in the Eurasian continent (IPCC, 2007). Vegetation phenology is highly sensitive to climate change, as a result of the recent increase in surface temperature, earlier spring phenological events have been observed in the high latitudes (Piao et al., 2007, Geerken, 2009, Jeong et al., 2011). Changes in plant phenology will affect future ecosystem structure and functions since they affect surface energy exchange, water cycling, and terrestrial carbon cycling (Richardson et al., 2010, Steven et al., 2011). Increasing attentions have been paid in how these phenological shifts may affect net primary productivity (NPP) (Day and Monk, 1977, Richardson et al., 2010), which is an important indicator of an ecosystem’s potential for storage of nutrients and energy. So improving our ability to accurately describe the response of NPP to phenological shifts will advance our understanding of terrestrial ecosystems respond to global change.
The Tibetan Plateau (TP), the highest plateau of the earth, located in the central part of the troposphere in the mid-latitude westerlies, is regarded as the Earth’s third pole. The mean annual temperature on the plateau is only 1.6 °C, more than 60% of the plateau is covered by alpine grasslands (alpine steppe and meadow). The TP has a large variety of ecosystem types, from subtropical rain forest in southeast to alpine desert in the northwest. TP is one of the most sensitive regions to climate change, over the past three decades the plateau has experienced significant warming trends (0.13 °C/year in winter, 0.04 °C/year in annual mean) (Liu and Chen, 2000, Du et al., 2004), and this warming is predicted to continue in the 21st century (IPCC, 2007). Thus, the plateau’s ecosystems and natural environment are inherently fragile and instable, making them especially vulnerable to global warming and leading diverse phenological responses to warming across the plateau. In recent years some studies applied time-series of the satellite-derived vegetation index (VI) to investigate phenological events and their connection to climate change in the TP (Piao et al., 2011, Shen et al., 2011). However, these studies revealed that the timing phenological events under the current climate warming differed in magnitude and even in direction (i.e., advance vs. delay in the date). Especially, due to the lack of ‘in situ’ observing data, vegetation phenology change and its linkage with NPP over the plateau remain poorly understood.
In this study, we aimed to investigating the temporal trends and spatial variability of vegetation phenology and NPP dynamics, as well as examining the mechanisms of NPP response to phenological shifts in the Tibetan Plateau during 2002–2012. Our purposes were to address the following key scientific questions: (1) What are the vegetation phenology and NPP pattern over the Tibetan Plateau during the recent decade? (2) How do relationships between vegetation phenology and NPP across the plateau? And (3) What are the driving mechanisms of NPP and phenological changes, especially, how do the driving factors control NPP and do the effects of phenology on NPP? Answers to these questions will improve our understanding of the relationships between vegetation phenology and NPP, help to quantify the magnitude and direction of NPP changes caused by phenological shifts, and thus provide an important scientific basis for the sustainable development.
Section snippets
NDVI Dataset from MODIS
The Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Terra and Aqua satellites provide near-daily repeated coverage of the earth’s surface with 36 spectral bands and a swath width of approximately 2330 km. For this study we selected the MOD 13C1 for vegetation-related information extraction. The MODIS products are distributed through NASA’s Earth Observing System Data and Information System (EOSDIS). The MODIS product (MOD 13C1), with a 0.05° spaital resolution and 16-d
Spatial patterns and interannual variations of phenology and NPP
The spatial patterns of phenological metrics (BGS, EGS, LGS) and NPP were depicted in Fig. 4(a–d). The BGS date gradually decreased from west to east in the TP, while LGS and NPP gradually increased from west to east. These characteristics reflected the spatial heterogenity of climate and terrain, and are in accordance with the gradients in humidity, temperature and elevation in the plateau. The BGS dates ranged from approximately 111 days for forests to approximately 122 days for steppe; EGS
Role of climate in NPP response to phenological metrics
To examine the role of climate factors in NPP response to phenological changes, we further analyzed the relationship between trends in NPP, phenology and climate within different temperature and precipitation zones determined based on historical climate data. We divided the TP into over 336 hydrothermal zones. The procedures to generate these hydrothermal zones are conducted in two steps. The first step is to divide the plateau’s rainfall into 24 rainfall zones along the annual average
Conclusion
Considering the importance of quantifying long-term phenological changes and its effects on NPP in the context of global climate changes, this study investigated the spatiotemporal variability of vegetation NPP and phenological dynamics in the TP, and further examined the response mechanisms of NPP to phenological dynamics. During 2002–2012, most of the plateau experienced significant trends in advancing of BGS, delaying of EGS and prolonging of LGS, and vegetation NPP also substantially
Acknowledgements
This study was supported by the Natural Science Foundation of China (Grant No. 41271426, No. 41428103 and No. 91547107), National Basic Research Program of China (No. 2011CB707100), and “1-3-5 Project” of Chinese Academy of Sciences. The authors are very grateful to Prof. Hanqin Tian and the anonymous reviewers for their constructive and critical comments on this manuscript.
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