Tree-Ring Inferred Drought Variations in the Source Region of the Yangtze, Yellow, and Mekong Rivers over the Past Five Centuries

Located in the northeastern Qinghai-Tibetan Plateau (NE-QTP), the source region of three major rivers (i.e., Yangtze River, Yellow River, and Mekong River) is an important component of the “water tower” of East Asia [1]. This region is also an important water resource reserve and ecological barrier in China and even in East Asia, which is characterized by rich alpine biological resources, a fragile ecological environment and sensitivity to climate change [2,3]. Changes in climate and its extremes over this region not only influence local ecosystems, environment and water resources, but also the security of food, energy and water in the downstream areas [4]. Therefore, there is an urgent need to conduct more scientific study on conservation techniques in this region because of its unique and significant strategic position with respect to national ecological security.

Climate change in this region exerts significant impacts on water resources not only at the regional scale but also on lower reaches of the rivers, thus influencing the life of a dense human population [5]. Both the regional climate and ecosystems show significant changes over the NE-QTP region due to global warming [6,7]. The rate of warming in this region from 1982 to 2015 was four times the global warming rate, which would accelerate vegetation growth in the region [8]. Both ecological restoration and warming climate change favorably impact regional runoff in the NE-QTP region [9]. It was reported that the water resources and precipitation in this region significantly declined in the 1990s, but returned to above normal during the last decade [10,11]. However, other studies reported that the regional runoff showed a continuous decreasing trend during recent decades [12,13], yet different spatial and seasonal characteristics were reported [14]. Historical changes in climate and ecology were found to cause significant reductions in mean and high flows over the Yellow River headwaters during 1979–2005, which potentially increased drought risk over its downstream areas [15]. Changes in temperature and precipitation alter the regional water cycle and ecological environment of the NE-QTP region, which have changed the spatial patterns of ecological services [16]. Recent research suggests that the NE-QTP region will become warmer and wetter in the future. Extreme precipitation will also increase at the 1.5 °C global warming level [17]. These reveal that the frequency and magnitude of hydroclimate extremes have increased in the context of global warming. However, how the hydroclimate extremes would respond to the 1.5 °C/2 °C warming, and how much ecological factors contribute are still unknown. These observations were short in length, making it difficult to obtain the full spectrum of regional hydroclimate variability. Thus, it is imperative to put the recent changes into the context of a longer time scale to better understand its temporal patterns of variability.

Reconstructions of past climate by high-resolution proxies, such as tree rings and ice cores, are useful approaches for evaluating present conditions in the context of the long-term past [18,19,20]. Several studies based on tree rings have been conducted to investigate the drought–wetness variations on the NE-QTP. Gou et al. (2007) [21] reconstructed the streamflow variations of the Yellow River over the past six centuries and found an increasing trend during much of the twentieth century but decreasing after the 1980s. It was also reported that precipitation has undergone a slowly increasing trend over the past five centuries in the middle of Qilian Mountain, and the probability of intensified rainfall events seemed to increase during the past two centuries [22]. Ham et al. (2023) [23] found that the variability of precipitation became larger under the ongoing climate warming. Nevertheless, a drying trend in recent decades was detected from the stable carbon isotope record of Qinghai spruce [24]. Zhang et al. (2011) [25] also pointed out that drought events became more frequent during the 20th century. Reconstructed precipitation over the past 1000 years in Delingha exhibited a decrease in magnitude after 1850 [26]. Liu et al. (2021) [27] found that the reconstructed runoff in the source region of the Yangtze River had periodic changes with multiple time scales of quasi 16–32 years and significantly correlated with the North Atlantic Oscillation (NAO) and Pacific Decadal Oscillation (PDO). However, the possible causes of hydroclimate extremes in the NE-QTP region are still not fully understood. To date, it remains unclear as to the long-term moisture variability and its potential causes, possibly due to the spatial complexity of the study areas.

In this study, we attempted to (1) establish a regional chronology based on tree rings sampled in the source region of the three great rivers (hereafter TRS-region for short), (2) reconstruct the regional drought–wetness variations for the past five centuries, and (3) check whether the frequency and severity of drought events have increased in the context of the long past, and identify their potential causes. The remainder of this paper is organized as follows: the materials and methods are introduced in Section 2, the results are presented in Section 3, the discussions are described in Section 4, and conclusions are provided in Section 5.