Abundance and δ¹³C of sedimentary black carbon indicate rising wildfire and C₄ plants in Northeast China during the early Holocene

Highlights

• Black carbon and δ¹³C_BC tracked fire and vegetation history since 34,000 cal yr BP in NE China.

• Fire was strong in the early Holocene and peaked around 7700 cal yr BP.

• C₄ plants were present during the early Holocene.

• Fire and C₄ plants were mainly controlled by temperature-mediated effective moisture.

• Fire risks are projected to increase as human-induced global warming continues.

Abstract

Global warming and human activity are thought to be the primary drivers of the globally rising occurrences of forest fires. Due to the importance of local to regional environmental drivers in mediating fire frequencies, reconstructing fire histories of representative geographic regions is important to understand fire regimes and predict future changes. The present study aims to establish a relatively high-resolution fire record of boreal forests in northeastern (NE) China and identify the primary environmental drivers. We analyzed the concentration and stable carbon isotopic composition of black carbon (BC and δ¹³C_BC) of a 34,000 cal yr BP sedimentary sequence from a crater lake (Tianchi). The results show that fire events and C₄ plants were scarce during the late Last Glacial but increased since the deglaciation. Both fire and the abundance of combusted C₄ plants reached their maxima during the early Holocene, which may be attributed to rising temperatures due to intensified solar radiation and summer monsoons. During mid-to-late Holocene, fire events remained relatively stable and were primary associated with C₃ plants burning. Fire was particularly infrequent and C₃ trees were abundant during the periods of 5500–3100 cal yr BP and 1500–150 cal yr BP, which can be attributed to a wet climate. Fires exhibited evident increases from 150 cal yr BP to the present, suggesting a substantial role of human activities. Both BC and δ¹³C_BC during the Holocene show a significant correlation with pollen-based temperatures but not with precipitation, suggesting that temperature rises played a primary role in stimulating forest fires via mediating effective moisture. Collectively, these findings suggest that forest fires and C₄ plants would likely increase in NE China in response to global warming and increasingly intensive anthropogenic disturbance, highlighting the importance of establishing a long-term understanding of fire regimes to create more sustainable, science-based fire management strategies.

Introduction

Massive forest fires are raging around the world, including the most recent California wildfires in 2018 and 2020, Amazon rainforest fires in 2019, and Australian forest fires that lasted by several months in 2019 (Nolan et al., 2020; Yu et al., 2020). Intensive, large-scale fires lead to life loss and economic damages, as well as posing a significant threat to the ecosystem and biodiversity. Global warming is thought to be the primary cause for forest fires (Coogan et al., 2019; Running, 2006) for leading to warm and dry conditions that are conducive to biomass burning (Zhou, 2005). Human activities also contribute to rising forest fires for increasing the frequency of ignition (Marlon et al., 2013). However, the extent and frequency of forest fires show large spatiotemporal variability that suggests the importance of regional environmental drivers (Marlon et al., 2013), and identifying these drivers requires a robust understanding of evolving fire-vegetation-climate dynamics through geological spans of time.

Black carbon (BC) produced from biomass burning and fossil fuel combustion contains a wide range of carbon-rich compounds (Bird and Ascough, 2012; Masiello and Druffel, 1998). BC is also known as biochar, charcoal, pyrogenic carbon and soot (Bird and Ascough, 2012). Due to the recalcitrant nature to chemical and microbial alterations (Masiello and Druffel, 1998; Schmidt and Noack, 2000), BC is usually well preserved in peats, soil, and lacustrine and marine sediments, and therefore has been regarded as a reliable proxy for reconstructing fire history (Bird and Cali, 1998; Zhou et al., 2007, Zhou et al., 2014, Zhou et al., 2017; Wang et al., 2013; Sun, 2016). The concentrations of BC in sediments are overall positively correlated to fire frequencies, and stable carbon isotopic compositions of BC (δ¹³C_BC) can indicate the type of vegetation contributing to the fire (Bird and Gröcke, 1997; Jia et al., 2003; Zhou et al., 2009, Zhou et al., 2014, Zhou et al., 2017; Sun et al., 2015, Sun et al., 2018; Zhang et al., 2015). The δ¹³C values of biomass undergo a fractionation of −1.6–0‰ during combustion (Bird and Gröcke, 1997), but they do not exhibit significant fractionations after burial (Bird and Gröcke, 1997; Bird and Ascough, 2012; Liu et al., 2013; Sun et al., 2018). The application of sedimentary BC and δ¹³C_BC allows reconstructing past changes in fire, vegetation, and climate, as well as their spatiotemporal coupling at different scales.

Northeastern (NE) China supports some of the most diverse forest ecosystems in northern Asia, and it is one of the highest-fire-risk areas in Asia, with more than 1100 forest fires reported in Heilongjiang province from 2002 to 2017 (Zhang et al., 2018). Situated at the boundary between East Asian summer monsoon and Westerlies, fire and vegetation of NE China are sensitive recorders of climate and environmental changes (Tan, 2019). However, long-term geological records of forest fire and vegetation in this area were established only in a low-resolution chronological framework (e.g., Li et al., 2003; Sun et al., 2018; Wu, 2009), and/or derived mainly from low-lying lake basin deposits whose paleoenvironmental interpretations are often subject to ambiguity due to complex sedimentary sources and post-burial human disturbance (e.g., Li et al., 2005; Li et al., 2012; Sun et al., 2018).

The present study sought to reconstruct a relatively high-resolution fire and vegetation evolution history over ~34,000 cal yr BP in NE China using sediments from a high-elevation crater lake. Crater lakes are considered to be excellent paleoenvironmental archives because they are usually hydrologically isolated (with little inflow or outflow) and thus create a stable sedimentary environment with relatively simple exogenous sedimentary sources (Liu et al., 2010; Liu et al., 2017; Liu et al., 2019; Zhou et al., 2016). Furthermore, they usually have a high elevation and thus relatively are less disturbed by anthropogenic sources and activities (Liu et al., 2019). Here, we establish a complete record of fire history of NE China since the late Last Glacial (after ~34,000 cal yr BP) using BC and δ¹³C_BC of sediments from the Tianchi Crater Lake. Our results provide new insight into the role of vegetation vs. climate in regulating forest fires at different temporal scales, laying the groundwork to further improve our abilities of managing and predicting fires in mountainous boreal forests.