Communication Asymmetrical Lightning Fire Season Expansion in the Boreal Forest of Northeast China Cong Gao 1, Ran An 1, Wenqian Wang 1, Chunming Shi 1,* , Mingyu Wang 2,*, Kezhen Liu 3, Xiaoxu Wu 1, Guocan Wu 1 and Lifu Shu 2 1 College of Global Change and Earth System Science, Beijing Normal University, Beijing 100875, China; [email protected] (C.G.); [email protected] (R.A.); [email protected] (W.W.); [email protected] (X.W.); [email protected] (G.W.) 2 Research Institute of Forest Ecology, Environment and Protection, Key Laboratory of Forest Protection of National Forestry and Grassland Administration, Chinese Academy of Forestry, Beijing 100091, China; [email protected] 3 Department of Public Security, Nanjing Forest Police College, Nanjing 210023, China; [email protected] * Correspondence: [email protected] (C.S.); fi[email protected] (M.W.) Abstract: All-season warming is assumed to advance snowmelt and delay snow accumulation; additionally, coupled with warming-induced drought stress, all-season warming could extend both the beginning and the end of the fire season. Using fire data updated for 1968–2018, we found an asymmetrical expansion of the lightning fire season in the boreal forest of Northeast China. Lightning fires have not advanced into the early fire season (May–June) but have largely extended into the snowless late fire season (July–September) since the late 1990s (mean end Julian date delayed by 51.1 days for 1998–2018 compared with 1968–1997, p < 0.001). Despite significant warming, the Julian days of snowmelt have changed only slightly, which has prohibited the fire season from advancing into early spring. The expansion of lightning fires into July–September was associated with a Citation: Gao, C.; An, R.; Wang, W.; warming-induced significant increase in evapotranspiration and a decrease in soil/fuel moisture. Shi, C.; Wang, M.; Liu, K.; Wu, X.; Wu, G.; Shu, L. Asymmetrical Keywords: lightning fire; fire season; boreal forest; asymmetrical expansion Lightning Fire Season Expansion in the Boreal Forest of Northeast China. Forests 2021, 12, 1023. https:// doi.org/10.3390/f12081023 1. Introduction In association with climate warming and frequent heatwaves, wildfire frequency, Academic Editor: Franco Biondi burned area, and duration have all increased [1–3], leading to a great loss of human lives and property, disturbance of forest ecosystems, and biodiversity changes [4–8]. The severity Received: 8 July 2021 of wildfires is a key factor in transforming forest ecosystems from net carbon sinks to net Accepted: 28 July 2021 Published: 31 July 2021 carbon sources [9,10] and has substantially contributed to greenhouse gas emissions [11]. Warming rates increase with increasing latitude [12], and boreal forests have experienced Publisher’s Note: MDPI stays neutral increases in drought stress, fuel dryness, and dry seasons duration [13–16]. Lightning is the with regard to jurisdictional claims in leading cause of fire ignition in some regions of boreal forest [17–19], and the coupling of published maps and institutional affil- lightning with increased warming-induced drought severity is responsible for the increase iations. in fires in these regions [20–22]. In addition to increasing fire frequency, warming has been shown to expand the fire season by both advancing the beginning and extending the end in North America, Europe, and Southwest China [23–25]. The global wildfire season length increased by 18.7% from 1979 to 2013 [26], and seasonal expansion has been and will continue to be Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. more pronounced in northern high latitudes [3]. Fire ignitions are highly dependent on This article is an open access article fuel dryness, which is controlled by precipitation, evapotranspiration [27], and interactions distributed under the terms and between snow cover and soil moisture conditions [28]. Snow cover naturally prohibits fire conditions of the Creative Commons occurrence, and strong spring-summer warming and earlier snowmelt theoretically reduce Attribution (CC BY) license (https:// soil and fuel moisture and thereby advance fire occurrence into early spring [29,30]. creativecommons.org/licenses/by/ The Greater Khingan forest, the only boreal forest in Northeast China, has experi- 4.0/). enced a rapid warming rate and the largest number of lightning fires among all forests in Forests 2021, 12, 1023. https://doi.org/10.3390/f12081023 https://www.mdpi.com/journal/forests Forests 2021, 12, x FOR PEER REVIEW 2 of 10 The Greater Khingan forest, the only boreal forest in Northeast China, has experi- Forests 2021, 12, 1023 enced a rapid warming rate and the largest number of lightning fires among all forests2 of 10in China [31]. Increased fire occurrence, larger burned areas, and an elongated fire season were observed in this region between 198 and 005 [32]. However, whether the seasonality Chinaof lightning [31]. Increasedfires has changed fire occurrence, in recent larger decades burned remains areas, unclear and an [17]. elongated This uncertainty fire season werearises observed from the indistinct this region time betweenintervals 198 and and old 005 versions [32]. However, of fire recor whetherds used the by seasonality previous ofstudies. lightning The firesfrequency has changed and severity in recent of lightning decades fires remains have unclearbeen shown [17]. to This be uncertaintydetermined arisesby moisture from the conditions, distinct timefuel intervalsstock and and dryness old versions [33–35], of however, fire records the usedpotential by previous mecha- studies.nisms linking The frequency fire seasonality, and severity snow cover, of lightning and soil fires moisture have been need shown to be tofurther be determined explored. by moistureIn this study, conditions, we used fuel stockthe long and- drynessterm fire [33 record,–35], however, climate, thesoil potential moisture, mechanisms and snow linkingcover data fire for seasonality, the Greater snow Khingan cover, forest. and soil The moisture dynamics need of tolightning be further fire explored. seasonality, fire frequency,In this snowmelt study, we and used snow the long-termaccumulation fire days, record, snow climate, cover, soil and moisture, key climate and factors snow betweencover data 1968 for and the Greater2018 were Khingan characterized. forest. The In addition, dynamics the of lightningconnections fire between seasonality, fire fireoc- currencefrequency, and snowmelt climate change, and snow snow accumulation cover, and days,moisture snow conditions cover, and were key explored. climate factors between 1968 and 2018 were characterized. In addition, the connections between fire 2.occurrence Data and and Methods climate change, snow cover, and moisture conditions were explored. 2.1. Study Area 2. Data and Methods 2.1. StudyThe study Area area is located in the Greater Khingan forest of Heilongjiang Province, China (Figure 1), which is the southern extension of the Russian Far East boreal forest into The study area is located in the Greater Khingan forest of Heilongjiang Province, China, and covers a rectangular region within 121°–127° E and 50°–53° N. The dominant China (Figure1), which is the southern extension of the Russian Far East boreal forest into tree genera are Larix and Pinus. The elevation ranges from 180 m to 1528 m above sea level China, and covers a rectangular region within 121◦–127◦ E and 50◦–53◦ N. The dominant (a.s.l.). tree genera are Larix and Pinus. The elevation ranges from 180 m to 1528 m above sea level (a.s.l.). Figure 1. Locations of lightning fires from 1968 to 2018. The black rectangle at the top right indicates the boundary of the map. The top left insert (a) shows the monthly mean temperature and precipita- tion of the study area (121◦–127◦ E and 50◦–53◦ N). The bottom right insert (b) shows the monthly mean snow cover and lightning fire ratio. Forests 2021, 12, 1023 3 of 10 The Greater Khingan forest is located in the cold-temperate continental monsoonal climate region. Mean temperature ranges from −4 to −2 ◦C. Mean annual precipitation is 400 to 550 mm (Figure1a). The mean number of frost-free days is 90 to 110. Almost all lightning fires (98.7%) occur between May and September (MJJAS) (Figure1b), which is defined as the lightning fire season. 2.2. Fire and Climate Data Detailed information on historical wildfires, including fire dates, coordinates, causes of ignition, burned areas, etc., was investigated and recorded by the local forest administra- tions. Monthly climate data, including mean temperature (Temp), maximum temperature (Tmax), and precipitation (Prec), were obtained from the Climatic Research Unit Time- series 4.03 dataset with a resolution of 0.5◦. The monthly and daily snow cover data were extracted from the Rutgers University Global Snow Lab database. The one-month standard- ized precipitation evapotranspiration index (SPEI) data were obtained from the Spanish National Research Council. The 0–10 cm soil moisture (SM) and evapotranspiration (Eva) data were compiled from the Common Land Model ERA-Interim dataset. Information on data sources, accessed dates, and source websites has been listed in Table A1. 2.3. Methods The fire season was separated into the early and late fire seasons of May to June (MJ) and July to September (JAS), respectively. The climate parameters, including Temp, Tmax, Prec, SPEI, Eva, SM, and snow cover, were averaged over the region of 121◦–127◦ E, 50◦–53◦ N to estimate the mean climate conditions of the study region. The burned area data were base-10 logarithm-transformed for convenience; the transformed data are plotted in the figures. To examine the fire season changes over time, we transformed the fire dates and the daily snow cover data into Julian days [36]. A previous study identified a negative to positive regime shift for both lightning fire frequency and summer temperature in approximately 1998 [37], so this year was used as a breakpoint in this study.