Abstract
Abstract. Live fuel moisture content (LFMC) plays a critical role in wildfire dynamics, but little is known about responses of LFMC to multivariate climate change, e.g., warming temperature, CO2 fertilization, and altered precipitation patterns, leading to a limited prediction ability of future wildfire risks. Here, we use a hydrodynamic demographic vegetation model to estimate LFMC dynamics of chaparral shrubs, a dominant vegetation type in fire-prone southern California. We parameterize the model based on observed shrub allometry and hydraulic traits and evaluate the model's accuracy through comparisons between observed and simulated LFMC of three plant functional types (PFTs) under current climate conditions. Moreover, we estimate the number of days per year of LFMC below 79 % (which is a critical threshold for wildfire danger rating of southern California chaparral shrubs) from 1960 to 2099 for each PFT and compare the number of days below the threshold for medium and high greenhouse gas emission scenarios (RCP4.5 and 8.5). We find that climate change could lead to more days per year (5.2 %–14.8 % increase) with LFMC below 79 % between the historical (1960–1999) and future (2080–2099) periods, implying an increase in wildfire danger for chaparral shrubs in southern California. Under the high greenhouse gas emission scenario during the dry season, we find that the future LFMC reductions mainly result from a warming temperature, which leads to 9.1 %–18.6 % reduction in LFMC. Lower precipitation in the spring leads to a 6.3 %–8.1 % reduction in LFMC. The combined impacts of warming and precipitation change on fire season length are equal to the additive impacts of warming and precipitation change individually. Our results show that the CO2 fertilization will mitigate fire risk by causing a 3.5 %–4.8 % increase in LFMC. Our results suggest that multivariate climate change could cause a significant net reduction in LFMC and thus exacerbate future wildfire danger in chaparral shrub systems.
Highlights
Historical warming and changes in precipitation have already impacted wildfires at a global scale (e.g., Stocks et al, 1998; Gillett et al, 2004; Westerling et al, 2003, 2006) and it is expected that accelerating future warming will continue to substantially affect global wildfires (e.g., Flannigan et al, 2009; Liu et al, 2010; Moritz et al, 2012)
Our results showed that Functionally Assembled Terrestrial Simulator (FATES)-HYDRO was able to capture variation in the live fuel moisture content (LFMC) for different plant functional types (PFTs) and soil water content at 5 cm depth (Figs. 2 and S3) as well as for chamise in 2018 (Fig. S5), we had limited observed LFMC data
The model was able to capture the seasonal dynamics of soil water content, LFMC, and LFMC below the threshold of 79 % in comparison to observed data (Figs. 2a, c, e and S3)
Summary
Historical warming and changes in precipitation have already impacted wildfires at a global scale (e.g., Stocks et al, 1998; Gillett et al, 2004; Westerling et al, 2003, 2006) and it is expected that accelerating future warming will continue to substantially affect global wildfires (e.g., Flannigan et al, 2009; Liu et al, 2010; Moritz et al, 2012). While previous studies provide great insights into fire risks with changes in climate, dead fuel moisture, fuel loads, and representation of live fuel moisture, there is still limited understanding of how climate change influences live fuel moisture content (LFMC) and the consequent wildfire risks. This is true for the combined impacts of warming temperature, altered precipitation, and increasing CO2 fertilization (Chuvieco et al, 2004; Pellizzaro et al, 2007; Caccamo et al, 2012a, b; Williams et al, 2019; Goss et al, 2020)
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