Future changes in fire weather, spring droughts, and false springs across U.S. National Forests and Grasslands

Abstract

Public lands provide many ecosystem services and support diverse plant and animal communities. In order to provide these benefits in the future, land managers and policy makers need information about future climate change and its potential effects. In particular, weather extremes are key drivers of wildfires, droughts, and false springs, which in turn can have large impacts on ecosystems. However, information on future changes in weather extremes on public lands is lacking. Our goal was to compare historical (1950–2005) and projected mid‐century (2041–2070) changes in weather extremes (fire weather, spring droughts, and false springs) on public lands. This case study looked at the lands managed by the U.S. Forest Service across the conterminous United States including 501 ranger district units. We analyzed downscaled projections of daily records from 19 Coupled Model Intercomparison Project 5 General Circulation Models for two climate scenarios, with either medium‐low or high CO2− equivalent concentration (RCPs 4.5 and 8.5). For each ranger district, we estimated: (1) fire potential, using the Keetch‐Byram Drought Index; (2) frequency of spring droughts, using the Standardized Precipitation Index; and (3) frequency of false springs, using the extended Spring Indices. We found that future climates could substantially alter weather conditions across Forest Service lands. Under the two climate scenarios, increases in wildfire potential, spring droughts, and false springs were projected in 32–72%, 28–29%, and 13–16% of all ranger districts, respectively. Moreover, a substantial number of ranger districts (17–30%), especially in the Southwestern, Pacific Southwest, and Rocky Mountain regions, were projected to see increases in more than one type of weather extreme, which may require special management attention. We suggest that future changes in weather extremes could threaten the ability of public lands to provide ecosystem services and ecological benefits to society. Overall, our results highlight the value of spatially‐explicit weather projections to assess future changes in key weather extremes for land managers and policy makers.

Risk to North American birds from climate change‐related threats

“…In addition, increased fire weather, a measure of drought indicative of wildfire potential, is projected to affect two-thirds of the country and over half of species (including nearly 80% of vulnerable breeding species) at the 3.0 C scenario. This pattern indicates that the majority of the conterminous US will have weather conditions that are indicative of wildfires and drought, a more than 233% increase from historical levels (Martinuzzi et al, 2019), during one-quarter of the year or more. Heavy rains and spring droughts are also expected to increase, with nearly one-quarter of the country and over 20 and 13% of breeding species affected by each threat, respectively.…”

“…Across the threats analyzed here, short-term intermittent threats had the greatest contribution to risk and the widest spatial coverage. Although these threats are historically uncommon, both their magnitude and frequency are anticipated to increase with climate change (Jentsch, Kreyling, & Beierkuhnlein, 2007;Martinuzzi et al, 2016Martinuzzi et al, , 2019. Indeed, extreme spring heat events occurring every 2 years or more often are set to affect 99% or more of species.…”

Recent bark beetle outbreaks influence wildfire severity in mixed‐conifer forests of the Sierra Nevada, California, USA

“…Drought‐related disturbances are projected to increase in frequency in the future (Christensen et al 2007, Intergovernmental Panel on Climate Change 2014, Seidl et al 2017, Martinuzzi et al 2019), leading to higher probability of disturbance interactions, where multiple disturbances occur on the same landscape within a short time period (Kane et al 2017). Severity of drought related disturbances will likely increase with a changing climate as well (Seager et al 2007, Allen et al 2010, Millar and Stephenson 2015, Bowman et al 2017), with severity defined here as the amount of biomass lost due to disturbance (Keeley 2009).…”

Wilderness areas in a changing landscape: changes in land use, land cover, and climate

“…False springs, as described by Martinuzzi et al (2016), captured the mean annual probability of false springs for each pixel during the historic record and during 2006-2050 in which reproductive effort by plants may have not been successful because of hard freezes occurring after blooming. The return interval of false springs, which is the average time between false spring occurrences, was calculated by dividing one by the annual probability (Martinuzzi et al 2016(Martinuzzi et al , 2019.…”