Wildfires and geochemical change in a subalpine forest over the past six millennia

Abstract

The frequency of large wildfires in western North America has been increasing in recent decades, yet the geochemical impacts of these events are poorly understood. The multidecadal timescales of both disturbance-regime variability and ecosystem responses make it challenging to study the effects of fire on terrestrial nutrient cycling. Nonetheless, disturbance-mediated changes in nutrient concentrations could ultimately limit forest productivity over centennial to millennial time scales. Here, we use a novel approach that combines quantitative elemental analysis of lake sediments using x-ray fluorescence to assess the geochemical impacts of high-severity fires in a 6200 year long sedimentary record from a small subalpine lake in Rocky Mountain National Park, Colorado, USA. Immediately after 17 high-severity fires, the sedimentary concentrations of five elements increased (Ti, Ca, K, Al, and P), but returned to pre-fire levels within three decades. Multivariate analyses indicate that erosion of weathered mineral material from the catchment is a primary mechanism though which highseverity fires impact element cycling. A longer-term trend in sediment geochemistry was also identified over millennial time scales. This decrease in the concentrations of six elements (Al, Si, K, Ti, Mn, and Fe) over the past 6200 years may have been due to a decreased rate of high-severity fires, longterm ecosystem development, or changes in precipitation regime. Our results indicate that highseverity fire events can determine elemental concentrations in subalpine forests. The degree of variability in geochemical response across time scales suggests that shifting rates of high-severity burning can cause significant changes in key rock-derived nutrients. To our knowledge, these results are the first to reveal repeated loss of rock-derived nutrients from the terrestrial ecosystem due to high-severity fires. Understanding the future of fire-prone coniferous forests requires further documentation and quantification of this important mechanism linking fire regimes and biogeochemical cycles.

Biogeochemical Responses to Holocene Catchment‐Lake Dynamics in the Tasmanian World Heritage Area, Australia

“…Long‐term disturbance from fire in fire‐sensitive vegetation, such as rainforest, can result in substantial ecosystem transformation, such as altered species composition, soil formation, and nutrient loss that can have important implications for associated water bodies (Ball et al, ; Huvane & Whitehead, ; Korhola et al, ; Leys et al, ; Morris et al, ; Smith et al, ). Fires in temperate environments, such as western Tasmania, mostly occur in the period from September to March (from spring to early autumn) and are often followed by heavy rain events (Bridle et al, ; Pemberton, ), which can remove the soil layer into water bodies, altering water geochemistry and nutrient availability (Beck, Fletcher, Kattel, et al, ; Boerner, ) and precipitating an aquatic ecosystem response (Beck, Fletcher, Gadd, et al, ).…”

Vegetation Response to Wildfire and Climate Forcing in a Rocky Mountain Lodgepole Pine Forest Over the Past 2500 Years

“…Anderson et al, 2015;Carter et al, 2013;Jiménez-Moreno et al, 2011; Jiménez-Moreno and Anderson, 2013;Minckley et al, 2012), while other datasets are centered around smaller timeframes to capture specific events like the Medieval Climate Anomaly, Little Ice Age, and other brief periods of climate variation during the late Holocene from about 7000 calibrated years before 1950 AD (cal yr BP) to present (e.g. Caffrey and Doerner, 2012;Calder and Shuman, 2017;Dunnette et al, 2014;Leys et al, 2016).…”

“…Reconstructing this temporal range of focus has been aided by new developments in paleoecological data analysis that allow for higher resolution reconstructions of fire, climate, and (Anderson et al, 2015;Leys et al, 2016) and the development of CHARAnalysis (Higuera et al, 2009) allows for the identification of local fire events by identifying peaks in charcoal against background data. Multiproxy studies can aid in improved interpretations of past fire activity, an example of which is when the results of CHARAnalysis are paired with Magnetic Susceptibility data, a lacustrine core-based proxy for catchment erosion pulses (Dunnette et al, 2014;Morris et al, 2015;.…”

“…(2014) and Leys et al (2016), which included a high-resolution record of fire history and biogeochemical change (i.e., 5-50 yr sampling interval), and a lower resolution record of vegetation change based on fossil pollen (i.e., 20-275 yr sampling interval). For the studies, two parallel, overlapping sediment cores were collected from Chickaree Lake at 7.9-meter water depth in August 2010 with a modified Livingstone piston corer (Wright et al, 1984).…”

“…Anderson et al, 2015;Calder and Shuman, 2017;Jiménez-Moreno et al, 2011), quantified past fire regimes (e.g. Caffrey and Doerner, 2012;Carter et al, 2017), assessed the impacts of fire on biogeochemical cycling (Dunnette et al, 2014;Leys et al, 2016;McLauchlan et al, 2014), and captured long-term changes in vegetation following the glacial retreat of the region (e.g. Brunelle et al, 2005;Lynch, 1996;MacDonald et al, 1991;Minckley et al, 2012).…”