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

Abstract

Wildfire is a ubiquitous disturbance agent in Rocky Mountain subalpine forests.Lodgepole pine (Pinus contorta), a dominant tree species in subalpine forests of western North America, is largely resilient to high-severity fires. However, the resilience of lodgepole forests may be compromised with predicted changes to climate and moisture availability. While the modern post-fire dynamics of these systems are well studied, less is known about post-fire responses of lodgepole forests over the past few centuries and millennia. Thus, I investigated fire occurrence and post-fire vegetation change in a lodgepole forest over the past two millennia to understand ecosystem responses to variability in wildfire activity and climate. I reconstructed vegetation composition over the past 2,500 years in the small basin of Chickaree Lake, Colorado, U.S.A., in Rocky Mountain National Park. Pollen samples (n=52) were analyzed to characterize both broad-scale (centennial to millennial) trends in pollen assemblages associated with climate, and short-term (decadal) changes associated with multiple high-severity fire events previously reconstructed through charcoal analysis. Pollen assemblages were dominated by Pinus throughout the record. The primary broadscale change in pollen composition characterized by an increase Artemisia and Rosaceae, and a decrease in mean Pinus and extra local pollen (Quercus, Salix, and Sarcobatus), which occurred around 1,155 calibrated years before present (cal yr BP). This change is coincident with a shift towards increased winter precipitation which characterizes modern climate identified from δO 18 and lake level data. Wildfire occurrence resulted in significant decreases in Pinus pollen following the zone break at 1,155 cal yr BP, suggesting that impacts of fire on the composition of lodgepole pine forests may depend on underlying climate conditions. Throughout the past 2,500 years, vegetation composition returned to pre-fire conditions within 75 years, indicating overall resilience of Rocky Mountain lodgepole forests to fire activity despite some variability in post-fire vegetation recovery. v

Disturbance ecology and the problem of n = 1: A proposed framework for unifying disturbance ecology studies to address theory across multiple ecological systems

“…Palaeoecological studies of disturbance regimes, such as forest fires (e.g. Chileen et al, 2020) are an excellent parallel to the microbial community disturbance studies mentioned above (e.g. Jacquet & Altermatt, 2020).…”

“…It is true that not all categories fit as comfortably as others, depending on the study; for example, while functional groups are reported and interpreted in Chileen et al (2020), they vary continuously across the time period. This makes reporting simple metrics like functional group diversity very difficult.…”

Considering regeneration failure in the context of changing climate and disturbance regimes in western North America

“…Studies across the western US have shown the persistence of mature cold mixed-conifer forests and that it is specific high-severity fires combined with periods of rapid climatic change that prompts forest conversion (Gavin et al 2007;Calder et al 2015). Multiple studies demonstrate how cold mixed-conifer forests with fire-intolerant species expanded during times of low fire frequency and shifted to more fire-tolerant assemblages such as ponderosa pine and Douglas-fir or even lodgepole pine during periods of warmer climate and higher fire frequency (Gavin et al 2007;Whitlock et al 2008;Chileen et al 2020). In a related paleoecological study, Crausbay et al (2017) documented that high-severity fire combined with changing climate often precipitated rapid vegetation change in cold mixed-conifer forests of western Washington.…”

Legacies of past human activities on one of the largest old-growth forests in the south-east European mountains

“…To test the simultaneity between fire and erosion events, thereby allowing identification of high-severity local catchment fires (Chileen et al 2020), we used Event Coincidence Analysis (ECA; Siegmund et al 2017). We determined erosion events as (i) peaks of magnetic susceptibility, which respond to increases in minerogenic input that often follow fire events as a result of erosion, and to in-washing of secondary ferromagnetic minerals that are formed when soils with a high concentration of convertible Fe are heated (Thompson and Oldfield 1986;Millspaugh and Whitlock 1995), and (ii) peaks of Cenococcum geophilum Fr.…”