THE BIOGEOCHEMICAL CONSEQUENCES OF LATE HOLOCENE WILDFIRES IN THREE SUBALPINE LAKES FROM NORTHERN COLORADO

Forest fire activity has been increasing in western North America over the past several decades. However, the biogeochemical effects of changing fire regimes are poorly understood. Lake sediment records can provide a long-term perspective into the biogeochemical impacts of fire. Provisional evidence suggests a feedback between fire disturbance and the availability of growth-limiting nutrients in soils could affect the ability of forests to regenerate. Here, we utilize three lake sediment records from northern Colorado to investigate the biogeochemical consequences of charcoal-inferred fire events over the past 2500 years. The study sites are small high-elevation lakes in subalpine forests that experience infrequent, high-severity fires. The dominant vegetation includes lodgepole pine (Pinus contorta), Engelmann spruce (Picea engelmannii), and subalpine fir (Abies lasiocarpa). We measured element (C, N, P, S, Al, Ca, K, Si, Ti) and stable isotope (δ¹³C, δ¹⁵N) values as proxies for biogeochemical processes. To characterize dominant modes of variability in the proxy records, we interpreted the data using principal component analysis. Principle component 1 was detrended by removing the 300-year moving average, and the residual series was then compared with reconstructed fire events to investigate fire impact on sediment biogeochemistry. This analysis suggests that fires produced variable biogeochemical responses within and among the study lakes. On average, fires are associated with increased carbon and nitrogen concentrations, while titanium and other metals found in mineral material decrease. Further investigation will help to quantify the cumulative impacts of fire on lake sediment biogeochemistry.