Biogeochemistry in changing Sierra Nevada ecosystems
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Authors
Bagcilar, Seren H
Issue Date
2023
Type
Dissertation
Language
Keywords
carbon , fire regimes , high severity fire , mineral-associated organic matter , montane meadow , Sierra Nevada
Alternative Title
Abstract
Forest and meadow ecosystems of the Sierra Nevada are experiencing change on varied temporal and spatial scales that may influence biogeochemical and ecosystem function. Repeated high severity fire within a relatively short time frame (10 �" 20 y) is a major driver of change in mixed-conifer forests, a dominant cover type of the northern Sierra Nevada and southern Cascades. Repeated high severity fire can lead to long-term forest conversion to montane chaparral, representing a “press” or long-term, continuous effect on mixed-conifer forests.Chapter 1 explores the potential soil biogeochemical impacts of multiple severe fires with short fire return intervals on forest soil using existing disturbance frameworks. This conceptual paper includes an empirical analysis of the reburn trends throughout the Sierra Nevada and Modoc (encompassing the neighboring southern Cascades) bioregions of California and a review of extant literature exploring this press effect on aboveground ecosystem components. Lastly, Chapter 1 poses a series of hypotheses on press effects to belowground ecosystem components. Our hypotheses were: 1) reduced recovery time between fire events will prevent the soil microbiome from returning to pre-fire conditions, leading to long-term changes in the microbial community; 2) reduced recovery time between fire events will prevent plant carbon (C) inputs from returning to pre-fire conditions, hindering soil organic matter (SOM) recovery; 3) particulate organic matter (POM) is more likely to experience continued decline with successive burning until a lower limit is reached than mineral-associated organic matter (MAOM); and 4): less aboveground biomass with repeated burning will produce smaller post-fire inorganic nitrogen (IN) pulses through ash deposition in each fire event, with the potential to influence ecosystem-level nitrogen (N) dynamics. Chapter 2 is an empirical study that addresses some of the hypotheses of Chapter 1 by comparing soil biogeochemical properties of a series of sites that have burned either once or twice in the past 20 years. We found that certain aspects of biogeochemistry, including inorganic N content and cycling and heterotrophic respiration (Rh), were affected by repeated burning while other aspects, including soil carbon C and N storage in soil organic matter fractions, do not. Chapter 2 also includes a burn table experiment to identify if soils from sites of varied burn history respond differently to an additional fire event, though the effects of the experiment were limited. Chapter 3 focuses on montane meadows, which are commonly degraded following past land use practices. As such, the main driver of change in montane meadows is currently hydrologic restoration aimed at restoring the ecosystem properties lost in degradation. Hydrologic restoration has been shown to increase soil C inputs from vegetation and increase soil C stocks. Chapter 3 aims to identify changes in soil C stability, or resistance to loss through time, with time since restoration. We found three lines of evidence that indicate soil C stability in montane meadows increases with restoration age: increased soil C storage in MAOM which has a longer residence time than that of POM, decreased susceptibility of MAOM-C to decomposition across the temperature manipulation experiment, and limited signs of MAOM utilization through stable isotope analyses. Overall, this dissertation identifies the resistance and resilience of forest and meadow soils to change and highlights soil as a persistent reservoir of ecosystem C and N.