Forest Response to Water Availability and Disturbance in the Western United States


Logan T. Berner

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Oregon State University


  • Earth’s atmosphere is unequivocally warming due to CO₂ and other greenhouse gas (GHG) emissions from human activities and this is having widespread impacts on forest ecosystems that provide important services to human societies. Forest ecosystems help regulate atmospheric CO2 concentrations by sequestering carbon in tree biomass and soils, which is a valuable ecosystem service that is sensitive to climate change and forest management. Rising air temperatures contributed to increased aridity and drought during recent decades among forests in the western United States and projections suggest that many parts of this region could become hotter and drier over the coming century barring significant reductions in GHG emissions. Managing regional forests and GHG emissions in a warming world requires better understanding of how forest carbon cycling is influenced by climate, including climate-mediated disturbance (e.g., fires). The objectives of this dissertation were to assess (1) forest response to water availability and (2) tree mortality from disturbance during recent decades in the western US. Forest response to water availability was assessed, in part, by quantifying changes in forest productivity and live biomass across sites that varied widely in average water availability. Bioclimatic relationships were developed using (1) field measurements from 12 sites in the eastern Cascade Mountains, (2) inventory and ancillary plot measurements from 1,953 sites in Washington, Oregon, and California (WAORCA), and (3) remote sensing measurements spanning 18 Mha of mature forest in the western US. In each case, forest productivity and live biomass increased markedly across sites as average water availability increased. For instance, median forest productivity increased from 2.2 to 5.6 Mg C ha⁻¹ yr⁻¹ between the driest and wettest 5% of sites in WAORCA, while live biomass increased from 26 to 281 Mg C ha⁻¹. These bioclimatic relationships illustrate that forests are widely sensitive to changes in water availability, suggesting that continued warming and drying could reduce carbon sequestration over the coming century in parts of the region.Tree mortality from fires, bark beetles, and timber harvest was quantified from 2003-2012 across the region using remote sensing, federal harvest statistics, and ancillary information. Tree mortality was quantified in terms of carbon storage in aboveground biomass killed by disturbance. Regional tree mortality from these disturbances together averaged 45.8±16.0 Tg C yr⁻¹ (±95% confidence interval), with harvest, beetles, and fires accounting for 50%, 32%, and 18% of mortality, respectively. Tree mortality from timber harvest was concentrated in the high-biomass forests of the Washington and Oregon. Tree mortality from bark beetles occurred largely in Colorado, Wyoming, and Montana, where tree defenses were suppressed by drought and beetle populations bolstered by rising winter temperatures. Tree mortality from fires was highest in California, Idaho, and Montana, which also experienced very dry conditions during this decade. Tree biomass killed by disturbance will gradually decompose and emit CO₂ to the atmosphere over decades to centuries, where it will act as a GHG. This analysis illustrates both opportunities and challenges to managing GHG emissions from forest ecosystems in the region. Swift and significant reductions in GHG emissions are needed to curtail adverse impacts of climate change on forest ecosystems and human societies.