Characteristics of climate-induced tree mortality in the US seven ecoregions

Abstract:

As temperature continues to increase, more extreme weather and drought events are expected to occur thereby resulting in worldwide tree mortality, which will speed up climate warming through a positive feedback loop. The types, frequency, and intensity of extreme weather events and climate would differ from region to region. It remains unknown what kind of extreme climate would cause what type of tree mortality. The cause-effect relationship between extreme climate and tree-mortality type might be different from region to region. In this report, we focus on the contiguous United States to test this hypothesis. We use the Monitoring Trends in Burn Severity (MTBS) map to establish the following seven ecoregions in the United States: Pacific Northwest, Pacific Southwest, Southwest, North Central, South Central, Northeast, and Southeast. Each ecoregion represents similar ecological conditions and its climate patterns are defined by the climate data extracted between 1988-2018 using the National Climatic Data Center (NCDC) on the National Oceanic and Atmospheric Administration (NOAA) website. Temperature, precipitation, and the Palmer Drought Severity Index (PDSI) are the three climate parameters used. In order to establish what the fundamental causes for tree mortality are for each ecoregion, we extracted tree mortality data and its causes of death across the seven ecoregions from published research papers between 1988-2018. The five causes of tree mortality used are drought, extreme weather, insect infestation, pathogen disease, and wildfires. Our initial results demonstrated that: (1) In the western parts of the United States, drought is a fundamental driver for insect infestation, pathogen disease, and wildfires; and (2) In the eastern parts of the United States, its consistent wet conditions fostered a favorable environment for insect and pathogen outbreaks to take place, therefore being the two major tree mortality categories.