Follow the Water: Extreme Drought and the Conifer Forest Pandemic of 2002–2003 Along the California Borderland

Abstract

California conifer forests suffer chronic low levels of bark beetle-caused tree mortality, but are subject to periodic episodes of widespread bark beetle outbreaks and extensive tree death. The extent and severity of the outbreaks are strongly influenced by precipitation, slope and aspect, stand density and stocking levels, fire, and soil characteristics as environmental factors. In addition, anthropogenic factors including fire suppression and stand management practices can also influence the susceptibility of forest stands to bark beetle activity. The mountains of southern California in the USA and Baja California Norte in Mexico were subject to a severe drought during 2002–2003. A major bark beetle outbreak accompanied the period of drought. Tree cover death was highest in Pinus coulteri (57.8 %), intermediate in Pseudotsuga macrocarpa (24.7 %) and mixed conifer forest (26.0–33.6 %), and lowest in subalpine forest (2.0 %) in Southern California mountains. The tree mortality in the adjacent mountains in Baja California was five times lower than what was observed north of the international border. The differences in forest management, particularly historical practices of fire suppression that resulted in stand densification and intense competition for available moisture in California may be the underlying mechanism driving the differences in severity of the bark beetle outbreaks on either side of the border.