Premature Seed Germination and High Seedling Mortality in a Chaparral shrub (Adenostoma fasciculatum) Caused by Severe Drought in Calfiornia

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Research Poster

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The Santa Monica Mountains (SMM) represents one of five Mediterranean-type environments in the world. With sustained summer droughts, this ecosystem is prone to frequent wildfires, a phenomenon that has affected the SMM with regularity once every 32 years. Adenostoma fasciculatum (chamise), is a chaparral shrub well adapted to wildfire cycles because typically its seeds are triggered to germinate by high fire temperatures, smoke, and charate. In the winter of 2015, we were surprised to discover seedlings of A. fasiculatum emerging at an unburned chaparral site without the occurrence of fire. We hypothesized that drought induced canopy dieback at some microsites, but not others, allowed high radiation loads to reach the soil surface, increasing soil temperatures that promoted premature seed germination, without fire. We tested this hypothesis by correlating seedling emergence and survival in 46 permanent quadrats (1 m2) with 1) solar radiation reaching the soil, 2) soil temperature, and 3) depth of the litter layer in each quadrate. Highest seedling emergence corresponded to high radiation loads, high soil temperature, and shallow litter layers, consistent with our initial hypothesis. Seedling mortality by the end of the summer (September 2015) achieved 100%. We conclude that severe drought and concomitant shade reduction can promote premature seed germination in A. fasciculatum without fire. However no seedlings survive because germination is late in the season, roots and shallow, not supporting seedling establishment. We suspect that severe drought in California is depleting soil seed banks in A. fasciculatum and may not allow full recovery from succeeding wildfires.

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