Conifer colonization of a 350-year old rock fall at Lassen Volcanic National Park in northern California

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Montane conifers of western North America commonly colonize mineral surfaces resulting from landslides, lahars, mudflows, and rock avalanches. This colonization can include shade-tolerant conifers that may eventually dominate the forest in a pattern termed “direct” succession. Documenting examples of this long-term successional process are useful for identifying alternative successional trajectories and indicating potential controlling mechanisms for subsequent experimental analysis. This study (1) analyzes the 1992 status of the conifer colonization on the coarse-textured surface of a 1650 AD rock avalanche in northern California and (2) measures individual growth and survivorship in permanent plots between 1992 and 2003. Increment cores of large trees indicated initial conifer colonization before 1700 AD with continuous subsequent colonization. Mean conifer density in 1992 was 725 (SD = 747) ha−1 with densities increasing with decreasing rock sizes. Densities were not correlated with distances to possible seed sources. Median heights were <1 m, and the mean proportional height growth rate for healthy individuals was 0.0166 year−1. The mortality rate for individuals ≥0.1-m tall was <0.007 year−1. The conifers were more numerous than shrubs, and there was little apparent evidence of facilitation or inhibition of conifers. The species assemblage is mostly (89%) Abies concolor, Pinus monticola, and Pinus ponderosa individuals dispersed across an elevation range of 1870–2040 m a. s. l. This is an atypical species mix for these elevations in this location, and this mix is not readily predicted from species properties such as seed mass, seed flight distance, or drought tolerance.

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