Abstract
Repeated observations of forest development using permanent plots can map pathways and rates of primary succession at the individual plant, the plot and the community level. This study re-measures the trees in 34 100 m2 plots that were first sampled in 1987 to document recent and to predict continued forest development for a mixed-conifer forest established on a volcanic mudflow formed at a 2000-m elevation in Lassen Volcanic National Park (LVNP) in 1915. In 1987 and 2008, trees ≥ 0.11-m tall were identified to species level, and measured for height (m) and basal area (m2). The most abundant species in both 1987 and 2008 were Pinus contorta, Abies magnifica and Pinus monticola, and there was no statistically significant difference in species composition despite a 20% increase in tree densities. From 1987 to 2008, the mean (± SE) proportional rates of increase for the number of trees per plot, the mean heights of trees and the total basal area per plot were, respectively, 0.009 ± 0.002 y-1, 0.023 ± 0.002 y-1, and 0.055 ± 0.004 y-1. Despite these increases, canopy closure has not occurred for most of the forest. This lack of closure, in conjunction with the continuing similarity of relative species abundances, suggests that abiotic factors such as snow damage and drought and not biotic interactions such as competition may still be the major limitation to tree growth and forest development. Projecting the current rates of basal area growth for the next 10 to 20 years suggests rapid forest development that includes canopy closure. In expectation of these rapid changes, data on tree positions within plots were collected to allow the fates of individual trees to be monitored and determined. Key words: Pinus contorta, conifers, primary succession, volcanism, mudflow.  
Highlights
This study reports on repeated observations of forest development using permanent plots that were originally established and studied in 1987 (Kroh et al, 2000) on a 1915 volcanic mudflow in Lassen Volcanic National Park in northern California, USA (40° 30’ N, 121° 26’ W)
The mudflow site is part of a larger area of volcanic disturbance termed the Devastated Area that was formed within LVNP by eruptions in May 1915
The forest appears to be becoming more uniform as suggested by the reductions in differences among transects in mean density, height and basal area from 1987 to 2008 (Table 2)
Summary
As trees increase in size and the canopy of the forest closes, the factors limiting the survival and growth of trees may shift from these abiotic extremes of physical factors to biotic factors including competition for light and nutrients. This shift in the primary limiting factors may cause changes in the relative success of different individuals and different species (Peet and Christiansen, 1980; Walker and Chapin, 1987; Walker and del Moral, 2003). For intraspecific competition for light in some conifers such as
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