Thirty years of conifer establishment in volcanic primary succession at Mount St. Helens: Patterns and factors affecting establishment

Background/Question/Methods

Massive, long-lived conifers dominate the forests on and around the Mount St. Helens volcano (US Pacific Northwest region). The 18 May 1980 eruption of Mount St. Helens created a mosaic of primary and secondary successions. The most devastated area is an approximately 60-km² primary successional habitat known as the Pumice Plain, which is largely isolated from surviving forests. This study investigated the spatial and temporal patterns of conifer establishment on the Pumice Plain during the first 30 years of primary succession and the extent to which these patterns were determined by environmental factors and seed availability. We quantified conifer species’ density and traits in 2002, 2007, and 2010, and reconstructed individual establishment dates in 177 permanent plots totaling 34.75 ha. We used surface topography features to assess landscape patterns of conifer abundance. For the two most common species, Abies procera and Pseudotsuga menziesii, we combined establishment data with time series of climate and cone production in a likely source population (Abies procera only) to explain temporal patterns of conifer establishment.

Results/Conclusions

Conifer density increased from 23.5 to 41.1 trees ha^-1 from 2002 to 2010. The community was dominated by Abies procera (49%), Pseudotsuga menziesii (31%), and Tsuga heterophylla (18%), with the remainder (2%) consisting of Pinus contorta, Pinus monticola, and Thuja plicata. For the two dominant species, trees at primary successional sites were lower in foliar pigments than those in secondary succession, consistent with low nutrient availability. Physiological stress (indicated by chlorophyll-a:carotenoid ratio) negatively affected radial growth but did not explain differences in abundance between plots. Incident radiation and heat load  (inferred from topography) negatively affected conifer abundance, while increasing surface roughness had a positive effect. Temporal patterns of establishment (1980-2000) for A. procera and Ps. menziesii were congruent, with peaks at ~3-yr intervals and a striking increase in establishment rate after 1992. Mean winter temperature negatively affected establishment by both species, while mean summer temperature positively affected A. procera establishment. Cone production was unrelated to establishment (A. procera).

Topographic factors interacted with climate to drive establishment of the two dominant conifer species, while seed availability did not, suggesting that both deterministic and stochastic abiotic environmental factors control one of the most important phases of succession, i.e. the establishment of the longest-lived and largest members of the regional biota. This temporal and spatial environmental heterogeneity results in an uneven-aged, spatially-aggregated conifer community.