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

Michael Keith Birchfield

Questions: What are the spatial and temporal patterns of establishment by dominant coniferous forest species in the first 30 years of primary succession? To what extent are these patterns determined by environmental stress and seed availability? Location: Pumice Plain, Mount St. Helens, Washington, USA (46°15N, 122°10W, elevation 1000-1250 m). 60-km2 primary successional habitat created by the 1980 eruption. Methods: We quantified conifer species’ density and traits in 2002, 2007, and 2010, and topographic features, and reconstructed individual establishment dates in 177 permanent plots totaling 34.75 ha. For the two most common species, Abies procera and Pseudotsuga menziesii, we combined these data with time series of climate and cone production in a likely source population (Abies procera only) to explain patterns of conifer establishment over 30 years. Results: 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. iv Physiological stress (indicated by chlorophyll-a : carotenoid ratio) negatively affected radial growth but did not explain differences in abundance between plots. Surface exposure (as inferred from topography) negatively affected conifer abundance, while increasing surface roughness had a positive affect. 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). Conclusions 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.

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

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