title: Spatially nonrandom tree mortality and ingrowth maintain equilibrium pattern in an old-growth Pseudotsuga-Tsuga forest.
abstract: Mortality processes in old-growth forests are generally assumed to be driven by gap-scale disturbance, with only a limited role ascribed to density-dependent mortality, but these assumptions are rarely tested with data sets incorporating repeated measurements. Using a 12-ha spatially explicit plot censused 13 years apart in an approximately 500-year-old Pseudotsuga-Tsuga forest, we demonstrate significant density-dependent mortality and spatially aggregated tree recruitment. However, the combined effect of these strongly nonrandom demographic processes was to maintain tree patterns in a state of dynamic equilibrium. Density-dependent mortality was most pronounced for the dominant late-successional species, Tsuga heterophylla. The long-lived, early-seral Pseudotsuga menziesii experienced an annual stem mortality rate of 0.84% and no new recruitment. Late-seral species Tsuga and Abies amabilis had nearly balanced demographic rates of ingrowth and mortality. The 2.34% mortality rate for Taxus brevifolia was higher than expected, notably less than ingrowth, and strongly affected by proximity to Tsuga. Large-diameter Tsuga structured both the regenerating conspecific and heterospecific cohorts with recruitment of Tsuga and Abies unlikely in neighborhoods crowded with large-diameter competitors (P < 0.001). Density-dependent competitive interactions strongly shape forest communities even five centuries after stand initiation, underscoring the dynamic nature of even equilibrial old-growth forests.
authors.forename: James A
authors.initials: JA
authors.lastname: Lutz
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Larson
authors.forename: Tucker J
authors.initials: TJ
authors.lastname: Furniss
authors.forename: Daniel C
authors.initials: DC
authors.lastname: Donato
authors.forename: James A
authors.initials: JA
authors.lastname: Freund
authors.forename: Mark E
authors.initials: ME
authors.lastname: Swanson
authors.forename: Kenneth J
authors.initials: KJ
authors.lastname: Bible
authors.forename: Jiquan
authors.initials: J
authors.lastname: Chen
authors.forename: Jerry F
authors.initials: JF
authors.lastname: Franklin
mesh_terms: D017753:Ecosystem; D008954:Models, Biological; D028224:Pseudotsuga / Q000502:physiology*; D014197:Trees / Q000502:physiology*; D028225:Tsuga / Q000502:physiology*