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

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