Different vital rates of Engelmann spruce and subalpine fir explain discordance in understory and overstory dominance

Andrus, Robert; Harvey, Brian; Chai, Rachel; Veblen, Thomas

Studies of forest dynamics commonly assume that species composition of the seedling bank reflects the composition of the future forest canopy. However, many forest types exhibit persistent differences in relative dominance of species in the seedling bank versus the forest canopy. Species-specific differences in tree vital rates (e.g., in-growth, mortality, height growth, canopy residence time) across canopy positions may explain this discord in dominance between seedling banks and forest canopies. We tested for differences in tree vital rates for two widely distributed, coexisting species in subalpine forests of the Rocky Mountains, North America. We quantified seedling bank dynamics (>950 aged seedlings) and vital rates in permanent plots (>2500 trees) from 1982 to 2017 to determine if differences in vital rates explained the shift from seedling bank dominance by subalpine fir (Abies lasiocarpa (Hook.) Nutt.) to codominance of the main canopy by subalpine fir and Engelmann spruce (Picea engelmannii Parry ex Engelm.). Higher rates of fir recruitment into the main canopy were balanced by equally high rates of mortality, whereas spruce exhibited higher rates of net population increase and longer residence time in the main canopy. Projections of future forest trajectories from seedling bank composition can be improved by considering species-specific differences in vital rates.