Simulation of Sapling Population Dynamics in Uneven-aged Picea abies Forests

Abstract

A series of mathematical equations describing the interrelationships between sapling recruitment, growth and mortality, and the density and ingrowth of saplings into the tree layer are presented. Assuming that average annual sapling recruitment, growth and mortality are constant over time, sapling density in a specified size class as well as ingrowth into the tree layer can be described as a geometric series of sapling recruitment. Sapling density and growth are readily measured, but not sapling recruitment and mortality. These processes can be estimated though. With no mortality the sum of the annual height increments in two adjacent size classes should be equal. With mortality among saplings, the sum should decrease over the size range. An equation for theoretical estimation of sapling mortality from the slope of decrease is suggested. With mortality known, sapling recruitment can also be estimated. To see how temporal changes in recruitment, growth and mortality affect sapling density and ingrowth into the tree layer over time, simulations were performed. For this a hypothetical sapling stand was constructed, based on data from permanent plots in central Sweden managed with single-tree selection. The simulations clearly demonstrated that sapling density by itself renders little information about the regeneration dynamics and cannot by itself be used to estimate ingrowth into the tree stratum. With annual mortality rates below 5% sapling growth had a large influence on both sapling density and ingrowth into the tree stratum. The equations suggested for estimating mortality and ingrowth gave reasonably accurate estimates. Results of the simulations are discussed with reference to recent studies of the stand dynamics of uneven-aged Norway spruce forests.